Glen R. Carnrick

Determination of selenium in biological materials with platform furnace atomic-absorption spectroscopy and Zeeman background correction

The presence of phosphate and iron provides a spectral interference for the determination of selenium at its primary resonance line at 196.0 nm that is avoided by using Zeeman background correction. Good quality pyrolytic graphite tubes, platform atomisation and integrated absorbance readings provide an acceptable quantitative environment for selenium and permit most analyses to be performed against a simple calibration graph using selenium in a nickel matrix modifier. Remaining problems appear to relate to loss of selenium prior to atomisation in the presence of certain matrices, notably the combination of sodium and sulphate. Charring in the presence of oxygen did not improve the analytical situation, nor did the use of silver, molybdenum or copper as a matrix modifier. We found a 2σ detection limit of 28 pg and a sensitivity of about 25 pg per 0.0044 As (A = absorbance units), expressed as characteristic amount. A preliminary direct method is presented for selenium in urine, with nickel, nitric acid and magnesium nitrate present, with a detection limit in urine of about 10 µg 1–1.

Graphite-tube effects on perchloric acid interferences on aluminum and thallium in the stabilized-temperature platform furnace

Abstract The determination of aluminum in the presence of perchloric acid is shown to depend upon the quality of the pyrolytic coating of furnace tubes. With new pyrolytically coated tubes, no interference was found from 0.5 M HClO4 on 0.5 ng Al and no decrease in signal or deterioration of the pyrolytic coating was found after more than 150 firings. Very little interference was found in the determination of thallium in perchloric acid. In both cases the literature reported severe interferences. The determination of thallium and aluminum in perchloric acid appears to be more sensitive to the quality of the pyrolytic graphite coating than any of the materials studied previously here.

Flow injection hydride generation electrothermal atomic absorption spectrometry with in-atomizer trapping for the determination of lead in calcium supplements

Lead hydride was generated from acid solution, containing potassium ferricyanide as an oxidizing agent, by the reaction with alkaline borohydride solution. The effects of reaction conditions (hydrochloric acid, ferricyanide and borohydride concentrations), and the lengths of reaction and stripping coils were studied. The effects of trapping temperature and argon flow rate were also investigated. Under the conditions giving the best peak area sensitivity, the detection limit (concentration giving a signal equal to three S.D. of the blank signal) was 0.12 mug l(-1) for a 1000 mul injection volume. The detection limit was improved to 0.03 mug l(-1) when the ferricyanide was purified by passage through a cation-exchange resin. Two calcium supplement materials were analyzed by the flow injection (FI)-hydride generation (HG)-electrothermal atomization atomic absorption spectrometry (ETAAS) method, giving values of 0.55 and 0.66 mug g(-1), in agreement with results obtained by previously validated methods. For a 500-mg sample the limits of detection and quantification were 0.006 and 0.02 mug g(-1), respectively.

Fast analysis with Zeeman graphite furnace AAS

Abstract Stabilized Temperature Platform Furnace (STPF) methods have been adapted and altered to reduce the analytical time to less than 1 min per sample with no loss of analytical precision or accuracy. It is shown that this could be further reduced to about 30 s per sample if certain changes in instrumentation are implemented, especially in the software and firmware that control the autosampler. The sample uptake rate for the autosampler should overlap the cooldown of the tube from the prior determination. Also, the sample should be deposited onto a heated platform. In this work the pyrolysis step and, in most cases, the use of a matrix modifier has been omitted. Since backgrounds were therefore larger, the use of Zeeman correction was usually required, but continuum background correction was not tried. To confirm that these fast analytical methods might be practical, more than 10 standard reference materials were analyzed for several elements including Pb, Cd, Cu, Ni, As and Cr. The paper is not primarily intended to provide routine and reliable methods; it is intended to test the feasibility of these fast methods.

Investigation of aluminum interferences using the stabilized temperature platform furnace

Abstract Al was determined in the stabilized temperature platform furnace with very few interferences. No interferences were found for several metal nitrates, sulfates or phosphates, or for NaCl. The Al absorbance signal was delayed in the presence of MgCl 2 but there was no interference. This led to the use of 50 μg Mg(NO 3 ) 2 as a matrix modifier for Al. There were no interferences for CaCl 2 but it was particularly important to use new pyrolytically coated tubes to avoid “aging” effects. CuCl 2 provided a very persistent interference that was reduced when the Mg(NO 3 ) 2 matrix modifier was used that permitted a char temperature of 1700°C. Perchloric acid interferences were severe with improperly coated graphite tubes but did not exist up to 0.5 M HClO 4 when the new pyrolytically coated tubes were used. A serum Al method was tested briefly and no problems were found. Al was determined in seawater with no influence from the salinity of the sample and less than 0.6 μg/1 Al in seawater could be detected.

