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Infrared spectroscopy
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==Practical IR spectroscopy== The infrared spectrum of a sample is recorded by passing a beam of infrared light through the sample. When the frequency of the IR matches the vibrational frequency of a bond or collection of bonds, absorption occurs. Examination of the transmitted light reveals how much energy was absorbed at each frequency (or wavelength). This measurement can be achieved by scanning the wavelength range using a [[monochromator]]. Alternatively, the entire wavelength range is measured using a [[Fourier transform]] instrument and then a [[transmittance]] or [[absorbance]] spectrum is extracted. This technique is commonly used for analyzing samples with [[covalent bond]]s. The number of bands roughly correlates with symmetry and molecular complexity. A variety of devices are used to hold the sample in the path of the IR beam These devices are selected on the basis of their transparency in the region of interest and their resilience toward the sample. {| class="wikitable" |+ Materials for containing IR samples<ref>{{cite book|title=TheChemist's Companion|page=179|author2=Richard A. Ford|author1=Arnold J. Gordon|publisher = Wiley|year=1972}}</ref> |- ! material !! transparency range (cm<sup>−1</sup>) !! comment |- | [[Sodium chloride]]|| 5000-650 || attacked (dissolved) by water, small alcohols, some amines |- | [[Calcium fluoride]] || 4200-1300 || insoluble in most solvents |- | [[Silver chloride]] || 5000-500 || attacked (dissolved) by amines, organosulfur compounds |} [[File:SolnIRcell.jpg|thumb|Typical IR solution cell. The windows are [[Calcium fluoride|CaF<sub>2</sub>]].]] ===Sample preparation=== <!-- A link on Liquid paraffin targets this paragraph. If you change the sub-heading, please change the link as well. Thanks. --> ==== Gas samples ==== Gaseous samples require a sample cell with a long [[Optical path length|pathlength]] to compensate for the diluteness. The pathlength of the sample cell depends on the concentration of the compound of interest. A simple glass tube with length of 5 to 10 cm equipped with infrared-transparent windows at both ends of the tube can be used for concentrations down to several hundred ppm. Sample gas concentrations well below ppm can be measured with a [[White cell (spectroscopy)|White's cell]] in which the infrared light is guided with mirrors to travel through the gas. White's cells are available with optical pathlength starting from 0.5 m up to hundred meters.{{citation needed|date=February 2024}} ==== Liquid samples ==== Liquid samples can be sandwiched between two plates of a salt (commonly [[sodium chloride]], or common salt, although a number of other salts such as [[potassium bromide]] or [[calcium fluoride]] are also used).<ref name=Har>{{cite book | first1 = Laurence M. | last1 = Harwood | first2 = Christopher J. | last2 = Moody | name-list-style = vanc | title = Experimental organic chemistry: Principles and Practice | edition = Illustrated | page = [https://archive.org/details/experimentalorga00harw/page/292 292] | isbn = 978-0-632-02017-1 | date = 1989 | publisher = Wiley-Blackwell | url = https://archive.org/details/experimentalorga00harw/page/292 }}</ref> The plates are transparent to the infrared light and do not introduce any lines onto the spectra. With increasing technology in computer filtering and manipulation of the results, samples in solution can now be measured accurately (water produces a broad absorbance across the range of interest, and thus renders the spectra unreadable without this computer treatment).{{citation needed|date=February 2024}} ==== Solid samples ==== Solid samples can be prepared in a variety of ways. One common method is to crush the sample with an oily [[mulling agent]] (usually mineral oil [[Nujol]]). A thin film of the mull is applied onto salt plates and measured. The second method is to grind a quantity of the sample with a specially purified salt (usually [[potassium bromide]]) finely (to remove scattering effects from large crystals). This powder mixture is then pressed in a mechanical [[Machine press|press]] to form a translucent pellet through which the beam of the spectrometer can pass.