National Institute for Occupational Safety and Health

“ 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. ”

“ Figure C1 illustrates the basic FTIR spectrometer configuration required for gas phase analyses. The infrared radiation emitted by the infrared (IR) source contains energy at all wavelengths between 2.0 and 20 microns; this is the portion of the electromagnetic spectrum usually referred to as the "mid-infrared" (MIR) . In the units typically employed in FTIR spectroscopy (wavenumbers, or cm-1) , this is the wavenumber range 5000 to 500 cm-1. The IR radiation passes through an interferometer, where the motion of an optical element-usually a mirror-optically modulates the infrared beam. ”

“ The calculation described above assumes that water is the only significant infrared absorber in the samples besides the analytes, and that only one analyte absorbs in any analytical region. If other analytes or interferants are present, a more conservative RSA may be estimated by adding the absorbance of the additional compounds to the difference spectrum using a set of suitable reference spectra, then subtracting their absorbance using a different set of reference spectra. ”

“ The instrument operator can also choose the apodization function to be used in the generation of FTIR spectra. Apodization is a mathematical alteration of the interferogram that can be performed before application of the FFT. Several standard alteration functions have been devised, and each affects the final absorption spectrum of the sample gas in a different way. As with the selection of instrument resolution, each choice has its advantages and drawbacks. The simplest choice, known as the "boxcar apodization" function, results in the lowest FWHM but also in relatively low S/N ratio. ”

“ For each residual spectrum R with discrete absorbance value for i = p to q, in the wavenumber range to , the residual squared area (RSA) is defined as:
Equation B2
The RSA has the dimensions (abs cm-1) , and serves as a measure of the integrated absorbance of spectral noise and water subtraction artifacts over the analytical region. The RSA is compared to the total absorbance of a compound in the same region to estimate the LOD for the compound in that region (see Appendix D, Section 9 and Appendix E, Section 1) . ”

“ 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. ”

“ Every compound-specific quantitative analytical technique, including FTIR spectrometry, can fail to provide accurate results when interfering compounds appear in a sample. However, the results of a mathematical FTIR spectral analysis designed for a particular set of compounds provides clues to its failure in the presence of interferants. An experienced analyst can often adjust the analysis to accommodate the interferants and provide accurate results. ”

“ A useful characteristic of extractive FTIR spectrometry is that it provides accurate field measurements for many compounds, but requires field calibration procedures involving only two compounds. The water vapor available in every ambient air sample serves to calibrate the wavenumber (x) axes of FTIR absorbance spectra; a single calibration transfer standard (CTS) gas serves to calibrate their concentration-related (y) axes. ”

“ Similarly, FTIR measurements provide only an approximation of the true absorbance spectrum of a mixture of gaseous compounds, though it is, under many circumstances, a sufficiently accurate approximation. It is the responsibility of the analyst to verify and ensure that the reference and sample spectra provide a sufficiently accurate quantitative analysis according to Beer's Law. ”

“ The water spectrum shown is typical of workspace air samples; carbon dioxide does not absorb in any of the spectrum. As a result, the limited spectral region shown in Figure E2 is actually of the greatest interest for an analysis of DFE and TFE in workspace air.
Shown in greater detail in Figure E2 are the three reference spectra and two possible analytical regions. Analytical region 1, from 1370 to 1295 cm-1, includes water bands which interfere with the TFE absorbance in the sample spectra. ”

“ FTIR system configuration—the set of parameters required to reproduce, as closely as possible, results from a particular FTIR system at a later time. This set includes (at least) the nominal MIL, the absorption pathlength, the apodization function, the gas temperature, the gas pressure, the zero filling factor, the measured wavenumbers of specific water absorption bands, the sources of the reference library spectra, the integration time, the detector type and serial number, the detector gain (including hardware and software settings) . ”

“ It is also important that the sampling system allow the operator to perform all the necessary calibration and sampling procedures without compromising the speed and flexibility of the analytical system. Figure C2 illustrates a sampling system con figuration that meets these requirements, though other configurations are possible. A computer controls the actions of the interferometer and records the voltage output of the IR detector throughout the motion of the interferometer's optical element. ”

“ The average of the last column in the table, which represents the absolute percent difference between the actual and calculated concentrations, the “fractional calibration uncertainty” (FCU; see Reference 2) , is 3.2%. This value represents the average error over the concentration range for these reference spectra to be expected from the linear analysis up to the maximum concentration-pathlength value used in the analysis (257 ppm-meter) . If additional accuracy is required, non-linear analyses or corrections such as those discussed above (Appendix B, Section 9) can be employed. ”

“ The analyst may choose to approve analytical results obtained when this ratio exceeds the recommended limit.
B5 System Response Time
Direct N2 or zero air through the entire sample interface and record spectra at approximately 30 second intervals. Abruptly replace the N2 or zero air gas flow with CTS gas and continue to record spectra. The system response time is the subsequent time required for the FTIR system to generate an absorbance spectrum in which the CTS compound's calculated concentration reaches 95% of the final (stable) concentration value indicated in later spectra. ”

“ The analyst must document criteria for the selection of the analytical region and any baseline correction procedures employed.
B2 Residual Squared Area
Note: If the following calculations are performed during testing or as part of the QC procedures (steps 14 through 17) , perform them using a workspace air spectrum instead of the “water vapor (absorbance) spectrum” described in the two paragraphs below.
Use the integration time selected for field testing in recording the spectra described below. Record a background spectrum of dry N2 gas or zero air. ”

“ The quantitative result for a single analyte may be based on data from more than one analytical region.
analyst—a person familiar with and experienced in performance of all aspects of this FTIR-based method. Analysts m ay perform any portion (s) of the method, and must perform certain portions of the method (see also "operator") .
analyte—a compound whose concentrations in a sample is of interest and must be accurately quantified (see also "interferant") . ”

“ In gases at atmospheric pressure, the commercially available thermal sources induce transition rates between quantum energy states which are small compared to those of the competing collisional relaxation processes, and cannot induce non-equilibrium energy state populations. ”

“ Prepare a test plan. The plan must include the following: a. The proposed system configuration, including the absorption pathlength and integration time for sample spectra. b. The data quality objectives, ana lytical region s, and expected Limit of Detection (LOD) values for each ana lyte. See Ta ble 2 a nd A ppe ndix E for exa m ple values and calculations. c. The names of all “operators” and “analysts” to be involved in the test. Analysts must be experienced in all aspects of the test procedures listed below, and may perform any or all of the test procedures. ”

“ FTIR (Fourier transform infrared) spectrometer—an analytical system that employs a source of mid-infrared radiation, an interferometer, an enclosed sample cell of known absorption pathlength, an infrared detector, optical elements that transfer infrared radiation between components, and a computer system. The time-domain detector response (interferogram) is processed by a Fourier transform to yield a representation of the infrared power vs. ”

“ REPORTING: 18. Reporting requirements include the analyte and interferant concentrations, the concentration uncertainties, the FTIR spectrometer configuration, the sampling locations and conditions, the source (s) of the reference spectra, the CTS spectral analysis results, the results of QA/QC procedures, and certificates of analysis for all standard ga ses . Any va riations of the test procedu res and origina l data qua lity objectives from those included in the test plan should also be documented and reported. ”

“ Appendices C, D, and E provide (respectively) a general description of FT IR spectrom etry, considerations for preparing reference libraries, and examples of calculations req uired for performance of this method.
NIOSH Manual of Analytical Methods, Fourth Edition ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 3 of 47 PRE-TEST PROCED URES: The following procedures (Steps 1 and 2) may be performed only by an experienced “analyst” (see Append ix A ) . 1. ”