Page 406 - Tandem Techniques

Page 390

Shum and Houk [39] employed both size exclusion chromatography and ion exchange chromatography
to separate some metal proteins, and two species of selenium, SeO 32- and SeO43-. A detection limit of about

15 pg of selenium was easily achieved. Atomic spectroscopy has become the preferred method for the determination of trace

elements due to its high sensitivity and selectivity. However, atomic spectroscopy cannot differentiate between the chemical
forms and oxidation states of the element, and these are important, as they determine the toxicity of the substance, and the
role the element plays in biosynthesis. If a separation technique is used to differentiate between the different species of the
element or its oxidation state, the mass spectrometer can then unambiguously identify the peaks that contain the element of
interest. In addition, if isotope ratioing is employed, a quantitative assay can also be accomplished.

The column used for the separation of the metal proteins was 25 cm long, 2 mm I.D., (a GPC column)
and that used for the separation of the selenium species was an ion exchange column, 10 cm long and
1.6 mm I.D. The eluent from the columns passed through a fused quartz capillary, 40 cm long, 50 µm
I.D., to a direct-injection nebulizer; the distance between the inner capillary and the nebulizer tip was
about 25 µm. The ICP/MS tandem instrument was the Elan Model 250 (Perkin Elmer Sciex). A sample
of blood serum containing metal proteins of lead, cadmium, zinc, barium, copper, iron and sodium was
separated on the size exclusion column, and the elements monitored by, the mass spectrometer. It
should be noted that results for all seven metals were obtained from a single injection.

The detection limits for iron, copper, zinc, cadmium and lead were reported to be 3 pg, 0.7 pg, 1 pg, 0.5
pg and 0.5 pg, respectively. It is clear that, although the elements were unambiguously identified, the
chemical nature of the eluents can only be assessed from the chromatographic data. It would be possible
to employ an electrospray interface to another mass spectrometer, in parallel with the ICP/Ms
instrument, and this could provide further information on the structure of the associated protein.