As speciation

SFC-ICP-MS requires rather expensive and complicated instrumental design [473,474]. Interfacing the SFC restrictor with the ICP torch follows different approaches for pSFC and cSFC [469]. Polar modifiers, however, do not have a serious deleterious effect on the ICP plasma, which enables the polarity of the mobile phase to be changed with no significant loss of sensitivity or resolution. This enables analysis of compounds which are too polar for adequate separation with pure C02 as the mobile phase. SFC is still in its infancy as far as speciation analysis of metal-containing additives is concerned. 

A number of organometallic compounds show promise for LCEC study, and a few have already been examined in detail (especially mercury alkyls) [9]. More highly conjugated organic compounds such as a,a-unsaturated ketones and imines are occasionally good candidates, but at this time UV detectors frequently outperform electrochemical detectors for such systems. At this writing there have been only a few reported LCEC studies of metal ions or metal complexes. Perhaps the major reason for this is that very little modern LC has been carried out on them using any detector. It is difficult to compete with atomic spectroscopy techniques for the determination of most elements, but as speciation becomes more important, it seems likely that more LCEC methods will be developed for metal complexes. 

Criscione and Blouin, 2004). Thus, our knowledge of life cycles not only enables an understanding of origins but also evolutionary processes such as speciation, coevolution and host switching. However, although the field is replete with stories as to how complex life cycles may have arisen, hypotheses often remain untested and untestable. Even hypothetical model-driven studies remain divorced from reality in the absence of an evolutionary framework (e.g. Parker et at., 2003). 

In common with other solid materials the determination of element speciation in soils presents a number of difficulties. Firstly, direct determination of speciation in the solid material, without prior separation of the species from the solid matrix, is generally limited to major component elements since few of the direct techniques available are sensitive enough for trace element studies. Resort to separation or extraction of element species presents the usual problem of maintaining the speciation unchanged during the extraction or separation procedure. Despite these difficulties, speciation studies related to nutrient element availability to crops have been a major topic in soil science for more than half a century, uncategorised, however, as speciation until the relatively recent adoption of this terminology. 

An operational definition is considerably more practical. Operationally determined species are defined by the methods used to separate them from other forms of the same element that may be present. The physical or chemical procedure that isolates the particular set of metal species is used to define the set. Metals extracted from soil with an acetate buffer is an operational definition of a certain class. Lead present in airborne particles of less than 10 pm is another. In water analyses, simply filtering the sample before acidification can speciate the analytes into dissolved and insoluble fractions. These procedures are sometimes referred to as fractionation, which is probably a more properly descriptive term than speciation, as speciation might imply that a particular chemical species or compound is being determined. When such operational speciation is done, careful documentation of the protocol is required, since small changes in procedure can lead to substantial changes in the results. Standardized methods are recommended, as results cannot be compared from one laboratory to another unless a standard protocol is followed [124], Improvements in methodology must be documented and compared with the currently used standard methods to produce useful, readily interpretable information. 

Several potential problems with As speciation have been described [407]. ArCl+ causes a spectral overlap in ICP-MS detection. Speciation can change between the time the sample is obtained and when it is measured. Field separations or preservations can be developed to prevent that problem. Particulates in drinking water can contain a significant portion of the As, so filtering the sample without also analyzing the composition of the particulates can lead to measurements that are too low [407]. 

Four forms of As, namely, inorganic As (III), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), and inorganic As (V), were detected in rice [25], but in another study only As(V) and MMA were found [24], As the concentration of As in the rice grain was low, As speciation was performed only on rice straw to predict the risk associated with feeding contaminated straw to the cattle. Speciation of As in tissues by HPLC-ICP-MS revealed that the predominant species present in straw was arsenate followed by arsenite and dimethylarsinic acid (DMA). As DMA was only present at low concentrations, it was considered unlikely this would greatly alter the toxicity of As present in rice [71]. 

Sample Preparation Methods Prior to As Speciation Analysis 618... 

SAMPLE PREPARATION METHODS PRIOR TO As SPECIATION ANALYSIS... 

Basically, As speciation regards nowadays four main groups of samples, namely water samples (drinking, irrigation, and wastewater) food samples (mostly... 

Water samples in As speciation have a place of their own in terms of sample preparation, as they usually feature the two inorganic As species, As(III) and As(V), the major chemical properties of which (e.g., redox stability) have been thoroughly studied in the recent past. The analyst is usually well aware of the importance of preserving the original ratio of inorganic As species. Hence, problems with low chromatographic recovery are rarely faced. 

Food samples, especially seafood (fish, shellfish, and seaweed), are particularly interesting from the viewpoint of As speciation as they contain most of the newly identified As species. On the other hand, the apparent contradiction between the relatively high total As content of seafood and the absence of the expected adverse effects of As intoxication was brilliantly solved in the 1970s [99]. Nowadays two general approaches of sample preparation should be mentioned in this field ... 

The Role of Organic Solvents and the Mixture of Methanol-Water in the Preparation of Solid Samples Prior to As Speciation... 

Arsenic speciation strongly depends on the ratio of methanol to water used in the extraction phase. Since the introduction of one of the first methanolic extraction protocols for As speciation [107], almost all organic samples for As speciation were prepared using this extraction medium with the aid of sonication or shaking. This method is easy to use because methanol can be quickly removed after extraction, for example, by evaporation or speed-vacuum instruments. On the other hand, As metabolism is dependent on proteins at each step of the As detoxification process. Arsenic is transported, methylated, and excreted via combination with proteins [108], Any kind of sample preparation irreversibly changing the protein structure may result in the release of As at each detoxification stage. 

On the basis of the experience accrued so far, the extraction efficiency can be affected by both the nature of the sample under test and the As species actually present in the sample. The lipid content of the sample and the presence of As species with hydrophobic residues play a key role in this context. As a consequence, it may be useful to remove or partition lipids with an organic solvent prior to the methanol-water extraction in order to increase the extraction efficiency. This approach is also viable to prevent emulsification of methanol and lipids, which could otherwise significantly reduce the extraction efficiency [26]. Thus, sample preparation prior to As speciation usually starts with the chloroform-methanol extraction, followed by centrifugation of the sample solution to remove solid particles. Subsequently, after the addition of methanol and water, the supernatant is separated in a funnel. The amount of As remaining in the organic phase is usually quantified as total As by wet digestion after solvent evaporation. Obviously, this entails the loss of all speciation information related to the lipid-soluble As species [2]. Speciation analysis is finally carried out on the methanol-water phase, usually after solvent evaporation (see Table 19.2). 

TABLE 19.2. Examples of As Speciation Studies Based on the Application of Methanolic Extraction as the Main Sample Preparation Method... 

In conclusion, it can be stated that extraction with the methanol-water mixture is still considered the leading sample preparation approach in As speciation, except for lipid-soluble As species. These last were out of the scientific focus for a long time, but nowadays are being investigated again as highlighted by Francesconi and Kuehnelt [124]. 

When comparing the enzymatic methods used prior to Se or As speciation, an unambiguous and essential difference comes out. In Se speciation, enzymatic... 

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