Article Updated on 17 March 2021
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Elemental analysis is widely used in analytical chemistry by scientists deciphering the chemical nature of our world. It can be qualitative – determining what elements are present – or quantitative, i.e. finding out how much of an element is present. Results can be used to determine the ratio of elements within a sample to infer a chemical formula, confirm the purity of a compound, or determine if the sample is what it states.
The 18th Century French nobleman and chemist Antoine Lavoisier is regarded as the inventor of elemental analysis as a quantitative means of assessing the chemical composition of a compound. His technique was based on the gravimetric determination of specific adsorbent materials before and after selective adsorption of combustion gases.
However, today’s fully automated systems are based on thermal conductivity and infrared spectrographic detection of gases.
Qualitative vs Quantitative Methods of Elemental Analysis
Quantitative analysis determines the mass of each element or compound present in a sample. Often gravimetry is used; the sample is dissolved and the element of interest precipitated and its mass measured, or the element of interest is volatized and mass loss measured.
Optical atomic spectroscopy using flame or graphite furnace atomic absorption can also be utilized to probe the outer electronic structure of the sample.
Neutron activation analysis relies on the activation of the sample matrix through neutron capture. The resulting radioactive target nuclei of the sample begin to decay, emitting gamma rays of specific energies that can identify the radioisotopes present in a sample.
Qualitative methods include chemical tests such as a sodium fusion test and Schöniger oxidation, in addition to mass spectrometric atomic spectroscopy to probe the mass of atoms, x-ray fluorescence and numerous additional spectroscopy methods to examine the inner electronic structure.
Each analysis is performed by determining the ratio of elements from within the sample and calculating a chemical formula that fits with those results.
CHNS Analysis
For organic chemists, establishing the mass fraction of carbon, hydrogen, nitrogen and heteroatoms such as halogens and sulfur in a compound is central to determining the structure of an unknown compound or the structure and purity of a synthesized compound.
The most commonly applied form of elemental analysis is CHNS, a combustion analysis technique. The sample is burned in an oxygen-rich environment, during which traps collect the combustion products – carbon dioxide, water and nitric oxide.
The masses of these combustion products are used to calculate the composition of the unknown sample. Some modern analyzers are also capable of determining sulfur within the same measurement run.
However, the technique is destructive and can only detect certain elements. Sensors could be used for a more comprehensive analysis: they are designed to detect and respond to input from the physical environment, which could be light, heat, motion, moisture, pressure, or other environmental triggers. They provide a means of deducing the elemental composition of a sample without destroying it, often offering up more useful data.
Such techniques are utilized in the water industry, for example, where it is vital to review the quality of drinking water to safeguard against health and safety occurrences. It is also used where crops are irrigated with industrial effluent; here it is necessary to monitor pollution levels in the water to limit the number of hazardous toxins potentially entering the food supply.
Soil Analysis and the Agricultural Industry
Elemental analysis of soil is critical to the agricultural sector. Analysis of carbon, hydrogen, silicon, iron, aluminum and oxygen in the soil is crucial to ensuring optimum crop yields.
Instead of traditional chemical analysis methods, which involves time-consuming soil sampling and chemical analysis in the laboratory, scientists are turning to neutron-gamma analysis. This non-destructive, in situ method requires no sample preparation, and is neither time consuming nor labor-intensive. It is quick, accurate, repeatable and reliable, and offers immediate results.
Click here for more information on the different types of sensors in the industry.
The method is based on the registering and analysis of gamma rays generated when neutrons interact with soil elements. It takes into account the penetration, moderation and interaction of fast neutrons with soil elements, gamma-ray appearance and propagation in soil, resulting in a gamma spectrum at the detector.
After a collision with a neutron, the nuclei of the soil elements undergo a specific reaction, emitting gamma rays of a specified energy. The intensity of the gamma-ray is directly proportional to the concentration of the component undergoing the particular reaction in the analyzed soil.
The technique can measure the average concentration of carbon in the upper four inches of any type of soil. The results can be used to create digital maps of soil element distribution.
Conclusion
Elemental analysis plays an essential role in many areas of science, including water and soil testing, to ensure public health. While traditional methods are still employed, techniques are becoming more sophisticated and require the use of more complex detectors and sensors to gather larger volumes and greater quality data.
References and Further Reading
Kavetskiy, A. et al. (2019) Physics of Neutron-Gamma Sensors for Soil Elemental Analysis [Online] ECSarXiv. Available at: https://ecsarxiv.org/srvxc/ (Accessed on 16 April 2020)
Yakubova G et al (2017) Measurements of Soil Carbon by Neutron-Gamma Analysis in Static and Scanning Modes Journal of Visualized Experiments doi: 10.3791/56270
Robinson, I. (2019) Using Sensors in Elemental Analysis [Online] AZoSensors. Available at: https://www.azosensors.com/article.aspx?ArticleID=1560 (Accessed on 16 April 2020).
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