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There are 118 elements on the Periodic Table, 94 of which occur naturally on Earth either as stable or unstable isotopes. Everything on Earth differs in its elemental composition and chemical properties, and knowing what they are made of can provide useful information about where they are from or their quality.
Trace elemental analysis, or TEA, enables scientists to determine small quantities of chemical elements in a sample; it is an integral part of understanding the natural world and can be used in a variety of fields, from environmental monitoring to toxicology, forensics and food safety.
As well as a huge variety of applications, several techniques can be employed, including atomic absorption spectroscopy, x-ray fluorescence, and proton-induced x-ray emission.
A trace element is a chemical element whose concentration is very low. The exact definition depends on the field of science:
- In analytical chemistry, trace elements have an average concentration of less than 100 parts per million or less than 100 micrograms per gram, as defined in the second edition of the IUPAC Compendium of Chemical Terminology.
- In biochemistry, trace elements are dietary essentials needed for proper growth, development, and physiology of an organism. They are elements required to perform vital metabolic activities.
- In geochemistry, a trace element has a concentration of less than 1,000 parts per million or 0.1% of a rock’s composition. The term is mainly used in igneous petrology.
Techniques for trace elemental analysis are responses to the increasing need for accurate measurements at extremely low quantities. Concentrations typically range between one and 100 parts per million, although today’s techniques can reach ultra-trace concentrations below parts per million.
When originally developed in the 19th century, many trace elements were still undetectable; as analytical technology has improved, the number of elements that can be detected has risen.
Although techniques now allow accurate, repeatable determination of elements, the term ‘trace’ is still used.
Trace elemental analysis might be employed to provide information on trace elements in coal fly ash, sand, water, nail varnish chips, and films, or lipsticks, for example. Techniques include:
- X-ray fluorescence (XRF) - This is a widely used technique utilized on geological materials, steels and cement, and archaeological, forensic or environmental samples. The method is non-destructive and can identify almost any element from fluorine upwards.
- Atomic absorption spectroscopy (AAS) - In flame AAS, a sample is introduced to a flame where it dissociates into its constituent atoms; electromagnet radiation is directed through the flame.
- Neutron Activation Analysis (NAA) - Neutrons are used to irradiate and activate a sample, and a nuclear reaction between the neutron and an isotope of the element of interest produces a radionuclide with a characteristic half-life which allows for the identification and quantification of elements. It has been used to analyze cancerous tissues, trace elements in human hair and drinking water.
- Proton-induced x-ray emission (PIXE) - This technique analyses x-ray emissions induced by heavy charge particles.
Each technique has its pros and cons, but they all suffer from some of the same issues. The presence of small levels of contaminants may influence the characteristics of materials in terms of the physical electrical and other properties, and could adversely affect the final product.
Contaminants can be introduced along many stages of processing and controlling and understanding how chemical adulteration impacts advanced materials is of the utmost importance. The systematic inspection of raw materials before production and testing throughout the key stages of manufacturing can identify and eradicate undesirable impurities in the resulting product.
The requirement of analytical science is becoming progressively demanding, and the boundaries of trace analysis are constantly being lowered. The challenge remains to accurately and repeatably quantify elements at incredibly small fractions; as techniques improve, these lower levels will easily be obtained.