Page:NIOSH Manual of Analytical Methods - 3800.pdf/39

ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 39 of 47

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Table E1. Results of RSA Analysis from Water Reference Spectra.

Analytical Region (cm-1)	Calculated Scaling Factor	Scaling Factor Uncertainty (1 σ)	Residual RMS Standard Deviation (absorbance)	RSA (absorbance -cm-1; Eq. B2)
1370-1295	1.636	0.003	0.00575	0.431
1215-1080	1.870	0.008	0.00069	0.093

An estimate of the LODs (ppm) for the two compounds—for the specified system configuration—can be made from the absorbance areas of the reference spectra over these analytical ranges (A_R,absorbance-cm^-1), the reference spectra concentration-path length products (CPP, ppm-m), the sample absorption pathlength L (meters), and the RSA values (absorbance-cm^-1) derived above. The absorbance areas were determined using a standard trapezoidal approximation without baseline correction. (For these spectra, baseline corrections lead to values which differ from those quoted by less than 3%, and have been neglected below.) Figure E6 shows the spectral features of TFE and DFE used to calculate the A_r for the reference spectra tfeav05.spc (CCP = 256.7 ppm-m) and dfea v05.spc (CCP = 1 97.8 ppm-m).

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. Mathematically, the LOD estimates are given by

$LOD={\frac {(CCP)(RSA)}{L{A_{R}}}}$ Equation E1

Table E2 lists the pertinent quantities and resulting LODs for DFE and TFE in the two analytical regions considered. The results indicate that the analytical region from 1215 to 1080 cm^-1 is likely to provide the most reliable TFE concentrations.

Table E2. LODs for TFE and DFE from RS A Analysis of Water Reference Spectra.

Analytical Region (cm-1)	Compound	Reference Spectra			System Configuration
		File Name	CCP (ppm-m)	Absorbance Area (abs-cm-1)	RSA (abs-cm-1)	L (m)	Estimated LOD (ppm)
1370-1295	TFE	tfe05.spc	256.7	16.03	0.431	10.0	0.69
1215-1080	TFE	tfe05.spc	256.7	13.97	0.093	10.0	0.17
1215-1080	DFE	dfe05.spc	197.8	8.72	0.093	10.0	0.21

NIOSH Manual of Analytical Methods, Fourth Edition