Chromatography Forum

Hello..Just wondering, if I prepare organophosphorus pesticide standard solutions (2000 ppm/50 mL) in acetone and store it in a freezer at -20 degrees celcius, how long will the standards last before it cannot be use anymore? How do I know that the standards will not evaporate when I open the cap during analysis? Just wondering...

Thanks.

I think you'll need to determine yourself, like run a study of old and freshly-prepared standards, and compare Peak Area v. concentration. You'll need to set in advance some type of success criteria, like within 3% RSD or something.

If you simply lose solvent, your concentration of the standard goes up and therefore your samples assay low.

If the pesticide degrades, your concentration of the standard would go down and therfore your samples assay high.

You could look up the stability of your pesticide in the literature. The standard solutions from Dr. Ehrentstorfer are usually certified for 2-3 yrs at room temperature in the dark. You should be on the safe side, if you use your standards for 1 year.

\

Depends on what compounds they are. Like Fenthion, it degrades very easy. Chlorpyrifos can last very long.

Thanx..really appreciate all your comments.

The expiration dates on pesticide standards are very conservative, assuming reasonable precautions. The only way to determine how long your standards will last is to do a study as stated above. One of the regional EPA labs did a study of standard stability in a -40C freezer, and standards showed no significant change over a period of 3 to 4 years if my memory is correct.

"How do I know that the standards will not evaporate when I open the cap during analysis?"

Just be very cautious with acetone standards. Recap punctured vials as soon as possible and mark your meniscus with a sharpie. I keep track of all my standard volumes this way.