Ewa Bulska

Capillary column gas chromatography for mercury speciation

Abstract The determination of methyl- and ethylmercury halides in environmental and biological samples typically involves gas chromatography with electron-capture detection. However, these organomercury halides are notorious for their poor chromatographic characteristics (severe tailing, decomposition, low column efficiencies) on packed columns. The problems can be temporarily alleviated by column passivation using a concentrated organic solution of mercury(II) chloride. Attempts to use capillary columns instead, to improve the chromatographic behaviour of organomercury halides, have met with mixed success, and the results presented generally show poorer performance than that obtained using packed columns, even after passivation. To eliminate the problem at its source (the polar mercury-halide bond), it is proposed to butylate the mercury species with a Grignard reagent to yield the non-polar dialkyl derivatives. As the electron-capturing halide moiety is absent from these derivatives, mercury-specific detection is necessary, and a microwave-induced plasma emission detector is utilized. In combination with capillary gas chromatography, unprecedented column and separation efficiencies for methyl- and ethylmercury are achieved. The practical utility of the method is illustrated in a preliminary application to the determination of mercury species in a fish tissue reference material after extraction and butylation.

Modifiers and coatings in graphite furnace atomic absorption spectrometry—mechanisms of action (A tutorial review)

Abstract A multitude of different and often contradictory mechanisms for the effects of modifiers and coatings have been proposed. Many of these proposals lack sufficient experimental evidence. Therefore, a series of statements based on our own investigations is given as ‘facts’. Another series of statements is made as ‘fictions’ related to erroneous proposals on the functioning of modifiers and coatings in the pertinent literature. Two basic concepts are developed for the sequence of processes leading to analyte stabilization for the two most important groups of modifiers: refractory carbide forming elements of the IVa–VIa groups of the periodic system on the one hand and Pt-group metals on the other hand. These concepts are based on the main reactions of graphite with elements and compounds: carbide formation and intercalation. Most important experimental results leading to this understanding are described: Penetration measurements for modifiers and analytes indicated the subsurface zone down to approximately 10 μm as the essential place for graphite–analyte–modifier interactions. The reason for this phenomenon is an open porosity of the pyrocarbon coating of 5–10% (v/v) into which liquids penetrate upon sample application. This also indicates that modifiers are best applied by impregnation or electrolysis whereas dense coatings are not advantageous. It is also shown that graphite tube assemblies are dynamic systems with a limited lifetime and carbon losses are an essential feature of tube corrosion. Most frequently found erroneous statements are discussed: (a) Particles on the tube surface are responsible for analyte stabilization and retention during pyrolysis. (b) Analyte stabilization is taking place by formation of intermetallic compounds or thermally stable alloys. (c) Experiments are performed with unrealistic concentrations of analytes and/or modifiers. (d) Dense coatings are advantageous. Finally, a functional schedule is given for the three steps of graphite furnace atomic absorption spectrometry (GFAAS): sample application and drying; pyrolysis; atomization. Contrary to the vast amount of literature on this topic it tried to provide the analyst working with GFAAS and in an increasing number working with Solid Sampling-GFAAS with a set of most important statements. This might spare the experimentalist a lot of useless optimization procedures but should lead him to a basic understanding of the complex phenomena taking place in his instrument and during his analytical work.

Application of palladium- and rhodium-plating of the graphite furnace in electrothermal atomic absorption spectrometry

Palladium- and Rh-plating of the graphite furnace has been evaluated as a method of introducing the metallic form of Pd and Rh for chemical modification in electrothermal atomic absorption spectrometry. It is shown that by electroplating Pd and Rh onto the inner surface of the tube, the pre-treated graphite surface may resemble the behaviour of the corresponding modifier. The resulting metallic layer is very effective in inhibiting the loss of volatile elements (e.g., As and Se), as well as reducing the influence of oxide and carbide formation (e.g., Si). The advantage of the proposed procedure of introduction of a solid chemical modifier is that the pre-treated tubes exhibit an extended analytical lifetime for the determination of As and Se, up to 80 and 160 firings in the presence of Pd and Rh, respectively. In the case of Si the Rh-plated graphite tube could last for about 100 firings.

Ectopic expression of Arabidopsis ABC transporter MRP7 modifies cadmium root-to-shoot transport and accumulation

Arabidopsis MRPs/ABCCs have been shown to remove various organic and inorganic substrates from the cytosol to other subcellular compartments. Here we first demonstrate that heterologous expression of AtMRP7 in tobacco (Nicotiana tabacum var. Xanthi) modifies cadmium accumulation, distribution and tolerance. Arabidopsis MRP7 was localized both in the tonoplast and in the plasma membrane when expressed in tobacco. Its overexpression increased tobacco Cd-tolerance and resulted in enhanced cadmium concentration in leaf vacuoles, indicating more efficient detoxification by means of vacuolar storage. Heterologous AtMRP7 expression also led to more efficient retention of Cd in roots, suggesting a contribution to the control of cadmium root-to-shoot translocation. The results underscore the use of AtMRP7 in plant genetic engineering to modify the heavy-metal accumulation pattern for a broad range of applications.

Noble metals as permanent modifiers for the determination of mercury by electrothermal atomic absorption spectrometry

Palladium, gold, rhodium and iridium have been investigated as chemical modifiers for mercury determination by ETAAS. The gold/rhodium mixture and palladium alone, thermally reduced in the graphite tube, and also electrodeposited palladium give the best analytical performance. Under those conditions a characteristic mass of 110 pg is obtained for mercury. The amount and the reduction temperature of the modifier, together with the pyrolysis, atomization and cleaning temperatures for mercury have been optimized in order to find best conditions for permanent modification. Experimental conditions for palladium and rhodium after thermal and electrodeposition as a permanent modifier have also been optimized. It is shown that thermally deposited palladium can serve for about 50 firings when the maximum temperature does not exceed 1800°C. The tubes pretreated with a thermally deposited gold/rhodium mixture or electrodeposited palladium exhibit extended analytical lifetime (up to 500 firings without loss of sensitivity) even when tubes are heated up to 2000°C.

