Page 524 - Tandem Techniques

Page 510

The inlet of the capillary was held at a positive potential, while the outlet was grounded. Typical
electrophoretic currents were 5-10 µA, and the volume of the capillary was about 4.3 µl. The basic
nebulizer was the Meinhard TR-30-A3, the nebulizing argon flow being accurately governed by a
precision flow controller. 4 to 5 cm of the external surface of the capillary was coated with silver paint
which served two purposes. First it provided an electrical connection to ground, and second it acted as a
cushion so that it fitted snugly inside the center tube of the pneumatic nebulizer. The end of the
electrophoresis tube was placed within 0.5 mm of the tip of the two tubes forming the concentric
pneumatic nebulizer. It was noted that under certain conditions contamination with silver ions could
occur. The spray chamber is usually employed to prevent large droplets entering the plasma, and to
restrict the aerosol transport to about 20 µl/min. The conical spray chamber with an impact bead is
typically used to prevent excessive aerosol loading. However, as the flow from the capillary was so
small, a spray chamber similar in shape but devoid of an impact bead was employed.

The interface was coupled to a standard ARL ICP torch and an ARL 34000 direct-reading optical
emission spectrometer. The outer argon gas flow was 15 l/min, and the intermediate flow was about 1
1/min. A power of 1.1 kW was used to form the plasma. The system was used to monitor chromium and
the total chromium and ion concentrations were about 100 µg/ml, and the electrolyte employed was
0.04 M sodium acetate at pH 8.2. The different elements were exclusively displayed by monitoring the
light emission at 259.9 nm (iron) and 267.7 nm (chromium) respectively. In addition, the
electrophoresis clearly separated the different species containing each of the two elements.

The separation of the two valency states of tin was also demonstrated in employing basically the same
interface but with the ICP torch coupled to a mass spectrometer. It was found possible to monitor 1 ppm
of tin in each valency state which was separated in 0.04 M sodium chloride, at pH 7.0. The
electrophoretic potential employed was 10 kV. The separation of the two valency states of tin was
complete in under a minute with more than baseline resolution, indicating that the system could be
developed to