A. Carlosena

Slurry sampling for direct analysis of solid materials by electrothermal atomic absorption spectrometry (ETAAS). A literature review from 1990 to 2000

The determination of trace metals in solid samples has traditionally been performed by acid digestion and subsequent measurement by a suitable instrumental technique. This dissolution step is time-consuming and it shows important drawbacks. For these reasons, in the past years many efforts have been focused on the direct analysis of solid samples. Among the developed methodologies, slurry sampling-electrothermal atomic absorption spectrometry combines the significant advantages of the solid and liquid sampling methods, and it can be already considered as a mature technique, that is widely utilized for metal determination in both organic and inorganic matrices, even for routine analysis. Accordingly, this work gives a retrospective view of the progresses of this technique during the past decade (1990-2000).

Searching for heavy metals grouping roadside soils as a function of motorized traffic influence

Ninety two soil samples were collected in four sampling seasons in La Coruña, NW of Spain, and its surroundings to study the impact of vehicular traffic onto the metallic content of roadside soils in a medium-size city. Samples exhibited different levels of exposure and, therefore, sample groups should arise as a function of this anthropogenic impact. Surprisingly, this was not so when all the nine metals initially considered were subjected to multivariate analyses. Then, different unsupervised multivariate statistical techniques were used to discover those elements able to group the samples according to the level of road traffic. Such an objective was achieved only after removing the natural variability. Accordingly, Pb, Cd, Cu and Zn were found to satisfactorily conduct to this end, Pb being the most discriminant one.

Antimony as a Tracer of the Anthropogenic Influence on Soils and Estuarine Sediments

The aim of this work was to evaluate antimony (Sb) as apotentially valuable tracer of the anthropogenic impact on soilsand estuarine sediments. Antimony was determined in fifteenroadsoils from a medium-size city, La Coruña (NW of Spain);and twelve sediments from two estuaries (La Coruña and Ares-Betanzos). Sb determination was accomplished in a rapid andreliable way by directly measuring the solid samples with anautomatic ultrasonic slurry sampling-ETAAS device. Soil contentsranged from 0.29 to 8.81 μg g-1. A relation between Sbconcentration and motor vehicle intensity was observed.Regarding estuarine sediments, Sb amounted from 0.22 to 1.51 μg g-1, for the La Coruña estuary and 0.24 to 0.71 μg g-1, forthe Ares-Betanzos estuary; higher Sb values were found forcoastal locations whereas lower contents corresponded to `innersamples. In order to confirm these findings, other pollutantswere studied: As, Cd, Co, Cr, Cu, Ni, Pb, Zn, aliphatichydrocarbons (AH) and polycyclic aromatic hydrocarbons (PAH).These values, along with the Sb ones, were subjected tomultivariate studies intended to elucidate whether Sb correlated(and to what extent) to other well-known anthropogenic pollutants. Thus, Sb became associated to Cd, Cu, Pb and Zn in road soils and to As, Cu, Zn, PAH, Pb and unresolved hydrocarbons, in sediments.

Evaluation of Various Sample Preparation Procedures for the Determination of Chromium, Cobalt and Nickel in Vegetables

Three methods for the determination of Cr, Co and Ni in vegetables were tested with a view to the evaluation of various sample preparation procedures involving dry ashing, the direct slurry technique and continuous flow microwave-assisted digestion. NIST SRM 1572 Citrus Leaves was used for this purpose. For the determination of the analytes by AAS, the temperature programmes of the graphite furnace were optimized and the use of some chemical modifiers was studied. Direct slurry analyses of vegetables (in samples prepared at a 2% m/v concentration in 1% HNO 3 containing 10% H 2 O 2 ) were generally accurate and precise for Co and Ni, but gave low recoveries (about 85%) for Cr. Continuous digestion of the samples in a microwave oven was necessary for the reliable determination of Cr (about 100% recovery) by use of 5% HNO 3 as the digesting reagent. The best method for each application was used for the determination of Cr, Co and Ni in a variety of vegetables, using direct calibration with aqueous standards.

A tutorial on multivariate calibration in atomic spectrometry techniques

Coupling multivariate regression methods to atomic spectrometry is an emerging field from which important advantages can be obtained. These include lower workloads, increased laboratory turnarounds, economy, higher efficiency in method development, and relatively simple ways to take account of complex interferences. In this paper four typical regression methods (ordinary multiple linear regression, principal components regression, partial least squares and artificial neural networks) are presented in a practice-oriented way. The main emphasis is placed on explaining their advantages, drawbacks, how to solve the latter and how atomic spectrometry can benefit from multivariate regression. Finally, a retrospective review considering the last sixteen years is made to present practical applications on: flame-, hydride generation-, electrothermal-atomic absorption spectrometry; inductively coupled plasma spectrometry and laser-induced breakdown spectrometry.

