Arsenic separation techniques

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. 

T. Guerin, A. Astruc, M. Astruc, Speciation of arsenic and selenium compounds by HPLC hyphenated to speci be detectors a review of the main separation techniques, Talanta, 50 (1999), 1D24. 

Besides these specific methods, the general arsenal of techniques described in sec. 3.7 remains available. So, optical and Volta potential measurements cure often invoked to obtain structural information on the monolayers. These techniques do not basically differ from the corresponding ones for Langmuir monolayers. However, surface rheology differs drastically because for Gibbs monolayers transport to and from the bulk is possible. Differences start to appear if the molecules are not very small and therefore diffuse with time scales comparable (or shorter) than those of the measurements. Therefore this theme will be devloped separately before surfactant monolayers are discussed, see sec. 4.5. 

Arsenic speciation in saline waters can be made using a tubular membrane as a gas-liquid separator for HG-ICP-MS.75 The detection limit achieved using this technique is at the picogram level. The separation step is very important during the sample process because of the presence of the chloride (estuarine and open ocean waters contain high levels of chloride). HC1 is commonly used in the HG processes, which result in a matrix extremely high in chloride. For the separation step to be performed prior to the ICP-MS technique, some studies are carried out using capillary electrophoresis (CE) as a separation technique,76 which includes the interface for CE and ICP-MS.76 77... 

When presented with a mixture of unknown organic compounds to identify, the modern chemist has an arsenal of tools available to assist her. The fist task is typically to separate the mixture into pure substances. Many separation processes have been developed for this purpose. These techniques may utilize differences in the molecules to separate them. Such differences may be in size, charge, chemical affinity, or physical properties. By repeatedly applying separation techniques, compounds can also be made increasingly pure. A few of the more common laboratory separation process are listed below ... 

Over the years a wide range of analytical techniques were employed for the speciation of arsenic, including different separation techniques. Which technique is used depends among others on the question to be answered and the species to be identified. 

In addition, it should be mentioned that the highest emphasis until now is on water-soluble arsenic species, which is reflected in the remainder of the article. But it should not been forgotten that lipid-soluble arsenic species (like arsenolipids) exist. Howevei they need different extraction protocols using mainly lipophilic solvents like methanol/chloroform and different separation techniques. 

The separation technique suited depends on the complexity of the sample matrix and the number of arsenic species present in the sample. 

It is not only necessary to have an inside knowledge of the separation power of the different techniques, more importantly, it is also necessary to know which arsenic species will be hidden by using the separation technique chosen. 

Capillary Electrophoresis. Capillary electrophoresis (ce) or capillary 2one electrophoresis (c2e), a relatively recent addition to the arsenal of analytical techniques (20,21), has also been demonstrated as a powerful chiral separation method. Its high resolution capabiUty and lower sample loading relative to hplc makes it ideal for the separation of minute amounts of components in complex biological mixtures (22,23). 

Arsenic and Boron. Arsenic and boron form volatile fluorides which are difficult to separate from high purity HF. Special equipment and techniques must be used to remove the arsenic. 

The SNMS instrumentation that has been most extensively applied and evaluated has been of the electron-gas type, combining ion bombardment by a separate ion beam and by direct plasma-ion bombardment, coupled with postionization by a low-pressure RF plasma. The direct bombardment electron-gas SNMS (or SNMSd) adds a distinctly different capability to the arsenal of thin-film analytical techniques, providing not only matrbe-independent quantitation, but also the excellent depth resolution available from low-energy sputterii. It is from the application of SNMSd that most of the illustrations below are selected. Little is lost in this restriction, since applications of SNMS using the separate bombardment option have been very limited to date. 

In addition to these advantages TLC also possesses other merits which ensure that it occupies a firm place in the arsenal of analytical techniques as a method for the separation of micro-, nano- and picogram quantities [5]. 

Haywood and Riley [17] found that arsenic (III) can be separated from arsenic (V) even at levels of 2 xg/l by extracting it as the pyrrolidine dithiocar-bamate complex with chloroform. They applied this technique to samples of seawater spiked with arsenic (V) and arsenic (III) and found that arsenic (V) could be satisfactorily determined in the presence of As111. 

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