Page 408 - Tandem Techniques

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the liquid chromatograph separated the different species of chromium, and the ICP/MS identified those
peaks that contained the chromium. The two oxidation states of chromium were well separated and both
the 52 and 53 isotopes could be clearly monitored and differentiated.

Dissolved organic materials in natural waters play an important role in the normal metabolic processes
that take place in aquatic ecosystems. Such material can act as a substrate in heterotrophic processes
(metabolic assimilation of decaying vegetable or animal tissue) or as enzymes, vitamins or toxins in
different organisms. In natural waters, most of the dissolved organic material comprises humic
substances. Humic substances contain a range of polar and dispersive functional groups and can,
therefore, interact strongly with heavy metal pollutants as well as PCBs and PAHs and pesticides. Thus
heavy metals can be transported by the humic substances over large distances, alternatively, they can
trap the heavy metals and act as a detoxification agent. It follows that the speciation of the different
heavy metals in water has very important ramifications. On the one hand, a particular species can
contribute to water toxicity, and on the other, it can help in the purification of water for drinking
purposes.

Rottman and Heumann [411 developed an apparatus to investigate the heavy metal interactions with
dissolved organic substances in natural waters, and employed a LC/MS tandem instrument with an ICP
interface and a special sample system. A diagram of their sampling arrangement is shown in Figure
9.53. The sample volume was 500 µl, and on injection, the sample passed through a guard column
packed with TopOffGel 3PW, and then through a TSK 3000 PW, glass, size exclusion, analytical
column. The eluent was monitored by a UV detector and the exit flow passed through a second sample
valve (used for calibrating the spike flow) to a Y junction where it was mixed with the spike flow. The
mixture then passed directly to the nebulizer of the ICP interface and was then monitored by the mass
spectrometer. Water from a bog, river and lake were examined. All the samples indicated clearly
fractionated organic substances. The components from the bog water are eluted fairly early in the
chromatogram, and as the separation was carried out on an exclusion column, this means that the
organic materials were of significantly higher