ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 30 of 47
D6. Recording CTS Spectra
The CTS (calibration transfer standard) gas is used to characterize the absorption pathlengths of the reference and field FTIR systems. Ethylene in nitrogen, at concentrations leading to between 100 and 300 ppm-meter concentration-pathlength values for the systems, is recommended; for ethylene, spectra with ppm-m values greater than 300 begin to exhibit non-linear absorbance, and must be carefully analyzed (see Sections D8 and D9 below). Standards of this gas with 2% accuracy are readily available, and both the EPA and NIST libraries contain ethylene spectra which can be used to link those libraries to measurements performed with field instruments. However, almost any other stable compound with appropriate IR absorption characteristics can be used in developing an independent reference library and for accurate field measurements.
In the field, CTS spectra must be recorded both before and after sample spectra are acquired using the same system configuration employed in recording the sample spectra (see Steps 7 and 11 above).
When developing reference spectra, the analyst must record CTS spectra at least daily; each reference spectrum should be associated with at least one CTS spectrum. The analyst must use the same system configuration employed in recording the reference spectra, with one exception: The integration time used for the CTS spectra may be less than that used for the reference spectra, if the quality of the resulting CTS spectra is still sufficient for an accurate determination of the absorption pathlength. It is highly recommended that the analyst store all the interferograms from which the CTS absorbance spectra are generated, including all background interferograms. The interferometric data provide the most direct method of verifying the FFT calculations and/or adding reference absorbance spectra for other apodization function choices.
D7. Recording Reference Spectra
Before recording reference spectra, verify that the requirements specified for system checks, system configuration parameters, gas standards, absorbance pathlength determinations, and CTS spectra described in the pertinent sections of this Appendix and Appendix B have been met. Additional checks of the sample gas pressure and temperature should also be made periodically as the spectra are recorded.
It is highly recommended that the analyst store all the interferograms from which the reference absorbance spectra are generated, including all background interferograms. The interferometric data provide the most direct method of verifying the FFT calculations and/or adding reference absorbance spectra for other apodization function choices.
Beer’s Law (Equation C1) describes the fundamental linearity of the infrared absorbance versus concentration. However, the resolution (and other) limitations of instruments typically used to generate field FTIR spectra often lead to non-linear behavior for many compounds, even at low absorbance levels. The expected accuracy of FTIR-based measurements results for a particular compound can be achieved only when 1) the reference absorbance is characterized at a sufficient number of concentration-pathlength values, up to some maximum value, and 2) the sample absorbance associated with any compound represents a value lower than that of the maximum concentration-pathlength value represented in the reference library for that compound.
For each analyte, the analyst should record two reference spectra at each of two concentration values (that is, at least four spectra) at a single absorption path length; the two concentrations should be separated by no more than a factor of ten, and a maxim um factor of five is recommended. After recording these spectra, the analyst must 1) permanently record the system parameters and the maximum measured concentration path length product and 2) characterize the linearity of the absorbance across the measured concentration-pathlength range (see Section D8). It may be necessary to record additional reference spectra for the analyte if either the absorbance linearity or maximum concentration-path length value proves unsuitable for later field measurements.
D8. Linearity Checks
The performance of linearity checks on a set of reference spectra is an important aspect of FTIR spectrometry, and is best illustrated by a practical example. The following example is based on a series of reference measurements performed in the 3M Environmental Laboratory on the compound tetrafluoroethylene, hereafter referred to as TFE.
NIOSH Manual of Analytical Methods, Fourth Edition
