Arsenic compounds analytical determination

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

Goessler, W. and Kuehnelt, D.D. (2002) Analytical methods for the determination of arsenic and arsenic compounds in the environment, in Environmental Chemistry of Arsenic (ed. W.T. Frankenberger Jr., Marcel Dekker, New York, 27-50. 

A review of the analytical chemistry of arsenic in the sea, including occurrence, analytical methods and the establishment of analytical standards, has been published The major organic arsenic compound in the environment is dimethyl arsinate. Specific and sensitive methods for the determination of this compound are needed, since the direct atomic spectrometric method does not distinguish between different organoarsenic species... 

The Trace Metals Project conducted a study to identify the type of container which would provide minimum losses of arsenic and mercury by precipitation, volatilization, adsorption, or diffusion. Solutions of organomercury and organoarsenic compounds added to petroleum feedstock were used. Because of the relative ease with which mercury and arsenic can be determined at sub-parts-per-million levels in a hydrocarbon matrix by instrumental neutron activation analysis (INAA), this technique was used for the analytical measurements. The solutions were stored in five different types of glass and/or plastic containers and sampled periodically over eight months (12). The results of the study are summarized in Tables 2.III and 2.IV. 

Analytical Methods for the Determination of Arsenic and Arsenic Compounds in the Environment... 

Nowadays, most of the arsenic-orientated analytical work is dealing with the determination of total arsenic concentrations in biological samples. The methods developed for this purpose allow the determination of trace and ultratrace concentrations simultaneously with other elements. The huge differences in the acute toxicities make it evident that total arsenic concentrations are not sufficient for risk assessment. Nowadays, methods for the determination of arsenic compounds are becoming more robust and are (or will be soon) routine methods. Scientists who investigate toxic and beneficial effects of arsenic must concentrate on the determination of arsenic compounds. 

The analytical chemists must augment the possibilities of their methods with respect to robustness, reliability, detection limits, and multielement capabilities. Methods for the extraction of arsenic compounds from environmental samples without changing the species must be improved, too. Moreover, techniques that allow the direct determination of arsenic compounds (without prior extraction) should be developed to get a clearer picture of arsenic in our environment. 

Finally, we wish to point out the limitations of the following data set. Analytical techniques for determining organoarsenic species have improved steadily since the first of such methods was reported in the 1970s. Nevertheless, most of the methods used today involve separations (or derivatizations) in aqueous media. Consequently, these methods are only capable of determining water-soluble arsenicals, and arsenic compounds that are not soluble in water remain unidentified. The presence of such compounds has often been ascertained by the difference between concentrations of total arsenic and water-soluble arsenic. Future work should profit from techniques capable of determining water-insoluble arsenic species. 

It is only relatively recently that analytical systems have been able to determine arsenic compounds at such low levels, and these techniques have subsequently been applied to some interesting environmental samples, such as earthworms and ants (130). 

This book contains conttibutions from world-renowned international scientists on topics that include toxicity of arsenic, analytical methods for determination of arsenic compounds in the environment, health and risk exposure of arsenic, biogeochemical conttols of arsenic, Peatment of arsenic-contaminated water, and microbial ttansformations of arsenic that may be useful in bioremediation. 

Heteropolyacids (HPA) are the unique class of inorganic complexes. They are widely used in different areas of science in biochemistry for the precipitation of albumens and alkaloids, in medicine as anticarcinogenic agents, in industry as catalysts. HPA are well known analytical reagents for determination of phosphoms, silica and arsenic, nitrogen-containing organic compounds, oxidants and reductants in solution etc. 

Certain volatile elements must be analyzed by special analytical procedures as irreproducible losses may occur during sample preparation and atomization. Arsenic, antimony, selenium, and tellurium are determined via the generation of their covalent hydrides by reaction with sodium borohydride. The resulting volatile hydrides are trapped in a liquid nitrogen trap and then passed into an electrically heated silica tube. This tube thermally decomposes these compounds into atoms that can be quantified by AAS. Mercury is determined via the cold-vapor... 

Gradually a tremendous arsenal of processes has been developed, allowing the analyst to respond to an increasing number of diverse demands. Furthermore, the study of modern chemical analysis techniques is far removed from traditional descriptive chemistry. Many analyses are conducted in non-specialised environments, either on site or at simple workbenches. The determination of compounds is currently quite remote from the use of chemical reactions, which are often avoided for many reasons. Former wet chemistry methods, at the origin of the term analytical chemistry, have become less important because they lack sensitivity, are lengthy and their precision can too easily be altered by the use of insufficiently pure reagents. Nonetheless, wet chemistry methods are still interesting to study. 

The most generally applied method for determination of an arsenical is by atomic absorption spectrometry (AAS) after reduction of the compound to AsH3. However, this only provides an indication of the presence of the element as against a natural background. Lewisite rapidly hydrolyzes to 2-chlorovinylarsonous acid (CVAA see Figure 7) in an aqueous environment such as blood plasma, and analytical methods have focused mainly on the determination of CVAA (see Chapter 16). 

---