Miguel A. Belarra

Direct solid sampling with electrothermal vaporization/atomization: what for and how?

In this article, the advantages and the disadvantages of solid sampling-graphite-furnace-based methods are critically examined, taking into account the latest research. A discussion of the different situations for which these methods are best suited within analytical chemistry is presented, together with a general methodology intended to maximize their advantages. The topics discussed include achieving a selective atomization/vaporization of the analyte, calibration, the use of alternative working conditions, and data treatment as a function of the specific goals of the analysis. The purpose of the article is to offer a critical, but tutorial, evaluation of the possibilities of these methods, making it easier for new users to approach them and take advantage of their possibilities.

Theoretical evaluation of solid sampling-electrothermal atomic absorption spectrometry for screening purposes

Obtaining results in the determination of metals by electrothermal atomic absorption spectrometry is conditioned by the non-homogeneity of the sample and the possible appearance of outliers, which make it difficult to obtain reliable results with only a few measurements. In order to establish the optimum conditions for using the technique as a screening method, 18 000 results corresponding to different possible situations were simulated by computer and subsequently analysed using information theory. In view of the results obtained, it can generally be claimed that the median can be used to better effect than the mean value as the former is less affected by the potential presence of outliers. Using the median and with normal working conditions, the minimum number of measurements that need to be carried out to guarantee a recall of over 0.95 in a screening method ranges from 5 to 20, involving between 15 min and 1 h work.

High-resolution continuum source graphite furnace atomic absorption spectrometry for direct analysis of solid samples and complex materials: a tutorial review

The purpose of this review is to examine the literature devoted to direct sample analysis using high-resolution continuum source atomic absorption spectrometry in a tutorial way, in an attempt to provide guidelines on the most critical issues to consider when developing a new method. This review discusses in detail the advantages and limitations of this technique, highlighting its benefits in comparison with classic line source atomic absorption spectrometry instrumentation in the context of direct analysis of solid samples, slurries and complex liquid samples, trying to establish in which situations the use of this technique can be particularly beneficial. Some of the aspects that are addressed comprise: (i) the improved potential to detect and correct for spectral interferences; (ii) the different options to adjust the sensitivity to the analyte content; (iii) strategies to minimize matrix effects and calibrate with aqueous standard solutions; (iv) possibilities to carry out multi-element analysis.

Discrimination of the causes of imprecision in the direct determination of metals in organic solid samples by electrothermal atomization atomic absorption spectrometry

The contribution of various factors (instrumental, working conditions, the use of subsamples of 1–2 mg and the direct atomization of solids) to the total variance on the determination of metals in organic solid samples by ETAAS has been studied using four different samples. The lack of homogeneity of the small subsamples used, which is not always improved by reducing particle size, appears to be the main cause of the high RSD values obtained, to such an extent that the influence of the other factors studied appears irrelevant. The technique, which provides results in accordance with the amount of analyte introduced into the atomizer, is perfectly suitable for controlling the homogeneity of certified reference samples or any other material, although its use as a conventional method of determining metals makes it necessary to carry out several measurements in order to obtain sufficiently reliable results.

Direct determination of phosphorus in biological samples using a solid sampling-high resolution-continuum source electrothermal spectrometer: comparison of atomic and molecular absorption spectrometry

Phosphorus is an element that is difficult to determine accurately by most analytical techniques. In particular, phosphorus determination by means of electrothermal atomic absorption spectrometry (ETAAS) has been traditionally hampered by (i) the practical inability of using phosphorus resonance lines; (ii) its complex chemical behaviour (e.g., formation of volatile compounds and a tendency to interact with graphite); and, particularly, (iii) the overlap observed between molecular and atomic phosphorus lines, which makes it very difficult to obtain reliable results when using line source-ETAAS. In this work, the potential of high resolution-continuum source ET spectrometry for the direct determination of phosphorus in biological solid samples is demonstrated. Two different methodologies, based on the monitoring of either atomic or molecular lines, were compared, paying special attention to the judicious selection of the chemical modifier and the temperature program that will be most adequate to the intended purpose. In order to enable atomic phosphorus monitoring, a high atomization temperature (2650 °C) and a combination of chemical modifiers consisting of palladium, ascorbic acid and tungsten (as permanent modifier) were used. On the other hand, when monitoring PO lines, tungsten coating and vaporization at only 1900 °C proved to be sufficient for attaining reliable results, although attention has to be paid to the possible effect of spectral interferences (e.g., NO lines). Both methods permitted the direct analysis of the samples under investigation, offering a high sample throughput (about 15–20 minutes per determination), good sensitivity (m0 around 5 ng and LODs at the µg g−1 level), RSD values in the 5–10% range, and straightforward calibration with aqueous standards.

Direct determination of metals in solid samples by graphite-furnace atomic absorption spectrometry: Does sample mass influence the analytical results?

In our previous experience on the determination of metals by direct analysis of the solid sample using graphite-furnace atomic absorption spectrometry, it was found that the use of too large or too small mass of sample may lead to erroneous results, even if the analyte content lies within the linear dynamic range. Few references, however, can be found in the literature dealing with this problem. The ANOVA performed on a total of 293 measurements of copper in a vitamin complex, proves that sample masses smaller than 0.3 mg or larger than 1.1 mg give overestimated and underestimated results, respectively.

