By Paul Livingstone
Wednesday, April 14, 2010
In other forms of spectroscopy, wavelength absorption generates measurement data. In the
infrared (IR) wavelength, however, the key phenomena for measurement are the vibrational
frequencies generated by the molecular bonds under analysis. The energy produced by these
vibrational modes varies widely over the IR spectrum, and this is reflected in the wide variety
of spectrometers that have been developed. Because the technique measures covalent bonds almost
exclusively, it is a good choice for characterizing complex molecular structures.
Like other spectrometers, IR instruments are typically built with a source, detector, and a
dispersive element, such as a prism or diffraction grating, to improve sensitivity over a wide
range of wavelengths. A monochromator is typically used to vary IR frequency, measuring over
time. Above 1000 nm limit, however, an interferometer is often preferred instead. This beam
splitter is placed between fixed and moving mirrors in the spectrometer. The signal is collected
all at once and a Fourier transform is performed to produce a conventional IR spectrum.
This technique, referred to as FTIR, is now common to most modern IR spectrometers primarily
because it is cheaper to construct an interferometer than a monochromator. There is no loss of
performance with FTIR instruments, and in fact spectra can be collected more quickly because all
spectra are sampled simultaneously. This means results can be averaged for greater
sensitivity.
Our list of common IR spectrometers is not meant as a comparison because applications for IR
spectroscopy vary so widely over the spectrum, and can include polymer analysis, forensic
studies, environmental measurement, chemistry, and semiconductor process control. In the
near-IR, for example, molar absorptivity is quite low compared to higher IR wavelengths.
Sensitivity suffers but depth of penetration improves and, as a result, bulk samples can be
characterized with little sample preparation.
Companies Included:
View PDF of the Infrared Spectrometer Specifications Guide
Published in R & D magazine: Vol. 52, No. 2, April, 2010, p.15.
