Chromatography Forum

Hey all,

I've been using a modified version of the USP method to quantitate natamycin for the past few months (really the only change from the USP method is no acetic acid in the methanol and less THF). There weren't any problems until we cracked open a new standard. Now all of my calibration peaks are twice the size they used to be and the y intercept has gone from +/- 20 to +/-500. I know logically that the y intercept shouldn't matter, but in this one instance it really makes a huge difference. Has anyone seen anything like this before? In addition the RT has slowly begun to drop sample by sample throughout a run.

Any help would be greatly appreciated.

Thanks!

Hello,

Can you verify the identity and purity of both the older and current standard.

Can I assume that you are using the same analytical column and the same mobile phase.

Do you use a column heater/chiller.

What happens when you inject a volume of the old standard, followed immediately after that run, by the same volume of the new standard.