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Due to its cis-trans isomerism, perfluoromethylcyclohexane elutes as a peak doublet around 31 minutes. Since the isomers cannot be resolved by the column and temperature regime, it is best to integrate the area under both peaks. Other features of a typical chromatogram include a peak around 30 sec due to the co-elution of the permanent gases (O2, CO2, CO, etc); a small peak before 11 minutes due to the hexane solvent; and peaks at 22 (doublet), 24 (single), 29 (single), 33.5 (single), and 35.5 minutes (single) from other perfluorocarbons (perfluoromethyldecalin, and perfluoro-tert-cyclohexane) occupying nearly 10% by volume of the original PFMD reagent.
The ECD is extremely sensitive to changing concentrations of PFMD. A vial of 10 ppm in the headspace is enough to saturate the detector. In most cases, the concentrations analyzed in samples will be sub ppm level. Injecting high levels of tracer will cause significant carryover between samples, especially if the A/S is used.
Figure 7: Typical chromatogram for PFMD
Figure 8: Chromatogram in which PFMD has saturated the detector
Figure 9: Typical PFD chromatogram. Cis-isomer elutes around 10.1 min, Trans-isomer 10.36 and the contaminant peak around 11.1 minutes. Expeditions 398, 399 and 400 used the Cis-isomer for PFD.
Analytical Batch
The analytical batch is a group of samples run together with a single set of QC parameters, such as calibration/calibration verification, blank, and other QA/QC samples.
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