Leonardo Lampugnani

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.

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.

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.

Cold vapour atomic fluorescence studies on the behaviour of mercury(II) and mercury(II)-thiol complexes. An alternative route for characterization of –SH binding groups

The behaviour of Hg II and Hg II -thiol complexes (RSH=L-cysteine, DL-penicillamine, propane-2-thiol, glutathione, thiosalicylic acid) following their reduction with alkaline sodium tetrahydroborate to give Hg 0 has been studied by using a continuous flow reaction system coupled with atomic fluorescence spectrometric (AFS) detection. The quantitative reduction of Hg II to Hg 0 takes place with a specific amount of sodium tetrahydroborate according to the stoichiometric reaction of mercury with alkaline NaBH 4 . The complete reduction of Hg II -thiol complexes to Hg 0 requires a molar excess of NaBH 4 of up to six orders of magnitude, depending on the type of complex. Under an appropriate excess of reductant, Hg II and its thiol complexes are not distinguishable giving the same AF molar response. The method allows the discrimination of Hg II from Hg II -thiol complexes without any preliminary separation. Applications to the indirect titration of thiols and to the determination of the number of accessible }}n1SH groups in pure ovalbumin samples are reported.

Electrothermal atomic absorption spectrometric determination of cadmium and lead with stabilized phosphate deposited on permanently modified platforms

Some drawbacks of the phosphate modifier such as reagent blank contribution and background absorption in electrothermal atomic absorption spectrometric determination of cadmium and lead are substantially alleviated by application of small amounts of phosphate, approximately 0.2 mumol (25 mug of NH(4)H(2)PO(4) or (NH(4))(2)HPO(4)), on the integrated platform of transversely heated graphite atomizer pre-treated with 2.7 mumol of Zr (250 mug) or W (500 mug) and 0.1 mumol of Ir (20 mug). Pyrolysis temperatures for Cd and Pb are up to 900 and 1100 degrees C for aqueous solutions and within 400-600 degrees C (Cd) and 750-850 degrees C (Pb) for biological fluids (urine, blood) and tissues (hair, liver, muscle) solubilized with tetraethylammonium hydroxide. The thermally stabilized phosphate on Zr-Ir or W-Ir treated platforms serves as a permanent modifier in analyses of environmental waters by multiple hot injections of large sample aliquots. Applications to water and biological certified reference materials are tabulated and show good agreement with certified values. Characteristic masses are 0.7-1.0 pg for Cd and 26-31 pg for Pb.

Masking agents in the determination of selenium by hydride generation technique

Abstract The effects of several masking agents in the determination of selenium by hydride generation was studied using a continuous flow hydride generator coupled with atomic absorption spectrometry. A miniature argon hydrogen diffusion flame was employed as the atomizer. The effects of masking agents (KI, NaSCN, thiourea, l -cysteine, 1,1,3,3 tetramethyl-2-thiourea) were studied both in the absence and in the presence of selected interfering species (Cu, Ag, Au, Ni, Co, Pd, Pt and Fe) and using different addition strategies of the masking agents to the reaction system: in batch mode, either to sample or NaBH4 reducing solution; in on-line mode, to the sample either before or after the HG step). The combined effect of some masking agents was also investigated. The addition mode of the masking agent to the reaction system could play a decisive role in the control of interfering processes both in the absence and in the presence of concomitants. The addition of NaSCN to the reducing solution of NaBH4 produced a moderate catalytic effect, similar to the one obtained in the presence of KI, and improved the tolerance limits for Cu, Ni, Co and Pd. Linearity of calibration graphs was unaffected by the on-line addition of 1,1,3,3 tetramethyl-2-thiourea to sample solution, while under similar conditions thiourea produced dramatic curvature of calibration graphs. The combined use of KI (added to reducing solution) and 1,1,3,3 tetramethyl -2-thiourea (added on-line to the sample) exhibited masking properties comparable but not superior to those of thiourea, except for Pt and Pd for which good tolerance limits were achieved. In the absence of KI in the reductant solution the masking efficiency of 1,1,3,3 tetramethyl-2-thiourea was considerably lowered. The addition of some masking agents such as thiourea, l -cysteine and 1,1,3,3 tetramethyl thiourea on-line to reaction solution after the NaBH4+KI reduction step, could be highly effective in the control of Cu and Ag interferences. The method was applied to determination of trace of selenium in pure copper standard reference materials.

