Fatty acids are important molecules in the human body because they are used as a source of fuel. There are many food sources of both “healthy” and “unhealthy” fatty acids. Many sources of dietary fatty acids come from fruits, vegetables, seeds, nuts, and animal fats. The QuEChERS methodology was developed to analyze pesticides in fruits and vegetables, however many scientists (including Restek) have adopted this approach to analyze pesticides in other commodities (tobacco, dietary supplements) or a further deviation of looking at other residues (PAHs, PCBs, PBDEs) in other food types (milk, tea, seafood). My colleague, Julie Kowalski, wrote a really nice 3 part series on QuEChERS for Separation Science that starts with the basics and finishes with using QuEChERS as a concept, or tool, instead of a direct method (A Primer, Beyond the Basics, The Concept).
Let’s get back to fatty acids. They are in foods, we want to test those foods, and they don’t play nicely with the GC injection port, and can completely overwhelm your target analytes. In the QuEChERS method, PSA (primary secondary amine) sorbent is used in the dispersive solid phase extraction (dSPE) cleanup to remove fatty acids. However, sometimes the capacity of the dSPE format is just not enough to provide an effective cleanup of the extract for analysis. I found this out first hand when I was trying to develop a method for determining PCBs and PBDEs in human milk using the QuEChERS concept. In my initial method development I was using GCxGC-ECD, but found that my target analytes were shifting retention times by almost 30 seconds! A quick look on the TOFMS and we confirmed that large amounts of fatty acids were still present in the extract.
We then moved to a 500 mg PSA cartridge SPE (cSPE) cleanup step to better remove the fatty acids present in human milk. The process was actually pretty simple. After a quick conditioning step with acetone, I loaded 2.5 mL of my extract on the cartridge, for this project my extract was in 1:1 hexane:acetone, and simply pulled it through the cartridge. After pulling vacuum for 2 min to dry the cartridge, I eluted with 5 mL hexane. Since human milk also has a significant amount of fat, I needed to further clean up the extract with a 500 mg silica cartridge. The cSPE PSA did a really great job of removing all of those fatty acids as you can see in the GCxGC contour plots of the NIST SRM below.