Chromatography Forum

Hello everyone,

I am working on the determination of a pharmaceutical in plasma, in the range of 10-2000 ppb. I had developed the method and I was ready to validate it. However, it suddenly appears that my analyte signal is being suppressed in low concentrations (below 100 ppb) by my internal standard (1 ppm), which is completely new, as I never had suppresion problems by my internal standard during method development.

If anyone can provide me with some helpful information, I would be grateful!

Also, is there some kind of test that I can do daily or weekly to check the mass accuracy and fragmentation of my mass spectrometer? (e.g. reserpine/ acid red 4?)

Can you reduce the internal standard to around 100-500 ppb (ng/mL), may be this can help you. Usually, I added the ISTD about 5-10% of maximum concentration of calibration.

Hello,

the problem just disappeared a week ago, suddenly as it occurred. Of course, I reduced the IS concentration to 250 ppb just to be sure. It appears that the instrument needed some time to perform well after the summer break, having done a quite thorough investigation this is the only reasonable explanation I can come up with.

Thanks anyway!

Stdev