Trimethylarsine determination

The compounds MMA, DMA, and TMAO are reduced in acidic aqueous media by borohydride solutions to methylarsine (MeAsH2, bp 2°C), dimethylarsine (Me2AsH, bp 35°C), and trimethylarsine (Me3As, bp 55°C), respectively. These products are useful derivatives for speciation analysis of arsenic because they are readily separated from complex sample matrices and may be further separated from each other by distillation (41) or by gas chromatography (42) prior to their determination by element-specific detectors. Consequently, arsine generation techniques are the most commonly used methods for determining MMA, DMA, and TMAO in marine samples. 

Gas phase proton affinities of phosphabenzene and arsabenzene have been determined by ion-cyclotron resonance techniques 94>. These confirm the qualitative solution phase data (see Fig. 5). Phosphabenzene (PA = 194.5 kcal/mol) has a proton affinity nearly 30 kcal/mol less than trimethylphosphine and only slightly greater than that of phosphine. Arsabenzene (PA = 188.0 kcal/mol) has a proton affinity 23 kcal/mol less than trimethylarsine. In the case of arsabenzene, protonation occurred on carbon rather than arsenic so the As-basidty may be even lower. By contrast, the proton affinity of pyridine (PA = 218 kcal/mol) is only slightly less than that of trimethylamine (PA = 222 kcal/mol) but considerably larger than ammonia (PA = 202 kcal/mol). 

Andreae [683] described a sequential volatilisation method for the sequential determination of arsenate, arsenite, mono-, di- and trimethylarsine, monomethylarsonic and dimethylarsinic acid, and... 

Later, the structure of the trimethylarsine-palladium bromide compound, [ (CH3)3As 2(PdBr2)2], was determined by x-ray crystal analysis (Mann and Wells, 1938) and found to possess the trans symmetric structure, XLIIIB, with a planar molecule 40), It should be added that each of the many bridged compounds prepared in these investigations appeared to be entirely homogeneous in the crystalline state. 

Let us assume for a moment, however, that we had reliable (and direct) thermochemical information about trimethylarsine and tetramethyldiarsine. How valid is it to assume all As—As bonds have comparable dissociation enthalpies Clearly we know enough not to assume the As—As bond in AS4 is the same as in tetramethyldiarsine. The former is immediately identified as strained and so we are to use the more stable allotrope with an infinite network of nonplanar hexagons. What about Sb—Sb bonds If we assume constancy of the dissociation enthalpies of these bonds (excepting the strained Sb4), how about using the bond enthalpy from elemental solid antimony The enthalpy of dimerization of antimonin. CsHjSb, to form the tricyclic Diels-Alder or [4-1-2] cycloaddition product has been determined to be — 30.5 +1.3 kJ mol by a careful study of the monomer-dimer equilibrium (equation 19). 

Andreae described a method for the sequential determination of arsenate, arsenite, mono-, di- and trimethyl arsine, MMAA, DMAA and trimethylarsine oxide in natural waters with detection limits of several ng/1. The arsines are volatilized from the sample by gas stripping the other species are then selectively reduced to the corresponding arsines and volatilized. The arsines are collected in a cold trap cooled with liquid nitrogen. They are then separated by slow warming of the trap or by gas chromatography, and measured with atomic absorption, electron capture and/or flame ionization detectors. He found that these four arsenic species all occurred in natural water samples. 

The basis of the second fairly simple routine method is transformation of arsenobeta-ine and arsenocholine into trimethylarsine and determination by headspace gas chromatography with a flame ionization or AAS detector. A detection limit below 10 /working conditions (Ballin et al., 1992 Ballin, 1992). 

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. 

Trimethylarsine oxide has been reported in several marine animals, where it is almost always a trace constituent. The one exception is the fish Kyphosus sydney-anus, which has trimethylarsine oxide as the major arsenical (25). That trimethylarsine oxide is not more widespread is perhaps surprising since it is likely to be a metabolite of the same pathway producing methylarsonate and dimethylarsinate, both of which are more commonly found. Trimethylarsine oxide chromatographs rather poorly on cation-exchange columns often used for determining arsenic species, and the resultant poor detection limits for this compound may partly explain the data indicating its apparent absence in many samples. Trimethylarsine oxide is usually only rarely reported in terrestrial organisms, but more recent work (with better detection limits) has shown it to be present in various terrestrial plants and two lichen samples (26). 

The extraction efficiency of arsenic from soil and sediments is low (11,119). Because most analytical techniques for determining arsenic species are performed on water-based solutions of the analytes (extractable arsenic), the results obtained for soil and sediments represent only a small proportion of the total arsenic present. Of this extractable portion, the inorganic species arsenate and arsenite dominate (1), although methylarsonate and dimethylarsinate are also found as natural constituents in some soils (43). These four arsenicals are also commonly found in sediments or in the interstitial water (porewater) of the sediments (45,134,135), and a trimethylated arsenic species, possibly trimethylarsine oxide, has also been detected in some sediment porewater samples (135). 

Parris et al- -, have described a procedure utilizing a commercial atomic absorption spectrophotometer with a heated graphite tube furnace atomizer linked to a gas chromatograph for the determination of trimethylarsine in respirant gases produced in microbiological reactions. The overall GC-AA system used by these workers is illustrated in Figure 188. 

It is common practice to compare lowest limits of detection or response to various analytes. A number of schemes have been introduced in order to provide such a threshold figure of merit, but perhaps the most reliable is that of Burrell,which employs an extension of the sensitivity concept. If measurement of are restricted to a series of n replicate analyses at a very low concentration, that is, where it is anticipated the signal-to-noise and precision become low, then a practical confidence interval can be imposed which will permit objective evaluation of a conservative detection limit, based on the t statistic appropriate to the determined for n observations. Skogerboe et al, have demonstrated the application of 6 in the form, where (1 - a) is the confidence interval required. Figure 192 also illustrates the procedure used by Parris et al, the open rectangles illustrate the range or standard deviation of five replicate runs made at three lowest concentrations of trimethylarsine. Consequently, for (n - 1) =4 and (1 - 0.05) = 0.95, t = 2.78 in most cases. 

---