Page:NIOSH Manual of Analytical Methods - 3800.pdf/33

ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 33 of 47 Figure D2. Normalized Reference Absorbance Spectra of TFE

Table D4 shows the results of a linear least-squares analysis using the average normalized spectrum as a single reference spectrum (with concentration-pathlength product 1.00 ppm-meters) over the analytical range 1050 to 1400 cm$-1$. The bias in the linear method is clearly evident in the fourth column of the Table, which indicates that the analysis yields concentrations which are slightly too high at the lowest concentration and slightly too low at the highest concentration. The average of the last column in the table, which represents the absolute percent difference between the actual and calculated concentrations, the “fractional calibration uncertainty” (FCU; see Reference 2), is 3.2%. This value represents the average error over the concentration range for these reference spectra to be expected from the linear analysis up to the maximum concentration-pathlength value used in the analysis (257 ppm-meter). If additional accuracy is required, non-linear analyses or corrections such as those discussed above (Appendix B, Section 9) can be employed.

NIOSH Manual of Analytical Methods, Fourth Edition