Evaluation of solid sampling-electrothermal vaporization-inductively coupled plasma mass spectrometry and solid sampling-graphite furnace atomic absorption spectrometry for the direct determination of Cr in various materials using solution-based calibration approaches

In this work, the possibilities for the direct determination of Cr in solid samples by means of graphite furnace atomic absorption spectrometry (SS-GFAAS) and by means of electrothermal vaporization-inductively coupled plasma mass spectrometry (SS-ETV-ICPMS) have been critically evaluated and compared, the goal always being to develop simple and fast methods, only relying on the use of aqueous standards for calibration. Four reference materials of very different nature (milk powder, lobster hepatopancreas, polyethylene and sewage sludge), covering a wide Cr concentration range (from 17.7 ng g−1 to 37.2 µg g−1), were selected for the study. The diverse volatility of their matrixes, compared with that of Cr, resulted in different degrees of difficulty for both techniques. SS-GFAAS offers a better limit of detection (2 ng g−1) than SS-ETV-ICPMS (30 ng g−1), mainly due to the occurrence of C-based polyatomic interferences with the latter technique. The samples that presented the least difficulties were those for which the matrix is mainly of an organic nature and can be easily removed during the pyrolysis step, allowing a selective atomization/vaporization of the analyte. Under these circumstances, external calibration against aqueous standards was feasible for SS-GFAAS in all cases. SS-ETV-ICPMS proved to be more sensitive to matrix effects, but the use of the palladium (added for its carrier properties) signal as internal standard also allowed the use of this straightforward calibration procedure. On the other hand, the analysis of sewage sludge (containing 85% of hardly volatile inorganic compounds) represented the most complicated situation for both techniques: for SS-GFAAS, the addition of Na2CO3 to effect an in-situ microfusion of the sample during the pyrolysis step greatly helped to improve the situation, while for SS-ETV-ICPMS the single standard addition method was necessary. In all cases, a good agreement with the certified values was obtained. The precision ranged between 6 and 22% relative standard deviation, depending on the homogeneity and the sample mass used. These values could be further improved, if needed, by increasing the number of replicates per determination (usually five). Every determination required approximately 15–20 min, except when the single standard addition method was used (30–40 min). Therefore, it can be stated that both techniques show the capability of providing a direct determination of this analyte in a variety of samples, at very different concentration levels, allowing the development of fast and reliable solid sampling procedures.

Direct determination of cobalt and zinc in samples of different volatility by means of solid sampling-graphite furnace atomic absorption spectrometry

Abstract In this work, a method has been developed for the direct determination of cobalt and zinc in two different real-life samples (a vitamin complex and an agricultural soil) by means of solid sampling–graphite furnace atomic absorption spectrometry. The selection of the elements and of the samples was based on their different volatility and complexity, the final aim being to check the maturity of solid sampling–graphite furnace atomic absorption spectrometry to deal with different situations, and to further contribute to the development of a general methodology that could be used for a large variety of cases. In order to achieve these goals a systematic study was carried out by selection of the most appropriate working conditions and optimization of the sample mass. It can be stressed that the maintenance of the Ar flow (250 ml min −1 ) during the atomization step was found to be especially useful to improve the signal to background ratio for those situations in which simultaneous release of the matrix and atomization of the analyte was unavoidable (Co determination in soil). Comparison of the results for both samples showed no significant differences (other than some disparity regarding the optimum mass range), proving that solid sampling–graphite furnace atomic absorption spectrometry can be as appropriate for soil analysis as it is for the analysis of organic samples, if appropriate working conditions are chosen. The method finally proposed shows interesting features for the determination of the analytes in the samples such as: the ability to use aqueous standard solutions for calibration, a high sample throughput (15–20 min analysis time per analyte in a sample), the fact that it is unnecessary to use hazardous reagents, a low sample consumption, adequate limits of detection (0.5 mg kg −1 for Co and 4.1 mg kg −1 for Zn in the soil sample), suitable precision values (R.S.D. ∼8%) and a reduced risk of analyte losses and contamination.

The use of chemical modifiers in the determination of cadmium in sewage sludge and tin in PVC by solid sampling-graphite furnace atomic absorption spectrometry

Abstract The effects of NH 4 H 2 PO 4 and Pd as chemical modifiers in SS-GFAAS are studied in the determination of Cd in sewage sludge and of Sn in PVC samples. Whereas for Cd both modifiers act equally well whether Cd is in solution or in the solid sample, the effect of Pd on tin is different. To make the Pd modifier as effective on Sn in PVC as on Sn in solution a two-step pyrolysis must be introduced into the temperature programme. It is also shown that the contribution of the application of chemical modifiers to the final imprecision of the determination is negligible compared to that introduced by the sample inhomogeneity.

Direct determination of Zn in individual Daphnia magna specimens by means of solid sampling high-resolution continuum source graphite furnace atomic absorption spectrometry

In this work, the capabilities of solid sampling high-resolution continuum source graphite furnace atomic absorption spectrometry (GFAAS) for the direct determination of the Zn body burden in individual specimens of Daphnia magna (small aquatic invertebrates of approx. 2 mm length that are widely used in ecotoxicological research) were investigated. It was concluded that this technique offers interesting features for this type of application, permitting the fast (approx. 2 min for the measurement of a single sample) and practically contamination-free analysis of these invertebrates, as the only preparation required is drying. Moreover, one of the traditional disadvantages of line source-GFAAS, the narrow linear range achieved using a particular set of experimental conditions, can be overcome with this technique by the judicious selection of the detector pixels used for signal quantification, such that the linear range can be extended simply by reprocessing the data, without the need to carry out any additional measurement. This feature is particularly interesting for this application, since every Daphnia magna specimen can be monitored only once, and thus, there is no possibility to alter the conditions and repeat the analysis for samples displaying a Zn content outside the linear range. In this case, the use of central and/or side pixels permits achieving a working range between 5 and 400 ng for the 307.590 nm Zn line, which is fit-for-purpose. Analysis of two biological certified reference materials (SRM 1549 Non-fat milk powder and SRM 1577b Bovine liver) was also investigated, and accuracy could be demonstrated when carrying out the calibration versus aqueous standard solutions.