ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 29 of 47
| integration time | Reference integration times (for both background and sample interferograms) should be greater than or equal to expected field integration times. For reference libraries, the longest practical integration time is recommended. |
| detector type and serial number | If an MCT or other potentially non-linear detector is employed in the reference system, IR attenuation may be required to ensure linearity; see Appendix B, Section 6; because each detector may exhibit linearity to a specific degree, the serial number of the detector should be included in any specification of the system configuration. |
D3. Wavenumber Standard Spectra
Except in extremely dry conditions, the IR absorption by water vapor in workspace air is detectable at even very short absorption pathlengths. The wavenumber positions of water vapor indicated by the reference system allow the analyst to confirm the wavenumber accuracy of sample spectra recorded on a different FTIR system in the field. Reference spectra used for this purpose are referred to here as the wavenumber standard spectra.
Every spectrum in a reference library should be associated with a wavenumber standard spectrum collected under the identical system configuration; even when the reference FTIR system is in a stable environment, it is recommended that a wavenumber standard spectrum be recorded daily with the system.
Before recording reference spectra, the analyst should check the most recent wavenumber standard spectrum against previous wavenumber standard spectra in the library; see Appendix B, Section 4 for recommended calculations. If the wavenumber reproducibility of the system is poor, the resulting reference spectra may not be suitable for quantitative use on other field systems.
D4. Obtaining and Preparing Standard Reference Gases
The accuracy of any FTIR quantitative analysis is limited by the accuracy of the concentrations of the gas standards used in preparing the reference library. It is therefore important to use gas standards of the highest available quality. NIST-traceable gravimetric standards are available from many commercial sources, and are quoted to 2% accuracy in many circumstances; EPA has also published guidance on the preparation of "Protocol 1" gases (see Reference 17), though these are available for only a limited number of compounds. Users should obtain documentation from the gas vendors regarding the analytical techniques applied and the stability limits (concentration and time) of the compounds in the cylinders. When possible, an alternative analytical method should be used to verify the quoted cylinder concentrations, especially for compounds which are reactive, corrosive, or have relatively high boiling points.
If cylinder standards are not available for a compound, the analyst may prepare reference gases based on gravimetric, barometric, or dynamic methods. In any case, the related mass, pressure, and flow measurements should be made with NIST-traceable equipment whenever possible. In general, methods which provide a stream of standard gas (dynamic methods) provide more reliable results than static methods, since the concentrations of static gases are more prone to change when reactions between the standards and the sampling system or absorption cell surface can occur.
D5. Determining the Reference Absorption Pathlength
The accuracy of any FTIR quantitative analysis is limited by the accuracy to which the pathlength of the absorption cell used in preparing the reference library is known. For single-pass absorption cells through which a collimated IR beam is passed, the pathlength can be physically measured to high accuracy. For multi-pass cells, the nominal pathlength can be estimated from the base pathlength and the number of passes. However, because focused beams and curved mirrors are required in multi-pass cells, the pathlength estimated in this fashion can differ significantly from the actual pathlength. Because of this, combinations of physical and spectroscopic measurements with multiple CTS gas standards (see below) and single-pass absorption cells should be employed to determine the actual pathlength of multi-pass absorption cells.
NIOSH Manual of Analytical Methods, Fourth Edition
