Dear people on the forum,

I am writing with a strange phenomenon for which I am eager to find a solution, but cannot make it myself.

We have a Waters Premier QTOF which we use routinely for accurate mass measurements with positive mode ESI and for GAG measurements with negative mode nanoESI. MassLynx 4.1 is installed on the IBM PC provided by Waters.

The problem is that we do not have proper signal in negative nanoESI. The test sample is 0.5 ng/uL LeuEnk. In ESI 10 uL/min and in nanoESI 0.6 uL/min flow rate is used.
Working with ESI source we have a signal of ca. 1500 cps in positive mode and 300 in negative mode. (Even this is not the best ratio...)
But working in nanoESI there is a 2000-3000 cps signal in positive mode and there is no peak for LeuEnk in negative mode. Yet, there are some signals, but only under 350 m/z. Over 350 m/z there seems to be only very low electronic noise, nothing else.

This problem is repeatable, changing from negative to positive and vice versa we see always the same signals and intensity.

I am curious if somebody had this problem before and could provide a solution or some ideas?

Thank you in advance!