An infrared spectrometer analyses a compound by passing infrared radiation, over a range of different frequencies, through a sample and measuring the absorptions made by each type of bond in the compound. This produces a spectrum, normally a ‘plot’ of % transmittance against wavenumber.
No two organic compounds have the same infrared spectrum and so individual, pure compounds can be identified by examination of their spectra. In the region, 7 – 11 microns (1430-910 cm-1) there are many absorption bands and even pairs of almost identical organic molecules show up differences here. This region is known as the “fingerprint region” and provided that a chemist has a copy of the spectrum, any unknown pure compound can be identified by making a simple comparison.
Other frequency ranges provide less distinct absorption bands (1.5 – 1.8; 2.1 – 2.6; & 3.1 – 3.6) but in many instruments are distinct enough to provide accurate measurements. Various mathematical algorithms have been developed to determine the makeup of a mix of hydrocarbons (C1 – C5).
Two approaches are available to implement spectroscopy.
Basically either monochromatic, collimated IR light is passed through a gas mixture. The light color is stepped across a frequency range and the resulting spectrum is analyzed to determine the mixture. Tunable lasers are the traditional way to implement this, however a new technique is being developed which offers a wider frequency span.
Alternatively broad spectrum collimated IR light is passed through the gas and various techniques are then used to obtain a spectrum. This is how the Fabry-Perot and the Michelson interferometer work.
Specrographics Instruments has spent the past two year developing the tools, the mathematical algorithms, gas chambers and finding a sensor that can provide the spectral measurements required. We have tested the use of 4 different sensor technologies two Fabry-Perot, one Michelson interferometer and one using a new technology that provides tune monchromatic light. We have determined that a Canadian sensor is the best but most expensive while an Egyptian one seems to be good enough, is a more mature product and is less expensive.