background MS

[ym3142]

I am running 1mM NaOAC 5% and ACN 95% through my LC MS. I saw a peak at 133. However, after I aquired this data, the MS is shown as below

[img][img]http://i35.tinypic.com/15g9kjr.jpg[/img][/img]

Any one can help me to assign these peak? Note there are 82 amu distance between adjacent checked set of peaks.

[Kostas Petritis]

The 82 Da difference is NaAc adducts which correlate pretty well with your mobile phase composition...

[ym3142]

Thanks, Kostas.

Interestingly, ACN happens to be 41. why not be adducts of ACN dimer?

What about the other two peaks e.g. say M=82+23=105, 2X82+23=187, 3X82+23=269 etc. then how to explain M-14 and M+24 peaks?

[Kostas Petritis]

ACN dimers have not reported in the literature to form adducts/clusters. It has been reported though that you can have ACN clusters with 41 difference which is not the case here. I do not see why you would get ACN dimers clusters instead of monomers?

Sodium acetate clusters have been reported thought in the literature...

Concerning the +23, -14 and +24... Another way to look at the data is that you also have a lot of 68 difference peaks that could potentially being assign to sodium formate clusters...maybe...

[lmh]

If you really want to convince yourself that it's not acetonitrile, just run the 1mM sodium acetate in methanol. You will still see the same peaks.

As a matter of interest, Bruker use clusters of this sort in their standard procedure for calibration of TOF instruments.

There are many possible explanations for the other peaks. For instance, the -14 peak may in fact be +68 (as you have an 82 difference), and if you have every used formic acid in your system, 68 is possibly an extra sodium formate.

[ym3142]

Thanks to both Kostas and lmh.

I am convinced that the base series is NaOAc. The -14 is about Na formate.

But I have not figured out what the +24 are yet.

The additional question is what caused the m/z 133 main peak in the tune page (Quattro Micro API of Waters) but this m/z 133 is absent in the spectrum I acquired.