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id="toc-Advantages" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Advantages"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Advantages</span> </div> </a> <button aria-controls="toc-Advantages-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Advantages subsection</span> </button> <ul id="toc-Advantages-sublist" class="vector-toc-list"> <li id="toc-Resolution" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Resolution"> <div class="vector-toc-text"> <span class="vector-toc-numb">5.1</span> <span>Resolution</span> </div> </a> <ul id="toc-Resolution-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Motivation" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Motivation"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>Motivation</span> </div> </a> <ul id="toc-Motivation-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Components" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Components"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>Components</span> </div> </a> <button aria-controls="toc-Components-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Components subsection</span> </button> <ul id="toc-Components-sublist" class="vector-toc-list"> <li id="toc-IR_sources" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#IR_sources"> <div class="vector-toc-text"> <span class="vector-toc-numb">7.1</span> <span>IR sources</span> </div> </a> <ul id="toc-IR_sources-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Detectors" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Detectors"> <div class="vector-toc-text"> <span class="vector-toc-numb">7.2</span> <span>Detectors</span> </div> </a> <ul id="toc-Detectors-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Beam_splitter" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Beam_splitter"> <div class="vector-toc-text"> <span class="vector-toc-numb">7.3</span> <span>Beam splitter</span> </div> </a> <ul id="toc-Beam_splitter-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Attenuated_total_reflectance" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Attenuated_total_reflectance"> <div class="vector-toc-text"> <span class="vector-toc-numb">7.4</span> <span>Attenuated total reflectance</span> </div> </a> <ul id="toc-Attenuated_total_reflectance-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Fourier_transform" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Fourier_transform"> <div class="vector-toc-text"> <span class="vector-toc-numb">7.5</span> <span>Fourier transform</span> </div> </a> <ul id="toc-Fourier_transform-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Spectral_range" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Spectral_range"> <div class="vector-toc-text"> <span class="vector-toc-numb">8</span> <span>Spectral range</span> </div> </a> <button aria-controls="toc-Spectral_range-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Spectral range subsection</span> </button> <ul id="toc-Spectral_range-sublist" class="vector-toc-list"> <li id="toc-Far-infrared" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Far-infrared"> <div class="vector-toc-text"> <span class="vector-toc-numb">8.1</span> <span>Far-infrared</span> </div> </a> <ul id="toc-Far-infrared-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Mid-infrared" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Mid-infrared"> <div class="vector-toc-text"> <span class="vector-toc-numb">8.2</span> <span>Mid-infrared</span> </div> </a> <ul id="toc-Mid-infrared-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Near-infrared" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Near-infrared"> <div class="vector-toc-text"> <span class="vector-toc-numb">8.3</span> <span>Near-infrared</span> </div> </a> <ul id="toc-Near-infrared-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Applications" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Applications"> <div class="vector-toc-text"> <span class="vector-toc-numb">9</span> <span>Applications</span> </div> </a> <button aria-controls="toc-Applications-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Applications subsection</span> </button> <ul id="toc-Applications-sublist" class="vector-toc-list"> <li id="toc-Nano_and_biological_materials" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Nano_and_biological_materials"> <div class="vector-toc-text"> <span class="vector-toc-numb">9.1</span> <span>Nano and biological materials</span> </div> </a> <ul id="toc-Nano_and_biological_materials-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Microscopy_and_imaging" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Microscopy_and_imaging"> <div class="vector-toc-text"> <span class="vector-toc-numb">9.2</span> <span>Microscopy and imaging</span> </div> </a> <ul id="toc-Microscopy_and_imaging-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Nanoscale_and_spectroscopy_below_the_diffraction_limit" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Nanoscale_and_spectroscopy_below_the_diffraction_limit"> <div class="vector-toc-text"> <span class="vector-toc-numb">9.3</span> <span>Nanoscale and spectroscopy below the diffraction limit</span> </div> </a> <ul id="toc-Nanoscale_and_spectroscopy_below_the_diffraction_limit-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-FTIR_as_detector_in_chromatography" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#FTIR_as_detector_in_chromatography"> <div class="vector-toc-text"> <span class="vector-toc-numb">9.4</span> <span>FTIR as detector in chromatography</span> </div> </a> <ul id="toc-FTIR_as_detector_in_chromatography-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-TG-IR_(thermogravimetric_analysis-infrared_spectrometry)" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#TG-IR_(thermogravimetric_analysis-infrared_spectrometry)"> <div class="vector-toc-text"> <span class="vector-toc-numb">9.5</span> <span>TG-IR (thermogravimetric analysis-infrared spectrometry)</span> </div> </a> <ul id="toc-TG-IR_(thermogravimetric_analysis-infrared_spectrometry)-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Water_content_determination_in_plastics_and_composites" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Water_content_determination_in_plastics_and_composites"> <div class="vector-toc-text"> <span class="vector-toc-numb">9.6</span> <span>Water content determination in plastics and composites</span> </div> </a> <ul id="toc-Water_content_determination_in_plastics_and_composites-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> <span class="vector-toc-numb">10</span> <span>See also</span> </div> </a> <ul id="toc-See_also-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">11</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-External_links" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#External_links"> <div class="vector-toc-text"> <span class="vector-toc-numb">12</span> <span>External links</span> </div> </a> <ul id="toc-External_links-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" title="Table of Contents" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet 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Available in 16 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-16" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">16 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/%D9%85%D8%B7%D9%8A%D8%A7%D9%81%D9%8A%D8%A9_%D8%A7%D9%84%D8%A3%D8%B4%D8%B9%D8%A9_%D8%AA%D8%AD%D8%AA_%D8%A7%D9%84%D8%AD%D9%85%D8%B1%D8%A7%D8%A1_%D8%A8%D8%A7%D8%B3%D8%AA%D8%AE%D8%AF%D8%A7%D9%85_%D8%AA%D8%AD%D9%88%D9%8A%D9%84_%D9%81%D9%88%D8%B1%D9%8A%D9%8A%D9%87" title="مطيافية الأشعة تحت الحمراء باستخدام تحويل فورييه – Arabic" lang="ar" hreflang="ar" data-title="مطيافية الأشعة تحت الحمراء باستخدام تحويل فورييه" data-language-autonym="العربية" data-language-local-name="Arabic" class="interlanguage-link-target"><span>العربية</span></a></li><li class="interlanguage-link interwiki-bn mw-list-item"><a href="https://bn.wikipedia.org/wiki/%E0%A6%AB%E0%A7%81%E0%A6%B0%E0%A6%BF%E0%A6%AF%E0%A6%BC%E0%A7%87-%E0%A6%B0%E0%A7%82%E0%A6%AA%E0%A6%BE%E0%A6%A8%E0%A7%8D%E0%A6%A4%E0%A6%B0_%E0%A6%85%E0%A6%AC%E0%A6%B2%E0%A7%8B%E0%A6%B9%E0%A6%BF%E0%A6%A4_%E0%A6%AC%E0%A6%B0%E0%A7%8D%E0%A6%A3%E0%A6%BE%E0%A6%B2%E0%A7%80%E0%A6%AC%E0%A7%80%E0%A6%95%E0%A7%8D%E0%A6%B7%E0%A6%A3" title="ফুরিয়ে-রূপান্তর অবলোহিত বর্ণালীবীক্ষণ – Bangla" lang="bn" hreflang="bn" data-title="ফুরিয়ে-রূপান্তর অবলোহিত বর্ণালীবীক্ষণ" data-language-autonym="বাংলা" data-language-local-name="Bangla" class="interlanguage-link-target"><span>বাংলা</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Espectrosc%C3%B2pia_infraroja_per_transformada_de_Fourier" title="Espectroscòpia infraroja per transformada de Fourier – Catalan" lang="ca" hreflang="ca" data-title="Espectroscòpia infraroja per transformada de Fourier" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/Infra%C4%8Derven%C3%A1_spektroskopie_s_Fourierovou_transformac%C3%AD" title="Infračervená spektroskopie s Fourierovou transformací – Czech" lang="cs" hreflang="cs" data-title="Infračervená spektroskopie s Fourierovou transformací" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/FTIR-Spektrometer" title="FTIR-Spektrometer – German" lang="de" hreflang="de" data-title="FTIR-Spektrometer" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Espectrofot%C3%B3metro_de_transformada_de_Fourier" title="Espectrofotómetro de transformada de Fourier – Spanish" lang="es" hreflang="es" data-title="Espectrofotómetro