J. M. Mir

Selenium speciation by high-performance liquid chromatography–fraction collection–electrothermal atomic absorption spectrometry: optimization of critical parameters

Chromatographic effluents were analysed by electrothermal atomic absorption spectrometry (ETAAS) using a sampling procedure based on fraction collection and hot injection onto a graphite furnace. In order to reduce analyte dilution and to attain the maximum sensitivity of the combined system, special attention was paid to the volume of fraction collected and the volume of each fraction injected onto the furnace. Total volumes of 60 µl (50 µl of sample + 10 µl of modifier) were introduced into the graphite furnace by hot injection, using a pre-heating temperature of 120 °C and an injection rate of 3.3 µl s–1. Under these conditions, the detection limit for determination of selenium was 1.21 µg l–1 and the repeatability 2.4% for 40 µg l–1. When effluent fractions were analysed with the hot injection programme, the use of fraction volumes of 0.50 ml or lower provided a detection limit below 1 ng of selenium with respect to the sample injected into the chromatograph. This procedure was applied to the speciation of selenium. The separation of trimethylselenonium, selenite and selenate was performed by anion-exchange chromatography using 0.01 mol l–1 ammonium citrate at pH 3.0 and 7.0 as eluent. The presence of such a concentration of citrate produced a negative interference on selenium determination and this signal suppression was reduced by 17% on increasing the amount of nickel (200 µg) and also adding magnesium nitrate (50 µg) and overcome by using matrix-matched standards. Detection limits were 1.67, 1.27 and 0.76 ng of Se for trimethylselenonium, selenite and selenate, respectively.

Study of interferences in the determination of lead by hydride generation-direct flame atomic-absorption spectrometry when oxidising agents are employed to increase the sensitivity

The greatest sensitivity in the determination of lead by hydride generation-direct flame AAS is obtained in the presence of ammonium persulphate (0.012 µg). A complete interference study is reported, comparing the results obtained when hydrogen peroxide and ammonium persulphate are employed. The oxidation to a metastable tetravalent state of lead by the prior action of the oxidising agent is considered to be the mechanism by which the atomic-absorption signal in increased.

Influence of Oxidising Agents in Lead Determination by Hydride Generation Direct Flame Atomic Absorption Spectroscopy

SummaryThe influence of Cr2O72−, MnO4−, Ce4+, H2O2 and S2O82− as oxidising agents in lead determination as volatile covalent hydride using NaBH4 is reported in this paper.The reaction conditions for every oxidising agents (pH, quantity and concentration of the oxidising agent, quantity and concentration of the NaBH4 and reaction time) are optimized, determining sensitivity and detection limit by measuring the peak height.The recovery of lead from solution has been measured by flameless atomic absorption spectroscopy.A linear relationship the logarithm of absorbance values and the redox potential of the system is obtained with a correlation coefficient of 0.999. The results and their interpretation are given in this paper.ZusammenfassungEs wird über den Einfluß der Oxidantien Cr2O7, MnO4−, Ce4+, H2O2 und S2O82− auf die Bleibestimmung als kovalentes Hydrid berichtet. Für jede oxidierende Substanz werden die Reaktionsbedingungen optimiert (pH, Menge und Konzentration des Oxidans, Menge und Konzentration von NaBH4 und Reaktionszeit), um Empfindlichkeit und Nachweisgrenze durch Peakhöhenauswertung zu bestimmen.Die Wiederfindungsrate wurde durch elektrothermale Atomabsorptionsspektrometrie von Blei in der Lösung bestimmt.Der Zusammenhang zwischen dem Logarithmus der Extinktion und dem Redoxpotential des Oxidans ergibt eine lineare Beziehung mit einem Korrelationskoeffizienten von 0.999. Die Ergebnisse und deren Interpretation werden in dieser Arbeit beschrieben.

Effect of nickel and palladium as chemical modifiers and influence of urine matrix on different chemical species of selenium in electrothermal atomic absorption spectrometry

The effect of nickel and palladium nitrates on the main selenium species in human urine, selenite and trimethylselenonium, was studied with respect to thermal stabilization in the graphite furnace. Special attention was also paid to the use of reduced palladium, either thermally or chemically. In all instances, maximum charring temperatures of 1200–1300 °C were attained for both species in aqueous solution. Trimethylselenonium was partially stabilized by reduced palladium, showing a similar behaviour to that using typical reducing agents that are found in urine, such as urea. The influence of the urine matrix on the selenium signal, using nickel as chemical modifier, was studied and minimized by optimization of the amount of modifier and charring and atomization steps. A selenium response independent of the urine matrix (selenium recoveries of 96%) was obtained for urine samples diluted 1+4 with water by adding 100 µl of nickel, using 900 and 2300 °C as charring and atomization temperatures, respectively, and including a cool-down step between charring and atomization.

