Dimiter L. Tsalev

On-line microwave sample pretreatment for the determination of mercury in water and urine by flow-injection cold-vapour atomic absorption spectrometry

Abstract A system for on-line treatment of liquid samples in a microwave oven and determination of mercury by cold-vapour atomic absorption spectrometry was designed and evaluated. The system consisted of an atomic absorption spectrometer, equipped with a mercury-hydride system and amalgamation accessory, a flow-injection system, an autosampler and a mircowave digestor. Urine and environmental water samples were stabilized with potassium dichromate-nitric acid and were mixed with a bromination reagent. The recoveries of eight mercury compounds from aqueous solutions and five compounds from dilute urine were studied. At an applied microwave power of 75 W, the recoveries of mercury(II) nitrate, methylmercury chloride, amidomercury chloride, phenylmercury chloride and diphenylmercury were between 92 and 102% for 1 + 2-diluted urine without amalgamation and between 94 and 111% for 1 + 5-diluted urine with amalgamation, respectively. The sample throughput was 30–40 h −1 without amalgamation and 24 h −1 in the amalgamation mode. Good agreement with certificate values was obtained for urine samples. A limit of detection (3σ) of 10 ng l −1 was obtained using 10-ml sample volumes of environmental waters (river, lake, rain) and the amalgamation technique. The results compared well with those from an external laboratory with correlation coefficients of 0.9302 and 0.9028 ( n = 22) for integrated absorbance and peak-height absorbance, respectively.

Permanent modification in electrothermal atomic absorption spectrometry-Advances, anticipations and reality

Abstract Permanent modification is an important recent development in chemical modification techniques which is promising in view of increasing sample throughput with ‘fast’ programs, reducing reagent blanks, preliminary elimination of unwanted modifier components, compatibility with on-line and in situ enrichment, etc. An overview of this approach based on the authors’ recent research and scarce literature data is given, revealing both success and failure in studies with permanently modified surfaces (carbides, non-volatile noble metals, noble metals on carbide coatings, etc.), as demonstrated in examples of direct electrothermal atomic absorption spectrometric (ETAAS) applications to biological and environmental matrices and vapor generation (VG)–ETAAS coupling with in-atomizer trapping of hydrides and other analyte vapors. Permanent modifiers exhibit certain drawbacks and limitations such as: poorly reproducible treatment technologies — eventually resulting in poor tube-to-tube repeatability and double or multiple peaks; impaired efficiency compared with modifier addition to each sample aliquot; relatively short lifetimes; limitations imposed on temperature programs, the pyrolysis, atomization and cleaning temperatures being set somewhat lower to avoid excessive loss of modifier; applicability to relatively simple sample solutions rather than to high-salt matrices and acidic digests; side effects of overstabilization, etc. The most important niches of application appear to be the utilization of permanently modified surfaces in coupled VG–ETAAS techniques, analysis of organic solvents and extracts, concentrates and fractions obtained after enrichment and/or speciation separations and direct ETAAS determinations of highly volatile analytes in relatively simple sample matrices.

Validation of reducing-difference procedure for the interpretation of non-polarized infrared spectra of n-component solid mixtures

Validation of reducing-difference procedure based on subtracting of non-polarized infrared (IR) spectra of n-component solid mixtures is presented. The accuracy and precision are established. The limits of detection are 3.0, 2.5, 1.5 and 1.0wt.% for 5-, 4-, 3-and 2-component mixtures, respectively. Smoothing procedures for IR spectral analysis, based on Savitzky-Golay or Fourier methods are applied as well. The mean values and relative standard deviations for peak position (nu(i)) and integral absorbance (A(i)) obtained by this data processing approach have been examined using Students t-test.

Thermally stabilized iridium on an integrated, carbide-coated platform as a permanent modifier for hydride-forming elements in electrothermal atomic absorption spectrometry. Part 3. Effect of L-cysteine

Iridium (2 µg) was deposited on a carbide-coated platform, pre-treated with about 1.2–1.3 µmol of Zr or W, and was evaluated as a permanent modifier in electrothermal atomic absorption spectrometry. The noble metal is not vaporized from the integrated platform of the transverse-heated graphite atomizer, provided that the atomization and clean-out temperatures do not exceed 2050–2100 and 2100–2200 °C, respectively. Under comparable conditions, Pd exhibits much worse thermal behaviour, being volatilized above 1300 and 1500 °C from Zr- and W-treated platforms, respectively. Pyrolysis–atomization curves were plotted for numerous hydride-forming and volatile analytes: Sb, As, Bi, Cd, Pb, Te, Tl, Sn and Se. The best characteristic masses for integrated absorbance measurements with the Ir–Zr-treated platforms are 92, 30, 176, 2.4, 35, 50, 65, 71 and 45 pg, respectively. Vaporization temperatures are generally above 1000 °C, except for Cd. The effect of atomization temperature on sensitivity in peak height and integrated absorbance measurements is discussed. Double peaks were observed for Bi and Te with Ir–W-treated platforms.

