MS ion's off by 1 m/z ?

[MarkRob]

has anyone had this problem?

I tuned as normal (DFTPP) but all my analytes are off by one (it added one to each fragment) eg. 180 --> 181

there's a lot of noise after my cleaning, other than that everything is the same. (Seeing a lot of 58 peak in noise)

any help is appreciated.

thanks!

[Don_Hilton]

My first impression is that your mass calibration is off - do you have an automatic PFTBA tune? IF so how does that look?

If you have a lot of m/z 58 in the background, an autotune may have located an incorrect mass to use as a reference. Residual hexane or other hydrocarbon solvent would leave masses 55, 57 and many other ions. If you did not bake the ion source parts after cleaning, you have two options: 1) let the isntrument pump down over night and see if it is clean enough or 2) break vacuum and take another shot at making sure the source is clean. If you did not use a hydrocarbon solvent, you may have a contaminant from another source.

You have not said what kind of instrument you have, and some tune quite differently from others. If applicable to yours, set to default tune parameters and autotune. See how it looks.

[MarkRob]

Hi,
thanks for the response.

I'm running an old 5972. I thought that maybe the column was breaking down.

I"ll try cleaning again. I also have a new column coming today.

thanks for your help.

-M