Effect of matrix components on chromium atomization processes in graphite furnace atomic absorption spectrometry

Abstract The effect of various organic and inorganic matrix components on chromium atomization in graphite furnace atomic absorption spectrometry is studied. The results are explained on the basis of chromiums atomization mechanisms. The two predominant mechanisms are the thermal dissociation of the oxide and of the carbide. Losses through molecular volatilization reduce the sensitivity when chromium chelate complexes are atomized. In this case, the atomization mechanism consists of the thermal dissociation of the chelate. The formation of chromium carbide from the carbon residue produced by decomposition of the organic solvents leads to a loss of sensitivity.

Determination of boron in waters by using methyl borate generation and flame atomic-emission spectrometry

An improved method is proposed for the determination of boron in waters. The esterification reaction between boric acid and methanol in a concentrated sulphuric acid medium and the vaporisation of the methyl borate formed (boiling-point, 68 °C) are used in the determination by boron by measuring the emission of the BO2 radical at 548 nm. This reaction is carried out in a simple and inexpensive generator, designed for this purpose, and the heat developed in it causes the rapid volatilisation of the methyl borate. Thus no collection systems or carrier gas are required. The proposed method gives an improved detection limit and it can be applied to the determination of boron in water samples. It is both rapid and highly selective.

On-line thermospray continuous volatilization of cobalt, aluminium and chromium volatile chelates and determination by heated quartz tube atomic absorption spectrometry

A thermospray of 24 cm in length and 0.5 mm inner diameter is used for on-line gas sample introduction of Co, Cr and Al chelate compounds into a heated quartz tube followed by atomic absorption spectrometric determination. The chelates are initially synthesised by means of flow injection (flow rate 6.5 ml min–1, injection volume 0.5 ml and reactor length 2m) and transported to the vaporizer by an air flow [60 psi (1 psi = 6895 Pa)]. The optimum heating temperature of the vaporizer is 10 °C higher than the volatilization temperature of the particular chelate. The sensitivity values obtained Co, 0.05; Al, 0.07; and Cr, 0.03 µg ml–1 show an improvement of 3- and 20-fold, linearity showing a similar improvement, although the reproducibility is lower, when compared with values obtained using a pneumatic nebulizer. The presence of other elements that also form chelates can interfere with the determination of the analytes.

Application of the ligand vapour technique to the volatilization of unstable chelate compounds (particularly iron(III) trifluoroacetylacetonate) in AAS

SummaryThis paper describes a method for volatilizing chelate complexes in AAS using a carrier gas containing ligand vapour in order to prevent chelate decomposition on heating. The method is evaluated with iron(III) trifluoroacetylacetonate. The sample containing chelate is injected into an electrically heated system, and is carried by nitrogen gas containing trifluoroacetylacetone H(TFA) to a silica tube where atomization occurs. This system increases the reproducibility of atomic absorption signals and decreases the high blanks which occur with the conventional method as a result of chelate decomposition. The characteristic mass of the combined analytical procedure is 1.30 ng Fe, the detection limit 12.0 ng Fe, and the relative standard deviation is 4.7% ng Fe (n=10). Sensitivity is improved 16-fold with respect to the system involving conventional nebulization and atomization of aqueous solutions in air-acetylene flame.

Determination of chromium by AAS using volatileβ-diketonate complexes

SummaryA new method is proposed for the determination of chromium, which is based on atomizing the volatile phase, generated from itsβ-diketonates, in a silica tube. For this purpose, an electrically-heated volatilization system was designed, in which the chelate is injected and volatilized, and then carried by nitrogen to the silica tube, which is heated with an air-acetylene flame. The system parameters were optimized and the analytical characteristics established, the following values being obtained: characteristic mass (measuring area under peak) 0.15 ng, reproducibility (expressed as the relative standard deviation) 5.6%, and linearity up to 25 ng. The results obtained were compared with those found when atomizing solutions ofβ-diketonates in an air-acetylene flame and in a graphite furnace. Atomization of chromiumβ-diketonates (after first volatilizing them) increases the sensitivity 54-fold as compared with the nebulization of aqueous solutions of dichromate with atomization in the flame.

Inter-laboratory note. Gas-liquid separator for automated hydride generation and atomic absorption spectrometry

A straightforward phase separator for continuous hydride generation was developed. The separator uses a small plastic syringe that minimizes the typical dead volume of conventional separators and avoids analyte losses by adsorption on glass. The device is simple and easy to fit to any FI system. It was used for the AAS determination of selenium, with a detection limit of 6.5 µg l–1. The method has a reproducibility of 2–3% expressed as the relative standard deviation.