GC recovery issues for pesticide SPME

[drlfb]

Hi! I'm one of the many folks who appear to be new to using GC.

I am extracting OCs and PCBs from a spiked chicken plasma using SPME via OASIS HLB cartridges. My final volume is .1 mL in DCM. I analyze them on an Agilent 7890 GC-ECD with dual column. I'm getting low recoveries (<70%) and am uncertain if it is an issue with the extraction procedure, standards, or how I calculate % recovery.

Background:
This is a new extraction procedure that my lab has selected to reduce the potential for loss.
Our standards were stored in clear glass vials in a dark cabinet at ambient temperature. We suspect that we need to make new ones and place in amber vials in cold storage. Thoughts?
We are uncertain if we need to use the nominal or measured standard values to calculate % recovery. Thoughts?
When the standards are run alone, the values are all over the place and do not match the nominal concentration. Is this normal?

Any other advice would be greatly appreciated. Thanks!

[rb6banjo]

Can you give us more of an idea of exactly what you do? It's not clear to me how you're marrying the SPME and the SPE (Oasis HLB) techniques. Is it just a solid-phase extraction followed by reconstitution of the extract in methylene chloride?

[MSCHemist]

You can't use dichloromethane with SPME as exposure to halogenated solvents destroy the fiber. OTOH any solvent other than water or methanol or a nonvolatile solvent (for headspace) will also bind to the fiber and displace analytes leading to low and inconsistent recovery. I once tried making a calibration standard in acetone and even though I ended up with only a few mcl of acetone in 2 ml of water in the vial I saw a 30% drop in area count of most of the more volatile analytes (up to about limonene).

I'm afraid this method need a complete redesign.