Here is a summary of main points that should be observed in using infrared absorption for quantitative measurements of gases.
Work in the infrared region where molecules have their strong fundamental bands.
Use fairly high spectral resolution. (0.5 cm-1 or 1.0 cm-1)
Use zero-filling to double the number of points in the spectra.
Use the mercury-cadmium-telluride detector.
Use the “White” multiple-pass cell, preferably with a glass cell body.
Do not bother to purge the spectrometer sample compartment.
Make water and carbon dioxide subtraction spectra on the same spectrometer that is used to make the sample spectra.
Use the region integration and subtraction (RIAS) method of quantitative analysis.
Here are five reasons for using RIAS in preference to other quantitative methods, such as CLS (classical least squares), ILS (inverse least squares), PCR (principal component regression) or PLS (partial least squares).
1. With RIAS there is no plotting of calibration curves and no creation of mathematical models.
2. RIAS does not require the preparation of standards; the standards are
already in the QASoft database.
3. With RIAS there is very little concern about baseline drift.
4. RIAS can make correct quantitative measurements even if there are unknown sources of absorbance in the spectrum.
5. When using RIAS, there is no need for matrix algebra or factor analysis.
Do not include any bands of high absorbance in the integration region chosen for measurement.
Analyze samples at room temperature and one atmosphere total pressure.
Make proper choices of pathlength, integration region and sample dilution so that the single reference spectrum from the database will cover a concentration range from one atmosphere down to 10-9 atmospheres (parts-per-billion).