Science Fair Project Encyclopedia
As with all spectroscopic techniques, it can be used to investigate the composition of a sample.
Infrared spectroscopy works because chemical bonds have specific frequencies at which they vibrate. These resonant frequencies are dependent 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. Bonds can vibrate in six different ways, symmetrical and asymmetrical stretching, scissoring, rocking, wagging and twisting. A useful flash animation of these can be found here
In order to measure a sample, a beam of monochromatic infrared light 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. The technique has been used for the characterization of very complex mixtures however.
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 infrared 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 bromide). 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.
A beam of infra-red light is produced and split into two separate beams. One is passed through the sample, the other passed through a reference which is often the substance the sample is dissolved in. 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.
A reference is used for two reasons:
- This prevents fluctuations in the output of the source affecting the data
- This allows the effects of the solvent to be cancelled out (the reference is usually a pure form of the solvent the sample is in)
Some absorptions due to typical bonds
|Bond||Organic Molecules found in||Absorption range|
|C-O||alcohols, esters||1000 - 1300 cm-1|
|C=O||aldehydes, ketones, carboxylic acids, esters||1680 - 1750 cm-1|
|O-H||carboxylic acids||2500 - 3300 cm-1(broad absorption)|
|N-H||primary amines||3100 - 3500 cm-1|
|O-H||hydrogen bonded in alcohols, phenols||3230 - 3550 cm-1|
|O-H||free||3580 - 3670 cm-1|
Uses and applications
Techniques have been developed to assess the quality of tea-leaves using infrared spectroscopy, this will mean that highly trained experts (also called 'noses') can be used more sparingly, at a significant cost saving.
Infrared 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 infrared 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 Infrared spectroscopy
Fourier transform infrared (FTIR) spectroscopy is a measurement technique for collecting Infrared spectra. Instead of recording the amount of energy absorbed when the frequency of the infra-red light is varied (monochromator), the IR light is guided through an interferometer. After passing the sample the measured signal is the interferogram. Performing a mathematical Fourier Transform on this signal results in a spectrum identical to that from conventional (dispersive) infrared spectroscopy.
FTIR spectrometers are cheaper than conventional spectrometers because building of interferometers is easier than the fabrication of a monochromator. In addition, measurement of single spectra is faster for the FTIR technique because the information of all frequencies is collected simultaneously. This allows multiple samples to be collected and averaged together resulting in an improvement in sensitivity. Because of its various advantages, virtually all modern infrared spectrometers are of the FTIR variety.
- Fourier transform spectroscopy
- Near infrared spectroscopy
- Vibrational spectroscopy
- Rotational spectroscopy
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