Arsenic analysis absorption spectrometry

See also Analytical Reagents Purification. Arsenic. Atomic Absorption Spectrometry Electrotfiermal. Atomic Mass Spectrometry Inductively Coupled Plasma. Dioxins. Elemental Speciation Oven/iew. Endocrine Disrupting Chemicals. Geochemistry Sediment. Humic and Fulvic Compounds. Isotope Dilution Analysis. Mass... 

It is seen by examination of Table 1.11(b) that a wide variety of techniques have been employed including spectrophotometry (four determinants), combustion and wet digestion methods and inductively coupled plasma atomic emission spectrometry (three determinants each), atomic absorption spectrometry, potentiometric methods, molecular absorption spectrometry and gas chromatography (two determinants each), and flow-injection analysis and neutron activation analysis (one determinant each). Between them these techniques are capable of determining boron, halogens, total and particulate carbon, nitrogen, phosphorus, sulphur, silicon, selenium, arsenic antimony and bismuth in soils. 

The determination of arsenic by atomic absorption spectrometry with thermal atomization and with hydride generation using sodium borohydride has been described by Thompson and Thomerson [117] and it was evident that this method could be modified for the analysis of soil. 

The most useful chemical species in the analysis of arsenic is the volatile hydride, namely arsine (AsH3, bp -55°C). Analytical methods based on the formation of volatile arsines are generally referred to as hydride, or arsine, generation techniques. Arsenite is readily reduced to arsine, which is easily separated from complex sample matrices before its detection, usually by atomic absorption spectrometry (33). A solution of sodium borohydride is the most commonly used reductant. Because arsenate does not form a hydride directly, arsenite can be analyzed selectively in its presence (34). Specific analysis of As(III) in the presence of As(V) can also be effected by selective extraction methods (35). 

In an interlab oratory study involving 160 accredited hazardous materials laboratories reported by Kimbrough and Wakakuwa [28], each laboratory performed a mineral acid digestion on five soils spiked with arsenic, cadmium, molybdenum, selenium and thallium. Analysis of extracts was carried out by atomic emission spectrometry, inductively-coupled plasma mass spectrometry, flame atomic absorption spectrometry and hydride generation atomic absorption spectrometry. 

An early method for the determination of arsenic in soils is that of Forehand et al. [23]. This method is based on the selective extraction of arsenic(III) by benzene and analysis of the extract by atomic absorption spectrometry. Firstly the soil is allowed to stand with 9.9 M hydrochloric acid for 12 hours, and then the arsenic is reduced from arsenic(V) to arsenic(III) with stannous chloride and potassium iodide. Following adjustment to pH 9 with hydrochloric acid, the aqueous phase is extracted with benzene. The benzene extract is then treated with water and the water extract analysed by atomic absorption spectrometry at 193.7 nm. An average recovery of 88% of the arsenic present in sandy soils was achieved by this procedure. 

The determination of arsenic by atomic absorption spectrometry with thermal atomisation and with hydride generation using sodium borohydride has been described by Thompson and Thomerson [29], and it was evident that this method couldbe modified for the analysis of soil. Thompson and Thoresby [30] have described a method for the determination of arsenic in soil by hydride generation and atomic absorption spectrophotometry using electrothermal atomisation. Soils are decomposed by leaching with a mixture of nitric and sulfuric acids or fusion with pyrosulfate. The resultant acidic sample solution is made to react with sodium borohydride, and the liberated arsenic hydride is swept into an electrically heated tube mounted on the optical axis of a simple, lab oratory-constructed absorption apparatus. 

Jiminez de Bias et al. [32] have reported a method for the determination of total arsenic in soils based on hydride generation atomic absorption spectrometry and flow injection analysis. The method gave good recoveries and had a detection limit below 1 ig/l for an injection volume of 160 pi... 

Grabinski [12] has described an ion exchange method for the complete separation of the above four arsenic species, on a single column containing both cation and anion exchange resins. Flameless atomic absorption spectrometry with a deuterium arc background correction is used as a detection system for this procedure. This detection system was chosen because of its linear response and lack of specificity for these compounds combined with its resistance to matrix bias in this type of analysis. 

