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The desorption ionization methods have enabled the widespread application
of mass spectrometry to large, nonvolatile and fragile molecules. Analysis
of large molecules and cluster ions is now possible. Mass spectrometers
can routinely detect compounds of masses greater than 10,000 Da. The
mass spectrum of bovine serum albumin (Figure 12) shows a molecular
ion cluster corresponding to the protonated trimer at m/z 199,290. Also
seen are ions representing the protonated dimer, the protonated molecule
and the doubly-protonated molecule.
As the desorption ionization methods were developed, there was a corresponding
need for mass spectrometers capable of analyzing ions of increasing
higher m/z values. Because electrospray ionization can put many charges
(z), usually in the form of protons, on amenable large molecules such
as proteins, it is an exception to this trend. Electrospray allows a
mass spectrometer with a very ordinary upper m/z range of 2000-4000
to analyze compounds of very high molecular mass. The spectrum in Figure
13 is of a protein with a molecular mass of about 29,000 Da. This spectrum
was obtained using a quadrupole mass spectrometer with a m/z range of
only 2000, thanks to the fact that the electrospray ionization process
added from 20 to 40 charges in the form of protons (z =20 to 40) to
the protein molecules. All the principal peaks in this spectrum result
from protein molecules of the same mass but with differrent numbers
of protons attached and hence different m/z values. Electrospray is
capable of ionizing biological materials with molecular masses of 10,000
to more than 1,000,000. Molecular mass can often be determined to a
precision on the order of one part in 10,000 or better. Because electrospray
is particularly compatible with liquid separation methods, it has become
a widely used method in biological and pharmaceutical analysis.
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