Analysis for metals
A wide variety of methods are available for the analysis of metallic elements at trace levels in biological matrices, including colorimetric and fluorimetric assays, electrochemical detection (anodic stripping voltammetry), flame atomic absorption spectrophotometry (AAS), electrothermal AAS (ETAAS) (also referred to as graphite furnace AAS (GFAAS)), inductively coupled plasma emission spectrometry (ICP-AES) (also referred to as ICP-optical emission spectrometry (ICP-OES)), and ICP-mass spectrometry (ICP-MS). AAS and ETAAS offer good sensitivity but the nature of the instrumentation means that only one element can be analysed at a time. This presents difficulties when specimen volumes are limited and the measurement of several elements is required because a certain volume of sample is required for the analysis of each element. If a large number of elements have to be analysed for, there may be insufficient sample available. Alternative technologies have been developed, such as anodic stripping voltammetry (ASV), but this needs careful specimen preparation (digestion) before specimens can be run. ICP-AES and ICP-MS offer the significant advantage of multi-element analysis with some instruments capable of analysing up to 75 elements simultaneously. This advantage comes at a significant cost in terms of instrument purchase and running costs relative to AAS and ETAAS but the multi-element capability makes screening for metals much quicker. ICP-MS is now the ‘gold standard’ for high sensitivity multi-element analysis (see Chapter 21). It is possible to analyse a small volume of sample for many elements simultaneously. In addition, it is possible to derive information on the relative isotopic abundance of some elements (e.g. lead), which can be used to link a biological sample to a suspect source material or scene residue. Heavier elements that are difficult to determine using ETAAS (e.g. platinum, uranium) are determined easily using ICP-MS. AAS, ICP-AES and ICP-MS can be linked to hydride-generation systems to analyse elements that form gaseous hydrides (such as arsenic and antimony); ICP-MS and -AES can also be connected to liquid chromatographic systems to study metal speciation in biological fluids and tissues. Interferences, including isobaric and polyatomic interferences, occur with ICP-MS which can give rise to problems in analysis for some elements, but newer instrument design and software capabilities have significantly reduced this problem. Nevertheless, the instrument operator must be aware of possible interferences. The high sensitivity of methods such as AAS, ETAAS, ICP-AES and ICP-MS means that strict precautions must be observed in sample collection and preparation and in the choice of sample containers. Aluminium is a particular case in point because it is ubiquitous in the environment and can readily contaminate samples. Colorimetric assays are also commercially available for the analysis of serum zinc, magnesium and copper, and are suitable for use on modern clinical laboratory equipment. Some screening tests that are simple to apply and that can be used to identify some metallic elements in stomach contents and scene residues are available. One such test is the Reinsch test, which can detect arsenic, antimony, bismuth and mercury. The test involves the use of copper foil, with inspection of the foil to determine its appearance after the test is applied. Interpretation can be difficult and is not entirely specific. However, the test may at least indicate that one or more of the above metals is present or otherwise and direct further, more specific, analyses. The Gutzeit test is a colorimetric test for the qualitative and semi-quantitative analysis of arsenic in urine, stomach contents, tissues, scene residues and contaminated water. It involves the reduction of inorganic arsenic to arsine gas, which then reacts with a solution of silver diethyldithiocarbamate to give a red complex. A specially designed Gutzeit apparatus can be purchased from commercial sources, and a detailed description of the test can be found in Flanagan et al. (1995). The sensitivity for arsenic is approximately 0.5 mg/L. A later modification of the basic technique has been applied to arsenic and other hydride-forming elements, such as antimony and selenium, with measure ment by AAS giving better sensitivity (Crawford and Tavares 1974; Kneip et al. 1977). Several convenient colorimetric methods can be applied to determine thallium in urine, stomach contents and suspect preparations. One of these methods (Flanagan et al. 1995) is based on measuring the absorbance of a chloroform-extractable pink–red thallium–dithizone complex from an alkaline solution that contains potassium, sodium and cyanide ions to mask interference from other metal ions. It indicates the presence of thallium in urine at concentrations of 1 g/L or more. However, the method is not specific and AAS, ETAAS, ICP-MS or ICP-AES are more reliable techniques.