Development of an analytical scheme for the direct determination of antimony in geological materials by automated ultrasonic slurry sampling-ETAAS

The ongoing application of ultrasonic probes for slurry homogenization gives new additional advantages with respect to the traditional slurry analyses by ETAAS, namely speed, automation, improved reproducibility, etc. Despite all these advantages, there are still difficulties in the optimization stages of the analytical procedures because of the large number of variables to take into account. Accordingly, several chemometric techniques were applied to help in the development of a systematic analytical scheme to determine Sb directly in soils and sediments by means of ultrasonic slurry sampling (USS)-ETAAS. They are intended not only to optimize the slurry preparation but also to diminish the amount of benchwork to be done. The chemometric techniques and their objectives were as follows: Plackett-Burman designs, to assess the influence of six analytical variables; optimization of the important analytical variables; a two-way ANOVA, to investigate the influence of sample cups and replicates; and control charts, to monitor the graphite tube performance. Additionally, two pipetting options and three quantification methods were evaluated. Analytical results for Sb determined in five certified reference materials confirmed the usefulness of the ultrasonic slurry sampler (USS-100) combined with ETAAS for a direct and almost fully automatic analysis of complex matrices.`mv-8\

Classification of edible vegetables affected by different traffic intensities using potential curves

Heavy metals in soils have largely been used to evaluate the impact of motorised traffic in the vicinity of motorways. Also in this field of work it is of paramount importance to analyse the vegetables grown in these areas since their consumption is one of the main sources of metal intake by people; not in vain human food is directly or indirectly derived from plants. Accordingly, a set of edible vegetable samples were first analysed and, then, classified employing different multivariate chemometric techniques; among them, SIMCA and a simplified mode of potential curves. The analytical variables were selected after a comprehensive study of roadside soil pollution where Pb, Cd and Cu were found to be the main metallic pollution tracers. Different groups of vegetables were obtained which were explained as a function of both the agricultural conditions (private versus commercial) and the different traffic intensities supported by the cultivated areas.

FLAME ATOMIC ABSORPTION SPECTROMETRY WITH FLOW-INJECTION ON-LINE ADSORPTION PRECONCENTRATION USING A KNOTTED REACTOR FOR CADMIUM DETERMINATION IN AQUEOUS SAMPLES

A knotted reactor flow injection on-line preconcentration system coupled to a flame atomic absorption spectrometer (FAAS) was developed for the direct determination of cadmium in aqueous samples. Diethyldithiocarbamate was used to complex the analyte and then, it was adsorbed on the walls of a knotted reactor (KR) made from PTFE tubing. All the FI and chemical variables were optimized. The complex elution was performed on-line using ethanol. The sample loading time and flow rate were set at 60 s and 6 mL/min, respectively. An enhancement factor of 33 was achieved, resulting in a sampling frequency (h−1) of 30. The limit of detection was 3.44 μg/L. The accuracy of the method was tested by the analysis of three certified reference materials (Certified Aqueous Standard 7878, Reference Water TM-24 and Trace Elements in Water 1643d). The results obtained are within the certified ranges at a 95% confidence level and a good precision was also achieved (< 2.7%, n = 6). The proposed procedure was applied to the Cd determination in rain and sea water samples.

Interpolation in the standard additions method

The standard additions method (SAM) has traditionally been performed by using extrapolation. This practice is suboptimal because predictions are affected by even slight departures of calibration points from a straight line. Despite this, most textbooks and papers in analytical chemistry still refer exclusively to extrapolation. In contrast, the use of interpolation is recommended in this paper as a way to get predictions on the central part of the regression line and thus minimize the bias in the prediction and the variance associated with the analytical result. Several scenarios were studied, with concentration errors simulated in different calibration solutions. It was found that translational effects due to variations at the central part of the calibration caused the lowest disturbances on the predicted concentrations. The differences between the interpolated and extrapolated predictions can be as large as ±30%. The confidence interval associated with the extrapolation result is wider than that due to interpolation by as much as 100%. It is shown that commonly used equations underestimate the correct confidence intervals. Both, absence of bias and improved precision, are of relevance in quality assurance, method validation and error propagation.

Linear PLS regression to cope with interferences of major concomitants in the determination of antimony by ETAAS

Most heavy metals are present in trace concentrations in many matrices whereas concomitants are, generally, several orders of magnitude higher. Thus, when Sb was determined in water samples by electrothermal atomic absorption spectrometry (ETAAS), typical major concomitant ions Ca2+, Fe3+, Na+, Mg2+, Cl−, PO43− and SO42− induced displacement, depletion and division of its atomic peak. These interferences were handled with partial least squares regression (PLS). Plackett–Burman experimental designs were implemented to develop the calibration matrix and assess which concomitants modified the atomic signal the most. Despite the concentration-dependent effects induced by the concomitants, linear PLS was a reliable way to predict the concentration of Sb in aqueous samples (standard error of prediction = 1.44 ng mL−1). Polynomial PLS regression was also studied but it did not outperform the linear models. The multivariate-derived figures of merit were calculated: sensitivity (0.014 absorbance/(ng mL−1)), characteristic mass (6.2 pg) and selectivity (using the net analyte signal concept, 83%); limit of detection (considering 5% of type-α and type-β risks, 5.6 ng mL−1) and quantification (10.6 ng mL−1), following recent IUPAC and ISO guidelines. The method was validated studying its robustness to current ETAAS problems and analysing several certified reference waters.