M.T.C. de Loos-Vollebregt

Electrothermal vaporization for sample introduction in atomic absorption, atomic emission and plasma mass spectrometry—a critical review with focus on solid sampling and slurry analysis

The aim of this work is to review and compare the present possibilities of electrothermal atomic absorption spectrometry (ETAAS), electrothermal vaporization-inductively coupled plasma-optical emission spectrometry (ETV-ICP-OES) and electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS) for analysis of solid samples, slurries and complex matrixes. The paper keeps in perspective the historical evolution of these techniques, providing an overview of the most relevant advances in instrumentation, methodology and fundamentals and highlighting the most relevant applications carried out during the last decade. The benefits of continuum source-AAS, the possible impact of diode lasers on this technique, the potential of ETV-ICP-methods for fractionation/speciation studies, as well as for resolving spectral interferences, and the influence of the different types of ICP-MS instrumentation currently available on the overall performance of ETV-ICP-MS will be some of the aspects discussed in more detail.

Coupling of HPLC and ICP-AES for speciation

Abstract The cross-flow nebulizer and the thermospray vaporizer can both be used as an interface for coupling high performance liquid chromatography with inductively coupled plasma-atomic emission spectrometry. The dispersion and the peak asymmetry are acceptable for both. With the thermospray system the sensitivity is threefold higher and the fraction of methanol in methanol/water mixtures introduced into the plasma can be as high as 75%, compared with 25% for the cross-flow nebulizer. Initial results obtained for the speciation of selenium in (CH 3 ) 3 Se + , SeO 3 2− and SeO 4 2− are promising.

On the use of line intensity ratios and power adjustments to control matrix effects in inductively coupled plasma optical emission spectrometry

In inductively coupled plasma optical emission spectrometry, matrix effects can be substantially reduced by applying robust operating conditions, i.e. a high rf power level and a low nebulizer gas flow. However, dissimilar line intensity changes are still observed, in particular with varying salt matrices. Calcium and, to a lesser extent, Mg induce stronger effects than Na and K. In 0.1, 0.3 and 1.0% Ca matrices, signal changes for 29 atomic as well as ionic lines have been determined with respect to the Ca-free solution. The changes range from –5 to –30% for the 1.0% Ca matrix. Starting from robust conditions in radial viewing, the rf power has been adjusted until the Cr ion-to-atom line ratio measured for the calcium solutions equalled the ratio determined during the calibration (0% Ca). By this power adjustment, the changes are within ±4%. No matrix effects originating from the sample introduction system are observed, probably due to the high acid concentrations used. The practical application of power adjustments is illustrated with results for certified sediment samples and with multiple line analysis for qualitative and semiquantitative analysis. The approach is an attractive alternative to matrix matching or standard additions. Internal standardization based on one atomic and one ionic line of the same element is indicated as another possibility.

Application of mathematical procedures to background correction and multivariate analysis in inductively coupled plasma-optical emission spectrometry

In inductively coupled plasma-optical emission spectrometry, continuous progress is made in acquiring and processing more spectral information than one gross analyte line intensity and two background intensities measured at both sides of the line. Compared to direct readers and slew-scan spectrometers, the recently introduced systems with echelle optics and charge transfer device detection provide more signals at a time, not only with respect to line and background, but also with respect to multiple lines for multiple elements. PC systems are increasingly powerful to apply more sophisticated mathematics for data reduction. Procedures developed over the last decade, based on convolution, differentiation, Fourier transforms, correlation, expert systems, neural networks, principal component analysis, projection methods, Kalman filtering, multiple linear regression and generalised standard additions are reviewed. The first four procedures aim at continuum or structured background correction in every spectral window measured, without prior knowledge about the appearance of the background and the composition of the sample. The other procedures utilise the information that is present for the sample and readily available in the spectrum. In addition to background correction, they allow signal averaging, noise separation, multiple line and multiple element analysis. Multiple line procedures concentrate on qualitative analysis and line selection, whereas multicomponent analysis allows quantitative analysis. Based on the characteristics of ICP-OES, the multicomponent analysis procedures are preferred.

Spectral interpretation and interference correction in inductively coupled plasma mass spectrometry

Abstract To tackle the several problematic polyatomic interferences in inductively coupled plasma mass spectrometry (ICP-MS), we have developed a software approach based on data reduction of the measured total mass spectrum through multicomponent analysis (MCA). The approach leans on a working knowledge of interferents that are likely to be encountered in a sample matrix, which composition is known by virtue of the total mass spectrum and knowledge of applied solvents. The full isotopic patterns for all elements and expected interferents are used in the modelling MCA matrix of 250 masses × 105 species at maximum. Polyatomic abundances are calculated by the software. Since all species are modelled fundamentally through their known natural abundances, the MCA matrix can be manipulated and reprocessed until interpretation of the mass spectrum and, hence, interference correction are optimal. The optimum is attained by use of the bar graph and calculation modes of the PC software and criteria for properly found isotopic patterns. With optimized models stored in the data base, the user may routinely process samples in one go, and operate the ICP-MS system in a true all-element mode. Use of elemental equations or measurement of large multivariate calibration sets and pure component solutions are superfluous. Data reduction is solely based on the information about the isotopic patterns, present in the measured mass spectrum itself. As a result, in the case of interferences, detection limits may be lowered by one to two orders of magnitude. The approach is illustrated with an industrial example of Hf determined in NdFeB, and with an environmental example. Here, a suite of elements over the 50–82 amu mass range has been determined in different salt matrices in ground water.

