V Vishwanath of Metrohm presents key trends in trace-contaminant analysis.
Ion Chromatography (IC) with conductivity detection has been successfully used to analyse low-molecular organic compounds such as amines and organic acids, as well as anionic and cationic substances. However, due to current toxicity concerns and consequently lowered maximum contaminant levels (MCL), analyses in complex matrices require improved sensitivity and selectivity.
The coupling of IC with multi-dimensional detectors such as an electrospray ionisation mass spectrometer (ESI-MS) or an inductively coupled plasma mass spectrometer (ICP-MS) solves even complex separation problems, simultaneously achieving outstanding sensitivity and selectivity. Additionally, these so-called hyphenated techniques provide valuable information for unambiguous peak identification and are less prone to matrix influences than conductivity detection. While IC-ICP-MS is used for element specific analysis, IC-MS allows the identification of parent compounds and corresponding metabolites. Examples of analysis of compounds and ions of great significance in drinking and treated water, affecting the environment and human health are discussed below:
IC-MS
The coupling of the separation power of an IC with the detection power of an MS has opened up new possibilities. Adequate chromatographic retention of the target compounds is combined with the sensitive detection of the MS, providing outstanding selectivity and sensitivity. Additionally, identification based on simultaneous retention time matching and isotope ratios reduces the risk of incorrect peak assignment.
Bromate in drinking water: Bromate, a potential carcinogen is generated by the oxidation of bromide traces during water desalination process and disinfection, for example by ozonisation. For drinking water and mineral water, current regulation stipulate a limit of 10 and 14 μg/L(ppb), respectively. Other Oxyhalides (Chlorite ClO3- & Chlorate ClO2-) and Standard Anions (Br-, NO3-, H2PO4-, HSO4-) can determined at the same time.
Perchlorate: Perchlorate is an environmentally stable, highly water soluble contaminant that is commonly used as an oxidant in solid fuel rocket propellants. Affecting humans, it has been shown to competitively inhibit the transport of iodine into the thyroid gland. The Environmental Protection Agency (EPA0 developed an IC/ESI-MS method (EPA322) for drinking water with a detection limit in the sub-μg/L range.
IC-ICP-MS
The zero tolerance policy concerning chromium, arsenic and selenium compounds in drinking water has led to IC-ICP-MS becoming one of the most prominent detection methods for trace metals in environmental samples. The advantage of this technique is that it can distinguish between difference oxidation states and chemical forma of a particular element. This approach is called speciation analysis. Individual concentration of element- containing species are, from the toxicological point of view, far more significant that total element concentrations as different valence states of an element have different properties. Chromium, arsenic and selenium species can either be identified by their retention times or isotopic ratios.
Chromium: Chromium compounds are used in dye and pigments production, in cement, in tanning, as mordants, wood preservatives, and as efficient corrosion inhibitors. While Chromium (III) is an essential trace element for humans, all forms of hexavalent chromium are regarded as highly toxic and carcinogenic. Accordingly, the EPA has fixed the MCL of Chromium (VI) at 100 μg/L.
Arsenic: Arsenic is the 20th most abundant element in the earth crust, ubiquitously found in a high number of minerals. It is used in pesticides, wood preservatives, and semiconductors. Its use as a weed killer and rat poison illustrates its high toxicity. Additionally, inorganic arsenical derivatives are considered to be carcinogenic and maybe teratogenic. Therefore, the EPA proposes a maximum allowable drinking water concentration of 10 μg/L.
In environmental and biological samples, more than 20 arsenic species have been identified. Depending on their binding characteristics, they have different toxicities and chemical behaviour. IC-ICP-MS allows the separation and unambiguous identification, based on structural date of different arsenic species in inorganic and organic forms.
Selenium: Because selenium compounds also differ strongly in their toxicity, speciation analysis is of paramount importance. Selenium is used in food supplements, photographic processes and electronics. Selenium is an essential trace element of humans and animals. Its uptake occurs mainly through food (grains, cereals and meat) and water. Elevated concentration, however are considered to adversely affect health (MCL- 50 μg/L). An increasing number of biogeochemical research papers point to the fact that selenium compounds have server impacts on aquatic wildlife.
