J.M. Andrade

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).

Statistical comparison of the slopes of two regression lines: A tutorial.

Comparing the slopes of two regression lines is an almost daily task in analytical laboratories. The usual procedure is based on a Students t-test although literature differs in whether the standard errors of the slopes or the standard errors of the regressions should be employed to get a pooled standard error. In this work fundamental concepts on the use of the Students test were reviewed and Monte Carlo simulations were done to ascertain whether relevant differences arise when the two options are considered. It was concluded that for small sample sets (as it is usual in analytical laboratories) the Students t-test based on the standard error of regression models must be used and special attention must be paid on the equality of the models variances. Finally, alternative approaches were reviewed, with emphasis on a simple one based on the analysis of the covariance (ANCOVA).

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.

The comparison of two heavy fuel oils in composition and weathering pattern, based on IR, GC-FID and GC-MS analyses: application to the Prestige wreackage.

This paper compares the weathering patterns of two similar fuel oils: a fuel oil spilled after a ship accident (Prestige-Nassau, off the Galician coast -NW Spain-) and a fuel designed to cope with the numerous quests for samples to carry out scientific studies (IFO). Comparative studies were made to evaluate the capability of common fingerprinting analytical techniques to differentiate the fuels, as well as their capabilities to monitor their weathering. The two products were spilled under controlled conditions during ca. four months to assess how they evolved on time. Mid-IR spectrometry and gas chromatography (flame ionization and mass spectrometry detectors) were used. IR indexes related to total aromaticity, type of substituents (branched or linear chains) and degree of aromatic substitution reflected well the differences between the fuels during weathering. Regarding the chromatographic measurements, the n-alkanes became highly reduced for both fuel oils and it was found that the PAHs of the synthetic fuel (IFO) were more resistant to weathering. Regarding biomarkers, the different profiles of the steranes, diasteranes and triaromatic steroids allowed for a simple differentiation amongst the two products. The %D2/P2 ratio differentiated both products whereas the %N3/P2 one ordered the samples according to the extent of their weathering.

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.

Monitoring photooxidation of the Prestige's oil spill by attenuated total reflectance infrared spectroscopy.

The recent release of ca. 70,000 tonnes of a heavy fuel oil from the Prestige-Nassau carrier along the Spanish northern coast, mainly along Galicia, was monitored using attenuated total reflectance-mid IR spectrometry. The fuel was characterized and differentiated from 10 products commonly transported along the Galician coast (and their series of weathered samples) using factor analysis. The Prestiges fuel was weathered under natural conditions and under infrared radiation to study its evolution on time. A correlation was established using the 1690-1700 cm(-1) carbonyl peak, where from it was deduced that IR radiation weathered the product two times faster than natural conditions. The use of 10 weathering indexes was carried out to confirm the main patterns given by factor analysis and to seek out which main functional groups and structures increased or decreased during weathering. It was found that the carbonyl and sulphoxide indexes varied greatly, as well as the total aromaticity and long chains ones. The substitution-related indexes pointed out that highly substituted aromatic structures increased although the total amount of isolated CH groups in aromatic structures reached a plateau.

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\

Comparison of the 2000 and 2005 spatial distributions of heavy metals in wild mussels from the North-Atlantic Spanish coast.

Spatial distributions of Cd, Cu, Hg, Pb and Zn in Mytilus galloprovincialis in 41 sampling sites in the North-Atlantic Spanish coast were monitored in 2000 and 2005 to assess the coastal environmental pollution. A pool of mussel soft tissue was prepared using 50 or more individuals representing the available size range. Samples were grounded, homogenised and quantified by atomic absorption spectrometry. Quality of the chemical analyses was assessed by participation in periodic interlaboratory exercises. Univariate and multivariate statistical studies, Principal Components Analysis and Procrustes rotation (a technique intended to compare several data spaces) were carried out. Some decreasing trends were observed for the concentrations throughout the 2000-2005 period, mainly for Hg and Pb. In general, the Northern area showed higher median concentrations (e.g., 3.79 and 4.28 mg Pb/kg dry weight, and 0.160 and 0.191 mg Hg/kg dry weight, for 2000 and 2005, respectively) than the Atlantic ones (e.g., 1.47 and 1.44 mg Pb/kg dry weight, and 0.097 and 0.084 mgHg/kg dry weight, for 2000 and 2005, respectively). Nevertheless, some Atlantic sampling points, located far from human inputs (cities, industries, rivers, etc.) exhibited high concentrations of Cd, which was attributed to the influence of the natural upwelling processes which occur annually in that geographical region.

Applicability of high-absorbance MIR spectroscopy in industrial quality control of reformed gasolines

Abstract Partial least squares (PLS), polynomial partial least squares (polynomial-PLS), locally weighted regression (LWR) and genetic inside neural network (GINN) algorithms were used to develop models for predicting motor octane number (MON) from non-leaded and catalytically reformed gasolines. Medium infrared (mid-infrared) spectra were obtained on liquid samples and chemometrically processed in order to get acceptable predictive models which allow their use for routine industrial quality monitoring. As MIR spectra currently present peaks with high absorbances, the presence and influence of nonlinearities was sought comparing the broadly-used PLS method with several other algorithms specially designed to cope with such influences (polynomial-PLS, local regression and neural networks). Their prediction abilities; i.e., stability and global prediction error when predicting new samples as well as their usefulness for routine industrial control were studied.