Page:NIOSH Manual of Analytical Methods - 3800.pdf/35

ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 35 of 47 APPENDIX E. Example Calculations.

This Appendix uses an analysis of tetrafluoroethylene (TFE) and 1,1-difluroethylene (DFE) in workplace air to illustrate the calculations of residual squared area (RSA) and limits of detection (LODs). The topics of designing, applying and verifying, and correcting a spectral analysis are also addressed below.

The FTIR system configuration assumed for this set of calculations is given below. These parameters are consistent with the LOD values presented in Table 2.

E1.

The analysis is to be carried out in workspace air, so the only potential spectral interferences are due to water and carbon dioxide. Figure E1 illustrates example reference spectra of TFE, DFE, and water. (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. Analytical Region 2 (1215 to 1100 cm$-1$) may be the best choice for analyzing samples for both TFE and DFE (though the weaker features near 3000 cm$-1$ remain an option for DFE); it avoids the TFE interference with water, but necessarily includes some spectral overlap between the two analytes. Until the relative TFE, DFE, and water concentrations in the samples are known, it is unclear which of the two regions will provide better concentration estimates for TFE, so calculations for both regions are described below.

Estimation of the LODs for TFE and DFE requires a calculation of the residual square area (RSA) in the actual sample spectra over the selected analytical regions. The RSA represents an estimate, based on the assumed sample matrix and available reference spectra, of the residual absorbance (see Equation A6) which will be used to calculate the concentration uncertainties from the actual sample spectra.

The reader should note that the RSA and derived LOD are only estimates. If the actual sample matrix differs substantially from that of the sample matrix assumed in the performance of these estimates, the actual concentration uncertainties and LODs may differ substantially from the RSA and LOD values obtained in the calculations described below. For workplace air samples, water is the only interfering compound which is certain to appear in any of the analytical regions considered below. A realistic NIOSH Manual of Analytical Methods, Fourth Edition