de transformada de Fourier" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D8%B7%DB%8C%D9%81%E2%80%8C%D8%B3%D9%86%D8%AC%DB%8C_%D8%AA%D8%A8%D8%AF%DB%8C%D9%84_%D9%81%D9%88%D8%B1%DB%8C%D9%87_%D9%81%D8%B1%D9%88%D8%B3%D8%B1%D8%AE" title="طیف‌سنجی تبدیل فوریه فروسرخ – Persian" lang="fa" hreflang="fa" data-title="طیف‌سنجی تبدیل فوریه فروسرخ" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Spectroscopie_infrarouge_%C3%A0_transform%C3%A9e_de_Fourier" title="Spectroscopie infrarouge à transformée de Fourier – French" lang="fr" hreflang="fr" data-title="Spectroscopie infrarouge à transformée de Fourier" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%ED%91%B8%EB%A6%AC%EC%97%90_%EB%B3%80%ED%99%98_%EC%A0%81%EC%99%B8%EC%84%A0_%EB%B6%84%EA%B4%91%EB%B2%95" title="푸리에 변환 적외선 분광법 – Korean" lang="ko" hreflang="ko" data-title="푸리에 변환 적외선 분광법" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/Spektroskopi_inframerah_transformasi_Fourier" title="Spektroskopi inframerah transformasi Fourier – Indonesian" lang="id" hreflang="id" data-title="Spektroskopi inframerah transformasi Fourier" data-language-autonym="Bahasa Indonesia" data-language-local-name="Indonesian" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-he mw-list-item"><a href="https://he.wikipedia.org/wiki/%D7%A1%D7%A4%D7%A7%D7%98%D7%A8%D7%95%D7%A1%D7%A7%D7%95%D7%A4%D7%99%D7%99%D7%AA_%D7%A4%D7%95%D7%A8%D7%99%D7%99%D7%94" title="ספקטרוסקופיית פורייה – Hebrew" lang="he" hreflang="he" data-title="ספקטרוסקופיית פורייה" data-language-autonym="עברית" data-language-local-name="Hebrew" class="interlanguage-link-target"><span>עברית</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/Fourier-transzform%C3%A1ci%C3%B3s_infrav%C3%B6r%C3%B6s_spektroszk%C3%B3pia" title="Fourier-transzformációs infravörös spektroszkópia – Hungarian" lang="hu" hreflang="hu" data-title="Fourier-transzformációs infravörös spektroszkópia" data-language-autonym="Magyar" data-language-local-name="Hungarian" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E3%83%95%E3%83%BC%E3%83%AA%E3%82%A8%E5%A4%89%E6%8F%9B%E8%B5%A4%E5%A4%96%E5%88%86%E5%85%89%E6%B3%95" title="フーリエ変換赤外分光法 – Japanese" lang="ja" hreflang="ja" data-title="フーリエ変換赤外分光法" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-sv mw-list-item"><a href="https://sv.wikipedia.org/wiki/FTIR" title="FTIR – Swedish" lang="sv" hreflang="sv" data-title="FTIR" data-language-autonym="Svenska" data-language-local-name="Swedish" class="interlanguage-link-target"><span>Svenska</span></a></li><li class="interlanguage-link interwiki-tr mw-list-item"><a href="https://tr.wikipedia.org/wiki/Fourier_D%C3%B6n%C3%BC%C5%9F%C3%BCml%C3%BC_K%C4%B1z%C4%B1l%C3%B6tesi_Spektroskopisi" title="Fourier Dönüşümlü Kızılötesi Spektroskopisi – Turkish" lang="tr" hreflang="tr" data-title="Fourier Dönüşümlü Kızılötesi Spektroskopisi" data-language-autonym="Türkçe" data-language-local-name="Turkish" class="interlanguage-link-target"><span>Türkçe</span></a></li><li class="interlanguage-link interwiki-zh mw-list-item"><a href="https://zh.wikipedia.org/wiki/%E5%82%85%E9%87%8C%E8%91%89%E8%BD%89%E6%8F%9B%E7%B4%85%E5%A4%96%E5%85%89%E8%AD%9C" title="傅里葉轉換紅外光譜 – Chinese" lang="zh" hreflang="zh" data-title="傅里葉轉換紅外光譜" data-language-autonym="中文" data-language-local-name="Chinese" class="interlanguage-link-target"><span>中文</span></a></li> </ul> <div class="after-portlet after-portlet-lang"><span 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<div class="vector-body-before-content"> <div class="mw-indicators"> </div> <div id="siteSub" class="noprint">From Wikipedia, the free encyclopedia</div> </div> <div id="contentSub"><div id="mw-content-subtitle"><span class="mw-redirectedfrom">(Redirected from <a href="/w/index.php?title=FTIR&redirect=no" class="mw-redirect" title="FTIR">FTIR</a>)</span></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Technique to analyze the infrared spectrum of matter</div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">"FTIR" redirects here. The term may also refer to <a href="/wiki/Frustrated_total_internal_reflection" class="mw-redirect" title="Frustrated total internal reflection">Frustrated total internal reflection</a>.</div> <p><b>Fourier transform infrared spectroscopy</b> (<b>FTIR</b>)<sup id="cite_ref-Griffiths_1-0" class="reference"><a href="#cite_note-Griffiths-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> is a technique used to obtain an <a href="/wiki/Infrared" title="Infrared">infrared</a> <a href="/wiki/Electromagnetic_spectrum" title="Electromagnetic spectrum">spectrum</a> of <a href="/wiki/Absorption_(electromagnetic_radiation)" title="Absorption (electromagnetic radiation)">absorption</a> or <a href="/wiki/Emission_(electromagnetic_radiation)" class="mw-redirect" title="Emission (electromagnetic radiation)">emission</a> of a solid, liquid, or gas. An FTIR spectrometer simultaneously collects high-resolution spectral data over a wide spectral range. This confers a significant advantage over a <a href="/wiki/Dispersion_(optics)" title="Dispersion (optics)">dispersive</a> <a href="/wiki/Spectrometer" title="Spectrometer">spectrometer</a>, which measures intensity over a narrow range of <a href="/wiki/Wavelength" title="Wavelength">wavelengths</a> at a time. </p><p>The term <i>Fourier transform <a href="/wiki/Infrared_spectroscopy" title="Infrared spectroscopy">infrared spectroscopy</a></i> originates from the fact that a <a href="/wiki/Fourier_transform" title="Fourier transform">Fourier transform</a> (a mathematical process) is required to convert the raw data into the actual spectrum. </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:FTIR_Spectrometer_%2B_ATR.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/1/1b/FTIR_Spectrometer_%2B_ATR.jpg/250px-FTIR_Spectrometer_%2B_ATR.jpg" decoding="async" width="220" height="170" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/1/1b/FTIR_Spectrometer_%2B_ATR.jpg/330px-FTIR_Spectrometer_%2B_ATR.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/1/1b/FTIR_Spectrometer_%2B_ATR.jpg/500px-FTIR_Spectrometer_%2B_ATR.jpg 2x" data-file-width="776" data-file-height="599" /></a><figcaption>An example of an FTIR spectrometer with an <a href="/wiki/Attenuated_total_reflectance" title="Attenuated total reflectance">attenuated total reflectance</a> (ATR) attachment</figcaption></figure> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Conceptual_introduction">Conceptual introduction</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=1" title="Edit section: Conceptual introduction"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1251242444">.mw-parser-output .ambox{border:1px solid #a2a9b1;border-left:10px solid #36c;background-color:#fbfbfb;box-sizing:border-box}.mw-parser-output .ambox+link+.ambox,.mw-parser-output .ambox+link+style+.ambox,.mw-parser-output .ambox+link+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+style+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+link+.ambox{margin-top:-1px}html body.mediawiki .mw-parser-output .ambox.mbox-small-left{margin:4px 1em 4px 0;overflow:hidden;width:238px;border-collapse:collapse;font-size:88%;line-height:1.25em}.mw-parser-output .ambox-speedy{border-left:10px solid #b32424;background-color:#fee7e6}.mw-parser-output .ambox-delete{border-left:10px solid #b32424}.mw-parser-output .ambox-content{border-left:10px solid #f28500}.mw-parser-output .ambox-style{border-left:10px solid #fc3}.mw-parser-output .ambox-move{border-left:10px solid #9932cc}.mw-parser-output .ambox-protection{border-left:10px solid #a2a9b1}.mw-parser-output .ambox .mbox-text{border:none;padding:0.25em 0.5em;width:100%}.mw-parser-output .ambox .mbox-image{border:none;padding:2px 0 2px 0.5em;text-align:center}.mw-parser-output .ambox .mbox-imageright{border:none;padding:2px 0.5em 2px 0;text-align:center}.mw-parser-output .ambox .mbox-empty-cell{border:none;padding:0;width:1px}.mw-parser-output .ambox .mbox-image-div{width:52px}@media(min-width:720px){.mw-parser-output .ambox{margin:0 10%}}@media print{body.ns-0 .mw-parser-output .ambox{display:none!important}}</style><table class="box-More_citations_needed_section plainlinks metadata ambox ambox-content ambox-Refimprove" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><a href="/wiki/File:Question_book-new.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/60px-Question_book-new.svg.png" decoding="async" width="50" height="39" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/120px-Question_book-new.svg.png 1.5x" data-file-width="512" data-file-height="399" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This section <b>needs additional citations for <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">verification</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Special:EditPage/Fourier-transform_infrared_spectroscopy" title="Special:EditPage/Fourier-transform infrared spectroscopy">improve this article</a> by <a href="/wiki/Help:Referencing_for_beginners" title="Help:Referencing for beginners">adding citations to reliable sources</a> in this section. Unsourced material may be challenged and removed.</span> <span class="date-container"><i>(<span class="date">June 2022</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table><figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:FTIR-interferogram.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/d/d4/FTIR-interferogram.svg/250px-FTIR-interferogram.svg.png" decoding="async" width="220" height="163" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/d/d4/FTIR-interferogram.svg/330px-FTIR-interferogram.