Coupled instrumental methods of analysis

A major concern in the development of coupled instrumental methods is the interface that links the separation module to the detector. Many factors must be addressed, including adjustments of the experimental conditions to accommodate the flow rate of gas or liquid from the chromatographic column. The nature of the liquid eluents is also important in the operation of the detector. Thus, the design of new and improved interfaces has been the subject of a number of reports. 

A direct injection nebuliser (DIN) was used to interface LC with ICP-MS (Shum et al., 1992a). The DIN transferred all of the sample into the inductively coupled plasma. Microscale LC separations in small packed columns were studied because the column flow rates of about 30 ml min 1 were compatible with the DIN. The low dead volume (less than 1 ml) of the interface prevented excessive band broadening. Eluents containing up to 85% methanol were accommodated. The analyte signal varied by about 20% as the eluent changed from 20% to 80% methanol in water. Detection limits for arsenic and tin species using the HPLC-DIN-ICP-MS system were 0.2-0.6 and 8-10pg, respectively. 

SFC has received attention as an alternative separation technique to liquid and gas chromatography. The coupling of SFC to plasma detectors has been studied because plasma source spectrometry meets a number of requirements for suitable detection. There have been two main approaches in designing interfaces. The first is the use of a restrictor tube in a heated cross-flow nebuliser. This was designed for packed columns. For a capillary system, a restrictor was introduced into the central channel of the ICP torch. The restrictor was heated to overcome the eluent freezing upon decompression as it left the restrictor. The interface and transfer lines were also heated to maintain supercritical conditions. Several speciation applications have been reported in which SFC-ICP-MS was used. These include alkyl tin compounds (Oudsema and Poole, 1992), chromium (Carey et al., 1994), lead and mercury (Carey et al., 1992), and arsenic (Kumar et al., 1995). Detection limits for trimethylarsine, triphenylarsine and triphenyl arsenic oxide were in the range of 0.4-5 pg. 

Capillary electrophoresis (CE) provides high resolution for separation of chemical compounds. Separations of metal ions, of metal ions in different oxidation states and of organometallic compounds are all possible with appropriate CE conditions. This technique is being investigated for speciation. Since sample volumes in CE are generally very small, a detector capable of very low detection limits is desirable. Thus, ICP-MS has been combined with CE to provide a means for studying metal speciation. CE-ICP-MS procedures have been described for the separations of platinum species (Michalke and Schramel, 1996), selenium species (Kumar et al., 1995 Michalke and Schramel, 1996) and arsenic species (Magnuson et al., 1997). Detection limits were about 1 mgl 1 (platinum species) and 10 and 24 pg for Sclv and Scvl, respectively. An application of CE-ICP-MS to platinum species in soils is described in Section 15.8.6. 

Research has continued on the speciation of aluminium in water and in soil related particularly to the effects of acid precipitation. Species of particular concern are Alm, Al(OH)2+ and Al(OH)4. These species are the most toxic with regard to fish and plants. The presence of fluoride, sulfate and organic compounds that can form complexes with aluminium result in a lower degree of toxicity. Consequently, the objectives of a number of investigations have been the relationship of 

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