Kalman filtering of data from overlapping lines in inductively coupled plasma-atomic emission spectrometry

Abstract In inductively coupled plasma-atomic emission spectrometry (ICP-AES) spectral interferences give rise to severe problems. Detection limits worsen considerably, and line selection depends on matrix composition. It is investigated whether the detection limits can be reduced towards values obtained from isolated lines by use of a multicomponent analysis method, such as the Kalman filter. Our Kalman filter approach uses as many data as are readily available prior to analysis. The flatness of the innovations sequence is shown to be a criterion for correction of the optical instability of the monochromator. Spectral scans have intentionally been taken from a medium resolution monochromator in a fast scanning mode. The spectral scans do not require background correction in advance. The filter can handle structured as well as unstructured background and allows for noise averaging. Detection limits are obtained which are even lower than those based on pure component solutions. This is illustrated by the measurement of Nd in the presence of Pr, P in the presence of Cu, Cd in the presence of Fe and In in the presence of Mo and W. These measurements give examples of overlapping lines with identical shape, with large or small peak separations and/or with large intensity differences. The results suggest that line selection is almost independent of matrix composition. The approach is fast, deals with multicomponent systems and with one-component systems, and yields accurate concentration values in all cases. Data reduction in ICP-AES can be fully based on Kalman filtering.

Theory of Zeeman atomic absorption spectrometry

Abstract A theoretical analysis is presented of the signals observed with different systems that employ the Zeeman effect for background correction in analytical atomic absorption spectrometry. Magnetic modulation of the primary source of radiation offers basically the same possibilities as the deuterium background correction system. Correction for wavelength dependent background absorption is possible only when the magnetic field is applied to the absorbing vapour. Similar expressions are obtained for constant or variable magnetic fields directed either perpendicular or parallel to the optical axis. However, mere magnetic modulation of either the source or the atomizer cannot correct for non-absorbed lines. It is demonstrated that simultaneous correction for non-absorbed lines and background absorption can be attained with a variable magnetic field applied to the atomizer, by taking measurements at three discrete, different field strengths.

Some spectral interference studies using Kalman filtering in inductively coupled plasma-atomic emission spectrometry

Abstract This paper continues the studies on the potential of Kalman filtering for data reduction in inductively coupled plasma-atomic emission spectrometry (ICP-AES). Two topics with respect to spectral interferences are discussed: line selection in dependence of the magnitude of the interfering signal; and the minimum peak separation required for quantitative resolution of analyte and interfering signals. In the presence of interferences, line selection is compulsory and depends on matrix composition. In many instances the interfering signal is of the same order of magnitude as the other contributions to the total background signal. With Kalman filtering line selection is not necessary, unless the interfering signal is substantially higher than the other background contributions. The minimum peak separation depends on the integration time per step in the spectral scan, the step size in the scan, and the number of lines in the spectral window. Two sets of filtering conditions for an analyte to interferent intensity ratio of 0.1 are given: a practical set leading to a minimum required separation of 3 pm; and an ultimate set for which the peaks may be separated down to only 1 pm at a spectral bandwidth of the monochromator of 17 pm. All results are illustrated with practical examples.

End-on observation of a horizontal low-flow inductively coupled plasma

Abstract End-on observation of a horizontal water-cooled low-flow plasma has been compared to side-on observation of a conventional high flow plasma. The analytical performance is similar to the conventional plasma. The detection limits are close to the detection limits measured with a Perkin-Elmer Plasma II conventional ICP. The linearity of calibration curves is also similar to the conventional ICP. However, a remarkable reduction of interferences has been observed for end-on observation of the low-flow plasma.

Ultratrace determination of Pb, Se and As in wine samples by electrothermal vaporization inductively coupled plasma mass spectrometry

The determination of Pb, Se and As in wine has a great interest due to health risks and legal requirements. To perform the analysis of wine, two considerations must be taken into account: (i) the low concentration level of the analytes; and (ii) the risk of interferences due to wine matrix components. The goal of this work is to evaluate electrothermal vaporization (ETV) sample introduction for ultratrace determination of Pb, Se and As in wine samples by inductively coupled plasma mass spectrometry (ICP-MS). The results obtained with ETV-ICP-MS were compared to those obtained with conventional liquid sample introduction in ICP-MS and electrothermal atomic absorption spectrometry (ETAAS). Analytical figures of merit of ETV sample introduction strongly depend on the amount of wine sample, on the modifier nature (i.e. Pd, ascorbic acid or citric acid) and concentration and on the temperature program. Wine matrix components exert a great influence on analyte transport efficiency. Due to this fact, the analysis of wine cannot be performed by means of external calibration but the standard addition methodology should be used. The determination of Pb and Se in wine by ETV-ICP-MS provides similar results as conventional liquid sample introduction ICP-MS. For As, the concentration values obtained with ETV sample introduction were between two and four times lower than with the conventional system. These differences are related to the lower intensity of polyatomic interferences (i.e. (40)Ar(35)Cl(+) vs. (75)As(+)) obtained for ETV sample introduction when compared to the conventional system. Finally, no differences for Pb determination were observed between ETV sample introduction and ETAAS. Unfortunately, the limits of detection for As and Se in ETAAS were not low enough to quantify these elements in the wine samples tested.