Simazine recovery issues

Discussions about GC and other "gas phase" separation techniques.

5 posts Page 1 of 1
I have been running EPA 525.2. It is a 1L water extraction with Millipore Sigma's Supelcoclean ENVI-18 SPE tubes (cat#505706). For a fortified matrix we are spiking 1L of water with 400uL of a 2.5 ppm standard made from Absolute cat#96675, lot#051321 (custom standard with all analytes of interest at 500 ppm). After NVAP we have a 1mL sample spiked at 1.00 ppb.

A calibration curve is prepared using Accustandard M-525-REF-EA-5x, lot#220101139 by preparing a 20 ppm stock and then using this to prepare a 9 point calibration curve ranging from 0.02 - 5.00 ppb.

GC-MS maintenance has just been performed (sources was cleaned with Al powder+MeOH mixtures and then rinsed with water, acetone, acetonitrile, then MeOH). A new Restek column was installed (RXI-5ms, 30m, 0.25mm ID, 0.50um DF) and tune check came back looking great.

Injection parameters are 1uL @275C, pulsed splitless, pulse pressure 30psi for 1min. Oven ramp is 70C for 1 min, ramp 25C/min till 200C then 10C/min till 320 then hold for 3 min.

All analytes in the absolute spike are coming back at 1ppb except simazine with is doubled. My only explanation is one of the standards I am using (accustandard or absolute) may not have the correct simazine concentration or has degraded. We are keeping both products away from light as much as possible and sonicating as suggested by the manufacturer prior to use.

If anyone has suggestions on why only simazine differs I would appreciate the discussion. Thank you.
Back in the 90's we ran a pesticide screen method using a GC with an atomic emission detector. We could do compound independant calibrations with pesticides containing hetero atoms. Simazine had variable concentrations from vial to vial. Out of a custom mix with 10 vials 1 or 2 could have less than 1/2 the concentration of the others. I don't recall if there was any resolution to the problem.
We found, and it was confirmed by the standard manufacturer, that Simazine will fall out of solution of you refrigerate the high concentration stock solutions. The manufacturer suggest storage at room temperature, while the method says to store in a freezer. Once we took the stock standards out of the freezer and began storing them in a drawer the problem went away.

Once Simazine fell out of solution in the freezer it didn't seem that any amount of sonication would bring it back.

Two separate auditors also told us that if the manufacturer suggest storage at ambient then it overrides the method suggestion of storage frozen. I have been running 525.3 for these analytes for a couple years now and had no trouble leaving the standards on the instrument for days as long as they are recapped right after injection.
The past is there to guide us into the future, not to dwell in.
Good to know.
Thanks James. We do currently store our standards in the freezer at -18C. I'll move them out and store new standards I order at the suggested temps the standard manufacturer suggests and we'll see if my simazine issues is resolved.
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