Trace Metal Analysis Methods

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Trace metals are found naturally in the environment and living things in minimal amounts, and are the metal subset of trace elements.

In chemistry, trace elements are considered as those where the average concentration is less than 100 parts per million (ppm). In geochemistry, a trace element has a concentration that is less than 1000 ppm of a rock's composition. In biochemistry, trace elements refer to small amounts of dietary elements required by living organisms.

Humans and animals do need a certain amount of some trace metals; however, if ingested at high levels, these trace metals can be toxic. The World Trade Organisation has developed international food standards to ensure that the maximum concentration levels of trace metals are not breached, and to deter fraud.

Trace metal analysis includes the detection, identification, and quantification of trace metals. Advances in analytical techniques (including spectroscopic, spectrometric, X-ray, and nuclear techniques) have made it possible to quantify trace metals and study their functions. Analysis of trace metals measures their levels and toxicity, and is often used in product testing in the pharmaceutical, environmental, chemical, medical, and cosmetic industries.

Types of Trace Metals

Trace metals in the human body occur naturally or can be ingested from food and drink as part of a normal diet. The types of trace metals in the human body include iron, nickel, cobalt, copper, chromium, vanadium, lithium, zinc, molybdenum, and manganese.

Trace metals in the environment include those found in the air, soil, and water. Trace metals in the environment exist both naturally and as a by-product of pollution and human activities. Trace metals found in the environment include arsenic, calcium, chromium, cobalt, copper, iron, lead, nickel, magnesium, selenium, sodium, potassium, and zinc.

The levels of trace metals in the environment can increase when they are released from rocks, through both natural processes and human activities. Natural processes include a breakdown of rocks, the spreading of mid-ocean ridges and volcanic activity. Human activities that cause an increase in trace metals include mining, smelting, burning coal, and wastewater disposal.

Trace Metal Analysis in Industry

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Analysis of trace metals is vital across many industries for both safety and regulatory purposes. Quality control is needed to identify and measure trace metal contaminants in drugs, fertilizers, cosmetics, packaging, medical devices, lubricants, and catalysts.

The industries that commonly use trace metal analysis include the food, environmental, pharmaceutical, petroleum, and chemical industries. Some of the primary uses of trace metal analysis in each industry are listed below:

  • Food - Testing food safety of toxic trace metals, the analysis of macro minerals, fraud, and quality control testing.
     
  • Environmental - The analysis of drinking water, wastewater, solid waste, soils, composts, and fertilizers.
     
  • Pharmaceutical - Testing nutritional supplements, patient toxicity testing, and analysis of the degradation of drugs.
     
  • Petroleum and chemical - Testing the concentrations of trace elements in petroleum and its derivatives, lubricant analysis, and the identification of contamination and wear of metals.

Techniques Used to Analyse Trace Metals

The main analytical techniques used to analyze trace metals include:

  • Neutron Activation Analysis - Used to detect trace impurities in materials by forming artificially radioactive isotopes and analyzing the nucleus.
     
  • X-Ray Fluorescence - Used to analyze geological materials, cement, archaeological samples, steels, forensic, and environmental samples by the interaction of X-rays with matter.
     
  • Atomic Absorption and Emission Spectroscopy - Used to quantitatively analyze roughly 70 different elements by the absorption and emission of optical radiation.
     
  • Inductively Coupled Plasma
    • ​Optical Emission Spectrometry (ICP-OES) - Used for the detection of trace metals in chemical and petroleum samples.
    • Mass Spectrometry (ICP-MS) - Used when ultra-trace quantification and high sensitivity is required.

Neutron Activation Analysis

Neutron activation analysis a technique where neutrons are used to irradiate and activate a sample. With neutron activation analysis, the sample requires little to no sample preparation, and it is a non-destructive technique. When a measurement is carried out without needing prior chemical separation, the technique is known as instrumental NAA (INAA).