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/d/d4/FTIR-interferogram.svg/500px-FTIR-interferogram.svg.png 2x" data-file-width="969" data-file-height="720" /></a><figcaption>An FTIR interferogram. The central peak is at the ZPD position ("zero path difference" or zero retardation), where the maximal amount of light passes through the <a href="/wiki/Michelson_interferometer" title="Michelson interferometer">interferometer</a> to the detector.</figcaption></figure> <p>The goal of <a href="/wiki/Absorption_spectroscopy" title="Absorption spectroscopy">absorption spectroscopy</a> techniques (FTIR, <a href="/wiki/Ultraviolet-visible_spectroscopy" class="mw-redirect" title="Ultraviolet-visible spectroscopy">ultraviolet-visible ("UV-vis") spectroscopy</a>, etc.) is to measure how much light a sample absorbs at each wavelength.<sup id="cite_ref-:0_2-0" class="reference"><a href="#cite_note-:0-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> The most straightforward way to do this, the "dispersive spectroscopy" technique, is to shine a <a href="/wiki/Monochromatic" class="mw-redirect" title="Monochromatic">monochromatic</a> light beam at a sample, measure how much of the light is absorbed, and repeat for each different wavelength. (This is how some <a href="/wiki/Ultraviolet%E2%80%93visible_spectroscopy" title="Ultraviolet–visible spectroscopy">UV–vis spectrometers</a> work, for example.) </p><p>Fourier transform spectroscopy is a less intuitive way to obtain the same information. Rather than shining a <a href="/wiki/Monochromatic" class="mw-redirect" title="Monochromatic">monochromatic</a> beam of light (a beam composed of only a single wavelength) at the sample, this technique shines a beam containing many frequencies of light at once and measures how much of that beam is absorbed by the sample. Next, the beam is modified to contain a different combination of frequencies, giving a second data point. This process is rapidly repeated many times over a short time span. Afterwards, a computer takes all this data and works backward to infer what the absorption is at each wavelength.<sup id="cite_ref-:0_2-1" class="reference"><a href="#cite_note-:0-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> </p><p>The beam described above is generated by starting with a broadband light source—one containing the full spectrum of wavelengths to be measured. The light shines into a <a href="/wiki/Michelson_interferometer" title="Michelson interferometer">Michelson interferometer</a>—a certain configuration of mirrors, one of which is moved by a motor. As this mirror moves, each wavelength of light in the beam is periodically blocked, transmitted, blocked, transmitted, by the interferometer, due to <a href="/wiki/Wave_interference" title="Wave interference">wave interference</a>. Different wavelengths are modulated at different rates, so that at each moment or mirror position the beam coming out of the interferometer has a different spectrum.<sup id="cite_ref-:0_2-2" class="reference"><a href="#cite_note-:0-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> </p><p>As mentioned, computer processing is required to turn the raw data (light absorption for each mirror position) into the desired result (light absorption for each wavelength).<sup id="cite_ref-:0_2-3" class="reference"><a href="#cite_note-:0-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> The processing required turns out to be a common algorithm called the <a href="/wiki/Fourier_transform" title="Fourier transform">Fourier transform</a>. The Fourier transform converts one domain (in this case displacement of the mirror in cm) into its inverse domain (wavenumbers in cm<sup>−1</sup>). The raw data is called an "interferogram". </p> <div class="mw-heading mw-heading2"><h2 id="History">History</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=2" title="Edit section: History"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The first low-cost <a href="/wiki/Spectrophotometer" class="mw-redirect" title="Spectrophotometer">spectrophotometer</a> capable of recording an <a href="/wiki/Infrared_spectroscopy" title="Infrared spectroscopy">infrared spectrum</a> was the <a href="/wiki/Perkin-Elmer" class="mw-redirect" title="Perkin-Elmer">Perkin-Elmer</a> Infracord produced in 1957.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> This instrument covered the wavelength range from 2.5 μm to 15 μm (<a href="/wiki/Wavenumber" title="Wavenumber">wavenumber</a> range 4,000 cm<sup>−1</sup> to 660 cm<sup>−1</sup>). The lower wavelength limit was chosen to encompass the highest known vibration frequency due to a fundamental <a href="/wiki/Molecular_vibration" title="Molecular vibration">molecular vibration</a>. The upper limit was imposed by the fact that the <a href="/wiki/Dispersion_(optics)" title="Dispersion (optics)">dispersing element</a> was a <a href="/wiki/Dispersive_prism" title="Dispersive prism">prism</a> made from a single crystal of rock-salt (<a href="/wiki/Sodium_chloride" title="Sodium chloride">sodium chloride</a>), which becomes opaque at wavelengths longer than about 15 μm; this spectral region became known as the rock-salt region. Later instruments used <a href="/wiki/Potassium_bromide" title="Potassium bromide">potassium bromide</a> prisms to extend the range to 25 μm (400 cm<sup>−1</sup>) and <a href="/wiki/Caesium_iodide" title="Caesium iodide">caesium iodide</a> 50 μm (200 cm<sup>−1</sup>). The region beyond 50 μm (200 cm<sup>−1</sup>) became known as the far-infrared region; at very long wavelengths it merges into the <a href="/wiki/Microwave" title="Microwave">microwave</a> region. Measurements in the far infrared needed the development of accurately ruled <a href="/wiki/Diffraction_grating" title="Diffraction grating">diffraction gratings</a> to replace the prisms as dispersing elements, since salt crystals are opaque in this region. More sensitive detectors than the <a href="/wiki/Bolometer" title="Bolometer">bolometer</a> were required because of the low energy of the radiation. One such was the <a href="/wiki/Golay_detector" class="mw-redirect" title="Golay detector">Golay detector</a>. An additional issue is the need to exclude atmospheric <a href="/wiki/Water_vapour" class="mw-redirect" title="Water vapour">water vapour</a> because water vapour has an intense pure <a href="/wiki/Rotational_spectrum" class="mw-redirect" title="Rotational spectrum">rotational spectrum</a> in this region. Far-infrared spectrophotometers were cumbersome, slow and expensive. The advantages of the <a href="/wiki/Michelson_interferometer" title="Michelson interferometer">Michelson interferometer</a> were well-known, but considerable technical difficulties had to be overcome before a commercial instrument could be built. Also an electronic computer was needed to perform the required Fourier transform, and this only became practicable with the advent of <a href="/wiki/Minicomputer" title="Minicomputer">minicomputers</a>, such as the <a href="/wiki/PDP-8" title="PDP-8">PDP-8</a>, which became available in 1965. Digilab pioneered the world's first commercial FTIR spectrometer (Model FTS-14) in 1969.<sup id="cite_ref-Griffiths_1-1" class="reference"><a href="#cite_note-Griffiths-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> Digilab FTIRs are now a part of Agilent Technologies's molecular product line after Agilent acquired spectroscopy business from <a href="/wiki/Varian,_Inc." title="Varian, Inc.">Varian</a>.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Agilent2010_5-0" class="reference"><a href="#cite_note-Agilent2010-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Michelson_interferometer">Michelson interferometer</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=3" title="Edit section: Michelson interferometer"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1251242444" /><table class="box-More_citations_needed_section plainlinks metadata ambox ambox-content ambox-Refimprove" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><a href="/wiki/File:Question_book-new.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/60px-Question_book-new.svg.png" decoding="async" width="50" height="39" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/120px-Question_book-new.svg.png 1.5x" data-file-width="512" data-file-height="399" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This section <b>needs additional citations for <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">verification</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Special:EditPage/Fourier-transform_infrared_spectroscopy" title="Special:EditPage/Fourier-transform infrared spectroscopy">improve this article</a> by <a href="/wiki/Help:Referencing_for_beginners" title="Help:Referencing for beginners">adding citations to reliable sources</a> in this section. Unsourced material may be challenged and removed.</span> <span class="date-container"><i>(<span class="date">June 2022</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951" /><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Michelson_interferometer" title="Michelson interferometer">Michelson interferometer</a></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:FTIR_Interferometer.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/a/a1/FTIR_Interferometer.png/330px-FTIR_Interferometer.png" decoding="async" width="280" height="271" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/a1/FTIR_Interferometer.png/500px-FTIR_Interferometer.