Editing Infrared spectroscopy (section)

===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.