Hello, we have standard solutions of VOCs (benzene, toluene, ethylbenzene, xylene, styrene) that come to us in ampules. The solvent is methanol.
Once we open these ampules, we transfer them to septum-capped vials and put them in the freezer. Every time we need an aliquot from them, we take them out of the freezer, let it come to room temp, then open the cap (we have also tried poking the septum with needles) and take it back to the freezer. However, we have noticed that the concentration of these vials drops by about 20% after one week of storage. Is this normal? What can we do to avoid that?
What do you mean :"that the concentration of these vials drops by about 20% after one week of storage." Do you use GC with an internal standard? The reason might be in different volumes of sample injected in the instrument. You need to use an internal standard. You might have a different flow rate of H2 for FID. And finally, check the septum in an injection port, it might leak,
Your analytes are highly volatile. Possibly, a significant headspace of your analytes is forming in your vial that is lost when you open the vial.
For your analysis to be viable long-term I think it would be best to aliquot your standard solution across a larger number of vials (e.g. with an insert) and use the vials only once. Also, the type of septum on your vials may impact the ability to retain the volatiles. Some septum types may absorb your analytes.
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