Chromatography Forum

Hi,

I am using LCQ-Deca quadrapole ion trap mass spectrometer.I want to know from all experts that it is true that we can get always molecular ion peak in LC-mass. We always get the main mass and it is never break in without attempting MS/MS.
it is always true???

Best regards

praveen

it's not always the case. the loss of water or formation of adducts with sodium may well happen...

You can do in-source fragmentation.
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"If your experiment needs statistics, you ought to have done a better experiment." Rutherford

You are lucky that you are using an instrument that is particularly good at ionising things gently without fragmenting them in the source. Nevertheless, source fragmentation is quite common, particularly for labile molecules.

Some molecules are so labile that it is almost impossible to ionise them in electrospray without extensive fragmentation. A common example is aspirin, which needs very gentle treatment if salicylate isn't to dominate the spectrum.

Different adduct ions have different stabilities. Typically sodium adducts are tougher than hydrogen adducts. This means that you might see a spectrum with strong sodium adduct ions in it, and strange low-molecular mass ions that coelute exactly with the sodium adduct, suggesting that they are derived from the same chemical, but are masses that you cannot create from the sodium adduct by fragmentation in the trap. Often they turn out to be fragments of the hydrogen adduct, which may have disappeared completely!

In your instrument you will also find that ammonium adducts are hard to trap, but can be quite labile.