How large a molecule can be analyzed?





      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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