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/a/a1/FTIR_Interferometer.png/960px-FTIR_Interferometer.png 2x" data-file-width="2131" data-file-height="2065" /></a><figcaption>Schematic diagram of a Michelson interferometer, configured for FTIR</figcaption></figure> <p>In a <a href="/wiki/Michelson_interferometer" title="Michelson interferometer">Michelson interferometer</a> adapted for FTIR, light from the polychromatic infrared source, approximately a <a href="/wiki/Black-body" class="mw-redirect" title="Black-body">black-body</a> radiator, is <a href="/wiki/Collimated" class="mw-redirect" title="Collimated">collimated</a> and directed to a <a href="/wiki/Beam_splitter" title="Beam splitter">beam splitter</a>. Ideally 50% of the light is refracted towards the fixed mirror and 50% is transmitted towards the moving mirror. Light is reflected from the two mirrors back to the beam splitter and some fraction of the original light passes into the sample compartment. There, the light is focused on the sample. On leaving the sample compartment the light is refocused on to the detector. The difference in optical path length between the two arms to the interferometer is known as the <i>retardation</i> or <i>optical path difference</i> (OPD). An interferogram is obtained by varying the retardation and recording the signal from the detector for various values of the <a href="https://en.wiktionary.org/wiki/retardation" class="extiw" title="wikt:retardation">retardation</a>. The form of the interferogram when no sample is present depends on factors such as the variation of source intensity and splitter efficiency with wavelength. This results in a maximum at zero retardation, when there is <a href="/wiki/Constructive_interference" class="mw-redirect" title="Constructive interference">constructive interference</a> at all wavelengths, followed by series of "wiggles". The position of zero retardation is determined accurately by finding the point of maximum intensity in the interferogram. When a sample is present the background interferogram is modulated by the presence of absorption bands in the sample.<sup id="cite_ref-:0_2-4" class="reference"><a href="#cite_note-:0-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> </p><p>Commercial spectrometers use Michelson interferometers with a variety of scanning mechanisms to generate the path difference. Common to all these arrangements is the need to ensure that the two beams recombine exactly as the system scans. The simplest systems have a plane mirror that moves linearly to vary the path of one beam. In this arrangement the moving mirror must not tilt or wobble as this would affect how the beams overlap as they recombine. Some systems incorporate a compensating mechanism that automatically adjusts the orientation of one mirror to maintain the alignment. Arrangements that avoid this problem include using cube corner reflectors instead of plane mirrors as these have the property of returning any incident beam in a parallel direction regardless of orientation. </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Interferometer_schematics.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/8/8e/Interferometer_schematics.jpg/250px-Interferometer_schematics.jpg" decoding="async" width="220" height="98" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/8/8e/Interferometer_schematics.jpg/330px-Interferometer_schematics.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/8/8e/Interferometer_schematics.jpg/500px-Interferometer_schematics.jpg 2x" data-file-width="1096" data-file-height="487" /></a><figcaption>Interferometer schematics where the path difference is generated by a rotary motion.</figcaption></figure> <p>Systems where the path difference is generated by a rotary movement have proved very successful. One common system incorporates a pair of parallel mirrors in one beam that can be rotated to vary the path without displacing the returning beam. Another is the double pendulum design where the path in one arm of the interferometer increases as the path in the other decreases. </p><p>A quite different approach involves moving a wedge of an IR-transparent material such as <a href="/wiki/Potassium_bromide" title="Potassium bromide">KBr</a> into one of the beams. Increasing the thickness of KBr in the beam increases the optical path because the refractive index is higher than that of air. One limitation of this approach is that the variation of refractive index over the wavelength range limits the accuracy of the wavelength calibration. </p> <div class="mw-heading mw-heading2"><h2 id="Measuring_and_processing_the_interferogram">Measuring and processing the interferogram</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=4" title="Edit section: Measuring and processing the interferogram"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The interferogram has to be measured from zero path difference to a maximum length that depends on the resolution required. In practice the scan can be on either side of zero resulting in a double-sided interferogram. Mechanical design limitations may mean that for the highest resolution the scan runs to the maximum OPD on one side of zero only. </p><p>The interferogram is converted to a spectrum by Fourier transformation. This requires it to be stored in digital form as a series of values at equal intervals of the path difference between the two beams. To measure the path difference a laser beam is sent through the interferometer, generating a sinusoidal signal where the separation between successive maxima is equal to the wavelength of the laser (typically a 633 nm <a href="/wiki/Helium%E2%80%93neon_laser" title="Helium–neon laser">HeNe laser</a> is used). This can trigger an <a href="/wiki/Analog-to-digital_converter" title="Analog-to-digital converter">analog-to-digital converter</a> to measure the IR signal each time the laser signal passes through zero. Alternatively, the laser and IR signals can be measured synchronously at smaller intervals with the IR signal at points corresponding to the laser signal zero crossing being determined by interpolation.<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> This approach allows the use of analog-to-digital converters that are more accurate and precise than converters that can be triggered, resulting in lower noise. </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Digitisation_of_the_interferogram.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/5/52/Digitisation_of_the_interferogram.png/330px-Digitisation_of_the_interferogram.png" decoding="async" width="300" height="208" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/5/52/Digitisation_of_the_interferogram.png/500px-Digitisation_of_the_interferogram.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/5/52/Digitisation_of_the_interferogram.png/960px-Digitisation_of_the_interferogram.png 2x" data-file-width="1230" data-file-height="854" /></a><figcaption>Values of the interferogram at times corresponding to zero crossings of the laser signal are found by interpolation.</figcaption></figure> <p>The result of Fourier transformation is a spectrum of the signal at a series of discrete wavelengths. The range of wavelengths that can be used in the calculation is limited by the separation of the data points in the interferogram. The shortest wavelength that can be recognized is twice the separation between these data points. For example, with one point per wavelength of a HeNe reference laser at <span class="nowrap"><span data-sort-value="6993633000000000000♠"></span>0.633 μm</span> (<span class="nowrap"><span data-sort-value="7004158000000000000♠"></span>15<span style="margin-left:.25em;">800</span> cm<sup>−1</sup></span>) the shortest wavelength would be <span class="nowrap"><span data-sort-value="6994126600000000000♠"></span>1.266 μm</span> (<span class="nowrap"><span data-sort-value="7003790000000000000♠"></span>7900 cm<sup>−1</sup></span>). Because of <a href="/wiki/Aliasing" title="Aliasing">aliasing</a>, any energy at shorter wavelengths would be interpreted as coming from longer wavelengths and so has to be minimized optically or electronically. The spectral resolution, i.e. the separation between wavelengths that can be distinguished, is determined by the maximum OPD. The wavelengths used in calculating the Fourier transform are such that an exact number of wavelengths fit into the length of the interferogram from zero to the maximum OPD as this makes their contributions orthogonal. This results in a spectrum with points separated by equal frequency intervals. </p><p>For a maximum path difference <span class="texhtml mvar" style="font-style:italic;">d</span> adjacent wavelengths <span class="texhtml mvar" style="font-style:italic;">λ<sub>1</sub></span> and <span class="texhtml mvar" style="font-style:italic;">λ<sub>2</sub></span> will have <span class="texhtml mvar" style="font-style:italic;">n</span> and <span class="texhtml mvar" style="font-style:italic;">(n+1)</span> cycles, respectively, in the interferogram. The corresponding frequencies are ν<sub>1</sub> and ν<sub>2</sub>: </p> <dl><dd><dl><dd><table> <tbody><tr> <td>d = nλ<sub>1</sub></td> <td>and d = (n+1)λ<sub>2</sub> </td></tr> <tr> <td>λ<sub>1</sub> = d/n</td> <td>and λ<sub>2</sub> =d/(n+1) </td></tr> <tr> <td>ν<sub>1</sub> = 1/λ<sub>1</sub></td> <td>and ν<sub>2</sub> = 1/λ<sub>2</sub> </td></tr> <tr> <td>ν<sub>1</sub> = n/d</td> <td>and