Arsenic Table speciation

The accuracy of the analytical method was established hy independent analysis of the three additional filters from each of the 5 10 and 20 yg/m3 generation runs using both NAA and XRF analyses. Because NAA and XRF analysis techniques provide only a total arsenic measurement, the IC-AAS speciation results obtained for MMA, DMA and p-APA were used to estimate the total amount of arsenic. Table X presents the total arsenic obtained by the three techniques. The accuracy ranged from 90-120 of the values obtained by NAA and XRF. 

Figure 2.4 Speciation of arsenic acid with pH. Curves calculated from acid dissociation constants listed in Table 2.10.

In thermospray interfaces, the column effluent is rapidly heated in a narrow bore capillary to allow partial evaporation of the solvent. Ionisation occurs by ion-evaporation or solvent-mediated chemical ionisation initiated by electrons from a heated filament or discharge electrode. In the particle beam interface the column effluent is pneumatically nebulised in an atmospheric pressure desolvation chamber this is connected to a momentum separator where the analyte is transferred to the MS ion source and solvent molecules are pumped away. Magi and Ianni (1998) used LC-MS with a particle beam interface for the determination of tributyl tin in the marine environment. Florencio et al. (1997) compared a wide range of mass spectrometry techniques including ICP-MS for the identification of arsenic species in estuarine waters. Applications of HPLC-MS for speciation studies are given in Table 4.3. 

Arsenic, chromium, mercury, selenium, and tin have been the object of numerous investigations. Because some of them are classified as probable human carcinogens23-25 (strictly speaking, some of their species), the accurate assessment of concentration and speciation in environmental matrices is enormously important. Unfortunately, such factors as chemical reactions between species, low concentration, microbial activity, redox conditions, as well as the presence of other dissolved metal ions, may cause the amounts and distributions of chemical species in a sample to vary. In response to these problems, analytical research efforts have focused on developing techniques enabling the original valence state of the metals to be preserved. Table 2.3 lists some of these stabilization methods. 

Speciation studies have been focused on relatively few elements, mainly aluminum, antimony, arsenic, chromium, lead, mercury, selenium, and tin. The primary species of these elements studied with ICP-MS detection are presented in Table 3 and they include different oxidation states, alkylated metal and/or metalloid compounds, selenoaminoacids, and selenopeptides. In addition, applications in studies of the pharmacokinetics of metal-based drugs (Pt, V, Au), metalloporphyrins (Ni, V, Fe, etc.), heavy metals in phytochelatins (Cd, Cu, Zn, Hg, etc.) and in humic substances should be mentioned. 

The measurement of standard reference materials (SRMs) provides the best method for ensuring that an analytical procedure is producing accurate results in realistic matrices. Many SRMs are available (Govindaraju, 1994 Rasmussen and Andersen, 2002). The most widely used are those supplied by National Institute of Standards and Technology (NIST). Arsenic and selenium concentrations have been certified in a range of natural waters, sediments, and soils (Tables 4 and 5). The SLRS range of certified standards from the National Research Council of Canada also includes several river waters with much lower arsenic concentrations than the NIST standards ( 0.2-l pgL ). Certified standards for As(III)/As(V) and Se(lV)/Se(VI) speciation are available commercially (e.g., SPEX Certiprep speciation standards). 

TABLE 19. Arsenic speciation from spiked and unspiked contaminated sediment interstitial water ... 

Table I. Summary of XAFS Studies of Arsenic Speciation in Metal Sulfides.

Case Examples. The effects of various oxoanions on EDTA-pro-moted dissolution of lepidocrocite (y-FeOOH) have been studied by Bondietti et al. (33). EDTA was chosen as a reference system because it is dissolution-active over a relatively wide pH range. Phosphate, arsenate, and selenite markedly inhibit the dissolution at near-neutral pH values. At pH <5 phosphate, arsenate, and selenite accelerate the dissolution. It is presumed that the bi-nuclear surface complexes formed at near-neutral pH values by these oxoanions (Table II) inhibit the dissolution. Figure 8a displays data on the effect of selenite on EDTA-promoted dissolution, and Figure 8b shows that calculations on surface speciation by Sposito et al. (35) support the preponderance of binuclear selenite surface complexes in the neutral-pH range. Mononuclear surface species prevail at lower pH values. 

Table 12.1 Applications of LC with ICP-MS detection for arsenic speciation analyses of toxicological and/or relevance... 

The NA pathways of inorganics and data necessary to characterize the process are summarized in Table 10.3. However, the processes that affect speciation, fate, and transport of arsenic and metals are not sufficiently understood. According to US EPA (2001), there is need to ... 

Table 2 summarizes literature data on arsenic species in blood. Limited speciation analyses revealed the presence of arsenobetaine (AB) and DMA(V) in human blood (106-108). Arsenobetaine and DMA(V) were detectable in serum of uremic (renal disease) patients, with mean values being 1 pg/L for DMA(V) and... 

HG in its simplest form allows the speciation of inorganic As and As For the speciation of samples containing more than these two hydride-active species, a combination with GC is necessary. Every arsenic species, with the exception of those carrying four carbon bonds, can be transformed into arsines by borohydride. Most of the arsines are volatile and can be separated by GC. Depending on the pH used during hydride formation a distinction between tri-and pentavalent arsenic is in most cases possible. Trivalent arsenic species form hydrides already at pH 7, whereas pentavalent ones are only reactive at pH 1 (Table 2). Measuring the same sample with both pHs... 

The instrumental conditions for the speciation analysis are shown in Table 18.3, and a plot of signal intensity versus time of the simultaneous separation of a 1 pg/L standard of arsenic, chromium, and selenium is shown in Figure 18.7. 

Table 4. Optimized experimental conditions of SI-ASV system for inorganic arsenic speciation in water samples.

Arsenic speciation by IC can be simple and reproducible (Table 8.3). One procedure, using a suppressed system was reported by Takamatsu et al. [37]. The method required two separate injections for the determination of As03 and AsO. The procedure was sensitive and there was no interference by ions such as NO3, HPO ", and SO that are present in the sediment extract. 

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