Page 528 - Tandem Techniques

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sample volume is not without sacrifice the reduction in sensor volume of nearly three orders of
magnitude is accompanied by a twenty-fold loss in sensitivity.

An elegant demonstration of the use of the CE/NMR tandem technique is shown in Figure 13.18. The
two-dimensional presentation has the electrophoretic separation time represented on one axis and the
chemical shift on the other. The sample separated is a mixture of arginine, glycine and cysteine. A
complete spectrum was acquired every 16 seconds, with each being eight scans of a 512 point, 4000-Hz
spectral width.

Synopsis

Capillary electrophoresis is capable of generating extremely high plate efficiencies but, at the same
time, also produces solute bands of extremely small volume. Consequently, the association of the
technique with a spectrometer demands special interfaces and techniques. Because of its readily
available high sensitivity, the mass spectrometer was one of the first tandem instruments to be
associated with the capillary electrophoretic system. An electrospray interface was used in conjunction
with a quadrupole mass spectrometer. The combination was reported to have a minimum detectable
mass of about 0.1 pg. The sensitivity of the system was subsequently improved by reducing the
diameter of the electrophoresis capillary. A number of concentration enhancement techniques were
tried, including micro-cartridge extractors and isotachophoretic concentration. Capillary electrophoresis
has also been satisfactorily associated with the ion trap mass spectrometer, the magnetic segment mass
spectrometer and the time of flight mass spectrometer, using both the electrospray interface and the
atmospheric ionization interface. Several modified jet systems have been examined for the CE/MS
interfaces, to optimize on sensitivity without denigrating the electrophoretic resolution. The basic
CE/MS tandem system has also been used with microbore columns, using electro-osmotic flow as an
alternative to hydraulic flow. The in-line association of capillary electrophoresis with a spectrometer
has a number of disadvantages, as only a small proportion of the solutes are ionized and reach the
spectrometer, and the in-line