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ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 38 of 47 estimate of the RSA is therefore available by forming the mathematical difference of two water spectra—of substantially different water concentrations—recorded using the same FTIR system configuration. For each analyte, this estimate assumes that only water and that analyte will absorb substantially in the actual sample spectra. Later analyses based on the actual sample matrix could lead to either lower or higher concentration uncertainty and LOD estimate ; examples of such analyses are detailed below.

Figures E3 and E4 illustrate two absorbance spectra recorded using the system configuration described above. They represent spectra of workspace air at approximately 20% and 40% relative humidity. The upper trace in each Figure is slightly offset for clarity.

Figure E5 shows two residual spectra formed by subtracting scaled versions of the lower-absorbance spectrum from the higher-absorbance spectrum. The scaling factors were determined in independent LSF analyses over the two analytical regions. Linear baseline corrections (offset and slope) were also determined during the analyses, so the mean value of each residual spectrum is zero. (These calculations were performed by using the ANOVA data analysis tool in Microsoft Excel 97). The residual squared area (RSA) values for the residual spectra are defined in Equation C2, and the related calculations are presented in Table E1. (The calculations also were performed in the program Excel by applying the function STD EV to the regression residuals.)

Figure E5. Water Residual Spectra NIOSH Manual of Analytical Methods, Fourth Edition