Page 504 - Tandem Techniques

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spectrometer. The concentrating system was tested by first separating a sample of peptides by simple
capillary electrophoresis. The results obtained are shown in Figure 13.4(A). The sample concentration
was about 10-5 M for each protein. If the sample was further diluted by a factor of 10 (10-6 M), it was
found that most of the proteins were not detectable. An example of the mass spectra obtained are shown
in Figure 13.4(C). The preliminary isotachophoretic concentration and subsequent capillary
electrophoretic separation was accomplished as follows. First the column was filled with the
background electrolyte followed by injection of 750 nl of the sample dissolved in ammonium acetate
buffer. The end of the column was then returned to the background electrolyte reservoir. The
ammonium ions (which have high mobility) moved ahead of the sample when the field was applied. At
this stage, the sample ions stacked behind the ammonium zone in a narrow band and began moving at a
constant velocity, as in isotachophoresis. The ammonium ions continued to move through the slower
background electrolyte, and consequently, the concentration of ammonium ions began to decrease
below the point necessary to sustain isotachophoretic migration. At this point the zones continued to
separate by capillary electrophoresis. The separation obtained is shown in Figure 13.4 (B). Each protein
is present at a concentration of 5 x 10 M. Consequently, by employing isotachophoretic concentration,
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the practical concentration range for this particular separation was reduced by one to two orders of
magnitude. It is clear that the technique is useful, and could be used with the majority of electrophoretic
equipment presently available.

Capillary Electrophoresis in Tandem with the Ion Trap Mass Spectrometer

Capillary electrophoresis has also been combined with the ion trap mass spectrometer for the separation
and identification of some isoquinoline alkaloids by Henion et al. [8]. A arrangement of their interface
is depicted in Figure 13.5 . The low dead volume T that delivers the sheath flow, was made from PEEK,
and fitted with finger-tight unions. PEEK tubing was also used to act as a seal round the capillaries, and
as a guide between the sheath flow tube and the CE capillary. The CE capillary was housed in a