USP Vitamin E Acetate Assay

[mjfrayer]

Hello

I am currently responsibly for getting the usp vitamin E acetate to work in our lab. I appear to get suitable chromatography but am having significant precision issues. On several occasions i am seeing multiple sample injections with a %RSD of greater than 5%. I have tried several different theories as to the cause of this problem including solution dynamics and light stability. Has anyone had similar problems with this assay?

Thanks

Matt

[Rafael Chust]

Are you doing Vitamin E by GC?

No wonder you are having trouble....

[mjfrayer]

Yes, the usp in its wisdom has this as a packed column internally standardized gc method with kind of a pick your own conditions sort of method.

[Consumer Products Guy]

Ah, yes, good old USP methods, nobody ever said these were the best, or were current. How many of us still have a GC that can accomodate packed columns???? Anyway, since it's packed-column, there is no inlet split to worry about. What is your inlet temperature, vitamin E acetate is a big molecule, you need the inlet hot. You may see a little free vitamin E eluting just before the E acetate. Vitamin E acetate is pretty stable, E tocopherol is not.

[mjfrayer]

A GC that can accomodate a packed column? I have 6 not to mention the two that are packed TCD instruments. I have been using a inlet temperature that is 275 C. The monograph specifies 10 degrees above the column temperature of 245-265. This would seem to be hot enough though maybe not.

[stappg]

Is your reproducability problem on peak area or peak area ratio? If it is on the PAR you indeed have a complex problem but if it is on PA I would suggest it is most likely more simple (maybe??). Try a new gas-tight syringe with a fixed needle. Does your inlet have a liner? If so replace it with a new one and a new septa. Good luck. I've just finished the same assay on an Agilent 6890N with a column about 5 years old and I get an RSD on PA of 0.7% for both peaks and an RSD on PAR of 0.1%.
Peter