Chromatography Forum

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.

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

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?

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...

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.

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.