ν<sub>2</sub> = (n+1)/d </td></tr> <tr> <td colspan="2">ν<sub>2</sub> − ν<sub>1</sub> = 1/d </td></tr></tbody></table></dd></dl></dd></dl> <p>The separation is the inverse of the maximum OPD. For example, a maximum OPD of 2 cm results in a separation of <span class="nowrap"><span data-sort-value="6999500000000000000♠"></span>0.5 cm<sup>−1</sup></span>. This is the spectral resolution in the sense that the value at one point is independent of the values at adjacent points. Most instruments can be operated at different resolutions by choosing different OPD's. Instruments for routine analyses typically have a best resolution of around <span class="nowrap"><span data-sort-value="6999500000000000000♠"></span>0.5 cm<sup>−1</sup></span>, while spectrometers have been built with resolutions as high as <span class="nowrap"><span data-sort-value="6997100000000000000♠"></span>0.001 cm<sup>−1</sup></span>, corresponding to a maximum OPD of 10 m. The point in the interferogram corresponding to zero path difference has to be identified, commonly by assuming it is where the maximum signal occurs. This so-called centerburst is not always symmetrical in real world spectrometers so a phase correction may have to be calculated. The interferogram signal decays as the path difference increases, the rate of decay being inversely related to the width of features in the spectrum. If the OPD is not large enough to allow the interferogram signal to decay to a negligible level there will be unwanted oscillations or sidelobes associated with the features in the resulting spectrum. To reduce these sidelobes the interferogram is usually multiplied by a function that approaches zero at the maximum OPD. This so-called <a href="/wiki/Apodization" title="Apodization">apodization</a> reduces the amplitude of any sidelobes and also the noise level at the expense of some reduction in resolution. </p><p>For <a href="/wiki/Fast_Fourier_transform" title="Fast Fourier transform">rapid calculation</a> the number of points in the interferogram has to equal a power of two. A string of zeroes may be added to the measured interferogram to achieve this. More zeroes may be added in a process called zero filling to improve the appearance of the final spectrum although there is no improvement in resolution. Alternatively, interpolation after the Fourier transform gives a similar result.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (September 2019)">citation needed</span></a></i>]</sup> </p> <div class="mw-heading mw-heading2"><h2 id="Advantages">Advantages</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=5" title="Edit section: Advantages"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>There are three principal advantages for an FT spectrometer compared to a scanning (dispersive) spectrometer.<sup id="cite_ref-Griffiths_1-2" class="reference"><a href="#cite_note-Griffiths-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p> <ol><li>The multiplex or <a href="/wiki/Fellgett%27s_advantage" title="Fellgett's advantage">Fellgett's advantage</a> (named after <a href="/wiki/Peter_Fellgett" title="Peter Fellgett">Peter Fellgett</a>). This arises from the fact that information from all wavelengths is collected simultaneously. It results in a higher <a href="/wiki/Signal-to-noise_ratio" title="Signal-to-noise ratio">signal-to-noise ratio</a> for a given scan-time for observations limited by a fixed detector noise contribution (typically in the thermal infrared spectral region where a <a href="/wiki/Photodetector" title="Photodetector">photodetector</a> is limited by <a href="/wiki/Generation-recombination_noise" class="mw-redirect" title="Generation-recombination noise">generation-recombination noise</a>). For a spectrum with <i>m</i> resolution elements, this increase is equal to the square root of <i>m</i>. Alternatively, it allows a shorter scan-time for a given resolution. In practice multiple scans are often averaged, increasing the signal-to-noise ratio by the square root of the number of scans.</li> <li>The throughput or Jacquinot's advantage (named after <a href="/wiki/Pierre_Jacquinot" title="Pierre Jacquinot">Pierre Jacquinot</a>). This results from the fact that in a dispersive instrument, the <a href="/wiki/Monochromator" title="Monochromator">monochromator</a> has entrance and exit slits which restrict the amount of light that passes through it. The interferometer throughput is determined only by the diameter of the collimated beam coming from the source. Although no slits are needed, FTIR spectrometers do require an aperture to restrict the convergence of the collimated beam in the interferometer. This is because convergent rays are modulated at different frequencies as the path difference is varied. Such an aperture is called a Jacquinot stop.<sup id="cite_ref-Griffiths_1-3" class="reference"><a href="#cite_note-Griffiths-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> For a given resolution and wavelength this circular aperture allows more light through than a slit, resulting in a higher signal-to-noise ratio.</li> <li>The wavelength accuracy or Connes' advantage (named after <a href="/wiki/Janine_Connes" title="Janine Connes">Janine Connes</a>). The wavelength scale is calibrated by a laser beam of known wavelength that passes through the interferometer. This is much more stable and accurate than in dispersive instruments where the scale depends on the mechanical movement of diffraction gratings. In practice, the accuracy is limited by the divergence of the beam in the interferometer which depends on the resolution.</li></ol> <p>Another minor advantage is less sensitivity to stray light, that is radiation of one wavelength appearing at another wavelength in the spectrum. In dispersive instruments, this is the result of imperfections in the diffraction gratings and accidental reflections. In FT instruments there is no direct equivalent as the apparent wavelength is determined by the modulation frequency in the interferometer. </p> <div class="mw-heading mw-heading3"><h3 id="Resolution">Resolution</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=6" title="Edit section: Resolution"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The interferogram belongs in the length dimension. <a href="/wiki/Fourier_transform" title="Fourier transform">Fourier transform</a> (FT) inverts the dimension, so the FT of the interferogram belongs in the reciprocal length dimension([L<sup>−1</sup>]), that is the dimension of <a href="/wiki/Wavenumber" title="Wavenumber">wavenumber</a>. The <a href="/wiki/Spectral_resolution" title="Spectral resolution">spectral resolution</a> in cm<sup>−1</sup> is equal to the reciprocal of the maximal retardation in cm. Thus a 4 cm<sup>−1</sup> resolution will be obtained if the maximal retardation is 0.25 cm; this is typical of the cheaper FTIR instruments. Much higher resolution can be obtained by increasing the maximal retardation. This is not easy, as the moving mirror must travel in a near-perfect straight line. The use of <a href="/wiki/Corner_reflector" title="Corner reflector">corner-cube</a> mirrors in place of the flat mirrors is helpful, as an outgoing ray from a corner-cube mirror is parallel to the incoming ray, regardless of the orientation of the mirror about axes perpendicular to the axis of the light beam. </p><p>A spectrometer with 0.001 cm<sup>−1</sup> resolution is now available commercially. The throughput advantage is important for high-resolution FTIR, as the monochromator in a dispersive instrument with the same resolution would have very narrow <a href="/wiki/Monochromator#Czerny-Turner_monochromator" title="Monochromator">entrance and exit slits</a>. </p><p>In 1966 <a href="/wiki/Janine_Connes" title="Janine Connes">Janine Connes</a> measured the temperature of the atmosphere of <a href="/wiki/Venus" title="Venus">Venus</a> by recording the <a href="/wiki/Rovibrational_coupling" class="mw-redirect" title="Rovibrational coupling">vibration-rotation spectrum</a> of Venusian CO<sub>2</sub> at 0.1 cm<sup>−1</sup> resolution.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> <a href="/wiki/Albert_Abraham_Michelson" class="mw-redirect" title="Albert Abraham Michelson">Michelson</a> himself attempted to resolve the hydrogen <a href="/wiki/H-alpha" class="mw-redirect" title="H-alpha">H<sub>α</sub> emission band</a> in the spectrum of a <a href="/wiki/Hydrogen" title="Hydrogen">hydrogen</a> atom into its two components by using his interferometer.<sup id="cite_ref-Griffiths_1-4" class="reference"><a href="#cite_note-Griffiths-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup> p25</sup> </p> <div class="mw-heading mw-heading2"><h2 id="Motivation">Motivation</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=7" title="Edit section: Motivation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>FTIR is a method of measuring infrared absorption and emission spectra. For a discussion of <i>why</i> people measure infrared absorption and emission spectra, i.e. why and how substances absorb and emit infrared light, see the article: <a href="/wiki/Infrared_spectroscopy" title="Infrared spectroscopy">Infrared spectroscopy</a>. </p> <div class="mw-heading mw-heading2"><h2 id="Components">Components</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=8" title="Edit section: Components"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Fourier_transform_spectrometer.