Early colorimetric methods for arsenic analysis used the reaction of arsine gas with either mercuric bromide captured on filter paper to produce a yellow-brown stain (Gutzeit method) or with silver diethyl dithiocarbamate (SDDC) to produce a red dye. The SDDC method is still widely used in developing countries. The molybdate blue spectrophotometric method that is widely used for phosphate determination can be used for As(V), but the correction for P interference is difficult. Methods based on atomic absorption spectrometry (AAS) linked to hydride generation (HG) or a graphite furnace (GF) have become widely used. Other sensitive and specihc arsenic detectors (e.g., AFS, ICP-MS, and ICP-AES) are becoming increasingly available. HG-AES, in particular, is now widely used for routine arsenic determinations because of its sensitivity, reliability, and relatively low capital cost. 

The differentiation of inorganic As(III) and As(V) can be achieved by exploitation of the pH sensitivity of the reduction of arsenic compounds by sodium borohydride, as adapted to analysis by atomic absorption spectrometry . An alternative to pH control of arsine production involves suppression of As(V) reduction by the addition of DMF . 

Ricci and coworkers have described a highly sensitive, automated technique for the determination of MMAA, DMAA, p-aminophenyl arsonate, arsenite and arsenate. This procedure is based on ion-chromatography on a Dionex column, with 0.0024 M NaHC03/0.0019 M NajCOj/O.OOl M Na2B407 eluent, when all the compounds except arsenite and dimethyl arsinite are separated effectively. For separation of the last two, a lower ionic strength eluent (0.005 M Na2B407) can be used in a separate analysis. The detection system utilizes a continuous arsine generation system followed by heated quartz furnace atomization and atomic absorption spectrometry. Detection limits of less than 10 ng/ml were obtained for each species. 

Figure 3.40 Schematic of an LC-HGAAS system (Reprinted from Analytica Chimica Acta, Niedzielski, P. The new concept of hyphenated analytical system Simultaneous determination of inorganic arsenic(lll), arsenic(V), selenium(IV) and selenium(VI) by high performance liquid chromatography-hydride generation-(fast sequential) atomic absorption spectrometry during single analysis, 55 (1-2), 199-206 T Copyright 2005 with permission from Elsevier).

De Bias OJ and Mateos NR (1996) Determination of total arsenic and selenium in plants by atomic absorption spectrometry with hydride generation and flow injection analysis coupled techniques. JAOAC Int 79 764-768. 

The detection limits of the old methods for the determination of arsenic (10) were too high to determine arsenic in uncontaminated biological samples. With the invention of instrumental techniques, such as flame atomic absorption (emission) spectrometry, graphite furnace atomic absorption spectrometry, neutron activation analysis, inductively coupled plasma atomic emission spectrometry, and inductively coupled plasma mass spectrometry, the ubiquity of arsenic in our environment was proven. The improvement of the analytical techniques has changed the reputation of arsenic from a poisonous substance to an essential trace element at least for warm-blooded animals (11). An arsenic requirement for humans cannot be deduced from these animal experiments. In recent literature, there are certainly more hints that arsenic might be an essential trace element for humans, but there is still a lot of future research work necessary to prove this. 

Figure 3 Instrumental methods for the determination of arsenic compounds (Abbreviations AAS, atomic absorption spectrometry APS, atomic fluorescence spectrometry CE, capillary electrophoresis GC, gas chromatography HG, hydride generation ICP-AES, inductively coupled plasma-atomic emission spectrometry ICP-MS, inductively coupled plasma-mass spectrometry INAA, instrumental neutron activation analysis LC, liquid chromatography MS, mass spectrometry).

Figure 3 Illustrates the problem faced by the IAEA in the broader context of their trace element laboratory intercomparison program. These data show the reported results of 16 laboratories for measurements of arsenic in the horse kidney intercomparison sample (H-8), based on various versions of atomic absorption spectrometry, optical emission spectrometry, neutron activation analysis, and Induced X-ray emission analysis. The objective of the horse kidney intercomparison was to assess (and refine) analytical methods for the determination of essential and toxic trace elements in this surrogate for human kidney (2). Kidney, as the main target organ which accumulates toxic elements, was of special Interest with respect to cadmium. Horse kidney, which contains similar levels of cadmium to the human kidney cortex, was selected for the development and maintenance of methods having a demonstrated level of quality to assure reliable biological monitoring of this element. Participants were Invited to analyze some 24 additional trace elements, however.

With careful sample preparation hydride generation is an extremely valuable analytical technique appropriate to ICP-AES and ICP-MS as well as atomic absorption spectrometry. Within the context of biomedical analysis, measurements of arsenic and selenium are currently the more important. 

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