<ref name="Har" /> A third technique is the "cast film" technique, which is used mainly for polymeric materials. The sample is first dissolved in a suitable, non-[[Hygroscopy|hygroscopic]] solvent. A drop of this solution is deposited on the surface of a [[Potassium bromide|KBr]] or [[Sodium chloride|NaCl]] cell. The solution is then evaporated to dryness and the film formed on the cell is analysed directly. Care is important to ensure that the film is not too thick otherwise light cannot pass through. This technique is suitable for qualitative analysis. The final method is to use [[microtomy]] to cut a thin (20–100 μm) film from a solid sample. This is one of the most important ways of analysing failed plastic products for example because the integrity of the solid is preserved.{{citation needed|date=February 2024}} In [[photoacoustic spectroscopy]] the need for sample treatment is minimal. The sample, liquid or solid, is placed into the sample cup which is inserted into the photoacoustic cell which is then sealed for the measurement. The sample may be one solid piece, powder or basically in any form for the measurement. For example, a piece of rock can be inserted into the sample cup and the spectrum measured from it.{{citation needed|date=February 2024}} A useful way of analyzing solid samples without the need for cutting samples uses ATR or [[attenuated total reflectance]] spectroscopy. Using this approach, samples are pressed against the face of a single crystal. The infrared radiation passes through the crystal and only interacts with the sample at the interface between the two materials.{{citation needed|date=February 2024}} ===Comparing to a reference=== [[File:IR spectroscopy apparatus.svg|thumb|upright=1.5|Schematics of a two-beam absorption spectrometer. A beam of infrared light is produced, passed through an [[monochromator]] (not shown), and then split into two separate beams. One is passed through the sample, the other passed through a reference. The beams are both reflected back towards a detector, however first they pass through a splitter, which quickly alternates which of the two beams enters the detector. The two signals are then compared and a printout is obtained. This "two-beam" setup gives accurate spectra even if the intensity of the light source drifts over time.]] It is typical to record spectrum of both the sample and a "reference". This step controls for a number of variables, e.g. [[infrared detector]], which may affect the spectrum. The reference measurement makes it possible to eliminate the instrument influence.{{citation needed|date=February 2024}} The appropriate "reference" depends on the measurement and its goal. The simplest reference measurement is to simply remove the sample (replacing it by air). However, sometimes a different reference is more useful. For example, if the sample is a dilute solute dissolved in water in a beaker, then a good reference measurement might be to measure pure water in the same beaker. Then the reference measurement would cancel out not only all the instrumental properties (like what light source is used), but also the light-absorbing and light-reflecting properties of the water and beaker, and the final result would just show the properties of the solute (at least approximately).{{citation needed|date=February 2024}} A common way to compare to a reference is sequentially: first measure the reference, then replace the reference by the sample and measure the sample. This technique is not perfectly reliable; if the infrared lamp is a bit brighter during the reference measurement, then a bit dimmer during the sample measurement, the measurement will be distorted. More elaborate methods, such as a "two-beam" setup (see figure), can correct for these types of effects to give very accurate results. The [[Standard addition]] method can be used to statistically cancel these errors. Nevertheless, among different absorption-based techniques which are used for gaseous species detection, [[Cavity ring-down spectroscopy]] (CRDS) can be used as a calibration-free method. The fact that CRDS is based on the measurements of photon life-times (and not the laser intensity) makes it needless for any calibration and comparison with a reference <ref>{{cite journal| first1 = Soran | last1 = Shadman | first2 = Charles | last2 = Rose | first3 =Azer P. | last3 = Yalin | name-list-style = vanc |title= Open-path cavity ring-down spectroscopy sensor for atmospheric ammonia |journal = [[Applied Physics B]]|volume = 122|issue=7 |pages = 194|date = 2016|doi = 10.1007/s00340-016-6461-5 |bibcode=2016ApPhB.122..194S| s2cid = 123834102 }}</ref> Some instruments also automatically identify the substance being measured from a store of thousands of reference spectra held in storage. ===FTIR=== {{Main|Fourier transform infrared spectroscopy}} [[File:FTIR-interferogram.svg|thumb|An interferogram from an [[FTIR]] measurement. The horizontal axis is