As with all spectroscopic techniques it can be used to investigate the composition of a sample.
Infra-red spectroscopy works because chemical bonds have specific frequencies at which they will vibrate. These resonant frequencies are dependant on the length of the bond, and the mass of the atoms at either end of it. Thus, the frequency of the vibrations can be associated with a particular bond type.
In order to measure a sample, a beam of infra-red light at a specific frequency is passed through the sample, and the amount of energy absorbed is recorded. By repeating this operation across a range of interest (usually no more than 4000-500cm-1), a chart can be built up. When looking at a chart for a substance, an experienced user can identify the substance from the information on the chart.
This technique works almost exclusively on covalent bonds, and as such is of most use in organic chemistry. Clear charts (or spectra) will be produced by samples with high levels of purity of one substance, however, the technique has been used for the characterisation of very complex mixtures. For example, techniques have been developed to assess the quality of tea-leaves using infra-red spectroscopy, this will mean that highly trained experts (also called 'noses') can be used more sparingly, at a significant cost saving.
Liquid samples can be sandwiched between two plates of high purity salt (as in sodium chloride, or common salt). The plates are transparent to the infra-red light and will not introduce any lines onto the spectra. The plates are obviously highly soluble in water, and so the sample and washing reagents and the like must be anhydrous (without water).
Solid samples can be prepared in two major ways. The first is to crush the sample with a mulling agent in a marble pestle and mortar. If the solid can be induced to dissolve, or at least be crushed into a *very* fine powder, then the results will be good.
The second method is to mix a quantity of the sample with a specially purified salt (usually potassium chloride). This powder mixture is then crushed in a pellet press in order to form a pellet through which the beam of the spectrometer can pass. This pellet must be crushed to high pressures in order to ensure that the pellet is translucent, but this can be achieved without powered machinery.
Uses and applications
Aside from smelling tea, infra-red spectroscopy is widely used in both research and industry as a simple and reliable technique for measurement, quality control, and dynamic measurement. The instruments are now small, and can be transported, even for use in field trials. 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). Some machines will also automatically tell you what substance is being measured from a store of thousands of reference spectra held in storage.
By measuring at a specific frequency over time, changes in the character or quantity of a particular bond can be measured. This is especially useful in measuring the degree of polymerisation in polymer manufacture. Modern research machines can take infra-red measurements across the whole range of interest as frequently as 32 times a second. This can be done whilst simultaneous measurements are made using other techniques. This makes the observations of chemical reactions and processes quicker, more accurate, and more precise.
Fourier Transform Infra-red spectroscopy
Fourier Transform Infra-red (FTIR) spectroscopy refers to an experimental technique for collecting Infra-red spectra. Instead of recording the amount of energy absorbed as the frequency of the infra-red light is varied, the sample is exposed to a single pulse of infra-red light and the resulting signal observed. By performing a mathematical Fourier Transform on this response, a spectrum identical to that from conventional infra-red spectroscopy can be derived.
FTIR spectrometers are cheaper than conventional spectrometers because it is not necessary to vary the frequency of the infra-red light in a carefully controlled manner. In addition sampling time is smaller for the FTIR technique. This allows multiple samples to be collected and averaged together resulting in an improvement in sensitivity. Because of its various advantages, virtually all modern infra-red spectrometers are of the FTIR variety.
(See also Fourier Transform Spectroscopy)