Arsenic atomic absorption spectrometer

Howard and Arbab-Zavar have described a technique for the determination of inorganic As(III) and As(V), methylarsenic and dimethylarsenic species which is based on the trapping of arsines and selective volatilization into a heated quartz atomizer tube situated in the optical path of an atomic-absorption spectrometer. Improved reproducibility is obtained by the use of a continuous flow reduction stage and detection limits are approximately 0.25 ng. For a typical sample volume of 10 ml this corresponds to a detection limit of 0.025 ngmU of arsenic. Interference effects encountered by earlier workers were investigated and depression of results was observed in the presence of... 

The optimum instrument parameters for arsenic measurement were established with a Perkin-Elmer model 403 atomic absorption spectrometer. Better sensitivity was obtained at the 197.2 nm arsenic line than at the 193.7 nm line. Some investigators have recommended the use of a quartz or silicon furnace for arsenic measurement (16,17,18). However, the hydrogen-argon entrained air flame in combination with the described arsine generation apparatus offers comparable sensitivity. A hollow cathode lamp was used throughout the development of the method. Subsequent studies have shown that a five-fold improvement in sensitivity can be obtained with an electrodeless discharge lamp. 

Atomic absorption spectrometer with selenium and arsenic hoUow cathode lamps. Wavelengths Se, 1%.0 ran As, 193.7 ran. Chart recorder readout. 

Using sodium borohydride, arsenic ions are reduced to arsenic hydride, transferred to a heated quartz cuvette with the aid of a current of inert gas, decomposed thermally, and the absorption of the atoms is measured in the beam of an atomic-absorption spectrometer. In the hydride technique, the element which is to be determined is volatilized as a gaseous hydride and separated off from the matrix. Interferences may occur if there is a considerable excess of elements such as antimony, tin, bismuth, mercury, selenium or tellurium, which may also be volatilized using this technique. 

A simple gas chromatographic method determines total arsenic in urine as arsine with a PN detector. The detection limit is 50 ng. The gas sample is separated on a 1.2 m glass column filled with Chromosorb 103 80-100 mesh at a furnace temperature of 30 C [153]. MMAA and DMAA are separated as thioglycolic acid methyl esters on a glass column (1.8 m, 2 mm ID) packed with Chromosorb G AW-DMCS coated widi 2.5% XE-60 for flame ionization detection. The detection limit is 10 ng [154]. Triphenylarsine formation is a more time-consuming method. But the combination of a gas chromatograph with a microwave emission spectrometric detector reaches a detection limit of SO ng/Iiter. The method is also applied for the determination of alkylarsenic acids [132,155], Atomic absorption spectrometers [134] and mass spectrometers [135] were also used as detectors of gas chromatographs. 

A novel technique of atomisation, known as vapour generation via generation of the metal hydride, has been evolved, which has increased the sensitivity and specificity enormously for these elements [5-7,9]. In these methods the hydride generator is linked to an atomic absorption spectrometer (flame graphite furnace) or inductively coupled plasma optical emission spectrometer (ICP-OES) or an inductively coupled plasma mass spectrometer (IPC-MS). Typical detection limits achievable by these techniques range from 3 pg/1 (arsenic) to 0.09 pgd (selenium). 

Atomic absorption spectrometer. See also Section Reagents and equipment under arsenic. 

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. 

These early GC-element-specific detection systems are described in several review articles [18-21]. An excellent summary of the applications of these and other sterns, to be discussed later, to the determination of mercury, lead, selenium, tin and arsenic compounds is provided in the review of this field by C.J. Cappon [9]. The detection limits obtained with these systems reach the low picogram levels under ideal conditions however, detection limits in the nanogram range, are more common. With elemental mercury or organic mercury compounds in a sample, 100 picograms of mercury were detected in a system consisting of a fused silica capillary gas chromatograph and a cold vapor atomic absorption spectrometer [22]. 

Common gas chromatographic detectors that are not element- or metal-specific, atomic absorption and atomic emission detectors that are element-specific, and mass spectrometric detectors have all been used with the hydride systems. Flame atomic absorption and emission spectrometers do not have sufficiently low detection limits to be useful for trace element work. Atomic fluorescence [37] and molecular flame emission [38-40] were used by a few investigators only. The most frequently employed detectors are based on microwave-induced plasma emission, helium glow discharges, and quartz tube atomizers with atomic absorption spectrometers. A review of such systems as applied to the determination of arsenic, associated with an extensive bibliography, is available in the literature [36]. In addition, a continuous hydride generation system was coupled to a direct-current plasma emission spectrometer for the determination of arsenite, arsenate, and total arsenic in water and tuna fish samples [41]. 

R.H. Fish, F.E. Brinckman, and K.L. Jewett. Fingerprinting inorganic arsenic and organoarsenic compounds in in situ oil shale retort and process waters using a liquid chromatograph coupled with an atomic absorption spectrometer as a detector. Environ. Sci. Technol., 16, 174 (1982). 

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. 

More recently, PS Analytical have introduced the PS A 10.003 and the Merlin Plus continuous flow vapour generation atomic absorption and atomic fluorescence spectrometers (AFS) [10-12]. These facilitate the determination of very low concentrations (ppt) of mercury, arsenic, and selenium in solution, enabling amounts down to 10-20 ppm of these elements to be determined in polymer digests. 

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