7
Figure 3. System Calibration Dynamic Range (0.05 – 1000 ppb).
Figure 2. Samples after QuEChERs Cleanup: From left to Right: Blank, Butter, Cheese, Egg, Milk and Fish.
QuEChERs procedure using a commercial kit (Phenomenex roQ Extraction Kit). An aliquot (500 uL) of the cleaned acetonitrile phase was transferred to an LC vial for analysis. Figure 2 displays an extraction blank and the fi ve sample types following sample preparation.
Table 1. PFAS Analyte List
Optional Solid Phase Extraction. A dispersive SPE cleanup was used to achieve a 10-fold lower level of quantitation. Four replicate samples of the egg matrix were spiked with the PFAS analyte mix at the 0.1 ng/g level and processed by the QuEChERs procedure. Following extraction, 500uL of the acetonitrile phase was diluted with 15 mL of water and loaded onto a preconditioned, weak- ion-exchange SPE tube (Phenomenex Strata-X-AW 200 mg). The analytes of interest were then eluted with 4 mL of 0.3%
NH4OH-acetonitrile.The eluate was evaporated to dryness, reconstituted with 500uL of acetonitrile and transferred to an LC autosampler vial for analysis.
LC-MS/MS Analysis. The chromatography was performed on an Agilent 1290 UHPLC system. The LC column employed was a Phenomenex Luna Omega 1.6 um PS C18 operating at 40ºC with a fl ow rate of 0.55mL/min and an injection volume of 20 uL. The mass spectrometer used was an Agilent 6460 QQQ. Various LC-MS/MS conditions were explored and an ammonium acetate/acetonitrile gradient (Table 2) proved to be optimum, resulting in a run time of approximately 4 minutes.
Results and Discussion
System calibration showed a linear dynamic response from 0.05 ppb – 1000 ppb with a lower limit of quantisation of 0.05 ppb as shown in Figure 3 and a calibration chromatogram at the 0.05 ppb level is shown in Figure 4. Recovery data for the fi ve matrix types is summarised in Figures 5 - 9. Four replicates of each matrix were spiked at the 1 ng/g level and prepared for analysis as described above (but were not subjected to the solid phase extraction process). Figure 10 presents the recovery data for four replicates of the egg matrix spiked at 0.1 ng/g and prepared as described above, but with the addition of the solid phase extraction step to increase method sensitivity.
The recovery data show good recovery for all fi ve matrices spiked at the 1ng/g level, with most analytes falling into the 80% - 120% recovery range. Precision is generally somewhat poorer for the higher fat dairy products than for the lower fat matrices. The recoveries on tuna fi sh are particularly good, considering the complexity of the matrix. In comparing the analyte recoveries from eggs at the 1 ng/g and 0.1ng/g levels (Figure 9 and Figure 10), both show comparable recoveries although, as expected, the higher spike level shows greater precision. Overall, the data suggest that the method has suffi cient accuracy and precision to potentially be used to assess environmental PFAS contamination of food products. Clearly, this is preliminary data and further development and multi-laboratory validation would be required to demonstrate such a purpose. However, the data clearly show that current sample preparation techniques, coupled with the power of advanced chromatography and triple-quad mass spectrometry represent a suitable workfl ow.
The Sequel
The earlier discussion showed the use of current analytical technology to address the challenge of environmental PFAS contamination of the food supply. However, care should be taken since experience with analytical chemistry teaches us that we will inevitably be facing further analytical challenges from the realm of the ‘unknown-unknowns’.
Figure 7. Tuna Recoveries (QuEChERs: 1 ng/g, n=4). Figure 6. Butter Recoveries (QuEChERs: 1 ng/g, n=4).
Figure 4. Chromatogram of 0.05 ppb Lower Limit of Quantization Standard.
Figure 5. Milk Recoveries (QuEChERs: 1 ng/g, n=4).
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