the position of the mirror, and the vertical axis is the amount of light detected. This is the "raw data" which can be [[Fourier transform]]ed to get the actual spectrum.]] '''[[Fourier transform]] infrared (FTIR) spectroscopy''' is a measurement technique that allows one to record infrared spectra. Infrared light is guided through an [[interferometer]] and then through the sample (or vice versa). A moving mirror inside the apparatus alters the distribution of infrared light that passes through the interferometer. The signal directly recorded, called an "interferogram", represents light output as a function of mirror position. A data-processing technique called [[Fourier transform]] turns this raw data into the desired result (the sample's spectrum): light output as a function of infrared [[wavelength]] (or equivalently, [[wavenumber]]). As described above, the sample's spectrum is always compared to a reference.{{citation needed|date=February 2024}} An alternate method for acquiring spectra is the "dispersive" or "scanning [[monochromator]]" method. In this approach, the sample is irradiated sequentially with various single wavelengths. The dispersive method is more common in [[Ultraviolet-visible spectroscopy|UV-Vis spectroscopy]], but is less practical in the infrared than the FTIR method. One reason that FTIR is favored is called "[[Fellgett's advantage]]" or the "multiplex advantage": The information at all frequencies is collected simultaneously, improving both speed and [[signal-to-noise ratio]]. Another is called "Jacquinot's Throughput Advantage": A dispersive measurement requires detecting much lower light levels than an FTIR measurement.<ref name=white>[https://books.google.com/books?id=t2VSNnFoO3wC&pg=PA7 ''Chromatography/Fourier transform infrared spectroscopy and its applications'', by Robert White, p7]</ref> There are other advantages, as well as some disadvantages,<ref name=white/> but virtually all modern infrared spectrometers are FTIR instruments. === Infrared microscopy === Various forms of [[infrared microscopy]] exist. These include IR versions of sub-diffraction microscopy<ref name="wiley12">{{cite encyclopedia|last1=Pollock|first1= Hubert M|title=Microspectroscopy in the Mid-Infrared |first2= S G|last2= Kazarian|editor-first=Robert A. |publisher = John Wiley & Sons Ltd |editor-last=Meyers|encyclopedia= Encyclopedia of Analytical Chemistry|year=2014|isbn=9780470027318|pages=1–26|doi=10.1002/9780470027318.a5609.pub2}}</ref> such as IR [[Near-field scanning optical microscope|NSOM]],<ref>H M Pollock and D A Smith, The use of near-field probes for vibrational spectroscopy and photothermal imaging, in Handbook of vibrational spectroscopy, J.M. Chalmers and P.R. Griffiths (eds), John Wiley & Sons Ltd, Vol. 2, pp. 1472 - 1492 (2002)</ref> [[photothermal microspectroscopy]], [[Nano-FTIR]] and [[AFM-IR|atomic force microscope based infrared spectroscopy]] (AFM-IR). === Other methods in molecular vibrational spectroscopy === Infrared spectroscopy is not the only method of studying molecular vibrational spectra. [[Raman spectroscopy]] involves an [[inelastic scattering]] process in which only part of the energy of an incident photon is absorbed by the molecule, and the remaining part is scattered and detected. The energy difference corresponds to absorbed vibrational energy.{{citation needed|date=February 2024}} The [[selection rule]]s for infrared and for Raman spectroscopy are different at least for some [[molecular symmetry|molecular symmetries]], so that the two methods are complementary in that they observe vibrations of different symmetries.{{citation needed|date=February 2024}} Another method is [[electron energy loss spectroscopy]] (EELS), in which the energy absorbed is provided by an inelastically scattered electron rather than a photon. This method is useful for studying vibrations of molecules [[Adsorption|adsorbed]] on a solid surface. Recently, [[High resolution electron energy loss spectroscopy|high-resolution EELS]] (HREELS) has emerged as a technique for performing vibrational spectroscopy in a [[Transmission electron microscopy|transmission electron microscope]] (TEM).