Page:NIOSH Manual of Analytical Methods - 3800.pdf/34

ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 34 of 47 D9.

The estimated LODs are the concentrations – for the given system configuration, reference spectra, and analytical region – at which each compound’s absorbance area is equal to the RSA. For a specified system configuration and analytical region, an estimate of the LOD for a compound can be made from the absorbance area of a reference spectrum in that region ($${A_R}$$, absorbance-cm$-1$), the reference spectrum concentration-pathlength product (CPP, ppm-m), the sample absorption pathlength ($$L$$, meters), and the RSA values (absorbance-cm$-1$) as

$$LOD=\frac{(CCP)(RSA)}{L{A_R}}$$ Equation D1

A simple trapezoidal approximation of $${A_R}$$ over the analytical region of interest, including baseline corrections when necessary, is suitable for the LOD estimate. The RSA is formed from the product of the FTIR system’s typical root-mean-square noise level in absorbance and the width of the analytical region in cm$-1$ (see Appendix B, Section 2); the RSA estimate should reflect the error incurred in subtracting known spectral interferences. Appendix E, Section 1 presents detailed examples of RSA and LOD calculations.

D10.

The analyst may use any of the publicly available reference libraries for quantitative analyses, but it is strongly recommended that quantitative results for analytes be based only on spectra from libraries which meet the requirements discussed in this Appendix. If the reference library consists of spectra recorded at a spectral resolution different from that of the field system, the analyst must 1) de-resolve the spectra mathematically to that of the field data and 2) ensure that the library data are suitably linear over the desired measurement range at the spectral resolution of the field system. NIOSH Manual of Analytical Methods, Fourth Edition