Page 521 - Tandem Techniques

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smoothing. Separation efficiencies were reported to be 130, 000 theoretical plates.

Off-line fraction collecting procedures produce excellent results and, due to the method of fraction
collection, the tandem arrangement is very similar to a disk transport interface. Until there has been
significant improvements made to the present in-line interface devices, to make them more amenable to
a wide range of CE operating conditions, the off-line procedure developed by Walker et al., might well
be the preferred starting point for those analysts interested in utilizing CE/MS tandem techniques.

Interface for Capillary Electrophoresis and Inductively Coupled Plasma Mass Spectrometry

Lu et al. [15], also employed a coaxial conductive liquid sheath as a return ground in capillary
electrophoresis, in a tandem apparatus incorporating an inductively coupled plasma mass spectrometer.
The overall tandem instrument was evaluated in the separation of metallothionein and ferritin. A
diagram of the interface is shown in Figure 13.16. The basic system was built round the ICP/MS
(Perkin Elmer Sciex ELAN 5000a), but the normal double-pass spray chamber, and cross-flow
nebulizer were replaced by a conical chamber, with an inner impact bead and a commercial glass
nebulizer. A Teflon adapter was machined to connect the torch box to the spray chamber outlet. Except
for nebulizer flow-rate adjustment, the operating parameters of the ICP-MS remained unchanged. In
normal CE/MS operation, the capillary tip is held at 3-5 kV to operate the electrospray ionization-
evaporation process. However, the capillary in the nebulizer must be held at ground potential, because
any electrical disturbance of the plasma will cause it to extinguish. The contact with ground potential
was achieved using an electrically conducting sheath liquid. A concentric glass nebulizer (manufactured
by Meinhard) was used and the CE capillary was inserted into the nebulizer central glass tube.

Since the glass tube tapered towards the tip of the nebulizer, the CE capillary did not reach the tip, and
terminated about 2 cm from the end.