<ref name=":0">{{cite journal | vauthors = Krivanek OL, Lovejoy TC, Dellby N, Aoki T, Carpenter RW, Rez P, Soignard E, Zhu J, Batson PE, Lagos MJ, Egerton RF, Crozier PA | display-authors = 6 | title = Vibrational spectroscopy in the electron microscope | journal = Nature | volume = 514 | issue = 7521 | pages = 209–12 | date = October 2014 | pmid = 25297434 | doi = 10.1038/nature13870 | bibcode = 2014Natur.514..209K | s2cid = 4467249 }}</ref> In combination with the high spatial resolution of the TEM, unprecedented experiments have been performed, such as nano-scale temperature measurements,<ref>{{cite journal | vauthors = Idrobo JC, Lupini AR, Feng T, Unocic RR, Walden FS, Gardiner DS, Lovejoy TC, Dellby N, Pantelides ST, Krivanek OL | display-authors = 6 | title = Temperature Measurement by a Nanoscale Electron Probe Using Energy Gain and Loss Spectroscopy | journal = Physical Review Letters | volume = 120 | issue = 9 | pages = 095901 | date = March 2018 | pmid = 29547334 | doi = 10.1103/PhysRevLett.120.095901 | bibcode = 2018PhRvL.120i5901I | doi-access = free }}</ref><ref>{{cite journal | vauthors = Lagos MJ, Batson PE | title = Thermometry with Subnanometer Resolution in the Electron Microscope Using the Principle of Detailed Balancing | journal = Nano Letters | volume = 18 | issue = 7 | pages = 4556–4563 | date = July 2018 | pmid = 29874456 | doi = 10.1021/acs.nanolett.8b01791 | bibcode = 2018NanoL..18.4556L | s2cid = 206748146 }}</ref> mapping of isotopically labeled molecules,<ref>{{cite journal | vauthors = Hachtel JA, Huang J, Popovs I, Jansone-Popova S, Keum JK, Jakowski J, Lovejoy TC, Dellby N, Krivanek OL, Idrobo JC | display-authors = 6 | title = Identification of site-specific isotopic labels by vibrational spectroscopy in the electron microscope | journal = Science | volume = 363 | issue = 6426 | pages = 525–528 | date = February 2019 | pmid = 30705191 | doi = 10.1126/science.aav5845 | bibcode = 2019Sci...363..525H | doi-access = free }}</ref> mapping of phonon modes in position- and momentum-space,<ref>{{cite journal | vauthors = Hage FS, Nicholls RJ, Yates JR, McCulloch DG, Lovejoy TC, Dellby N, Krivanek OL, Refson K, Ramasse QM | display-authors = 6 | title = Nanoscale momentum-resolved vibrational spectroscopy | journal = Science Advances | volume = 4 | issue = 6 | pages = eaar7495 | date = June 2018 | pmid = 29951584 | pmc = 6018998 | doi = 10.1126/sciadv.aar7495 | bibcode = 2018SciA....4.7495H }}</ref><ref>{{cite journal | vauthors = Senga R, Suenaga K, Barone P, Morishita S, Mauri F, Pichler T | title = Position and momentum mapping of vibrations in graphene nanostructures | journal = Nature | volume = 573 | issue = 7773 | pages = 247–250 | date = September 2019 | pmid = 31406319 | doi = 10.1038/s41586-019-1477-8 | arxiv = 1812.08294 | bibcode = 2019Natur.573..247S | s2cid = 118999071 }}</ref> vibrational surface and bulk mode mapping on nanocubes,<ref>{{cite journal | vauthors = Lagos MJ, Trügler A, Hohenester U, Batson PE | title = Mapping vibrational surface and bulk modes in a single nanocube | journal = Nature | volume = 543 | issue = 7646 | pages = 529–532 | date = March 2017 | pmid = 28332537 | doi = 10.1038/nature21699 | bibcode = 2017Natur.543..529L | s2cid = 4459728 }}</ref> and investigations of [[polariton]] modes in van der Waals crystals.<ref>{{cite journal | vauthors = Govyadinov AA, Konečná A, Chuvilin A, Vélez S, Dolado I, Nikitin AY, Lopatin S, Casanova F, Hueso LE, Aizpurua J, Hillenbrand R | display-authors = 6 | title = Probing low-energy hyperbolic polaritons in van der Waals crystals with an electron microscope | journal = Nature Communications | volume = 8 | issue = 1 | pages = 95 | date = July 2017 | pmid = 28733660 | pmc = 5522439 | doi = 10.1038/s41467-017-00056-y | arxiv = 1611.05371 | bibcode = 2017NatCo...8...95G }}</ref> Analysis of vibrational modes that are IR-inactive but appear in [[inelastic neutron scattering]] is also possible at high spatial resolution using EELS.<ref>{{cite journal| vauthors = Venkatraman K, Levin BD, March K, Rez P, Crozier PA |date=2019|title=Vibrational spectroscopy at atomic resolution with electron impact scattering |journal=Nature Physics|volume=15|issue=12|pages=1237–1241 |doi=10.1038/s41567-019-0675-5 |arxiv=1812.08895|bibcode=2019NatPh..15.1237V|s2cid=119452520}}</ref> Although the spatial resolution of HREELs is very high, the bands are extremely broad compared to other techniques.<ref name=":0" /> === Computational infrared microscopy === By using computer [[simulation]]s and [[normal mode]] analysis it is possible to calculate theoretical frequencies of molecules.<ref>{{cite journal | vauthors = Henschel H, Andersson AT, Jespers W, Mehdi Ghahremanpour M, van der Spoel D | title = Theoretical Infrared Spectra: Quantitative Similarity Measures and Force Fields | journal = Journal of Chemical Theory and Computation | volume = 16 | issue = 5 | pages = 3307–3315 | date = May 2020 | pmid = 32271575 | pmc = 7304875 | doi = 10.1021/acs.jctc.0c00126 }}</ref>
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