Chromatography Forum

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

Are you doing Vitamin E by GC?

No wonder you are having trouble....

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.

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.

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.

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