Applications of laser-ablation-inductively-coupled plasma-mass spectrometry in chemical analysis of forensic evidence

Abstract

Since the first coupling of laser-ablation inductively-coupled plasma to mass spectrometry (LA-ICP-MS) in 1985, applications of this analytical technique in various scientific fields have been developed. LA-ICP-MS is an elemental and isotopic microanalytical technique that is increasingly used for routine analysis in forensic laboratories around the world due to its great discriminating power between samples and its capacity for analyzing solid evidence of small size. In addition, its quasi-non-destructive character is especially welcome for this type of sample.

The main forensic applications of LA-ICP-MS have been developed for glass and paint samples. However, applications to other samples have also been reported [e.g., documents (ink and paper), fibers, cannabis, gems, Australian ocher and porcelain, brick stones, and gold and silver]. In the forensic field, the adjustment of instrumental analytical parameters, the study of interferences produced by fractionation and polyatomic ions, and the statistical treatment of data are considered of great importance. This review is intended as a practical, useful guide to help forensic chemists incorporate this attractive, powerful technique into their laboratories.

Introduction

After the first source of coherent microwave radiation, named microwave amplification by stimulated emission of radiation (MASER) [1], the first source of coherent light radiation, known as light amplification by stimulated emission of radiation (LASER), was developed in 1960. The capacity and the precision of LASER for the collection of data directly from solid materials attracted the attention of researchers [2]. Shortly after the creation of lasers, laser micro-optical emission spectrometry (LM-OES) and laser-ablation mass spectrometry (LA-MS) were investigated for identification and elemental quantitative analysis of solids [3]. More recently, the coupling of laser ablation with inductively coupled plasma-MS (LA-ICP-MS) has shown great potential for the direct elemental analysis of solid samples [4].

The first ICP-MS commercial instruments were available in 1983. They were initially employed on daily routine analysis of aqueous solutions. However, for the analysis of solid samples, the transformation of the sample in a solution was necessary, usually by sample digestion with strong acids and oxidizers. Obviously, any such sample preparation involves the risk of sample contamination, the loss of volatile elements, and, besides, it creates a complex aqueous matrix, which can be source of several spectroscopic and non-spectroscopic interferences. The coupling of ICP-MS with an LA system for sample introduction [4], [5] enabled direct, reliable, sensitive, and quasi non-destructive analysis of major and trace elements of solid samples. In the past 15 years, there has been rapid development of this technique, which has been widely applied to several fields of science (e.g., geology, medicine, science of materials and, more recently, to forensic sciences, where solid samples constitute a large part of the physical evidence submitted to forensic chemistry laboratories).

As a consequence of the increasing interest on LA-ICP-MS, several reviews have been written on the technique itself and its applications to the elemental analysis of different types of samples. Those that stand out are from the Günther Group [3], [6], [7], Durrant [4], Russo et al. [8], and Mokgalaka and Gardea-Torresdey [9]. Recently, Trejos and Almirall [10] published quite an interesting chapter in the Encyclopedia of Analytical Chemistry, entitled “Laser Ablation Inductively Coupled Plasma Mass Spectrometry in Forensic Science”, where they explain the technique, its main characteristics, and give a general view of its potential for the analysis of a wide number of forensic samples, ranging from glass to biological and environmental samples.

This review is a practical guide to help forensic scientists in the use of LA-ICP-MS as a versatile tool for the chemical investigation of common physical evidence found in forensic laboratories [e.g., glass, paints, documents (inks and papers), fibers, cannabis, gems, porcelains, brick stones, and gold and silver]. The review, based on the most important articles published in the past 12 years in forensic chemistry, comprises a basic introduction to the technique, a detailed overview of its applications to the analysis of these samples, and future perspectives. Biological and environmental-forensic samples do not fall within the scope of this review.