Chromatography Forum

Hi, (fairly new to MS/MS so please be patient with me)

We have a Waters Quattro premier and have been running several compounds with daughter scans.
However two compounds we have have been difficult to determine a suitable MRM.
Using the daughter scan with infusion (in methanol) we get no fragmetation at low collision energies, and as soon as the energy is taken over a certain level (roughly 20) the compound fragments into about 10+ other compounds. Overall this means that our MRM is less sensitive than an SIR.
We are primarily interested in sensitivity (<500pg/ml) as our LC method is very selective for the compounds.

Is there anything i can do to produce only one or two fragments to increase MRM sensitivity?

If you only want one fragment then monitor the parent ion - exploit the selectivityof the LC rather than relying on the MS-MS.

Peter

Tayzyboy,

There are compounds like the ones that you are analyzing and there is no much you can do about it. From the moment that you investigated different collision energies (with a step of 1 or 2) and you are not able to get any major fragment there are no other MS settings that you can play to increase your MRM sensitivity.

You could try to make a complex/adduct and then fragment it to recover your molecular ion but it would be fairly complicated and there might issues with the quantitation...

If you mean absolute counts, then splitting into 10 signals in MS/MS is not good. But in MS/MS, often the Signal/noise ratio can increase even though the absolute signal decreases. Depends on the chemical noise from the instrument from solvents, column bleed, etc..

You didn't say whether using M+H or some other adduct. Addition of ammonium acetate might give you the ammonium adduct which could give you different fragmentation?

Also, you might take a look at the compound in negative ion mode.