PFAS X:2 FTS isotope issues

[RSWest]

We have had issues during PFAS analysis where the isotopes for the FTS analytes (13C2 4:2FTS, 13C2 6:FTS and 13C2 8:2 FTS) will either drop half response, or will drift high - sometimes up to 200%. The associated analytes will still pass CCV criteria, but this drift in the isotopes will cause the CCV to fail low or high. This will cause issues especially for UCMR5 analysis. In the past I have cleaned the source, thinking it was the problem, and then recalibrated. A few days later the isotopes often will go back to where they were initially - if they started failing high - then they drop back low, or vice-versa. Is very frustrating. Has anyone else seen this happen? If so, any ideas why? And what are you doing about it?
Thanks for any input.

[bcd_GCLCMSMS]

We perform an initial calibration at the start of every batch to lessen variation within a batch. The other issue is that the FTS isotope dilution analogs are spiked at a very high concentration versus analyte concentrations in the EPA method.

We have found that polypropylene sample vials need to be vortexed before the start of every batch, otherwise we can have reproducibility problems with FTS analytes (and others). The samples are in methanol/water and should be entirely homogeneous, so I attribute the observed variation in our laboratory to analytes "creeping" (partitioning?) up the walls of the vials at the surface of the liquid. Vortexing all of the vials as we load a batch into the tray solves the problem for us.

[RSWest]

We calibrate quite often, and with UCMR typically with every sequence. We vortex standards before going on the tray. We vortex EIS before we spike samples. We vortex spikes before spiking QC. We vortex NIS before we spike the extracts. We vortex the extracts after adding NIS before they go on the tray. We vortex everything! And are still seeing this issue. We will vortex the standards ran during the sequence, which most run overnight, and the opening CCVs will pass as do the mid-sequence CCVs and ending CCVs, but the samples ran between the CCVs will have FTS EIS issues. Lately it is just 8:2FTS EIS going high. The NIS recoveries will be 100%, so it is not the NIS going low that is biasing it high. Is befuddling. If you don't mind me asking, what instrument are you running on? We are running a Sciex 5500+. Also, how are you prepping? Are you using a block, or an automated system. We started on blocks and typically didn't have issues. But we have switched to an automated system (promochrom) and have had problems with 533. 537 on the PC look great! 1633 on the PC looks good. Our issues seem to center on 533. Thanks for any response/input!

[bcd_GCLCMSMS]

We are using a Sciex 6500+ and automated SPE on "PFAS-free" converted Dionex (now Thermo) Autotraces, although we have run them manually on the method recommended extraction manifold in the past.

"We will vortex the standards ran during the sequence, which most run overnight, and the opening CCVs will pass as do the mid-sequence CCVs and ending CCVs, but the samples ran between the CCVs will have FTS EIS issues"

^^^^^ Sounds like something from the extraction process, as only extracted samples have the issue. Is it possible the EIS is slightly high in the FTS components?

Have you tried preparing an unextracted "reagent blank" of the final 96:4 reconstitution solution with both spikes to compare the raw area counts? We have had spiking solutions concentrate over time and drift up from the calibration curve.

[RSWest]

We have since determined that it is the mobile phase chemistry that is biasing the 8:2FTS EIS high. We were using the 95/5 2mM ammonium acetate with acetonitrile organic as used in 1633. We are trying to standardize the mobile phases across the lab. However we did some tests and switched to 20mM ammonium acetate with methanol and the 8:2FTS EIS is fine. We will be calibrating using this mobile phase and see how the curve looks. This is the mobile phase we are using on our Agilent system and we have never had any issues with the 8:2FTS EIS.

We do have sporadic issues with low recoveries for PFUnA, PFDA and PFNA EISs, but I believe that is matrix issues.

Thanks for your input!

[ajc92]

We have had issues during PFAS analysis where the isotopes for the FTS analytes (13C2 4:2FTS, 13C2 6:FTS and 13C2 8:2 FTS) will either drop half response, or will drift high - sometimes up to 200%. The associated analytes will still pass CCV criteria, but this drift in the isotopes will cause the CCV to fail low or high. This will cause issues especially for UCMR5 analysis. In the past I have cleaned the source, thinking it was the problem, and then recalibrated. A few days later the isotopes often will go back to where they were initially - if they started failing high - then they drop back low, or vice-versa. Is very frustrating. Has anyone else seen this happen? If so, any ideas why? And what are you doing about it?
Thanks for any input.

The FDA's 2024 analyte and matrix method extension uses labeled standards for the X:2FTS with deuteriums from CIL to get around this issue. Might be worth a try.