Hyphenated ICP-MS Techniques for Speciation Analysis

March 1, 2009

The authors discuss speciation analysis methods that enable scientists to identify and measure the quantities of one or more individual chemical species in a sample.

Traditionally, inorganic chemical analysis has been used to investigate the presence and concentration of elements and trace elements in a sample. However, total elemental determination does not provide the physicochemical characteristics, biological activity, toxicity, mobility, and bioavailability of elements in the sample because such information can only be derived through the analysis of chemical species. Speciation analysis identifies and measures the quantities of one or more individual chemical species in a sample, thereby enabling scientists to determine the chemical form in which the elements occur as well as their distribution.

According to the International Union of Pure and Applied Chemistry (IUPAC), a chemical species is a specific form of an element defined with regard to isotopic composition, electronic or oxidation state, and complex or molecular structure. The speciation of an element is defined as the distribution of the element among defined chemical species in a system. Speciation analysis is the activity within the framework of analytical chemistry of identifying and measuring the quantities of one or more individual chemical species in a sample. Speciation analysis is meant to provide information assuring product quality and safety for consumers, process efficiency with respect to raw materials, energy and waste production, safety for the production plant and for the workplace environment, compliance with rules and legislation, and absence of risks for the environment and its inhabitants.

Dr. Shona McSheehy

Speciation Analytical Techniques

Inorganic analysis and especially trace metal analysis have evolved historically through the development of atomic spectrometry. Elemental speciation analysis has only developed during the last two decades with the evolution of instrumental detection powers coupled with the knowledge that speciation leads to a better understanding of elements in our environment.

Dr. Michael Sperling

Atomic spectrometry sources and aggressive sample preparation techniques destroy the molecular information associated with originally present chemical species. Some techniques, such as X-ray, electron spectroscopy, and mass spectrometry (MS), currently are capable of offering species-related information in solid samples, while certain electroanalysis, magnetic resonance spectroscopy, and nuclear spectroscopy techniques can provide molecular information directly. However, hyphenated techniques have emerged as the most viable methods for the separation, detection, identification, and quantification of elemental species.

Speciation via ICP-MS

Separation modules such as high performance liquid chromatography (HPLC) and gas chromatography (GC) are combined with inductively coupled plasma-mass spectrometry (ICP-MS) to provide accurate and dependable species-specific detection. Quantification can be achieved using ICP-MS methodology even without compound standards, while defensible results can be obtained by isotope dilution methodology.

Using commercially available methods such as GC–ICP-MS and HPLC–ICP-MS, analysts have achieved significant improvements in the area of quality assurance and control, resulting in better accuracy and traceability. Improved techniques are applied for sample preparation, leaving the actual species intact, whereas new definitive methods such as species-specific isotope dilution analysis overcome artifacts.

Legal Requirements

Most existing rules and legislation force analytical laboratories to perform total element determinations. While the European Water Framework Directive (2000/60/EC) specifies that the species of cadmium (Cd), lead (Pb), mercury (Hg), nickel (Ni), and tributyltin (TBT) must be controlled in water, there are very few national rules and standards implemented that regulate species-related measurements.

Speciation analysis is an essential tool for helping companies comply with current legislation, including REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals, Europe), TSCA (Toxic Substances Control Act, U.S.), RoHS (Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment, Europe), and WEEE (Waste Electrical and Electronic Equipment, Europe). For example, the amount of hexavalent chromium is regulated by RoHS for concentration in electrical and electronic equipment and by TSCA for its use in water treatment in industrial cooling towers.

Real-World Application Benefits

ICP-MS hyphenated techniques are applicable in an extended range of industries, offering a wealth of short- and long-term benefits. In the case of the chemical and petrochemical industries, the application of these techniques results in the optimization of production methods, efficient production control to avoid damage of the production facilities, and timely product specification to avoid risks for consumers and the environment.

In the food and drink industry, ICP-MS hyphenated methods achieve enhanced characterization of food products with respect to their nutritional value and toxic contaminants while also enabling the development of food products with additional health benefits. In addition, the techniques provide improved characterization of fresh water with respect to requirements for water treatment. Process control and elimination of toxic contaminants from drinking water also are achieved.

The pharmaceutical and food industries benefit from using ICP-MS hyphenated techniques, which perform superior characterization of food supplements and pharmaceuticals with respect to their bioavailability and biological activity. Side effects of drug products can be reduced considerably.

The methods also are applicable in the waste management industry for risk-assessment analyses to define the mobility of pollutants, degradation, transformation, and potential toxicity and for optimization of remediation strategies.

Future Developments

A major downfall of ICP-MS hyphenated techniques is that they cannot identify unknown species. One way to address this issue is to perform comparisons of retention times between the unknown species and a standard. Confidence can be enhanced by using different separation methods; however, the availability of pure compounds limits the possibility of identification. Multidimensional separation techniques also can be applied, yet species must be sufficiently stable to survive time-consuming analysis under different conditions and changing media. A third method involves MS for separated compounds by using soft ionization sources such as electrospray ionization (ESI) and matrix-assisted laser desorption–ionization (MALDI). Unfortunately, with MS, results can be jeopardized by the presence of salts and other components at concentration levels above 10 nM. Also, ESI-MS is in many cases significantly less sensitive than ICP-MS, especially when both techniques are operated under their individual optimum conditions. Future strategies include more powerful and stable separation protocols with, for example, HPLC stationary phase particles becoming smaller and more reproducible for improved species resolution and thus surer retention time matching. GC strategies also will improve with techniques such as large volume injection (LVI) with cryo-cooling, 2D-GC for improved resolution, and matrix separation and FAST GC for higher throughput.

Dr. Shona McSheehy has 10 years experience with quadrupole ICP-MS and specializes in coupling chromatographic systems for speciation analysis. She has been working as an applications specialist at the Thermo Fisher Scientific site in Bremen for the last two years. Prior to that she spent two years as a Thermo Fisher Scientific post-doc researcher developing speciation methodologies at the Centre of Excellence based in the University of Pau, France. She spent three years in Pau earlier in her research career while preparing her Ph.D. thesis and had a three-year postdoctoral fellowship at the NRC in Ottawa, Canada.

Dr. Michael Sperling is Chief Executive Officer of the European Virtual Institute of Speciation Analysis (EVISA), an association aiming at the promotion of speciation analysis especially by fostering interdisciplinary cooperation, transfer of know-how, education and exchange of information. He gained his broad experience in the field of atomic spectrometry, trace element and speciation analysis as the head of the laboratory for applied research of a renowned instrument manufacturer. His work, often in cooperation with international scientists, has been published in more than 75 contributions within highly ranked scientific journals. More recently, Michael Sperling joined the Institute of Inorganic and Analytical Chemistry of the University of Münster, where he is investigating techniques and methods for speciation analysis.