Page 518 - Tandem Techniques

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electrophoretic migration. The mass spectra indicated a high abundance of sodium and potassium
adducts. These adducts significantly reduce the intensity of the simple protonated species. In fact, in
order to detect the small amount of peptide injected (3 pmol) it was found necessary to decrease the
resolution of the mass spectrometer to increase its sensitivity.

In theory, at least, the combination of the separating power of chromatography with that of
electrophoresis used in conjunction with the mass spectrometer, should provide a very powerful tandem
analytical technique indeed. In practice, however, it would appear that there is some way to go before
the real potential of this particular tandem system is fully realized.

Off-line Matrix Assisted Laser Desorption Ionization Time of Flight Mass Spectrometric
Monitoring of Capillary Electrophoresis Separations

Although several in-line electrophoresis/mass spectrometer tandem systems have been developed and
described, the in-line approach, employing a suitable interface, has some distinct disadvantages as well
as advantages. First, in most in-line systems, only a relatively small proportion of the sample is ionized,
and reaches the mass spectrometer, thus the sensitivity of the overall system is often relatively poor.
Second, the buffer solutions that can be used with in-line systems are limited, both in type and
concentration, including micellar additives. As a consequence, the optimum performance of many
forms of capillary electrophoresis cannot be realized when using in-line interfaces. In order to eliminate
some of these disadvantages, Walker et al. [13] introduced an off-line fraction collecting system, to
allow the total sample to be ionized by matrix assisted laser desorption ionization (MALDI), in
conjunction with a time of flight mass spectrometer (TOF-MS). The collection device was miniaturized,
and ultimately provided mass spectra from as little as 25 fmol of protein. In assessing this sensitivity in
standard analytical terms, however, it should be noted that the molecular weights of proteins generally
are very high. Theoretically, the combination of (MALDI) with (TOF-MS) can be made to provide a
very high sensitivity, and can