It is that time of year again when laboratories are fulfilling accreditation requirements for the methods that they offer. One of the requirements that must be met for each method is called proficiency testing (PT). If you are not familiar with proficiency testing, it is a sample purchased from an approved vendor to evaluate the ability of a lab to meet the acceptance criteria of the method. If the labs’ results are out of the PT samples acceptance criteria, they can redo the testing.
Throughout my career, I have worked with many organic methods that have required proficiency testing. I understand the process and know that with many techniques and instrumentation through the workflow, mishaps happen, even if you follow the method and the PT vendors instructions. The method that we receive the same question on every year is EPA method 1664 by solid-phase extraction (SPE). The question I am asked all the time with both manual and automated SPE is always the same, “we are running method 1664 and we cannot pass a PT sample”. Customers have followed their method and the PT vendors instructions and have taken great care in their workflow to avoid mistakes but continue to receive recoveries in the failing criteria.
As a customer, this can be very frustrating and for the lab devastating if they cannot demonstrate compliance. With everything from perfect techniques to following the vendors PT instructions, you would expect passing results – who wouldn’t?
With the number of approved vendors offering EPA Method 1664 proficiency testing samples I have seen many instructions and they are consistently the same way. That sounds great, standardizing instructions, right? Yes, if you run method 1664 by liquid-liquid extraction (LLE). Yes, I would say most of the instructions packaged with PT samples are tailored for LLE and not SPE, so how does that affect the PT sample and result in consistent failures?
If you apply the LLE instructions to a SPE extraction process you will fail a PT sample every time. With LLE one is instructed to pour the entire content of the PT sample bottle out into a separatory funnel. Then rinse the sample bottle with the extraction solvent and pour the extraction solvent into the separatory funnel.
If one applies the same thinking to SPE one would open the sample bottle and pour extraction solvent to rinse the sample bottle, then process the contents of the sample bottle with SPE. This would result in the water sample and extraction solvents interacting with the analytes. This interaction prior to loading the sample will result in analytes of interest and the solvent going right through the disk and into the water waste yielding little to no recovery.
To effectively run this procedure with manual SPE you need to follow these steps. Condition your disk with the appropriate solvents, load your sample on the disk and, airdry the disk. Then pour the extraction solvent into the PT sample bottle and rinse the interior of the bottle maximizing recovery of analytes, then pour the extraction solvent from the bottle onto the disk to soak and elute. You can additionally rinse the inside of the sample cap and pour that on the disk during the elution step as well. With automated SPE the only difference is you will need to manually rinse the sample bottle cap and pour the extraction solvent into the disk holder during the soak and elution step.
By applying these steps one can be successful in processing PT samples and avoid mishaps and delayed accreditations. If you have any questions or comments about performing EPA Method 1664B or conducting Oil and Grease extractions, please reach out as we are happy to help with your laboratory workflow. For more information on tips for improving your oil & grease recoveries, read our previous blog post.