HARDWARE REQUIREMENTS FOR QASOFT
In presenting new software, one usually states the “hardware requirements”. For QASoft the hardware requirements are:
1. A Fourier transform spectrometer with spectral resolution of 0.5 cm-1 or 1.0 cm-1.
2. A nitrogen-cooled mercury-cadmium-telluride detector.
If either of these requirements is not met, QASoft will operate, but with diminished capabilities.
There are two reasons for working at fairly high resolution. The first reason is that one should take advantage of the detailed fine structure in the bands of the gaseous molecules. Most gases have sharp “spikes”, abrupt absorption band “shoulders”, small bundles of lines, or arrays of individual lines. In studying mixtures of gases, these features allow the operator to distinguish between molecules with overlapping bands, and also allow the software to correctly measure the components of complex gas mixtures. The use of fairly high resolution also allows one to reach between water lines to measure such molecules as NO, NO2, and SO2, whose main bands are badly mixed in with the water lines.
The second reason for working at fairly high resolution is to widen the range over which the absorption law is valid. SEE THE TOPIC ABSORBANCE LAW, FAILURE.
There are also two reasons for using the nitrogen-cooled MCT detector. The first and most obvious reason for using MCT is that its lowering of the spectrum noise level lowers the minimum detection limits for all compounds.
The second reason for using MCT is not quite so obvious, but it is at least as important. The lowering of the noise level widens the working range for the quantitative analysis software. For example, when working with molecules that have fine structure in their spectra, quantitative analysis by the region integration method is only valid from 0.1 absorbance units down to the noise level. If that noise level is 0.01 A.U., then the range of measurable concentration only spans a factor of 10. If, however, the noise level is down to 0.0001 A.U., as may be the case with MCT, then the range of measurable concentrations spans a factor of 1000.