The biouptake and toxicity of arsenic species on the green microalga Chlorella salina in seawater

The present study focuses on the biouptake, biotransformations, and toxicity of arsenic species on the marine green alga Chlorella salina in seawater from the Bulgarian Black Sea coast. Exposure to equal concentrations of As(III) or As(V) led to equivalent levels of toxicity and total intracellular arsenic content. Biouptake and toxicity of methylated arsenic species, monomethylarsonate (MMA) and dimethylarsinate (DMA), were approximately three orders of magnitude lower than those for inorganic arsenic species. Seawater enrichment with phosphate (up to 1.3 mg P L(-1)) resulted in a significant reduction of both intracellular As content and toxicity due to As(III) and As(V). In contrast, the toxicity and intracellular content of MMA and DMA were unaffected by the presence of phosphate. We measured the distribution and excretion of intracellular arsenic species, and demonstrated that the release of As(V) and/or As(III), together with the bio-reduction of As(V) and the subsequent methylation of As(III) may be a detoxification mechanism for these algae. The implications of the results with respect to arsenic species bioavailability and toxicity in marine water are further discussed.

Thermally stabilized iridium on an integrated, carbide-coated platform as a permanent modifier for hydride-forming elements in electrothermal atomic absorption spectrometry. Part 2. Hydride generation and collection, and behaviour of some organoelement species

The in situ collection of volatile hydrides in an electrothermal atomizer with an integrated platform pre-treated with 110 µg of Zr or 240 µg of W and 2 µg of Ir for permanent modification was studied. An optimization study of the performance characteristics of an automated FI-HG–ETAAS system based on an FI hydride generator interfaced with a transverse-heated graphite atomizer and longitudinal Zeeman-effect background correction was elaborated. The HG step for AsIII, AsV, BiIII, SbIII, SbV, SeIV, SnIV and TeIV, as well as for several alkylated species of As and Sn, was optimized by means of a full factorial 32 design, the factors being the concentrations of acid and tetrahydroborate (or their supply rates in µmol s–1).The corresponding regression equations are tabulated, and representative response surfaces and contour diagrams are plotted. All inorganic hydrides except for SnH4, are generated and collected with high efficiency at tetrahydroborate concentrations of 0.25–0.4% m/v, sample acidity of 1.5–3 mol l–1 HCl, trapping temperatures of 400 °C and a purge gas flow of argon of 100–130 ml min–1. The optimum conditions for stannane and alkyltin hydrides are: pH 1–4, tetrahydroborate concentrations of 0.2–0.4% m/v, trapping temperatures between 400 and 600 °C and argon flow rates of 60–120 ml min–1. Arsine, monomethylarsine and dimethylarsine are effectively collected on both coatings at temperatures between 400 and 500 °C and purge gas flow rates of 70–120 ml min–1. Optimum HG conditions differ strongly for AsIII, AsV, monomethylarsonate and dimethylarsinate species with this FI system, unless L-cysteine is added. Organoelement species of As, Sn and Se are thermally stabilized in a similar manner on both Ir–Zr- and Ir–W- treated platforms, the least stable species being selenornethionine and trimethylselenonium. The best levelling-off effect on the integrated absorbance for different analyte species (isoformation) is observed for As and the worst for organotins, particularly for trialkylated species such as tributyltin, trimethyltin and trimethylselenonium. Relatively better isoformation is achieved for organotins on Ir–W- and for organoselenium on Ir–Zr-treated platforms. The long-term stability of the Ir–Zr and Ir–W modifier coatings during at least 600–700 thermal cycles is demonstrated. The Ir–Zr treatment is preferred to Ir–W for hydride trapping, owing to lower atomization temperatures, longer lifetime of the atomizer and an absence of double peaks. Such peaks persist for Bi and Te on Ir–W-treated platforms. The best characteristic masses in integrated absorbance measurements with Ir–Zr-treated platforms are close to those for the direct injection mode, viz., 35, 107, 83, 43, 104, 48, 31, 32, 153, 146, 148, 145 and 152 pg for ASIII, BiIII, SbIII, SeIV, SnIV, TeIV, monomethylarsonate, dimethylarsinate, monomethyltin, dimethyltin, trimethyltin, diethyltin and monobutyltin, respectively. Analytical results for As, Sb and Se in certified reference materials (water and autoclave-decomposed sediments) are in good agreement with the certified contents.

