ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 42 of 47
| Synthetic Sample Spectrum File Name | TFE Synth. Conc. (ppm) | TFE LSF Result (ppm) | TFE LSF 3σ Uncert. (%) | DFE Synth. Conc. (ppm) | DFE LSF Result (ppm) | DFE LSF 3σ Uncert. (%) | H2O LSF Result (arb) | H2O LSF 3σ Uncert. (%) |
|---|---|---|---|---|---|---|---|---|
| S001.spc | 0 | 0.02 | 52.5 | 0 | 0.07 | 13.7 | 1.12 | 1.23 |
| S002.spc | 0 | 0.03 | 32.2 | 0 | 0.14 | 8.95 | 2.24 | 0.80 |
| S551.spc | 25.0 | 24.41 | 0.26 | 25.0 | 24.30 | 0.29 | 0.87 | 11.8 |
| S511.spc | 25.0 | 24.43 | 0.25 | 5.00 | 4.88 | 1.43 | 0.87 | 11.5 |
| S151.spc | 5.00 | 4.88 | 0.41 | 25.0 | 24.33 | 0.09 | 1.07 | 3.04 |
| S552.spc | 25.0 | 24.43 | 0.26 | 25.0 | 24.35 | 0.29 | 1.99 | 5.20 |
| S512.spc | 25.0 | 24.45 | 0.26 | 5.00 | 4.95 | 1.43 | 1.99 | 5.11 |
| S152.spc | 5.00 | 4.90 | 0.43 | 25.0 | 24.39 | 0.10 | 2.19 | 1.56 |
| Average Abs. % Concentration Difference[1] | 2.24 | 2.34 | ||||||
| Average Abs. % Concentration Uncertainty[1] | 0.31 | 0.61 | 6.38 |
The LSF results for the non-zero TFE and DFE spectra are consistently good, being different from the synthetic concentrations less than 3% in every case. Listed in the Table for TFE and DFE are the average percent differences between the synthetic concentrations and those derived from the LSF analysis, which are 2.24% and 2.34%, respectively.
The concentration uncertain ties listed in Table E4 are the statistically determined 3σ values from the LSF analyses. Included in the last row of the Table are the average percent concentration 3σ uncertainties for the non-zero TFE and DFE spectra. These percent uncertainty parameters and the visual appearance of the LSF residual spectra are important indicators of the quality of the least squares analysis, and are discussed further in the following section.
Though they are not included in the averages presented in the last two rows of Table 3, the TFE and DFE results for the two synthetic spectra which contain no absorption features of TFE and DFE (S001 and S002) are of interest and importance. The LSF concentration results are small. (Their maximum is 0.14 ppm). They are smaller in each case than the LOD estimates of Table E2, and their corresponding 3σ uncertainties from this LSF analysis are smaller still. Unfortunately, no consensus exists on the exact mathematical relationship between the LODs calculated as described in this document and the 3σ concentration uncertainties. This statement is supported by the contents of Section A2 in Reference 3—a consensus document —which prescribes three different methods for calculating LODs (or, in the terminology of Reference 3, "minimum detectable concentrations"). These three prescriptions include one similar to the LOD method prescribed in this document, and one which is based on the concentration uncertainties derived from spectra similar to S001 and S002. In this limited example, two of the three prescriptions included in Reference 3 provide very different results, and they indicate that the LOD calculation described in this document provides the most conservative estimate—that is, the highest estimate—of the three LOD prescriptions of Reference 3.
NIOSH Manual of Analytical Methods, Fourth Edition
