13. Calculations
13.1 Measure the response, Rix (peak height or area, integrator counts), for each peak common to both the chromatogram of the test specimen being analyzed and that of the relevant standard obtained under the same chromatographic conditions. Calculate the concentration of PCB resulting in each peak, i, in the chromatogram of the sample being analyzed from the following equation.
Calculate the total PCB content, C, by summing the concentrations associated with each peak in the chromatogram, as follows:
C = ∑i Ci
13.1.1 Standard and appropriate ranges of peak retention times (a ≤ i ≤ b) are described in 13.2 and 13.3.
NOTE 6 - (V x d) may be used in place of W x. See 12.3.3.
13.2 When the chromatogram of a test specimen being analyzed clearly shows it to contain only a single Aroclor (1242 or 1254, or 1260), calculate the PCB content using the response, Ris, found in the chromatogram of a comparable single Aroclor standard and the values of PCB content associated with the same peaks in the chromatogram of that standard (Table 1, Table 2, or Table 3). The relevant peaks for Aroclor 1242 have relative response times of 11 ≤ i ≤ 146; for Aroclor 1254, 47 ≤ i ≤ 232, and for Aroclor 1260, 70 ≤ i ≤ 528.
13.2.1 The higher resolving power of mega-bore columns may result in additional peaks beyond those identified within the Webb & McCall paper. Except in those specific instances where an identified peak is obviously resolved into two similarly sized peaks requiring grouping together to address the entire assigned mass, daughter or satellite peaks may be ignored without significant impact on the final calculated value. The assumption is made that by assigning the entire mass to the major or parent peak and ignoring smaller peaks, a multi-level calibration will generate more consistent results.
13.2.2 A simplified, but more approximate calculation may be made when the test specimen contains only a single Aroclor. Calculate PCB content as follows:
where Rpx and Rps are the responses of the larger or more cleanly separated of the peaks in the chromatograms of the test specimen being analyzed and of the standard. The total PCB content calculated in this way may be incorrect, because the PCB content reflected by any individual peak has been reduced or relatively enhanced by specific PCB removal processes. The response of that particular peak may have been enhanced by unremoved impurity, or the response of that particular peak may have been affected by some instrumental anomaly. The reported result should be the average of that calculated for a minimum of three peaks in the chromatogram of the test specimen being analyzed. This simplified calculation should not be used in circumstances where maximum accuracy is required.
13.3 The PCB content of test specimens containing mixtures of Aroclors should be calculated using standards of all three Aroclors. The PCB concentrations measured by peaks i = 11 through 78 should be calculated in accordance with 13.2 using values of Mi and Ris derived from an Aroclor 1242 standard; those measured by peaks i = 84 through 174 using values derived from an Aroclor 1254 standard; and those measured by peaks i = 203 through 528 using values derived from an Aroclor 1260 standard. The total PCB content is the summation of the concentrations measured by all the peaks in the chromatogram as follows:
C = ∑i Ci
where:
i = 11 to 78 + 84 to 174 + 203 to 528.
13.3.1 The retention-time windows are convenient for the purpose of quantifying total PCB content in mixtures. Peaks in the chromatogram of the unknown test specimen are then compared with comparable peaks in the most relevant standard chromatogram. However, the PCB content in the window i = 11 to 78 is not the total content of Aroclor 1242 because Aroclor 1242 also contains PCBs having longer retention times. Similarly, the Aroclor 1254 and 1260 concentrations are not defined by the PCB contents resulting from the two longer retention-time windows. More complex proportionating procedures are needed to calculate individual Aroclor concentrations in test specimens containing mixtures. This method is directed toward determining the total PCB content.
13.3.2 A skilled analyst may readily recognize the components of a mixture of Aroclors found in an oil test specimen. However, calculation of the individual concentrations of the components is inherently somewhat imprecise because of the overlap of peaks in the chromatograms of the several Aroclors. It is recommended that the total PCB content be calculated to the nearest part per million and the relative ratios (1:1, 3:1, 1:2, etc.) of the individual Aroclors present be noted. An impression of undue accuracy in the determination of individual Aroclors is avoided.
NOTE 7 - The response factors (Mi/Ris) for peaks with i = 117, 146, and 174 in the chromatograms of Aroclor 1254 and 1260 are somewhat different. Calculation of the concentrations of peaks 84 ≤ i ≤ 174 should be based on the use of Aroclor 1260 as the standard if Aroclor 1254 is clearly a minority component (that is, if peak (shoulder) 117 is distinct; if peak 98 is indistinct; if the height of peak 104 is distinctly less than that of peak 84, etc.) and if maximum accuracy is required.
NOTE 8 - Calculation of PCB content based on a mixed standard (11.2 (Note 7)) is useful in the routine analysis of mixtures containing Aroclor 1254 and 1260. The differences due to different response factors are minimized using the mixed standard.