Arsenic compounds detectors

The simplest analytical method is direct measurement of arsenic in volatile methylated arsenicals by atomic absorption [ 11 ]. A slightly more complicated system, but one that permits differentiation of the various forms of arsenic, uses reduction of the arsenic compounds to their respective arsines by treatment with sodium borohydride. The arsines are collected in a cold trap (liquid nitrogen), then vaporised separately by slow warming, and the arsenic is measured by monitoring the intensity of an arsenic spectral line, as produced by a direct current electrical discharge [1,12,13]. Essentially the same method was proposed by Talmi and Bostick [10] except that they collected the arsines in cold toluene (-5 °C), separated them on a gas chromatography column, and used a mass spectrometer as the detector. Their method had a sensitivity of 0.25 xg/l for water samples. 

Fig. 3. Typical separation of four arsenosugars and DMA by HPLC/ICP-MS using an ODS reversed-phase column at pH 3.2 under conditions described in Ref. 60. The sensitivity and specificity of the detector allows the determination of arsenosugars and other arsenic compounds to be conducted on dilute aqueous extracts of the marine samples.

The application of high-sensitivity ICP-MS detectors coupled to HPLC has enabled the detection of trace arsenic compounds present in marine animals. Thus, arsenocholine has been reported as a trace constituent (<0.1% of the total arsenic) in fish, molluscs, and crustaceans (37) and was found to be present in appreciable quantities (up to 15%) in some tissues of a marine turtle (110). Earlier reports (46,47) of appreciable concentrations of arsenocholine in some marine animals appear to have been in error (32). Phosphatidylarsenocholine 45 was identified as a trace constituent of lobster digestive gland following hydrolysis of the lipids and detection of GPAC in the hydrolysate by HPLC/ICP-MS analysis (70). It might result from the substitution of choline with arsenocholine in enzyme systems for the biogenesis of phosphatidylcholine (111). 

A number of techniques have been used for the speciation of arsenic compounds. The most important has been the formation of volatile hydrides of several species, separation by gas chromatography and detection by AAS. HPLC has been used to separate arsenic species. Several types of detectors have been studied for the determination of arsenic species in the column effluent. These have included AAS both off- and on-line, ICPAES and ICP-MS. An important comparative study of coupled chromatography-atomic spectrometry methods for the determination of arsenic was published (Ebdon et al., 1988). Both GC and HPLC were used as separative methods, and the detectors were FAAS, flame atomic fluorescence spectrometry (FAFS) and ICPAES. The conclusions were (1) that hydride generation and cryogenic trapping with GC-FAAS was the most... 

The speciation of arsenic compounds, using IPC and ICP-MS as a detector, has received much attention [33-39]. Beauchemin et al. [33,34] used IPC for the analysis of arsenic species in dogfish muscle. The ion-pair reagent was 10 iM sodium dodecylsulfate in a 5% methanol, 2.5% glacial acetic acid mobile phase at pH 2.5 with a C18 column. The toxic inorganic species, As (III) and As (V), as well as the less toxic organoarsenic species, monomethylarsenic (MMA), dimethylarsenic (DMA), arsenobetaine (AB), and arsenocholine (AC), were separated. AB was the dominant species and constituted 84% of the total arsenic concentration with a detection limit of 300 pg (as As). 

A set of guidelines for the separation of arsenic compounds using ion-exchange chromatography and ICP-MS as element-specific detector has been published (58). 

There is still a lot of work to do to make CZE a robust method for the determination of arsenic compounds in environmental samples. The proposed methods are too matrix sensitive and the difficulties of connecting CZE to element-specific detectors, such as ICP-MS, are not yet solved. At the moment, liquid chromatography in connection with element-specific detectors is certainly favored over CZE. 

The HG-F(QF)AAS method is a widespread detector for the determination of arsenic compounds because it is inexpensive. The disadvantage of this detection is that it is restricted to hydride-forming arsenic compounds. There are numerous articles published using postcolumn UV photo-oxidation (67,80,81) or microwave-assisted oxidation (81,82) to convert the non-hydride-active arsenic compounds, such as AB, AC, and TETRA, to the hydride-active compounds. 

Atomic fluorescence spectrometry is certainly an excellent detector for the determination of arsenic compounds. Necessary low detection limits can be obtained only after hydride generation. This is certainly a drawback as the nonhydride-forming arsenic compounds have to be converted into the volatile hydride-forming ones. 

Among the detectors discussed thus far, ICP-MS is certainly not the cheapest one. The advantage of ICP-MS lies in its multielement capabilities, excellent detection limits, and wide linear range. Moreover, low detection limits are not restricted to the hydride-forming arsenic compounds. The application of ICP-MS as an element-specific detector changed the knowledge about arsenic compounds... 

The identification of arsenic compounds using element-specific detectors in general is based on matching the retention time with known standards. This works perfectly as long as known standards are available. When unknown signals are obtained in a chromatogram, ICP-MS cannot provide any structural information and other detectors must be applied. 

M Slekovec, W Goessler, KJ Irgolic. Inorganic and organic arsenic compounds in Slovenian mushrooms Comparison of arsenic specific detectors for liquid chromatography. Chem Spec Bioavailab 11 115-123, 1999. 

W Goessler, A Rudorfer, EA Mackey, PR Becker, KJ Irgolic. Determination of arsenic compounds in marine mammals with high performance liquid chromatography and an inductively coupled plasma mass spectrometer as element specific detector. Appl Qrganomet Chem 12 491-501, 1998. 

Anion chromatography (Hamilton PRP-XlOO column) of arsenic compounds (each at lo ppb) at pH 5.8 in 15 mM KjHPO /KHjPO,. with atomic fluorescence detection of As. Structures of the parent acids are shown. Arsenic compounds were converted to AsHj, which decomposes to As atoms in the detector. [Courtesy Y. Cai and L. Yehiayan, Florida International University.)... 

Concentrations of total arsenic in soil and water samples contaminated with old Arsenical Munitions are not very useful to characterize the potential risks. Knowledge of which arsenic compounds are present in such samples is absolutely necessary to define toxicity. The identification of arsenic compounds requires a separation step combined with a detection step. For separation, gas chromatography and high performance liquid chromatography are widely used. Atomic absorption spectrometers, inductively coupled plasma optical emission spectrometers, and inductively coupled plasma mass spectrometers may serve as arsenic-specific detectors. 

The first GC-microwave-induced plasma emission system was reported in 1965 [23]. During the past two decades GC-plasma emission systems have gained in popularity and have been used for the identification and quantification of mercury, lead, tin, selenium, and arsenic compounds [13]. The most frequently used plasma source is the microwave-induced plasma operated either at reduced pressure or at atmospheric pressure with helium or argon as the plasma gases at powers of 100 to 200 W The Beenakker cylindrical resonance cavity introduced in 1976 [24], and since then modified to achieve better detection limits, is most frequently used in the GC-microwave-induced plasma emission systems that are easily adaptable to capillary GC operation. These microwave-induced plasma detectors respond to non-metals (H, D, B, C, N, O, F, Si, F S, Cl, As, Se, Br, I) and metals, with absolute detection limits in... 

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