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/1/12/Fourier_transform_spectrometer.png/330px-Fourier_transform_spectrometer.png" decoding="async" width="280" height="261" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/1/12/Fourier_transform_spectrometer.png 1.5x" data-file-width="391" data-file-height="365" /></a><figcaption>FTIR setup. The sample is placed right before the detector.</figcaption></figure> <div class="mw-heading mw-heading3"><h3 id="IR_sources">IR sources</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=9" title="Edit section: IR sources"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>FTIR spectrometers are mostly used for measurements in the mid and near IR regions. For the mid-IR region, 2−25 μm (5,000–400 cm<sup>−1</sup>), the most common source is a <a href="/wiki/Silicon_carbide" title="Silicon carbide">silicon carbide (SiC)</a> element heated to about 1,200 K (930 °C; 1,700 °F) (<a href="/wiki/Globar" title="Globar">Globar</a>). The output is similar to a blackbody. Shorter wavelengths of the near-IR, 1−2.5 μm (10,000–4,000 cm<sup>−1</sup>), require a higher temperature source, typically a tungsten-halogen lamp. The long wavelength output of these is limited to about 5 μm (2,000 cm<sup>−1</sup>) by the absorption of the quartz envelope. For the far-IR, especially at wavelengths beyond 50 μm (200 cm<sup>−1</sup>) a mercury discharge lamp gives higher output than a thermal source.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Detectors">Detectors</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=10" title="Edit section: Detectors"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Far-IR spectrometers commonly use <a href="/wiki/Pyroelectric_detector" class="mw-redirect" title="Pyroelectric detector">pyroelectric detectors</a> that respond to changes in temperature as the intensity of IR radiation falling on them varies. The sensitive elements in these detectors are either deuterated triglycine sulfate (DTGS) or lithium tantalate (LiTaO<sub>3</sub>). These detectors operate at ambient temperatures and provide adequate sensitivity for most routine applications. To achieve the best sensitivity the time for a scan is typically a few seconds. Cooled photoelectric detectors are employed for situations requiring higher sensitivity or faster response. Liquid nitrogen cooled mercury cadmium telluride (MCT) detectors are the most widely used in the mid-IR. With these detectors an interferogram can be measured in as little as 10 milliseconds. Uncooled indium gallium arsenide photodiodes or DTGS are the usual choices in near-IR systems. Very sensitive liquid-helium-cooled silicon or germanium bolometers are used in the far-IR where both sources and beamsplitters are inefficient.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (June 2022)">citation needed</span></a></i>]</sup> </p> <div class="mw-heading mw-heading3"><h3 id="Beam_splitter">Beam splitter</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=11" title="Edit section: Beam splitter"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Beam-splitter.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/7/7e/Beam-splitter.png/330px-Beam-splitter.png" decoding="async" width="280" height="190" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/7/7e/Beam-splitter.png/500px-Beam-splitter.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/7/7e/Beam-splitter.png/960px-Beam-splitter.png 2x" data-file-width="4209" data-file-height="2859" /></a><figcaption>Simple interferometer with a beam-splitter and compensator plate</figcaption></figure> <p>An ideal beam-splitter transmits and reflects 50% of the incident radiation. However, as any material has a limited range of optical transmittance, several beam-splitters may be used interchangeably to cover a wide spectral range. </p><p>In a simple Michelson interferometer, one beam passes twice through the beamsplitter but the other passes through only once. To correct for this, an additional compensator plate of equal thickness is incorporated. </p><p>For the mid-IR region, the beamsplitter is usually made of KBr with a germanium-based coating that makes it semi-reflective. KBr absorbs strongly at wavelengths beyond 25 μm (400 cm<sup>−1</sup>), so <a href="/wiki/Caesium_iodide" title="Caesium iodide">CsI</a> or <a href="/wiki/Thallium_halides" title="Thallium halides">KRS-5</a> are sometimes used to extend the range to about 50 μm (200 cm<sup>−1</sup>). ZnSe is an alternative where moisture vapour can be a problem, but is limited to about 20 μm (500 cm<sup>−1</sup>). </p><p>CaF<sub>2</sub> is the usual material for the near-IR, being both harder and less sensitive to moisture than KBr, but cannot be used beyond about 8 μm (1,200 cm<sup>−1</sup>). </p><p>Far-IR beamsplitters are mostly based on polymer films, and cover a limited wavelength range.<sup id="cite_ref-Griffiths2_9-0" class="reference"><a href="#cite_note-Griffiths2-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Attenuated_total_reflectance">Attenuated total reflectance</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=12" title="Edit section: Attenuated total reflectance"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951" /><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Attenuated_total_reflectance" title="Attenuated total reflectance">Attenuated total reflectance</a></div> <p><a href="/wiki/Attenuated_total_reflectance" title="Attenuated total reflectance">Attenuated total reflectance</a> (ATR) is one accessory of FTIR spectrophotometer to measure surface properties of solid or thin film samples rather than their bulk properties. Generally, ATR has a penetration depth of around 1 or 2 micrometers depending on sample conditions. </p> <div class="mw-heading mw-heading3"><h3 id="Fourier_transform">Fourier transform</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=13" title="Edit section: Fourier transform"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The interferogram in practice consists of a set of intensities measured for discrete values of retardation. The difference between successive retardation values is constant. Thus, a <a href="/wiki/Discrete_Fourier_transform" title="Discrete Fourier transform">discrete Fourier transform</a> is needed. The <a href="/wiki/Fast_Fourier_transform" title="Fast Fourier transform">fast Fourier transform</a> (FFT) algorithm is used. </p> <div class="mw-heading mw-heading2"><h2 id="Spectral_range">Spectral range</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=14" title="Edit section: Spectral range"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading3"><h3 id="Far-infrared">Far-infrared</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=15" title="Edit section: Far-infrared"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The first FTIR spectrometers were developed for far-infrared range. The reason for this has to do with the mechanical tolerance needed for good optical performance, which is related to the wavelength of the light being used. For the relatively long wavelengths of the far infrared, ~10 μm tolerances are adequate, whereas for the rock-salt region tolerances have to be better than 1 μm. A typical instrument was the cube interferometer developed at the <a href="/wiki/National_Physical_Laboratory_(United_Kingdom)" title="National Physical Laboratory (United Kingdom)">NPL</a><sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> and marketed by <a href="/wiki/Grubb_Parsons" title="Grubb Parsons">Grubb Parsons</a>. It used a stepper motor to drive the moving mirror, recording the detector response after each step was completed. </p> <div class="mw-heading mw-heading3"><h3 id="Mid-infrared">Mid-infrared</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=16" title="Edit section: Mid-infrared"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>With the advent of cheap <a href="/wiki/Microcomputer" title="Microcomputer">microcomputers</a> it became possible to have a computer dedicated to controlling the spectrometer, collecting the data, doing the Fourier transform and presenting the spectrum. This provided the impetus for the development of FTIR spectrometers for the rock-salt region. The problems of manufacturing ultra-high precision optical and mechanical components had to be solved. A wide range of instruments are now available commercially. Although instrument design has become more sophisticated, the basic principles remain the same. Nowadays, the moving mirror of the interferometer moves at a constant velocity, and sampling of the interferogram is triggered by finding zero-crossings in the fringes of a secondary interferometer lit by a <a href="/wiki/Helium%E2%80%93neon_laser" title="Helium–neon laser">helium–neon laser</a>. In modern FTIR systems the constant mirror velocity is not strictly required, as long as the laser fringes and the original interferogram are recorded simultaneously with higher sampling rate and then re-interpolated on a constant grid, as pioneered by <a href="/wiki/James_W._Brault" title="James W. Brault">James W. Brault</a>. This confers very high wavenumber accuracy on the resulting infrared spectrum and avoids wavenumber <a href="/wiki/Calibration" title="Calibration">calibration</a> errors. </p> <div class="mw-heading mw-heading3"><h3 id="Near-infrared">Near-infrared</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=17" title="Edit section: Near-infrared"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951" /><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Near-infrared_spectroscopy" title="Near-infrared