Selenium hydride atomization, fate of free atoms and spectroscopic temperature in miniature diffusion flame atomizer studied by atomic absorption spectrometry

The mechanism of hydride atomization and the fate of free atoms was investigated in the miniature diffusion flame. Selenium hydride was used as a model for the other hydrides. Mercury vapor was employed as an analyte to study physical processes, such as macroscopic movements and free atom diffusion, controlling the distribution of free analyte atoms in the observation volume, separately from chemical reactions of the free atoms. Free atoms were detected by atomic absorption spectrometry. Spectroscopic temperature measurements based on atomic absorption at 196.1 and 204.0 nm Se lines were used to determine the temperature distribution. The spatial temperature distribution was highly inhomogeneous ranging from 150°C to 1300°C. The whole flame volume is actually a cloud of hydrogen radicals maintaining analyte in the free atom state since hydrogen radicals formed in outer zone of the flame diffuse to its cooler inner parts.

Studies in hydride generation atomic fluorescence determination of selenium and tellurium. Part 1 — self interference effect in hydrogen telluride generation and the effect of KI

Abstract The effects of tetrahydroborate (0.02–1%) and iodide (0–3 M) were investigated in determination of tellurium and selenium by hydride generation atomic fluorescence spectrometry. The effect of tetrahydroborate and iodide concentration were tested on the shape of calibration curves in concentration range of 1–1000 ng ml−1 analyte. Reductant deficiency resulted in a moderate sensitivity depression for tellurium but dramatically reduced the useful dynamic range down to 50 ng ml−1. On the contrary, selenium calibration curves retained a linear character even under conditions generating strong sensitivity depression. Curvature and rollover of tellurium calibration curves has been addressed to a self-interference effect caused by the formation of finely dispersed elemental tellurium. Iodide ions were found to have beneficial or no negative effects in the hydrogen telluride generation. Addition of iodide on-line to the sample has been proved effective in the control of the self-interference effect and allows to work in mild reaction conditions. Moreover, it allows a good control of Cu(II) interference and eliminates Ni(II) and Co(II) interferences. The method has been successfully applied to determination of tellurium in copper and lead ores certified reference materials.

Determination of antimony by continuous hydride generation coupled with non-dispersive atomic fluorescence detection

A sensitive method for the determination of Sb at ultratrace levels was developed by coupling continuous hydride generation with non-dispersive atomic fluorescence detection. A miniature argon–hydrogen diffusion flame was employed as the atomizer and a commercially available electrodeless discharge lamp as the light source. One of the main problems was the scattering signal generated by small droplets of solution which markedly deteriorated the signal-to-noise ratio. A simple way to remove the scattering signal was to operate under mild reaction conditions in order to minimize droplet formation. Under the optimized conditions, a limit of detection of 22 pg cm–3 of Sb (3s of the blank) was achieved, with a precision of 1.2% at the 5 ng cm–3 level and the calibration graphs were linear over more than 4 decades of concentration. L-Cysteine was employed both in the pre-reduction step and in the control of the interference effects arising from concomitant elements and acid mixtures. The analytical procedure was applied to the determination of Sb in certified reference materials of sediments, metallic copper and riverine water.

Permanent iridium modifier deposited on tungsten and zirconium-treated platforms in electrothermal atomic absorption spectrometry: vaporization of bismuth, silver and tellurium

The stabilizing role of permanent Ir modifier deposited on W-treated (WTP) and Zr-treated (ZrTP) platforms of transversely heated graphite atomizer (THGA) was studied in detail by electrothermal at. absorption spectrometry (ETAAS) and different surface techniques in model expts. for Ag, Bi and Te. The comparison of the stabilizing efficiency of permanent Ir modifier on WTP and ZrTP and each of the single components, reveals the better effect of Ir on WTP and Ir itself. The extent of analyte losses during pre-atomization and the strength of analyte assocn. with the modifier were estd. by the plotting of differential vaporization curves. The existence of double peaks of Ag, Bi and Te in WTP and Ir on WTP was confirmed and possible reasons for their formation are discussed. The absorbance profiles presented as differential curves reveal an existence of at least two different types of precursors detg. processes of atom generation. The obsd. differences in the behavior of Ir permanent modifier on WTP and ZrTP, resp., were explained by the different extent of Ir-W and Ir-Zr interaction and surface distribution. XRF, ESCA and SEM studies reveal nonuniform distribution of the modifier on the graphite substrate and the presence of oxide contg. species on the surface. [on SciFinder (R)]