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Using a Merlin MicroSeal Septum to Reduce Endrin and DDT Breakdown for EPA Method 8081b – Organochlorine Pesticides by Gas Chromatography

10 Sep 2011

EPA Method 8081b requires that Endrin and DDT breakdown percentages are less than 15% each to assure that the GC system is inert and ready for action.  Breakdown can be calculated by analyzing a 50/100 pg/µL Endrin/DDT standard and determining production of DDE, DDD, Endrin aldehyde, and Endrin ketone.  Breakdown is exacerbated by activity in the inlet liner, whether it comes from poor deactivation of the liner and its wool packing or from contamination by non-volatile residue from analyzed samples.  In addition, metal fittings in the inlet can cause breakdown of these sensitive analytes, especially under hotter inlet temperatures.

Inlet liner lifetime in organochlorine pesticide analysis as regards Endrin and DDT breakdown percentages is directly correlated with low breakdown to begin with, and sample cleanliness (or dirtiness).  The best way to achieve longer lifetimes is to start with as low a breakdown percentage as possible, which means choosing an inert liner and bottom seal (if present) and making sure the inlet is free of non-volatile residue.

What may not be obvious is that the autosampler injection process and the GC septum can contribute significantly to Endrin and DDT breakdown.  In an experiment using an Agilent 7683 autosampler with a dual taper 23/26s syringe needle (to minimize septum coring) on an Agilent 6890 GC-ECD fitted with a 4mm single taper with wool inlet liner and a Restek Thermolite septum, I saw a rapid increase in Endrin breakdown when repeatedly injecting an Endrin/DDT standard from a vial with a PTFE septum (NOT PTFE/silicone, but ALL PTFE).  Applying some detective work and getting some photomicrographs with the kind help of Mike Goss, I was able to attribute the degradation to pieces of metal and septum in the liner.  The metal comes from the autosampler syringe needle striking the metal needle guide on the septum nut and “cutting” pieces from the guide.  I was absolutely shocked at how much metal was atop the GC septum after only 100 injections!

Replacing the silicone septum with a Merlin MicroSeal septum is an excellent way to circumvent this problem and maintain Endrin/DDT integrity for longer periods of time, as seen in the graph below.  The MicroSeal septum is a wear-resistant fluorinated elastomer with an efficient dual-sealing mechanism that is not pierced like a silicone septum.  This eliminates the problem of septum pieces ending up in the liner.  In addition, I did not see any metal pieces in the liner when using the Microseal.  Note that a 23g blunt, conical needle autosampler syringe is specified for injecting through a Merlin MicroSeal.

While dirty samples can quickly corrupt the liner and lead to shorter lifetimes, when those lifetimes are measured by Endrin and DDT breakdown, you’ll still have more uptime if you switch from a silicone septum to a Merlin MicroSeal.

Additional reading:

GC Troubleshooting: Using “No Injection Instrument Blanks"

GC Inlet Liner Troubleshooting: When the problem isn’t your liner

The Solvent Shield: The Role of the Solvent in Blocking Active Sites


inlet seal close up
New septum nut for an Agilent GC splitless inlet before any injections have been made.


inlet seal close up
Used septum nut for an Agilent GC splitless inlet showing chips from metal caused by the autosampler syringe needle striking the guide.


metal pieces
Metal pieces on a GC septum that were "cut" from the septum nut needle guide by the autosampler syringe during injection.


wool
GC inlet liner wool contaminated with metal and silicone septum pieces from the autosampler injection process.


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