In my last blog I gave a quick intro on OTM-45, the new PFAS in source air method proposed by the EPA. While the real meat of the method is in the sample collection and extraction sections, it’s worth taking some time to look at the chromatography. After all, without good chromatography sample collection and preparation are hard to judge.
OTM-45 gives very little guidance for the LC-MS/MS method. It states that it is fashioned after EPA 533 (one of the PFAS in drinking water methods), and gives an example solvent gradient to use (the same gradient from 533, to be exact), shown in Table 1, but otherwise instruct you to optimize resolution and peak shape.
|Time (min)||% 20 mM Ammonium acetate||% Methanol|
Table 1 – OTM-45 example HPLC gradient conditions.
In an era of conveniences, like next-day shipping, a 35-minute run just seems too long for me. Luckily, it’s easy enough to reduce that. Figure 1 shows the conditions used in this work, which are very similar to our SW-846 8327 method (a PFAS in water/wastewater method, https://www.restek.com/en/pages/chromatogram-view/LC_EV0578), with a slightly extended time at 95% B to catch some later eluters.
Fig. 1 – HPLC-MS/MS conditions.
Figure 2 shows the chromatogram and retention times for the method. The Force C18 column gives good peak shapes, even for the early eluting PFBA.
Fig. 2 – Chromatogram and compound retention times for OTM-45 analysis.
The PFBA peak shape requires careful attention to solvent choice and injection volumes, however. While OTM-45 states in section 10.3 that increasing aqueous content of the extract to better focus early eluters is not permitted, it also states in section 11.2.2 to bring up samples to 2mL after concentration using either DI water or 5% ammonium hydroxide in methanol. As Figure 3 shows, pure methanol injections can lead to split peaks for PFBA. Using an 80:20 methanol:water mix instead (matching the final solvent mix of EPA 533) gives some fronting at 5 µL, while dropping down to 3 µL gave a nice, symmetrical peak.
Fig. 3 – Trace of PFBA at 5 µL 100 % MeOH (left), 5 µL 80:20 MeOH:Water (middle), and 3 µL 80:20 MeOH:Water (right).
There is one downside to this method, and that’s the pressure required. The 1.8 µm Force column has a peak pressure of about 7000 psi, which might be too much for some HPLC systems. Luckily, switching to a 3.0 µm Force is an option, giving you pressures closer to 3000 psi, as shown in Figure 4.
Fig. 4 – Pressure for 3.0 µm Force column (top, maximum pressure ~ 3000 psi) and 1.8 µm Force column (bottom, maximum pressure ~ 6000 psi).
The change from 1.8 to 3.0 µm has little effect on the retention times. Figure 5 shows that early compounds have their retention time extended by about 6% or so, but it quickly drops off to be almost equal.
Fig. 5 – Retention time ratio of 3.0 µm:1.8 µm Force columns.
The price you pay for the larger particle size is a slight decrease of the average peak height. Figure 6 shows a comparison of relative peak heights from a 20 ppb standard, and while some compounds are higher on the 3 µm, the majority show better peak height, and therefore increased signal/noise on the 1.8 µm column. If your system can take the pressure, the 1.8 µm column will likely help you gain a bit of sensitivity.
Fig. 6 – Peak height ratio of 3.0 µm:1.8 µm Force columns.
While the focus of this blog series is OTM-45, the method demonstrated here should be applicable to a number of PFAS methods that have a large compound list, such as the new draft 1633 method. In part 3 of this series I intend to cover the other key part of the instrumental analysis, calibration, so stay tuned for that.