Quality-assurance procedures for graphite-furnace atomic-absorption spectrometry.

A procedure is described for quality-control in graphite-furnace atomic-absorption spectrometry. It uses an NBS standard reference material to avoid errors in standard preparation, and very simple instrumental conditions, with no matrix modifier or pyrolysis step. The characteristic mass and the Zeeman ratio are calculated for Ag, Cu, and Cr, and deviations from the expected values for these quantities are correlated with potential instrumental malfunctions.

Analysis of solid samples by graphite furnace atomic absorption spectrometry using Zeeman background correction

Although graphite furnace atomic absorption spectrometry is one of the most sensitive techniques for trace element determinations, the analysis of solid materials can be challengingly difficult. As no dissolution is used, none of the sample matrix is removed before the introduction of the sample into the furnace, and this can result in severe vapour-phase interferences. Typically, quantification has been achieved by the use of standard additions or by comparison with a known reference material. Using stabilised temperature platform furnace technology and simple aqueous standards, we have determined Cr in plastic film, Pb in flexible PVC and Cu in NBS Standard Reference Material bovine liver. Agreement was obtained with the values provided by the sample suppliers. The precision was found to be about 4–6%(relative standard deviation).The solid materials analysed contained relatively high levels of analyte, requiring the use of alternative wavelengths and in two instances an increased gas flow during atomisation. The effect of gas flow on sensitivity was examined and 19 wavelengths were characterised for use with Zeeman background correction.

Properties of the cadmium determination with the platform furnace and Zeeman background correction

Abstract Cadmium, previously a difficult determination in inorganic matrices with the graphite furnace, is shown to be easy to determine with the combination of the stabilized temperature platform furnace, appropriate matrix modification and Zeeman background correction. The optimized conditions were suitable for many matrices; seawater and urine were explored in some depth. Natural water samples were analyzed for Cd. The matrix modifier included ammonium phosphate and either Mg(NO3)2 or NaCl. Simple working curves were used. Problems associated with char losses were studied and recovery experiments were shown to expose such problems. The concept of characteristic amount, the mass of analyte that produced a 1 % absorption signal (0.0044 abs), especially using integrated absorbance data, was shown to be a very powerful diagnostic tool for graphite furnace work. The characteristic amount for Cd was 0.35 pg/0.0044 abs-s, and it varied little with variations in experimental conditions. The effect of background signals that were evolved rapidly was to disrupt the Zeeman signal. Guidance is provided in identifying and controlling these problems. The advantage of the Zeeman technique in providing improved detection limits using bright sources was explored.

Spectral interferences using the Zeeman effect for furnace atomic absorption spectroscopy

Abstract Using a transverse, a.c. Zeeman system, with the magnet on the analyte, background correction is performed at the exact analyte wavelength. As a result, nearly all of the spectral interferences associated with continuum correction are eliminated. Errors may occur, though, using Zeeman correction if coincident or nearby absorption lines or molecular absorption bands exhibit Zeeman splitting. We have found an example of overcorrection in the determination of Cd at the alternate 326.1-nm line that we believe is due to splitting of PO bands. We have also confirmed errors from Ft in the determination of Fe at the alternate 271.9-nm line and from Co in the determination of Hg at 253.6 nm.

Signal processing and detection limits for graphite furnace atomic absorption with Zeeman background correction

Abstract The signal handling requirements for graphite furnace atomic absorption are much more demanding than those for flame atomic absorption. Graphite furnace signals change rapidly, background levels are higher, and signal interpretation needs are more extensive. We have identified a number of signal generation and processing factors that are important for success in graphite furnace analyses. These include: use of the transverse, a.c. Zeeman technique with the magnet on the analyte for background correction; production of a series of signal integrals at line frequency to accurately represent the shape of the furnace peak; use of interpolation techniques to better correct for rapidly changing background levels; use of integrated peak absorbance (A.s) signals rather than peak height absorbance for quantitative measurements; use of baseline correction to improve the accuracy of integrated peak absorbance signals; and use of graphical techniques to facilitate data interpretation and methods development. Examples are presented that illustrate the contribution of these factors to precision and detection limit performance. It is possible to improve detection limits over those previously reported by choosing appropriate signal handling parameters.