GCMS standards question

[JMACIORO]

Hi Everyone;

I'm running EPA 524.2 method and have and issue. I'm making 50 ppm intermediate standards - one for gases (dichlorodifluoromethane, chloromethane, vinylchloride, bromomethane, chloroethane, trichlorofluoromethane) and another for other 524.2 compounds

I have made new intermediates on 2/19 and calibrated the instrument. The intermediate standards are stored in the freezer.

With every run I have been running a 5 ppb check made from the same intermediates.
First 2 runs are of my 5 ppb check are great - everything passes (ie recoveries are within 70-130 %)
but now my gases are failing badly. They are at about 50% recovery. I'm trying to figure out what the problem could be as it is the 1st time I'm having issue with this.
I'm running the analysis using purge and trap system with the K trap from Tekmar.
Any advice will be appreciated.

[James_Ball]

How do you store the standards?

Do you open the cap of the standard when you make the dilution into water for the check standard?

How long is the cap open if you do open it?

Volatiles standards will lose components to evaporation in relation to their boiling points, the gasses will go away faster than the methanol solvent, and the high boiling point analytes like toluene and xylenes will remain longer, sometimes even concentrating as the methanol evaporates.

The method states that you should make up the intermediate gases standard every two weeks, but it can last longer if you store it in a sealed vial with a septum and only pierce the septum to remove the aliquot for dilution. Before storing it again in the freezer(both intermediates should be stored at -18C for best life time), replace the cap with one with a new septa.

For the longest life of an intermediate I use these

https://www.sigmaaldrich.com/catalog/se ... partialmax

They have a teflon cap with a slide valve and septa so that you only open it enough to insert a needle to measure the aliquot to make the standard then slide closed to seal and prevent loss of analytes.

Even with that it is difficult to get more than a month from an intermediate standard, especially the gasses.