With neutron activation analysis, a sample is exposed to a neutron flow, and the sample forms radioactive isotopes. When the radioactive isotopes decay to their lower energy state, delayed gamma rays are emitted that have characteristic energies for each element. The intensity of the gamma rays allows for quantitative measurement of the concentrations of the various trace elements.

X-Ray Fluorescence (XRF)

X-Ray Fluorescence (XRF) includes using the emission of X-rays from a material that has been excited after being hit with high-energy X-rays. When the X-ray radiation strikes the atom, it dislodges electrons from their inner orbitals. The spaces in the lower orbitals are filled by electrons from an outer shell, which have higher energy than the replacing electrons. As a result, energy is released in the form of X-rays.

Electronic energy levels for each element are different, so the energy from the X-ray fluorescence peaks can be correlated to specific elements.

XRF is a popular method for the analysis of trace elements in solid mineral and environmental samples. The direct quantitation of metal species in solid materials is possible, meaning that the elution step in sample preparation is not required and so sample handling is reduced. The sensitivity of XRF is dependent upon the energy of the radiation, geometry of the instrument, and efficiency of the detector.

Spectroscopic Techniques

Atomic Absorption Spectroscopy (AAS) works by measuring the absorption of light by free metallic ions in a gaseous state. AAS can be split further into Flame Atomic Absorption Spectrometry (FAAS) and Graphite Furnace Atomic Absorption Spectrometry (GFAAS), depending on the atomizer used.

Atomic Emission Spectroscopy (AES) measures the intensity of light emitted from either a flame, plasma, arc, or spark at a particular wavelength to determine the quantity of the trace element in a sample. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) is a type of emission spectroscopy that uses plasma as the light source.

AAS and AES are simple and easy to use, but a key reason that these spectroscopic techniques have fallen out of favor is due to their lack of sensitivity, compared to other techniques now available. Unlike techniques such as neutron activation analysis, sample pre-treatment is required, which is time-consuming and can be problematic.

Inductively Coupled Plasma Atomic Emission Spectrometry

Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) is also known as inductively coupled plasma optical emission spectrometry (ICP-OES). Samples are introduced into the plasma in liquid form, so solid samples require preparation digestion prior to injection.

The inductively coupled plasma is created by argon gas that is ionized in an electromagnetic field. A stable plasma is then generated due to collisions between the neutral argon atoms and the charged particles.

When the sample is introduced to the plasma, it collides with the electrons and charged ions, to be broken down into charged ions. The ions then become excited within the plasma, and the electrons jump from the lower to higher energy level. When the electrons relax back their initial ground state, energy is emitted in the form of photons.

The emitted photons possess wavelength characteristics of specific elements. The intensity of the emitted photons is indicative of the concentration of the element within the sample.

ICP-OES/AES has similar limits of detection to other spectroscopic techniques, but it is much more expensive and can suffer from interferences.

Inductively Coupled Plasma Mass Spectrometry

ICP-MS is an analytical technique for determining trace multi-elemental and isotopic concentrations in liquid, solid, or gaseous samples. It combines the ion-generating argon plasma source with the sensitive detection limit of mass spectrometry detection.

With ICPMS, the sample is ionized with the same type of plasma used in ICP-AES/OES. The ions are then released from the plasma and travel into the mass spectrometer, where the metal ions are isolated according to their atomic mass-to-charge ratio by a quadrupole or magnetic sector analyzer.  With ICP-MS, it is the metal ions that are detected and not the light that they emit.

ICP-MS is an expensive way to analyze trace metals, but it provides high sensitivity and limits of detection, as well as only needing a small sample volume and having the added ability for isotopic determination. It does suffer from the same interference problems as ICP-AES/OES, but these can be overcome by using high-resolution mass spectrometers to resolve the elements and interferences.

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Louise Saul

Written by

Louise Saul

Louise pursued her passion for science by studying for a BSc (Hons) Biochemistry degree at Sheffield Hallam University, where she gained a first class degree. She has since gained a M.Sc. by research and has worked in a number of scientific organizations.

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