Editing Infrared spectroscopy (section)

===Sample preparation===
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==== 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&nbsp;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&nbsp;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}}