Trace Metal Analysis
Trace metal analysis is the determination of metallic elements present at low concentrations in samples such as environmental, pharmaceutical, food, geological, clinical, and forensic materials, and it can be carried out using techniques ranging from gravimetric analysis and volumetric titration to spectroscopic methods, atomic absorption spectroscopy, and ICP spectroscopy, each offering a different balance of cost, sensitivity, and sample throughput.
Estimation of trace metals in different studies such as environmental samples, drinking water, pharmaceuticals, foods and beverages, minerals, geological samples, clinical samples and forensic samples are assuming greater importance due to increased awareness on both the nutritional values and harmful effects of trace metals.
Before going into the techno-commercial considerations it is necessary to go into the merits and limitations of common available options:
Gravimetric Analysis
Gravimetric analysis is based on estimation of elemental composition of the isolated compound by precipitation, purification and final ignition to constant weight. It is useful when a high degree of sensitivity is needed. It is a very time consuming technique but without high investment cost. The main sources of error in gravimetric analysis are co-precipitation and solubility of precipitates
Volumetric Titrations
Volumetric determinations are less time consuming in comparison to gravimetric estimations.. The accuracy of determinations is dependent upon correct volumetric readings and observation of endpoint colour changes. Electrometric endpoint detection titrations generally overcome this source of error.
Spectroscopic Methods
Spectroscopic methods are mostly based on development of coloured complexes of the trace metals with ligand groups on the reagents. Spectroscopic estimations consume much less time in comparison with the gravimetric and volumetric determinations but the accuracy of results is dependent on various factors such as pH of the solutions, selection of absorption wavelength, temperature variations and inherent errors in absorbance readings.
Atomic Absorption Spectroscopy
Atomic Absorption Spectroscopy is by far the most common and affordable technique used for analysis of trace metals. It affords estimations ranging upto ppm levels using flame technique and ppb levels for graphite furnace or hydride generation techniques. The limiting factor is that only one element can be analysed at a time and for multielement detection a different light source is required and on an average about 3 min are required for each element determination.
ICP Spectroscopic Techniques
ICP techniques afford simultaneous detection of a number of elements in a sample in the same time as it would take to analyse a single element in AAS. Both ICP – OES and ICP – MS techniques are versatile but ICP – MS in addition provides isotopic ratio of same element present in the sample. Both ICP – OES and ICP – MS are costly techniques requiring greater operational costs as well. However, both the techniques provide sub – ppb level determinations and very large sample throughputs. Due to these advantages it is possible to recover the cost on initial investment over a short period of time
As you can see several options are available for trace metal estimations and you have to choose from the available options based on your analysis requirements. The choice of the estimation technique is ultimately governed by a number of factors such as:
- Cost of the instrument
- Cost of analysis in terms of the reagent costs and cost of operating utilities such as gases
- Number of samples to be analysed
- Required limits of detection and concentration range over which determinations are to be made
- Availability of sample amount
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