B. Welz

Determination of cadmium, lead and copper in water samples by flame atomic-absorption spectrometry with preconcentration by flow-injection on-line sorbent extraction

Cadmium, lead and copper were determined in synthetic sea-water, drinking water and the NBS 1643b Trace Elements in Water standard reference material at mug/l. levels by flame atomic-absorption spectrometry after on-line preconcentration by sorbent extraction with a flow-injection system. Bonded silica with octadecyl functional groups packed in a micro column of 100 mul capacity was used to collect diethylammonium diethyldithiocarbamate complexes of the heavy metals in the aqueous samples. The sample loading time was 20 sec at a flow-rate of 3.3 ml/min. Ethanol or methanol was used to elute the adsorbed analytes into the spectrometer. The sample loading rate, elution rate and pH were optimized. Enrichment factors of 19-25 for Cd, Pb and Cu were achieved at sampling frequencies of 120/hr with precisions of 1.4, 1.0 and 1.3% rsd (n = 11), respectively. The detection limits (3sigma) for Cd, Pb and Cu were 0.3, 3 and 0.2 mug/l., respectively. Determination of Cd, Pb and Cu in NBS SRM 1643b showed good agreement with the certified values. Recoveries of Cd and Pb added to sea-water were 95 and 102%, respectively.

Mutual interactions of elements in the hydride technique in atomic absorption spectrometry: Part 1. Influence of selenium on arsenic determination

Abstract The interference from selenium(IV) on the determination of arsenic(III) and -(V) by hydride-generation atomic absorption spectrometry is removed if copper is added to the sample solution. This prevents the evolution of selenium hydride but has no influence — up to certain copper concentrations — on the formation of arsine. The possible interference mechanism is discussed. Interferences on selenium determinations are also described.

Determination of antimony, arsenic, bismuth, selenium, tellurium, and tin in metallurgical samples using the hydride AA technique—I: Analysis of low-alloy steels

Abstract Interferences in the gas phase are very unlikely and have not yet been observed with the hydride AA technique. Gas/liquid interferences between the volatile hydride forming elements and the major and minor components of low-alloy steels can be eliminated by working in the presence of higher acid concentrations and/or in acid mixtures containing nitric acid. Conditions could be found that allow the interference-free determination of the hydride forming elements in low-alloy steels directly against acid standards with detection limits around or below 0.0002%. The influence of the valence state on the determination of antimony and arsenic was investigated.

Differential determination of arsenic(III) and total arsenic using flow injection on-line separation and preconcentration for graphite furnace atomic absorption spectrometry

Abstract Arsenic(III) can be quantitatively extracted using sodium diethyldithiocarbamate (NaDDTC) as the complexing agent and C18 reversed phase packing as the column material for solid phase extraction. Arsenic(V) must be reduced to its trivalent oxidation state prior to extraction. A mixture of sodium sulphite, hydrochloric acid, sodium thiosulphate and potassium iodide was found to be optimum for on-line reduction. When the sorbent extraction is carried out without and with the addition of the reduction mixture, arsenic(III) and total arsenic can be determined sequentially by graphite furnace atomic absorption spectrometry with detection limits (3 σ) of 0.32 ng for As(III) and 0.43 ng for total arsenic. A 7.6-fold enhancement in peak area compared to direct injection of 40 μl samples was obtained after 60 s preconcentration. Results obtained for sea water standard reference materials, using aqueous standards for calibration, agree well with certified values. A precision of 5.5% RSD was obtained for total arsenic in a sea water sample (1.65 solμg l As). Results obtained for synthetic mixtures of trivalent and pentavalent arsenic agreed well with expected values.

Influence of the valency state on the determination of selenium in graphite furnace atomic absorption spectrometry

Abstract Several matrix modifiers have been investigated with respect to their stabilizing power for selenium in a graphite furnace. It was found that the different valency states of selenium are not stabilized to the same extent by frequently applied matrix modifiers like nickel or copper. This may be an explanation for pre-atomization losses and high characteristic mass data reported previously. Copper, mixed with magnesium nitrate, was found to be the best matrix modifier for selenium. Careful selection of the time-temperature program for thermal pretreatment in the graphite furnace is essential to avoid pre-atomization losses. A characteristic mass ( m 0 )of 18–20 pg/0.0044 A.s was obtained for selenium under optimized conditions.

