Silicone Autosampler Vial Septa Cause Endrin Breakdown and Sample Contamination

In the previous blog on Endrin and DDT breakdown, I attributed problems to metal and silicone septum pieces originating from the GC inlet (the septum nut needle guide and the silicone septum) that ended up in the liner.  But you can also see breakdown and contamination arising from autosampler vial septa, even those that are PTFE-lined.  It’s when you do multiple injections from the same vial that you start seeing problems.

To highlight this issue, I analyzed an Endrin/DDT standard using GC-ECD with repeated injections from a 2 mL amber autosampler vial with a 50 µL insert (about 50 µL standard in the insert) and a regular 2 mL amber autosampler vial (containing about 1 mL standard).  Both of the vials were closed with a PTFE-silicone vial septum cap.

After 20 punctures through the vial septum over the 50 µL insert, there are many extra contaminant peaks and Endrin ketone has become very large, indicating a high degree of breakdown.   Silicone pieces from the vial septum not only ended up in the sample, thus contaminating it and leading to Endrin breakdown, but an “extracted” residue was injected onto the liner that caused increased Endrin breakdown even when a new standard in a fresh vial was analyzed.

For the standard in the 2 mL vial, the 20^th puncture and injection chromatogram is similar to the 1^st injection chromatogram.  Although the vial septum coring was likely the same for both samples, since the 2 mL vial contained much more sample, any deleterious effects from the septum pieces in the sample are less pronounced.  Still, it is not good practice to (1) puncture a sample vial septum multiple times, or (2) store samples with punctured vial septa.  I’ve discussed some options for protecting your valuable samples by using limited volume inserts in another blog, and my colleague Amanda Rigdon did an excellent piece, The Forgotten Septum - How to Correctly Diagnose the Source of Bleed Contamination.