spectroscopy">Near-infrared spectroscopy</a></div> <p>The near-infrared region spans the wavelength range between the rock-salt region and the start of the <a href="/wiki/Visible_spectrum" title="Visible spectrum">visible</a> region at about 750 nm. <a href="/wiki/Overtones" class="mw-redirect" title="Overtones">Overtones</a> of fundamental vibrations can be observed in this region. It is used mainly in industrial applications such as <a href="/wiki/Process_control" class="mw-redirect" title="Process control">process control</a> and <a href="/wiki/Chemical_imaging" title="Chemical imaging">chemical imaging</a>. </p> <div class="mw-heading mw-heading2"><h2 id="Applications">Applications</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=18" title="Edit section: Applications"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>FTIR can be used in all applications where a dispersive spectrometer was used in the past (see external links). In addition, the improved sensitivity and speed have opened up new areas of application. Spectra can be measured in situations where very little energy reaches the detector. Fourier transform infrared spectroscopy is used in <a href="/wiki/Geology_applications_of_Fourier_transform_infrared_spectroscopy" title="Geology applications of Fourier transform infrared spectroscopy">geology</a>,<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> chemistry, materials, botany<sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> and biology research fields.<sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Nano_and_biological_materials">Nano and biological materials</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=19" title="Edit section: Nano and biological materials"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>FTIR is also used to investigate various <a href="/wiki/Nanomaterials" title="Nanomaterials">nanomaterials</a> and proteins in hydrophobic membrane environments. Studies show the ability of FTIR to directly determine the polarity at a given site along the backbone of a transmembrane protein.<sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-15" class="reference"><a href="#cite_note-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> The bond features involved with various organic and inorganic nanomaterials and their quantitative analysis can be done with the help of FTIR.<sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Microscopy_and_imaging">Microscopy and imaging</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=20" title="Edit section: Microscopy and imaging"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>An infrared <a href="/wiki/Microscope" title="Microscope">microscope</a> allows samples to be observed and spectra measured from regions as small as 5 microns across. Images can be generated by combining a microscope with linear or 2-D <a href="/wiki/Array_(data_type)" title="Array (data type)">array</a> detectors. The spatial resolution can approach 5 microns with tens of thousands of <a href="/wiki/Pixels" class="mw-redirect" title="Pixels">pixels</a>. The images contain a spectrum for each pixel and can be viewed as maps showing the intensity at any wavelength or combination of wavelengths. This allows the distribution of different chemical species within the sample to be seen<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (February 2024)">citation needed</span></a></i>]</sup>. This technique has been applied in various biological applications including the analysis of tissue sections as an alternative to conventional <a href="/wiki/Histopathology" title="Histopathology">histopathology</a>,<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (February 2024)">citation needed</span></a></i>]</sup> examining the homogeneity of pharmaceutical tablets<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (February 2024)">citation needed</span></a></i>]</sup>, and for differentiating morphologically-similar <a href="/wiki/Pollen" title="Pollen">pollen</a> grains.<sup id="cite_ref-18" class="reference"><a href="#cite_note-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Nanoscale_and_spectroscopy_below_the_diffraction_limit">Nanoscale and spectroscopy below the diffraction limit</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=21" title="Edit section: Nanoscale and spectroscopy below the diffraction limit"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The spatial resolution of FTIR can be further improved below the micrometer scale by integrating it into <a href="/wiki/Near-field_scanning_optical_microscope" title="Near-field scanning optical microscope">scanning near-field optical microscopy</a> platform. The corresponding technique is called <a href="/wiki/Nano-FTIR" title="Nano-FTIR">nano-FTIR</a> and allows for performing broadband spectroscopy on materials in ultra-small quantities (single viruses and protein complexes) and with 10 to 20 nm spatial resolution.<sup id="cite_ref-19" class="reference"><a href="#cite_note-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="FTIR_as_detector_in_chromatography">FTIR as detector in chromatography</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=22" title="Edit section: FTIR as detector in chromatography"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The speed of FTIR allows spectra to be obtained from compounds as they are separated by a gas chromatograph. However this technique is little used compared to GC-MS (gas chromatography-mass spectrometry) which is more sensitive. The GC-IR method is particularly useful for identifying isomers, which by their nature have identical masses. Liquid chromatography fractions are more difficult because of the solvent present. One notable exception is to measure chain branching as a function of molecular size in polyethylene using <a href="/wiki/Gel_permeation_chromatography" title="Gel permeation chromatography">gel permeation chromatography</a>, which is possible using chlorinated solvents that have no absorption in the area in question. </p> <div class="mw-heading mw-heading3"><h3 id="TG-IR_(thermogravimetric_analysis-infrared_spectrometry)"><span id="TG-IR_.28thermogravimetric_analysis-infrared_spectrometry.29"></span>TG-IR (thermogravimetric analysis-infrared spectrometry)</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=23" title="Edit section: TG-IR (thermogravimetric analysis-infrared spectrometry)"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Measuring the gas evolved as a material is heated allows qualitative identification of the species to complement the purely quantitative information provided by measuring the weight loss. </p> <div class="mw-heading mw-heading3"><h3 id="Water_content_determination_in_plastics_and_composites">Water content determination in plastics and composites</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=24" title="Edit section: Water content determination in plastics and composites"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>FTIR analysis is used to determine water content in fairly thin plastic and composite parts, more commonly in the laboratory setting. Such FTIR methods have long been used for plastics, and became extended for composite materials in 2018, when the method was introduced by Krauklis, Gagani and Echtermeyer.<sup id="cite_ref-20" class="reference"><a href="#cite_note-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup> FTIR method uses the maxima of the absorbance band at about 5,200 cm−1 which correlates with the true water content in the material. </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=25" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Discrete_Fourier_transform" title="Discrete Fourier transform">Discrete Fourier transform</a> – Type of Fourier transform in discrete mathematics − for computing periodicity in evenly spaced data</li> <li><a href="/wiki/Fourier_transform" title="Fourier transform">Fourier transform</a> – Mathematical transform that expresses a function of time as a function of frequency</li> <li><a href="/wiki/Fourier_transform_spectroscopy" class="mw-redirect" title="Fourier transform spectroscopy">Fourier transform spectroscopy</a> – Spectroscopy based on time- or space-domain data<span style="display:none" class="category-annotation-with-redirected-description">Pages displaying short descriptions of redirect targets</span></li> <li><a href="/wiki/Least-squares_spectral_analysis" title="Least-squares spectral analysis">Least-squares spectral analysis</a> – Periodicity computation method − for computing periodicity in unevenly spaced data</li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=26" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-Griffiths-1"><span class="mw-cite-backlink">^ <a href="#cite_ref-Griffiths_1-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Griffiths_1-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Griffiths_1-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-Griffiths_1-3"><sup><i><b>d</b></i></sup></a> <a href="#cite_ref-Griffiths_1-4"><sup><i><b>e</b></i></sup></a></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite id="CITEREFGriffithsde_Hasseth2007" class="citation book cs1">Griffiths, P.; de Hasseth, J. A. 