Temporal and spatial temperature distributions in transversely heated graphite tube atomizers and their analytical characteristics for atomic absorption spectrometry

Abstract The important role which temperature plays in atomic absorption spectrometry (AAS) for the formation and detection of atoms in the absorption volume is discussed and the literature is reviewed. Non-homogeneous temperature distribution in the absorption volume is in contradiction to one of the prerequisites for the application of Beers law used in AAS to convert absorbance into analyte concentration or mass, and is particularly troublesome for an “absolute analysis” envisaged for electrothermal atomic absorption spectrometry (ETAAS). Coherent anti-Stokes Raman scattering (CARS) is used to study the gas-phase temperature distribution in a state-of-the-art transversely heated graphite tube atomizer (THGA). The effect of the internal gas flow on the size of the heated atmosphere is studied by steady-state temperature measurements. Temporally and spatially resolved measurements make it possible to study the temperature field within the atomizer volume in all three dimensions during the rapid heating of the furnace to final temperatures in the range 2173–2673 K. The role of the integrated platform of the THGA on the temperature field is investigated by temperature measurements of the gas phase in the presence and absence of the platform. The platform is identified as the major source of temperature gradients inside the tube volume, which may be as high as 1000 K in the radial direction during rapid heating. These gradients are most pronounced for heating cycles starting at room temperature and gradually decrease with increasing starting temperature. Shortly after the tube wall reaches its final temperature, the gas-phase temperature equilibrates and approaches the wall temperature. Because of the unavoidable contact with the cold environment at the open ends of the tube, minor temperature gradients are observed in the gas phase also in longitudinal direction, which can be further reduced by restricting the openings with end caps. The results obtained for the THGA are compared with those obtained earlier for a longitudinally heated graphite tube atomizer (HGA), including some analytical applications of these two furnace types. Both the temperature characteristics and the resulting analytical characteristics substantiate the superiority of the THGA in comparison to the HGA as an atomizer for ETAAS.

Corrosion of transversely heated graphite tubes by mineral acids

Abstract Corrosive changes of transversely heated graphite atomizer (THGA)-tube and platform surfaces were studied by scanning electron microscopy in combination with tube lifetime measurements under recommended conditions for vanadium determination. This was done for the four mineral acid matrices HNO3, HF, HCl and HClO4. Rising corrosion and reduced tube lifetime are observed for these matrices in the sequence HNO3

Determination of phosphorus in steel with a stabilized-temperature graphite furnace and zeeman-corrected atomic absorption spectrometry

Abstract The determination of phosphorus in steel by graphite furnace a.a.s. is plagued by a spectral interference from the iron matrix which results in overcompensation when a continuum-source background corrector is used. Zeeman background correction using an alternating transverse magnetic field at the furnace eliminates this problem and allows a routine determination of phosphorus down to 0.002% in steel. Lanthanum is an effective matrix modifier for the phosphorus determination, but its enhancing effect depends largely upon the tube material used and the sample matrix. A 0.2% lanthanum solution was found to be optimum. The stabilized-temperature platform furnace concept allows an interference-free determination of phosphorus in steel, down to 0.002%, directly against aqueous standards. Atomizing the sample from a pyrolytic graphite platform in an uncoated graphite tube provides the optimum environment for a phosphorus determination.

Spatially and temporally resolved detection of analytical signals in graphite furnace atomic absorption spectrometry

Abstract Spatial non-uniformities in analyte, atomizer gas phase temperature and radiant intensity distributions characteristic for graphite furnace atomic absorption spectrometry are briefly summarized and their effect on the analyte detection is analysed. It is shown that conventional detection of analyte based on the use of photomultiplier tubes provides excellent temporal resolution, sufficient wavelength isolation but totally ignores the spatial aspects of the interaction of the probing radiation beam with the analyte in the atomizer. A new, spatially resolved, method of analyte detection based on the use of a solid state detector located along the monochromator slit is presented. The approach is illustrated by the temporally and spatially resolved detection of Cd atomization and NaCl vaporization. It is shown that severe non-uniformities in atomic and/or background absorbance may be a potential source of analytical error. Advantages of spatially resolved detection as compared with conventional detection are discussed.

Determination of selenium in nickel-based materials by hydride-generation atomic absorption spectrometry

Abstract Nickel seriously interferes with the determination of selenium by the hydride-generation technique, but is readily removed by precipitation with sodium hydroxide. After filtration and acidification of the solution, selenium can be determined down to 0.1 mg kg −1 in nickel-based samples.