Sequencing peptides by PSD analysis in MALDI–TOF is less straightforward (and in a
large percentage of experiments is unsuccessful) than tandem MS on a quadrupole ESI or
ion trap instrument. At anygiven setting of the reflector/ion mirror,charged fragmentsof
a particular range ofm/zare focussed in the reflector (Fig. 9.16). Fragment ions ofm/z
above and below this narrow range are poorly focussed. Therefore, since only fragment
ions of a limited mass range are focussed for a givenmirror ratioin the reflector, a
number of spectra are run at different settings andstitchedtogether to generate a
composite spectrum.Types of MALDI sample plates
MALDI sample plate types that are available include 100-well stainless steel flat
plates. These are good for multiple sample analysis where closeexternal calibration
is used, that is the use of a compound or compounds of known molecular mass placed
on an adjacent spot to calibrate the instrument. It is also easier to see crystallisation of
the matrix on this type of surface.
Four-hundred-spot Teflon-coated plates have particular application for concen-
trating sample for increased sensitivity. Due to the very small diameter of the spots, it
is difficult to spot accurately manually but these plates are good for automated sample
spotting. Only in the centre of each spot is the surface of the plate exposed therefore
the sample does not ‘wet’ over the whole surface but concentrates itself into the centre
of each spot as it dries. Gold-coated plates with wells (2 mm diameter, see Fig. 9.10b)
are good surfaces on which to contain the spread of sample and matrix when used
with highly organic solvents, e.g. tetrahydrofuran (THF) preparations for polymers.
They also allow on-plate reactions within the well with thiol-containing reagents that
bind to the gold surface.9.3.11 Novel hybrid instruments
There are a number of commercial developments of hybrid MS instruments that involve
coupling an electrospray, ion trap or a MALDI ion source with a hybrid quadrupole
orthogonal acceleration time-of-flight mass spectrometer (Fig. 9.17). This potentially
leads to improved tandem MS performance from MALDI phase samples.
The intention of the development of these instruments is to combine the best
features of both types of ion source with the best features of all types of analyser in
order to improve tandem MS capability and increase sensitivity. Hybrid magnetic
sector instruments are also manufactured where the first mass spectrometer is a two-
sector device and the second mass spectrometer is a quadrupole.9.3.12 Fourier-transform ion cyclotron resonance MS
The recent development ofFourier-transform ion cyclotron resonance(FT-ICR)
mass spectrometry has great potential in analysis of a wide range of biomolecules.
It is potentially the most sensitive mass spectrometric technique and has very high375 9.3 Mass analysers