Chromatography, arsenic compounds

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. 

It is also possible to separate the organic arsenic compounds by column chromatography first, and then reduce them to arsines later [16]. 

L. S. Milstein, A. Essader, E. D. Pellizzari, R. A. Fernando, and O. Akinbo. Selection of a Suitable Mobile Phase for the Speciation of Four Arsenic Compounds in Drinking Water Samples Using Ion-exchange Chromatography Coupled to Inductively Coupled Plasma Mass Sectrometry. Environ, lnt., 28(2002) 277-283. 

Odanake et al. [1] have reported the application of gas chromatography with multiple ion detection after hydride generation with sodium borohydride to the determination of mono and dimethyl arsenic compounds, trimethyl arsenic oxide and inorganic arsenic in soil and sediments. Recoveries in spiking experiments were 100-102% (mono and dimethyl arsenic compounds and inorganic arsenic) and 72% (trimethyl arsenic oxide). 

B.6 Speciation of Arsenic Compounds by Ion-Exchange High-Performance Liquid Chromatography with Hydride Generation Atomic Fluorescence Detection. 

Hansen, S.H., Larsen, E.H., Pritzi, G. and Cornett, C. (1992) Separation of seven arsenic compounds by high performance liquid chromatography with on-line detection by hydrogen-argon flame atomic absorption spectrometry and inductively coupled plasma mass spectrometry./. Anal. At. Spectrom., 1, 629-634. 

Le, X.C. and Ma, M.S. (1997) Speciation of arsenic compounds by using ion-pair chromatography with atomic spectrometry and mass spectrometry detection./. Chromatogr. A., 764, 55-64. 

Saverwyns, S., Zhang, X.R., Vanhaecke, F., Cornelis, R., Moens, L. and Dams, R. (1997) Speciation of six arsenic compounds using high-performance liquid chromatography inductively coupled plasma mass spectrometry with sample introduction by thermospray nebulisation./. Chromatogr. A, 779, 299-306. 

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... 

Magnuson, M.L., Creed, J.T. and Brocklioff C.A. (1996) Speciation of arsenic compounds by ion chromatography with inductively coupled plasma mass spectrometry detection utilizing hydride generation with a membrane separator./. Anal. At. Spectrom., 11, 893-898. 

Gudzinowicz, B. J., Driscoll, J. L. Separation of alkyl/aryl and perfluorinated organo-arsenic compounds by gasliquid chromatography. J. GasCromatog. 1, (5) 25 (1963). 

There have been few elemental speciation studies in the literature involving cation-exchange chromatography (CEC) coupled to ICP-MS. A cation-exchange column was used by Larsen et al. [57,69] for arsenic speciation in several seafood sample extracts. The chromatography was optimized for the separation of arsenocholine, trimethylarsinic, trimethylarsine oxide, inorganic As, and two unknown cationic arsenic compounds. A mobile phase of 20 mM pyridinium ion, at pH 2.65, was used to perform the separation (Fig. 10.10). 

U. Kohlmeyer, J. Kuballa, E. Jantzen, Simultaneous separation of 17 inorganic and organic arsenic compounds in marine biota by means of high-performance liquid chromatography/inductively coupled plasma mass spectrometry, Rapid Commun. Mass Spectrom., 16 (2002), 965D974. 

S. N. Pedersen, K. A. Francesconi, Liquid chromatography electrospray mass spectrometry with variable fragmentor voltages gives simultaneous elemental and molecular detection of arsenic compounds, Rapid Common. Mass Spectrom., 14 (2000), 641-645. 

E. H. Larsen, G. Pritzl, S. H. Hansen, Speciation of eight arsenic compounds in human urine by high-performance liquid chromatography with inductively coupled plasma mass spectrometric detection using antimonate for internal chromatographic standardization, J. Anal. Atom. Spectrom., 8 (1993), 557-563. 

Coelho, N.M.M., CoeUio, L.M., de Lima, E.S., Pastor, A., de la Guardia, M. Determination of arsenic compounds in beverages by high-performance liquid chromatography-inductively coupled plasma mass spectrometry. Talanta 66, 818-822 (2005)... 

Several analytical methods for speciating arsenic have been reported. They include chromatographic techniques such as electrophoresis and ion-exchange (17), paper chromatography (18) and HPLC (19) selective volatilization of arsenic compounds to analogous arsines followed by GC-MES (20) boiling point separation/spectral emission (21) and atomic absorption (22). The above techniques have been applied to samples such as commercial pesticides (20),coal and fly ash (23),rocks, sediments, soils and minerals (24, 22),plant tissue (18), bovine liver (23),and water samples T25). 

FIGURE 30. Thin-layer chromatography of three arsenic compounds. Reprinted with permission from Reference 215. Copyright (1968) American Chemical Society... 

Talmi and Bostick and Talmi and Norvell improved the method by using gas chromatography. Although the method is highly sensitive, it has some disadvantages. It cannot discriminate all arsenic compounds in the biological samples because the same arsines can be produced by reduction from different organoarsenicals. Furthermore, the collection efficiency is incomplete for the very volatile arsine (b.p. — 55°C), and two reduction steps are necessary to discriminate between As(III) and As(V). 

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