A fast ultrasound-assisted extraction procedure for trace elements determination in hair samples by ICP-MS for forensic analysis

https://doi.org/10.1016/j.forsciint.2009.08.003Get rights and content

Abstract

An ultrasound-assisted extraction method is proposed for the determination of trace elements in hair samples by inductively coupled plasma-mass spectrometry (ICP-MS) for forensic investigation. Prior to analysis, 25 mg of hair samples were accurately weighed into (15 mL) conical tubes. Then, 2 mL of 20% HNO3 is added to the samples, sonicated at 2 min (50 W, 100% amplitude), and then further diluted to 10 mL with Milli-Q water. Resulted diluted slurries are centrifuged and the analytes are directly determined in the supernatant. Calibrations against aqueous solutions were carried out with rhodium as internal standard. The method was successfully applied for the extraction of Al, As, Ba, Be, Cd, Co, Cr, Cu, Mn, Pb, Tl, U, V and Zn with a method detection limit (3 s, n = 20) of 0.1, 0.4, 0.2, 0.09, 0.08, 0.04, 0.1, 2.9, 1.0, 0.9, 0.04, 0.05, 0.1 and 4.2 ng/g, respectively. Method accuracy is traceable to Certified Reference Materials (CRMs) 85 and 86 human hair from the International Atomic Energy Agency (IAEA). Additional validation data are provided based on the analysis of hair samples from the trace elements intercomparison program operated by the Institut National de Sante’ Publique du Quebec, Canada. The proposed method is very simple and can be applied for forensic purposes with the elimination of sample digestion step prior to analysis. Then, a considerable improvement in the sample throughput is archived with the use of the proposed method.

Introduction

Hair is a biological specimen that is easily and noninvasively collected, with minimal cost, and it is easily stored and transported to the laboratory for analysis. These attributes make hair an attractive substrate for the evaluation of toxic elements poisoning. However, its validity has been repeatedly challenged. Specimen contamination and other analytical problems, mainly during sample preparation, render this test highly controversial [1].

During the analysis, sample preparation step is often the most time consuming step and involves some potential problems, such as incomplete dissolution, precipitation of insoluble analyte, loss of some elements during the heating.

Atomic absorption spectrometry (AAS), inductively coupled plasma-atomic emission spectrometry (ICP-AES), or inductively coupled plasma-mass spectrometry (ICP-MS) are the most frequent techniques used for trace elements determination in hair samples [2], [3], [4], [5], [6]. ICP-MS is the method with the highest potential with respect to its detection limits, sensitivity, precision and multi-elemental determinations and speed [7]. However, finding the correct procedure for the complete dissolution of solid samples by chemical digestion is the most critical aspect of ICP-MS analysis. The common dissolution techniques are the acid decomposition in open vessel, microwave-assisted digestion and the alkali fusion.

Over the last 20 years, microwave-assisted sample dissolution has been employed extensively for shortening the time required for sample dissolution, as well as to avoid analyte losses and contamination. Although microwave ovens of different designs are widely used in the analytical laboratories throughout the world, some problems which are not commonly described have arisen.

From the authors’ experience with microwave digestion systems the main drawbacks are: (i) commercial microwave ovens for analytical tasks are expensive; (ii) the short lifetime of the digestion vessels operated at high pressures; (iii) although there are several well established analytical procedures, occasional explosions have been observed during the development of methods; (iv) the long time required for cooling the digestion vessel after the microwave operation (or requirement of a special cooling system); (v) sample throughput is not very high; and (vi) the constant supervision required of the digestion procedure.

Then, the ideal method for the analysis of hair samples would eliminate the sample dissolution, minimize the sample preparation and improve the analytical results.

Based on this, some authors have reported that several analytes were totally or partially extracted into the liquid phase when ground solid samples were sonicated before being sampled [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. This process increases the sample throughput and is very simple. Quantitative metal recoveries depend on the analyte/matrix interaction, reagents employed for extraction and the performance of the ultrasonic processor. Controlling these variables in order to obtain a quantitative extraction of the analyte into the aqueous phase can result in an advantageous procedure to avoid the problems related with sample digestion procedures.

In this paper a fast and simple ultrasound-assisted extraction procedure is proposed for the determination of 14 elements in hair samples, in order to avoid the problems related with sample digestion. The main goal obtained with the proposed method is a higher sample throughput when compared with typical solid decomposition procedures prior to analysis which is very attractive for forensic purposes.

Section snippets

Instrumentation

All measurements were made with an ICP-MS (Elan DRC II PerkinElmer, Norwalk, CT) operating in a standard model with high-purity argon (99.999%, White Martins, Brazil). A Meinhard concentric nebulizer (Spectron/Glass Expansion, Ventura, CA, USA) connected to a cyclonic spray chamber was used for sample introduction. A radiofrequency (rf) of 1200 W power was selected in pulse mode with autolens one. Sample data were acquired by using 20 sweeps/reading, 1 reading/replicate and a dwell time of 50 ms.

Results and discussion

Ultrasound-assisted extraction method has been applied to simplify sample preparation handling to increase the extraction rate, avoid sample contamination, and prevent analyte losses that may occur if the sample is decomposed before analysis by wet-decomposition or dry-ashing procedures. These extractions are based on the leaching of metal ions from powdered materials in slurries containing an appropriate solvent, generally a diluted acid solution. Quantitative metal recoveries depend on the

Conclusion

Quantitative recoveries of Ag, Se, Mo in hair reference materials were not achieved with the proposed ultrasound-assisted extraction procedure. Probably, these elements are stronger bond to the hair structure. However, the proposed method showed to be reliable for the determination of 14 elements with potential interest in forensic investigations in hair samples by inductively coupled plasma-mass spectrometry. The good agreement between found values and the certified or reference values of CRMs

Acknowledgements

The authors are grateful to Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support and fellowships.

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