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- Posts: 14
- Joined: Fri Feb 22, 2019 4:34 pm
I've been working for the last several months (year, really) developing an EPA 525.2 method for my laboratory, and I feel like I could benefit from some guidance. My problems at this point are two-fold:
1) My IS/SS recoveries are generally somewhere between 50-80%, which on the high end is sufficient but not something I consider functional. My worst recoveries tend to be for Acenapthene-d10, and 1,4-DM2NB.
2) On any extracted sample I get substantial background. It consists mostly of poorly-resolved "abstract art" rather than sharp peaks, and is scattered across the whole chromatogram (but more on the back end than the front).
Preparing my samples consists of taking ~1 L of H2O and making sure pH is <2 and no Chlorine is present, dechlorinating and/or adjusting pH as necessary if they aren't. I then add 100 uL of 50 ppm IS/SS mix to 5 mL MeOH, and then adding that to the sample. I add the IS/SS mix to the MeOH because the IS/SS mix is suspended in EtOAc, and I observed that if I added it EtOAc directly to the water sample then it could bead and not mix into the water, whereas MeOH has little trouble doing so.
My extraction method is modeled on the EPA method, though my volumes are in general larger because the automated extractors I use cannot reliably measure volumes smaller than 8 mL. It is:
* Condition with 8 mL 1:1 DCM/EtOAc, Soak 60 sec, drain completely
* Condition with 16 mL MeOH, Soak 60 sec, start drain but immediately move on to next step
* Condition with 8 mL Reagent H2O (right now HPLC water), Soak 0 sec, drain 2 sec but then move on so that liquid remains above disk
* Load sample
* Wash sample container with 8 mL Reagent H2O, Soak 0 sec, move on immediately
* Wash sample container with 8 mL Reagent H2O, Soak 10 sec, drain completely
* Dry disk for 150 sec
* Wash sample container with 12 mL EtOAc, Soak 60 sec, drain at higher vacuum to pull solvent completely through disk even in the presence of residual H2O, for 10 sec to remove most of the EtOAc
* Wash sample container with 12 mL DCM, Soak 60 sec, drain 8 sec to remove most of the DCM
* Wash sample container with 12 mL 1:1 DCM/EtOAc, Soak 0 sec, drain completely and keep vacuum running to remove any residual solvent
After the extraction process I take the 36 mL of eluate and pour it through Sodium Sulfate (in a funnel stopped up with glass wool) and into an evaporation tube to dry it, then blow it down under heat until it's at a final volume of 0.75 mL. I transfer that volume to a GC vial, and rinse the evaporation tube with ~0.4 mL of EtOAc before transferring 0.25 mL of that to the GC vial (though generally 0.25 is the whole volume I get at the end because some sticks to the glass and some evaporates while I'm working with it).
Right at the end of the process I add 10 uL of 500 ppm p-Terphenyl-d14 to my sample.
Then, I run it on my GCMS. The chromatography/spectroscopy side of things works beautifully for calibration standards and other unextracted samples (RSDs at 5-15%, R^2's generally at 0.999+), so I'm not too worried that my instrument side might be giving me problems. What I'm worried about is the extraction side of things.
What I think I lack is simply experience with this method. Because of the way I was brought on, I had no one in the lab with experience with this method to ask for hints and pointers, to check if I'm doing things right, and so on. If there are people out there who could help, that would be very much appreciated. The method is straightforward enough, but that doesn't mean that I'm 100% confident I'm not missing some small-but-critical detail.
So, what do y'all think?