First injections give low respons on MS

[PMDOC]

Hi

I have a peculiar problem. The first samples of the days consistently give only 50% of the respons (in CPS). Slowly, taking about 2-3 hours, the response improves and thereafter remains stable and predictable.

This phenomena does decrease sensitivity for samples ran the first hours, but as all substances and stable isotope standards(I measure) are affected equally this doesn't seem cause quantification errors.

I think I have narrowed it down to an MS problem. Actually, the response takes time to build up even when bypassing the whole HPLC system, and doing a continuous flow injection. There is no carryover when doing HPLC.

The pressure of the MS is stable. Before I run any test I usually warm the MS instrument for 30-60 minutes. My substances are mostly pretty non-polar steroids. The instrument is an MSSCIEX API 4000, otherwise working without problems.

Have anyone has such an experience? Is this normal? What is the cause? How do I troubleshoot or fix this?

[sassman]

It is normal for a mass spec to take a few runs to stabilize. 2-3 hours is a pretty long time though. I normally just run a few blanks at the beginning and I'm good to go. You said that you let the MS "warm up" for 30-60 minutes before use. Does that mean that you turn it off daily? It is best to leave the MS on all the time. In this case, it doesn't need to warm up (except for the source which warms very quickly).

It also could be related to LC conditions slowly stabilizing over time. What steroids are you looking at (androgens, estrogens, etc.) and what are your LC conditions (mobile phase, gradient, flow rate, etc.)? Also be sure to adequately purge your LC system with your mobile phase before starting. I have found that on our system, it takes two purge cycles to get the mobile phase from the solvent bottles to the pumps.