Arsenic compounds in marine samples

Occurrence and Distribution of Arsenic Compounds in Marine Samples... 

Attempts have been made to use this difference in reactivity to distinguish between the two arsenic-containing compounds in marine samples. Although some success was achieved on synthetic materials, the results from marine samples (47) have been misleading and have been questioned (32). 

In chemical combination, arsenic can exist in oxidation state III or V and can have a coordination number of 3, 4, 5, or 6. In marine samples, arsenic is mainly found in the V oxidation state, although, usually as a consequence of biological factors, arsenic (III) compounds can also occur and may at times be predominant. The properties and analysis of the various arsenic-containing compounds of significance in marine arsenic research are briefly discussed, and information is provided on their synthesis. For ease of reference, the arsenic compounds frequently mentioned by name (or abbreviation/acronym) are listed in Table IV together with their structure numbers. 

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. 

A number of arsenic-containing ribosides, also referred to here simply as arsenosugars, occur in marine samples. Most of the arsenosugars are dimethylarsinoylribosides (Fig. 2, compounds 9 to 25). This group of compounds was unknown prior to 1981, when 9 and 12 were isolated from a brown alga (55). Structures for the two compounds were origi-... 

Included here are novel arsenic compounds reported in environmental samples over the last five years. Dimethylarsinoylacetate was identified as a naturally occurring arsenical in marine reference materials of mussel, oyster, and lobster hepatopancreas (36). This compound had been proposed as a possible intermediate in the formation of arsenobetaine (31). More recently, however, arsenobetaine was found to degrade to dimethylarsinoylacetate under aerobic microbial conditions (37), and such a biotransformation suggests an alternative hypothesis for the presence of dimethylarsinoylacetate in marine samples. 

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]... 

Marine biological material, radioactivity of, 3 315-316 Marine samples arsenic in, 44 148-151 compounds found, 44 151-162 occurrence and distribution, 44 149-151, 162-169... 

Fig. 3. Typical separation of four arsenosugars and DMA by HPLC/ICP-MS using an ODS reversed-phase column at pH 3.2 under conditions described in Ref. 60. The sensitivity and specificity of the detector allows the determination of arsenosugars and other arsenic compounds to be conducted on dilute aqueous extracts of the marine samples.

The concentrations of the three arsenicals (75-77) were determined in 37 marine organisms comprising algae, crustaceans, bivalves, fish and mammals by high-performance liquid chromatography/inductively coupled plasma mass spectrometry (HPLC/ICPMS) [170]. All three organoarsenics, which occurred at pg/kg concentrations, were detected in 25, 23 and 17 of the 37 samples analyzed, respectively. The limits of detection were 2-3 pg/kg dry mass. The data illustrate that all three compounds are common minor constituents in practically all marine samples. 

The implications are that these organoarsonic acids are natural products and hence have a biogeochemical origin in the oil shale taphonomy (fossilization) process. It is also interesting to note that no examples of biophenylation have been reported, whereas biomethylation of arsenic compounds is a well known reaction. (31) How the phenylarsonic acid forms will have to be answered with the examination of precursors to the oil shale such as freshwater marine algal mats as well as other biogeochemical samples ... 

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). 

---