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(b)

Laser

beam

+20 kV

Sample plate

Variable

grid

(a)

Plume of matrix and

sample ions

337 nm

Ground grid

Fig. 9.10MALDI ionisation mechanism and MALDI–TOF sample plate. (a) The sample is mixed, in solution,

with a ‘matrix’ – the organic acid in excess of the analyte (in a ratio between 1000 : 1 to 10000: 1) and transferred

to the MALDI plate. An ultraviolet laser is directed to the sample (with a beam diameter of a few micrometres) for

desorption. The laser radiation of a few nanoseconds’ duration is absorbed by the matrix molecules, causing rapid

heating of the region around the area of laser impact and electronic excitation of the matrix. The immediate region of

the sample explodes into the high vacuum of the mass spectrometer, creating gas phase protonated molecules of

both the acid and the analyte. The laser flash ionises matrix molecules: neutrals (M) and matrix ions (MH)þ,(MH)

and sample neutral fragments (A). Sample molecules are ionised by gas phase proton transfer from the matrix:

MHþþA>MþAHþ:

ðMHÞþA>ðAHÞþM:

The matrix serves as an absorbing medium for the ultraviolet light converting the incident laser energy into molecular

electronic energy, both for desorption and ionisation and as a source of Hþions to transfer to, and ionise, the analyte

molecule. (b) A MALDI sample plate.

Flight tube

Detector

15–25 kV gradient

Sample

on plate

Laser

+ + + + +

Fig. 9.11Principle of time-of-flight (TOF). The ions enter the flight tube, where the lighter ions travel faster

than the heavier ions to the detector. If the ions are accelerated with the same potential at a fixed point

and a fixed initial time, the ions will separate according to their mass to charge ratios. This time of flight can

be converted to mass. Typically a few 100 pulses of laser light are used, each of around a few nanoseconds’

duration and the information is accumulated to build up a good spectrum. With the benefit of a camera that is

used to follow the laser flashes one can move or ‘track’ the laser beam around the MALDI plate to find so called

sweet spotswhere the composition of co-crystallised matrix and sample is optimal for good sensitivity.

368 Mass spectrometric techniques