Sequential (Multistage) Mass Spectrometry (MSn)
In the low energy (soft) ionisation methods (ESI or MALDI) spectra
often only contain peaks representing the intact molecular species
with very little fragmentation. Consequently the spectra are of little
use for structural characterisation. In these cases, induced
fragmentation is required using a technique like collision induced
dissociation (CID) with tandem (MS/MS) and sequential mass
spectrometry (MS
n). Sequential mass spectrometry MS
n
is a technique, that can only be
performed on ion-trap and FT-ICR type instruments (which allows the
re-fragmentation of product ions from MS/MS). The figure shows a
schematic of MS
n. In the first stage,
the normal mass spectrum is produced in the usual way. The isolation
of
the precursor ion is then performed which is fragmented by CID in
exactly the same way is in MS/MS. In MS/MS, the experiment would end
here as the product ions are consumed by the detector, in MS
n
though, the product ions are trapped allowing another isolation and
fragmentation to be performed resulting in the MS
3
spectrum.
This process can be repeated a number of times, resulting is a series
of MS
n spectra where 'n' represents the number of times the
isolation-fragmentation-measurement cycle has been carried out.