Analytical Techniques for Trace Element Determination

TLDR: In the second half of the 20th century, together with rapid increase of detection capabilities of analytical techniques, a new term of ultratrace elements appeared as mentioned in this paper, which concerns elements at mass fraction below 1 ppm.

Abstract: A lot of elements occur in different matrices at low levels of content, and a lot of these elements were not detectable by analytical methods for a long time. The knowledge about their presence appeared with the development of analytical technology and caused the origin of the term “trace elements.” Trace element defined by IUPAC [1] is any element having an average concentration of less than about 100 parts per million atoms or less than 100 mg/kg. In the second half of the 20th century, together with rapid increase of detection capabilities of analytical techniques, a new term of ultratrace elements appeared. Even though the term exists and is commonly used, there is no rigid definition. Ultratrace concerns elements at mass fraction below 1 ppm. The knowledge of trace and ultratrace elements is very important in various fields of science, industry, and technology. Ultralow concentrations of elements might be as well essential as hazardous doses for organisms; some traces can dramatically change properties of designed devices. Therefore, the need for accurate measurements at low amount of contents is required and very important. The common use of extremely sensitive instrumentation needs the adequate control of contamination and verification of the accuracy of determination. The gain of analytical sensitivity multiplied contamination as well as other problems. Therefore, correct precautions should be taken to determine trace elements in the parts per billion concentration range and below. Errors during trace and ultratrace elemental analysis can be caused by improper sampling, storage, sample preparation, and, finally, by analysis itself. Therefore, an accuracy of an analytical determination should be always established. Collecting a representative sample without contaminating is a key to the meaningful analysis and Thiers’ words from 1957 “unless the complete history of any given sample is known with certainty, the analyst is well advised not to spend his time analyzing it” [2] is always up to date. Nowadays, there are a large number of available analytical techniques allowing for trace and ultratrace analysis of elemental composition. For the trace elements that are present in parts per million concentration range, the most widely used technique is probably atomic absorption spectrometry with flame atomization. For ultratrace elements present in concentration of parts per billion and below, the number of suitable techniques drops due to the required analytical sensitivity. The determination of trace elements is commonly held with potentiometry, voltammetry, atomic spectrometry, X-ray, and nuclear methods. Electrochemical methods can measure either free ions in solution (potentiometry) or free ions together with ions bound in labile complexes (voltammetry), and they can also provide analysis of the oxidation state of some of the elements. Atomic spectrometric techniques are very sensitive and can be used to measure the total element content within a sample; however, accuracy of these techniques can be affected by the matrix of the sample. X-ray and nuclear techniques provide very low limit of detections and matrix insensitivity and are used for comparison of results due to their principles fundamentally different from those of the other analytical techniques. Therefore, they are less likely to be prone to the same systematic biases. Benefits and losses of each technique should concern the number of analytes possible to measure with the use of the technique, occurrence of interferences and difficulties, detection limits, throughput of samples, and expenses. The determination of trace elements and contaminants in complex matrices often requires extensive sample preparation and/or extraction prior to instrumental analysis. A large number of samples that need to have determined the concentration of essential and toxic elements belong to food [3, 4], environmental [5, 6], clinical and biological [5–7, 7–9] samples. Routinely, the determination of trace metals has been carried out by inductively coupled plasma atomic emission spectrometry (ICPAES), inductively coupled plasma mass spectrometry (ICPMS), electrothermal atomic absorption spectrometry (ETAAS), and flame atomic absorption spectrometry (FAAS). However, matrix of many samples (biological, clinical, environmental, etc.) is complex and consists of high amounts of soluble solid substances and large amounts of inorganic compounds (i.e., salts of Ca, K,