Persistent organic pollutants (POPs) like dioxins, furans, polychlorinated biphenyls (PCBs), and polybrominated diphenyl ether (PBDEs) are lipophilic and therefore may be found in fatty matrices like fish. In order to remove the high lipid coextractives, lengthy cleanup procedures that require multiple cleanup cartridges are often used. I recently presented work at the Dioxin meeting where we took a shortcut and analyzed PBDEs and other halogenated flame retardants (HFRs) in fish using a quick QuEChERS type extraction, and a PSA pass through cleanup (Get the poster here!). While the PSA pass through does a good job at removing a large amount of fat (we reduced the fat by 50-70%), there is still a considerable amount of nonvolatile residue remaining in the final extract. This is especially evident in the mackerel sample that we analyzed where 38 mg/mL of nonvolatile residue remains after the PSA cleanup.
In order to combat the negative effects of injecting so much nonvolatile material onto the column, such as decreased response of high MW compounds and increased inlet activity, we used a 10:1 split injection. A split injection (aka shoot-and-dilute GC) takes advantage of the sensitive detectors we have (ECD, MS/MS, HRMS) and keeps the system up at least twice as long when compared to a splitless injection. The flow through the liner is much faster when compared to a splitless injection and the majority of the sample is swept into the split vent and not onto the analytical column. Just remember to clean that split vent once in a while (it can get pretty gross)!
The chromatograms below highlight the advantage of using a split injection for fatty samples like fish. In all of the experiments I used a Sky Precision liner with wool and a 15m x 0.25mm x 0.10µm Rtx-1614 column. The sequence was the following: split injection of HFR standard, splitless injection of HFR standard, split injection of mackerel sample, split injection of HFR standard, splitless injection of HFR standard, 2 x split injection of mackerel sample, followed by the standards again. I repeated this sequence with increasing numbers of the mackerel sample injections in between standard injections until the system failed (I could no longer quantify my results). As you can see the response for the last two eluting peaks, syn and anti-Dechlorane Plus, are dramatically decreased after only three mackerel injections. The split injection standard did not fail until 15 sample injections.
The increased ruggedness of using shoot-and-dilute GC was also highlighted by Jack Cochran in this blog post where he made repeated injections of used motor oil. So if you have dirty samples and sensitivity to spare, try the shoot-and-dilute GC approach and think about all those extra things you could be doing instead of GC maintenance!