Page:NIOSH Manual of Analytical Methods - 3800.pdf/17

ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 17 of 47

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$RSA={\frac {[{w_{p}}-{w_{q}}]}{q-p+1}}{\sqrt {\sum _{i=p}^{i=q}{\frac {{(R_{i})}^{2}}{q-p}}}}$ 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 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.

B3 Minimum Instrumental Linewidth (MIL)

Evacuate the absorption cell to a pressure below 100 mm Hg and record a background spectrum. Obtain a workspace air sample at an absolute pressure of approximately 300 mm Hg. Record the absorbance spectrum of this low-pressure sample. Measure at the FW HM (full width at half maxim um) linewidth, in absorbance, of at least two isolated water vapor lines (for example, the lines near 1918 cm^-1 and 2779 cm^-1). The MIL is the mean of these FW HM measurements.

B4 Wavenumber Reproducibility

Note: If this calculation is performed during testing or as part of the QC procedures (see Steps 6 and 10), perform these determinations using a workspace air spectrum instead of the water vapor absorbance spectrum described in Section B2.

Using a water vapor spectrum recorded as described in Section B2, determine the center wavenumber values w_S1 and w_S2 of two isolated water vapor absorption features; the peaks near 1918 cm^-1 and 2779 cm^-1 are suggested, though any other pair of isolated lines separated by 500 cm^-1 or more is suitable. Compare these results to those center wavenumber values w_R1 and w_R2 and for the same absorbance features in the water vapor wavenumber standard associated with the reference library to be used in quantitative analyses as follows: Calculate the relative wavenumber accuracy (RW A) in percent for each of the two absorption bands according to

$RWA=ABS(w_{Rt}-w_{st})$ for i = 1, 2. Equation B3

Compare the maximum of these two values to the MIL for the FTIR system (see Section B3). If the ratio RWA/MIL exceeds 2%, adjustment of the wavenumber scale for the sample spectra may be required.

Mathematical wavenumber adjustments may be made locally by shifting or stretching the wavenumber scale, or globally stretched by changing the laser wavenumber during the FFT. However, large shifts (on the order of 5% or more of the MIL) indicate that the system requires physical adjustments, such as re-alignment of the laser system responsible for control of the interferometer's moving element. In addition, mathematical wavenumber adjustments require some sort of interpolation procedure in conjunction with the quantitative spectral analysis, and those procedures may result in spectral mismatches whose effects on the accuracy of the analysis are not easily quantified.

The necessity of such wavenumber adjustments depends, in part, on the widths of the absorption peaks of the compounds involved in the spectral analysis. Because many of the absorption bands of water—a nearly ubiquitous interferant in workspace air IR analysis—are very narrow, an accurate analysis usually requires the relatively stringent limits placed above on the RWA to MIL ratio. However, it is possible to obtain accurate results when this ratio exceeds the recommended limit, especially when only broad absorbance features are actually employed. The analyst may choose

NIOSH Manual of Analytical Methods, Fourth Edition