Arsenic compounds, derivatization

The reaction product of arsenic trichloride (see Table 1) with 3,4-dimercaptotoluene, 2-chloro-5-methyl-l,3,2-benzodithiarsole, still contains an active chlorine atom, rendering its determination by GC/MS difficult. The derivatization reaction can also be carried out with 2-chlorovinylarsenic oxide (lewisite oxide, CAS 3088-37-7), which is one of the degradation products of lewisite 1. Thus, the highly reactive arsenous compounds can be detected as less reactive derivatives amenable to GC/MS. 

Thiomethylation only gives derivatives of arsenic-(III) whereas pentavalent arsenic compounds are reduced by the thiol with formation of the disulfide (SGM)2. The yields for the TGM-derivatization depend on the number of introduced thiomethyl groups. The determined yields and recovery rates are in accordance with the publication of Schoene (Schoene et al., 1995). The derivatization is subjected to sometimes strong matrix influence which can be corrected by determination of the yield of proper reference compounds. Multiple derivatized compounds show often thermal instability, thus the use of cold on-column injection or temperature programmed cold injection systems is required. 

Gas chromatography is not commonly used for the determination of arsenic compounds. The reason for this is that not all arsenic compounds are easily volatilized. A rapid method for the determination of arsine, methylarsine, dimethylar-sine, and trimethylarsine in air based on gas chromatography-mass spectrometry (GC-MS) was recently published (65). In another smdy, DMA and MA present in urine were derivatized with thioglycol methylate and extracted with a 100 qm solid-phase microextraction fiber in 40 minutes. Thereafter, the two arsenic compounds were determined with GC-MS (66). The combination of purge and trap gas chromatography with atomic fluorescence spectrometry was used for the determination of arsenous acid, arsenic acid, MA, and DMA in a mushroom sample (67). Low-temperature gas chromatography coupled to ICP-MS was used to determine the volatile arsenic compounds in intraoral air (68). This method is also applicable to the determination of volatile arsenic compounds in landtill gases. 

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. 

Figure 4. Information about groups other than organic is lost in this derivatization procedure. It should also be noted that this is a technique designed to "find" the arsenic compounds belonging to a specific class of compounds. Other arsenicals, which do not form volatile hydrides on treatment with borohydride, are "hidden" to this methodology.

For the determination of organotin compounds (tributyltin, triphenyltin, triethyltin, and tetra-ethyltin) a MAE is proposed before the normal phase (NP) HPLC/UV analysis [35], In organotin and arsenic speciation studies, hydride generation is the most popular derivatization method, combined with atomic absorption and fluorescence spectroscopy or ICP techniques [25,36], Both atmospheric pressure chemical ionization (APCI)-MS and electrospray ionization ESI-MS are employed in the determination of butyltin, phenyltin, triphenyltin, and tributyltin in waters and sediments [37], A micro LC/ESI-ion trap MS method has been recently chosen as the official EPA (Environmental Protection Agency) method (8323) [38] it permits the determination of mono-, di-, and tri- butyltin, and mono-, di-, and tri-phenyltin at concentration levels of a subnanogram per liter and has been successfully applied in the analysis of freshwaters and fish [39], Tributyltin in waters has been also quantified through an automated sensitive SPME LC/ESI-MS method [40],... 

Lewisite 1 per se is never found in the environment. Figure 18 shows that this compound hydrolyzes rapidly on contact with moisture to 2-chlorovinyl arsonous acid, which in turn slowly dehydrates to lewisite oxide (syn. 2-chlorovinyl arsenous oxide) (16). Both 2-chlorovinyl arsonous acid and lewisite oxide are nonvolatile. The most frequently used method for the identification of CWC-related chemicals is based on gas chromatography (GC) in combination with mass spectrometry (GC/MS). Indirect GC/MS analysis of lewisite 1 requires sample preparation, which involves conversion of lewisite oxide to 2-chlorovinyl arsonous acid in an acidic environment, followed by derivatization (12). The obtained species is both volatile and thermally stable, and thus amenable to GC analysis. Often, a mercaptan reagent is used as a derivatization agent. The reaction with 3,4-dimercaptotoluene is shown in Figure 19. 

Liquid chromatography, a seemingly simpler procedure than GC in the speciation analysis of arsenic because it does not require the derivatization stage, also generates a number of potential errors. The selection of chromatographic method involves preliminary evaluation of the solubility of compounds to be separated. The... 

The molecular characterization of methylarsonic acid, phenylarsonic acid and the inorganic anion, arsenate, found in a methanol extract of a Green River Formation Oil shale sample was accomplished by HPLC-GFAA analysis In addition, derivatization of the acids, HPLC purified, by reaction with 3-methylcatechol to form the five coordinate organoarsenic catecholates as well as reaction, via trimethylsilylation, of the ammonium salt of arsenate to form tristrimethylsilylarsenate was followed by GC-EIMS analysis to provide unequivocal evidence for the presence of these organometallic and inorganic compounds of arsenic as natural products in oil shale ... 

As mentioned above in the context of the analysis of hgnin degradation products, gas chro-matography/mass spectrometry and related methods have been developed as extremely powerful tools for the identification of phenolic compounds. Use of high-pressure liquid chromatography in combination with mass spectrometry adds to the analytical arsenal with respect to the detection of polar, non-volatile compounds but, in particular, the advent of modem ionization techniques, such as ESI and MALDI mass spectrometry, have continued to broaden the analytically governable field of organic chemistry. The latter methods diminish the need of derivatization of polar phenolics to increase the volatility of the analyte. In this section, a more or less arbitrary selection of examples for the application of mass spectrometric techniques in analytical chemistry is added to the cases already discussed above in the context of gas-phase ion chemistry. 

A recent study published in the Chinese Journal of Instrumental Analysis, Fenxi Ceshi Xuebao, showed a detection limit of 500 ng of Sulfur Mustard (HD) by using accelerated solvent extraction-gas chromatography (ASE-GC) coupled with a flame photometric detector (EPD) in the sulfur mode, in soil. In this case, the study showed evidence that ASE results in better recoveries and sensitivity than liquid solid extraction (LSE) [50]. In 1996, a paper was published on a method for the analysis of Lewisite through derivatization of the compound before introduction into a gas chromatograph. In order to simplify the derivatization process, a tube packed with absorbent was used for collection of airborne vapors. If a positive response occurs when screening analytes using a GC coupled with an FPD, then the same sample can be analysed using a GC equipped with an AED for confirmation based on the elemental response to arsenic (in the case of Lewisite) and sulfur (in the case of Sulfur Mustard) within the appropriate GC retention time window [54]. 

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