The role of subcellular distribution of cadmium and phytochelatins in the generation of distinct phenotypes of AtPCS1- and CePCS3-expressing tobacco

Exposure to Cd2+ leads to activation of phytochelatin synthase (PCS) and the formation of phytochelatins (PCs) in the cytosol. Binding of Cd by PCs and the subsequent transport of PC-Cd complexes to the vacuole are essential for Cd tolerance. Attempts to improve Cd detoxification by PCS overexpression have resulted in contrasting plant phenotypes, ranging from enhanced Cd tolerance to Cd hypersensitivity. In the present paper, changes in the subcellular phytochelatin, glutathione, gamma-glutamylcysteine and cadmium vacuolar and cytosolic distribution underlying these phenotypes were examined. Cadmium and PCs levels were determined in protoplasts and vacuoles isolated from leaves of Nicotiana tabacum expressing either of two phytochelatin synthase genes, AtPCS1 and CePCS (differing in their level of Cd tolerance; being Cd hypersensitive or more Cd-tolerant as compared to wild-type plants, respectively). We showed that Cd hypersensitivity of AtPCS1-expressing tobacco results from a significant decrease in both the cytosolic and vacuolar pool of PCs, indicating a decreased cadmium detoxification capacity. By contrast, enhanced Cd tolerance of CePCS plants was accompanied by an increased cytosolic and vacuolar SH of PC/Cd ratio, suggesting more efficient Cd detoxification. Surprisingly, the substantially reduced level of PCs did not influence Cd accumulation in vacuoles of AtPCS1-transformed tobacco (relative to the wild-type), which suggests the important role of mechanisms other than PC-Cd transport in Cd translocation to the vacuole. Our data suggest that the key role of the PCs in Cd tolerance is temporary binding of Cd2+ in the cytosol, and contrary to the current view, their contribution to cadmium sequestration seems to be less important.

Simultaneous determination of Se and As by hydride generation atomic absorption spectrometry with analyte concentration in a graphite furnace coated with zirconium

Abstract Conditions for the simultaneous determination of selenium and arsenic at ng l −1 level were developed. Simultaneous determination of these elements was possible through the multielement capabilities of hydride generation, with in situ trapping and atomization in a graphite tube coated with zirconium and measurements using a dual channel atomic absorption spectrophotometer. The zirconium coating employed in this work was relatively stable; once formed it could stand ≈80 firings without any significant change in the efficiency of hydride collection. This appears to be an advantage over the palladium coating, which is usually formed individually before each measurement because of its thermal instability during the atomization step of the furnace temperature programme. As a result, two determinations of the two elements could be performed in 2.5 min. Under the optimized conditions, concentration detection limits of 17 and 13 ng l −1 for a 7.1 ml sample volume were obtained for selenium and arsenic, respectively (absolute detection limits 120 and 92 pg for Se and As).

Microcolumn sorption of antimony(III) chelate for antimony speciation studies

Selective sorption of the Sb(III) chelate with ammonium pyrrolidine dithiocarbamate (APDC) on a microcolumn packed with C16-bonded silica gel phase was used for the determination of Sb(III) and of total inorganic antimony after reducing Sb(V) to Sb(III) by l-cysteine. A flow injection system composed of a microcolumn connected to the tip of the autosampler was used for preconcentration. The sorbed antimony was directly eluted with ethanol into the graphite furnace and determined by AAS. The detection limit for antimony was significantly lowered to 0.007 μg l−1 in comparison to 1.7 μg l−1 for direct injection GFAAS. This procedure was applied for speciation determinations of inorganic antimony in tap water, snow and urine samples. For the investigation of long-term stability of antimony species a flow injection hydride generation atomic absorption spectrometry with quartz tube atomization (FI HG QT AAS) and GFAAS were used for selective determination of Sb(III) in the presence of Sb(V) and total content of antimony, respectively. Investigations on the stability of antimony in several natural samples spiked with Sb(III) and Sb(V) indicated instability of Sb(III) in tap water and satisfactory stability of inorganic Sb species in the presence of urine matrix.

Optimization of electrochemical deposition of noble metals for permanent modification in graphite furnace atomic absorption spectrometry

Abstract Palladium, rhodium and iridium were investigated as chemical modifiers following their electrodeposition on the graphite surface. Conditions for electrodeposition were optimized to find the best analytical performance. Cadmium was used as a test element in this study. Using modified tubes, maximum pyrolysis temperatures were 800°C with Ir as modifier, 900°C with Pd, 1000°C with Pd+Rh, and 1100°C with Rh alone. The modified tubes exhibit a long-term performance, which indicates an advantage in use of the electrodeposition of noble metals as permanent modification.

Flow-injection speciation of aluminium

Abstract This review discusses recent trends in chemical speciation of aluminium in natural waters. Several difficulties which arise from the complexity of aluminium species present in natural samples as well as its participation in dynamic equilibrium are highlights. Taking this into account various approaches for speciation of Al described in the literature are critically evaluated. The strategy of fractionation procedures based on the rate of reaction, sorption efficiency, chromatographic behaviour of the aluminium compounds or size of the molecules are described in the paper. Besides conventional procedures involving various measuring and separation techniques, a special emphasis is focused on mechanised flow injection methodologies. This was found to be very important in order to ensure precise control of the reaction time, which is essential in aluminium species equilibrium system. The analytical performance of several separation procedures based on flow injection approaches as well as the detection methods are discussed and compared.