Hyphenated vapour generation atomic absorption spectrometric techniques

1.  Aims and scope2.  Modern flow techniques in VGAAS (FI-VGAAS and CF-VGAAS)3.  On-line decompositions (FI-MWD-VGAAS, CF-MWD-VGAAS, FI-UV-VGAAS, etc.)4.  On-line pre-reduction5.  On-line preconcentration and separation (FI-VG-ETAAS, FI-ion-exchange-VGAAS, HG-CT-AAS, etc.)5.1  VG-ETAAS6.  Hyphenated techniques for speciation analysis (VG-CT/GC-AAS, HPLC-VGAAS, HPLC-UV-VGAAS, HPLC-MWD-VGAAS, etc.)6.1  On-line speciation by non-chromatographic HGAAS techniques6.2  Coupling with GC6.3  Coupling with HPLC6.3.1  Scope, sensitivity and LODs6.3.2  Hardware complexity6.3.3  Mercury speciation6.3.4  Availability and validation7  Conclusion8  References

On-line microwave sample pre-treatment for hydride generation and cold vapour atomic absorption spectrometry. Part 1. The manifold

An analytical system for automated on-line pre-treatment of liquid samples in a microwave oven for flow injection cold vapour and hydride generation atomic absorption spectrometry has been designed and evaluated. The system is based on a focused microwave oven and a new manifold with two coils. A reaction coil and a ballast-load coil are placed and oriented within the digestor in a manner providing a reliable long-term operation and increased reaction time. Samples are mixed with an appropriate oxidation reagent and loaded on an autosampler; all further operations of sample uptake, digestion, measurement, calibration and data processing are performed automatically with a sample throughput rate of 20–40 h–1. For the determination of mercury at ng l–1 levels using amalgamation the sample throughput decreased to 7–20 h–1. Introducing hot sample digests into the mercury/hydride system resulted in an increased sensitivity, particularly for peak height absorbance.

Speciation determination of arsenic in urine by high-performance liquid chromatography–hydride generation atomic absorption spectrometry with on-line ultraviolet photooxidation†

A coupled system for arsenic speciation determination based on high-performance liquid chromatography (HPLC), on-line UV photooxidation and continuous-flow hydride generation atomic absorption spectrometry (HGAAS) was built from commercially available modules with minor modifications to the electronic interface, the software and the gas-liquid separator. The best results were obtained with strong anion-exchange columns, Hamilton PRP X-100 and Supelcosil SAX 1, and gradient elution with phosphate buffers containing KH2PO4-K2HPO4. The on-line UV photooxidation with alkaline peroxodisulfate, 4% m/v K2S2O8-1 mol l-1 NaOH, in a PTFE knotted reactor for 93 s ensures the transformation of inorganic AsIII, monomethylarsonate, dimethylarsinate, arsenobetaine, arsenocholine, trimethylarsine oxide and tetramethylarsonium ion to arsenate. About 32-36 HPLC-UV-HGAAS runs could be performed within 8 h, with limits of detection between 2 and 6 micrograms l-1 As, depending on the species. The method was applied to the analysis of spot urine samples and certified urine reference materials (CRMs). Upon storage at 4 degrees C, reconstituted CRMs are stable for at least 2 weeks with respect to both their total arsenic content and the individual species distribution.

On-line microwave sample pre-treatment for hydride generation and cold vapour atomic absorption spectrometry. Part 2. Chemistry and applications

A system for on-line treatment of liquid samples in a microwave oven digestor was evaluated for use with cold vapour (CV) and hydride generation (HG) atomic absorption spectrometry (AAS). Various oxidation mixtures were tested and those based on bromination (bromate–bromide–acid) and peroxodisulfate (persulfate–acid–complexing agent) were found to be compatible with and most appropriate for CVAAS and HGAAS. The composition of reagents and the analytical conditions were optimized for the determination of mercury, arsenic, bismuth, lead and tin in urine and environmental waters. The limits of detection were 0.01 and 0.2 µg l–1 for mercury, with and without amalgamation, respectively, 0.5 µg l–1 for arsenic, 0.07 µg l–1 for bismuth and 0.1 µg l–1 for tin and the sample throughput was between 13 and 30 h–1.