HPMC assay method - recovery issues

[Geof235]

Does anyone have any experience with hpmc assay?

I ran six sample preparations of a 2208 sample and got acceptable results but when I ran a 2910 sample I had high methoxy results.

The usp method is a little vague in that the injection volume can be between 1ul - 2ul. The amount of adipic acid can be between 60-100mg.

I tried both 1ul and 2ul injections and 2 appears to give me more consistent results. I read a technical document from Dow where they restrict the amount of adipic acid to 85+\-5mg (which I did do) but would the amount of adipic acid cause abnormally higher results for 1 substitution type but acceptable for another? Is there some other technique I should consider?

[Geof235]

I also noticed that the new USP method for HPMC has a 40:1 split. The old USP method used a Packed column and did not list a split ratio, assuming it was a splitless injection. The new method lists a DB-1 30m X 0.53mm, 3um column with a 40:1 split. A document I found online from DOW (before they updated the USP method) states that if you want to use an equivalent capillary method they recommended a DB-5 30m X 0.53mm, 3.0um with a 5:1 split. Besides the differences between DB1 and DB5, the USP recommendation of 40:1 split seems a little high for a 0.53mm column. Would such a high split ratio affect reproducibility of individual peaks?

It appears that my internal standard (n-octane) is lower for my samples than my standards, though I understand that is the point of an internal standard to account for injection variability.