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href="/w/index.php?title=Fourier-transform_infrared_spectroscopy&action=edit&section=27" title="Edit section: External links"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC441765/pdf/jcinvest00449-0191.pdf">Infracord spectrometer</a> photograph</li> <li>The Grubb-Parsons-NPL cube interferometer <a rel="nofollow" class="external text" href="https://books.google.com/books?id=XTolQEkzSR0C&pg=PA81">Spectroscopy, part 2 by Dudley Williams, page 81</a></li> <li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20101114040356/http://infrared.als.lbl.gov/content/web-links/58-irwindows">Infrared materials</a> Properties of many salt crystals and useful links.</li> <li><a rel="nofollow" class="external text" href="http://www.bristol.ac.uk/earthsciences/research/petrology/facilities/ftir-lab/">University FTIR lab example</a> from the <a 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title="Spectroscopy">Spectroscopy</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Infrared_spectroscopy" title="Infrared spectroscopy">Vibrational (IR)</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a class="mw-selflink selflink">FT-IR</a></li> <li><a href="/wiki/Raman_spectroscopy" title="Raman spectroscopy">Raman</a></li> <li><a href="/wiki/Resonance_Raman_spectroscopy" title="Resonance Raman spectroscopy">Resonance Raman</a></li> <li><a href="/wiki/Rotational_spectroscopy" title="Rotational spectroscopy">Rotational</a></li> <li><a href="/wiki/Rotational%E2%80%93vibrational_spectroscopy" title="Rotational–vibrational spectroscopy">Rotational–vibrational</a></li> <li><a href="/wiki/Molecular_vibration" title="Molecular vibration">Vibrational</a></li> <li><a href="/wiki/Vibrational_circular_dichroism" title="Vibrational circular dichroism">Vibrational circular dichroism</a></li> <li><a href="/wiki/Nuclear_resonance_vibrational_spectroscopy" title="Nuclear resonance vibrational spectroscopy">Nuclear resonance vibrational spectroscopy</a></li> <li><a href="/wiki/Vibrational_spectroscopy_of_linear_molecules" title="Vibrational spectroscopy of linear molecules">Vibrational spectroscopy of linear molecules</a></li> <li><a href="/wiki/Thermal_infrared_spectroscopy" title="Thermal infrared spectroscopy">Thermal infrared spectroscopy</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">UV–Vis–NIR "Optical"</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Ultraviolet%E2%80%93visible_spectroscopy" title="Ultraviolet–visible spectroscopy">Ultraviolet–visible</a></li> <li><a href="/wiki/Fluorescence_spectroscopy" title="Fluorescence spectroscopy">Fluorescence</a> <ul><li><a href="/wiki/Cold_vapour_atomic_fluorescence_spectroscopy" title="Cold vapour atomic fluorescence spectroscopy">Cold vapour atomic</a></li></ul></li> <li><a href="/wiki/Vibronic_spectroscopy" title="Vibronic spectroscopy">Vibronic</a></li> <li><a href="/wiki/Near-infrared_spectroscopy" title="Near-infrared spectroscopy">Near-infrared</a></li> <li><a href="/wiki/Resonance-enhanced_multiphoton_ionization" title="Resonance-enhanced multiphoton ionization">Resonance-enhanced multiphoton ionization</a> (REMPI)</li> <li><a href="/wiki/Raman_spectroscopy" title="Raman spectroscopy">Raman</a> <ul><li><a href="/wiki/Coherent_anti-Stokes_Raman_spectroscopy" title="Coherent anti-Stokes Raman spectroscopy">Coherent anti-Stokes</a></li></ul></li> <li><a href="/wiki/Raman_optical_activity" title="Raman optical activity">Raman optical activity</a></li> <li><a href="/wiki/Laser-induced_breakdown_spectroscopy" title="Laser-induced breakdown spectroscopy">Laser-induced breakdown</a></li> <li><a href="/wiki/Atomic_spectroscopy" title="Atomic spectroscopy">Atomic</a> <ul><li><a href="/wiki/Atomic_emission_spectroscopy" title="Atomic emission spectroscopy">emission</a> <ul><li><a href="/wiki/Glow-discharge_optical_emission_spectroscopy" title="Glow-discharge optical emission spectroscopy">Glow-discharge optical</a></li></ul></li> <li><a href="/wiki/Atomic_absorption_spectroscopy" title="Atomic absorption spectroscopy">absorption</a></li></ul></li> <li><a href="/wiki/Cavity_ring-down_spectroscopy" title="Cavity ring-down spectroscopy">Cavity ring-down spectroscopy</a></li> <li><a href="/wiki/Saturated_absorption_spectroscopy" title="Saturated absorption spectroscopy">Saturated absorption spectroscopy</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">X-ray and Gamma ray</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/X-ray_spectroscopy" title="X-ray spectroscopy">X-ray</a> <ul><li><a href="/wiki/Energy-dispersive_X-ray_spectroscopy" title="Energy-dispersive X-ray spectroscopy">Energy-dispersive</a></li> <li><a href="/wiki/X-ray_emission_spectroscopy" title="X-ray emission spectroscopy">Emission</a></li> <li><a href="/wiki/Extended_X-ray_absorption_fine_structure" title="Extended X-ray absorption fine structure">Extended X-ray absorption fine structure</a></li></ul></li> <li><a href="/wiki/Gamma_spectroscopy" title="Gamma spectroscopy">Gamma</a></li> <li><a href="/wiki/M%C3%B6ssbauer_spectroscopy" title="Mössbauer spectroscopy">Mössbauer</a> <ul><li><a href="/wiki/Conversion_electron_M%C3%B6ssbauer_spectroscopy" title="Conversion electron Mössbauer spectroscopy">Conversion electron</a></li></ul></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Electron</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Photoemission_spectroscopy" title="Photoemission spectroscopy">Photoelectron/photoemission</a> <ul><li><a href="/wiki/X-ray_photoelectron_spectroscopy" title="X-ray photoelectron spectroscopy">X-ray</a></li> <li><a href="/wiki/Ultraviolet_photoelectron_spectroscopy" title="Ultraviolet photoelectron spectroscopy">UV</a></li> <li><a href="/wiki/Angle-resolved_photoemission_spectroscopy" title="Angle-resolved photoemission spectroscopy">Angle-resolved</a></li> <li><a href="/wiki/Two-photon_photoelectron_spectroscopy" title="Two-photon photoelectron spectroscopy">Two-photon</a></li></ul></li> <li><a href="/wiki/Auger_electron_spectroscopy" title="Auger electron spectroscopy">Auger</a></li> <li><a href="/wiki/Electron_phenomenological_spectroscopy" title="Electron phenomenological spectroscopy">phenomenological</a></li> <li><a href="/wiki/Electron_paramagnetic_resonance" title="Electron paramagnetic resonance">paramagnetic</a></li> <li><a href="/wiki/Beta_spectroscopy" class="mw-redirect" title="Beta spectroscopy">Beta spectroscopy</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Nucleon</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Alpha-particle_spectroscopy" title="Alpha-particle spectroscopy">Alpha</a></li> <li><a href="/wiki/Inelastic_neutron_scattering" class="mw-redirect" title="Inelastic neutron scattering">Inelastic neutron scattering</a> <ul><li><a href="/wiki/Neutron_spin_echo" title="Neutron spin echo">Neutron spin echo</a></li></ul></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Radiowave</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Nuclear_magnetic_resonance_spectroscopy" title="Nuclear magnetic resonance spectroscopy">NMR</a> <ul><li><a href="/wiki/Two-dimensional_nuclear_magnetic_resonance_spectroscopy" title="Two-dimensional nuclear magnetic resonance spectroscopy">2D</a></li></ul></li> <li><a href="/wiki/Terahertz_spectroscopy_and_technology" title="Terahertz spectroscopy and technology">Terahertz</a></li> <li><a href="/wiki/Electron_paramagnetic_resonance" title="Electron paramagnetic resonance">ESR/EPR</a></li> <li><a href="/wiki/Ferromagnetic_resonance" title="Ferromagnetic resonance">Ferromagnetic resonance</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Others</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks hlist navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%">Data collection, processing</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Fourier-transform_spectroscopy" title="Fourier-transform spectroscopy">Fourier-transform spectroscopy</a></li> <li><a href="/wiki/Hyperspectral_imaging" title="Hyperspectral imaging">Hyperspectral imaging</a></li> <li><a href="/wiki/Spectrophotometry" title="Spectrophotometry">Spectrophotometry</a></li> <li><a href="/wiki/Time_stretch_analog-to-digital_converter" class="mw-redirect" title="Time stretch analog-to-digital converter">Time-stretch</a></li> <li><a href="/wiki/Time-resolved_spectroscopy" title="Time-resolved spectroscopy">Time-resolved spectroscopy</a></li> <li><a href="/wiki/Video_spectroscopy" title="Video spectroscopy">Video spectroscopy</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Measured phenomena</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Acoustic_resonance_spectroscopy" title="Acoustic resonance spectroscopy">Acoustic resonance spectroscopy</a></li> <li><a href="/wiki/Circular_dichroism" title="Circular dichroism">Circular dichroism spectroscopy</a></li> <li><a href="/wiki/Deep-level_transient_spectroscopy" title="Deep-level transient spectroscopy">Deep-level transient spectroscopy</a></li> <li><a href="/wiki/Dual-polarization_interferometry" title="Dual-polarization interferometry">Dual-polarization interferometry</a></li> <li><a href="/wiki/Hadron_spectroscopy" title="Hadron spectroscopy">Hadron spectroscopy</a></li> <li><a href="/wiki/Inelastic_electron_tunneling_spectroscopy" title="Inelastic electron tunneling spectroscopy">Inelastic electron tunneling spectroscopy</a> <ul><li><a href="/wiki/Scanning_tunneling_spectroscopy" title="Scanning tunneling spectroscopy">Scanning tunneling spectroscopy</a></li></ul></li> <li><a href="/wiki/Photoacoustic_spectroscopy" title="Photoacoustic spectroscopy">Photoacoustic spectroscopy</a></li> <li><a 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