Septum Life, Overview:
A nerve-wracking question GC analysts might have asked themselves at some point is, “Will my septum last through the entire weekend autosampler run?” In this blog we’ll look at some of the factors that help answer that question, but starting with, what is injection life anyway?
The simple answer is that septum life is the number of injections you can get out of a septum before you notice any degradation of your analytical results. I word it this way because in modern GCs, if an inlet leak is detected, which occurs depending on the brand at, say, 3 or 4 mL/min, it will shut down the instrument, ending the run but at least keeping un-analyzed samples in their intact vials. But before shutdown the septum could have been leaking up to 3 mL/min, which may or may not be noticeable in the results but isn’t good, and I’d call it clearly past its life.
A different failure mode is when septum particles, or “cores”, are punched or abraded into the inlet, causing volatile contamination that show up as “junk” peaks. This can happen well before leakage of carrier gas.
A third type of failure, rare these days, can occur from over-compression, where the tightness of the septum causes the needle to bend or jam. Even without jamming, over-compression can lead to unusually-high particle generation.
Fourth, fifth and sixth failure modes (well, not modes really, but causes of early failure) can be: condition or type of the needle or needle tip, alignment of the autoinjector “foot” with the needle guide (assuming non-manual injections), quality or not of the septum material, and in some cases, length of time in the inlet or extreme operating temperatures, either high or low.
Whehh… After all that you might say, “Okay, but how long will my septum last?!”. We can’t assign specs because of the tremendous number of variables involved, but IMHO, I would trust any of our premium septa, with reasonable equipment and rounded-tip needles, with a 200-sample run. That’s well below the average life we get in product testing - septa frequently achieve 1000 injections leak-free - but the average is, well, an average, and for a critical run I want close to certainty.
Early failures have been mostly engineered out of existence, however, and we’ll discuss that in future blogs!
Follow CRS BLOGables for more to come on the Septa Life Series.