Spotlight Environmental Analysis & Water Testing


ICP-MS is a popular method for fast and accurate measurements of trace (ppb-ppm) and ultra-trace (ppq-ppb) elemental concentrations in a wide range of geological and environmental applications. Having recognised the unparalleled sensitivity, precision and reliability of ICP-MS, environmental protection regulatory bodies across the world have mandated the use of ICP-MS for a variety of applications.


ICP-MS is based on the coupling of an inductively coupled plasma, producing ions and a mass spectrometer that separates and detects the ions. Argon gas (Ar) is used for the production of the plasma. Following dispersion into a separate stream of argon gas (nebulisation), samples are carried to the inductively coupled plasma where they are vaporised, atomised and ionised. The resulting ions are then passed through a series of apertures (cones) into a high vacuum mass spectrometer where they are separated according to their mass-to-charge ratio (m/e) and counted. The isotopes of the elements are identified by their m/e. The intensity of a specific peak in the mass spectrum is proportional to the amount of that isotope in the original sample.


Using CRC-based ICP-MS for Fast and Accurate Multi-Elemental Analysis of Environmental and Geological Sample Matrices


ICP-MS BENEFITS


The increasing popularity of the technique is attributed to the important benefits that it offers, including its multi- elemental capability compared to conventional elemental techniques, high sensitivity, superior detection limits, increased dynamic range and isotopic ratioing capabilities. ICP-MS can be used to analyse both solid and liquid samples, requiring only small sample quantities thanks to its analytical range from the ppt (parts per trillion) to the ppm (parts per million) regions. Used in conjunction with varying nebulisation and calibration techniques, ICP-MS is applicable to both liquid and solid materials.


APPLICATIONS


This highly sensitive and powerful technique performs reliable trace element distribution analysis of geological materials, identification of ultra-trace cesium (Cs) and rubidium (Rb) in hydrothermal pore waters and monitoring of transition, trace and heavy metal contamination in environmental water samples. Further applications include preliminary investigation into the determination of osmium (Os) in sea water, lead isotopic ratio determination in mud cores taken from salt marshes and analysis of uptake fluxes of trace redox-sensitive elements from sea-water in sediments.


REGULATORY REQUIREMENTS


“CRC-based ICP-MS has cemented its place in routine environmental laboratories due to its wide elemental coverage, high sensitivity and rapid analysis”


Author Details:


By Julian Wills, Shona McSheehy, Tomoko Oki, Meike Hamester, Thermo Fisher Scientific, Bremen, Germany and Bill Spence, Thermo Fisher Scientific, Winsford, UK


Most countries have strict legislation in place governing a wide range of methods for use in environmental industries, the purpose of which is to restrict pollution and to protect public health. Legislation is often based on international standards, for example environmental guidelines set out in EC directives and by the World Health Organisation (WHO). In the EU, there are no specific prescribed or approved analytical methods for regulatory drinking water analysis, however all methods must be proven to meet the performance requirements set out in the relevant EC directive.


In the United Kingdom, the Drinking Water Inspectorate (DWI) enforces the EC Directive 98/83 of November 1998. These regulations list a total of 53 parameters, including colour, turbidity, organic compounds, anions and inorganic elements, which must be monitored in drinking water. For each parameter a prescribed concentration or value is given. This relates to the maximum or minimum concentration, which must not be exceeded. EC Directive 98/83 sets out stringent standards for the elements Cu, As, Ni, Pb, Sb and B, making these regulations difficult to meet with a single instrument.


Under the Directive, each laboratory is required to performance test the analytical systems for each parameter before that analytical system can be used for routine analysis of compliance samples. The design of the performance testing and calculation of the performance characteristics should be in accordance with the guidelines laid down in the publication - ‘NS30, a manual on analytical quality control in the water industry.’


Techniques used for analyses include AA, ICP and ICP-MS. ICP and ICP-MS are more common in typical European environmental labs because their multi-element analysis ability allows more efficient operation. Although very powerful and widely recognised by regulatory agencies, ICP-MS presents a few limitations, including spectroscopic interferences.


INTERFERENCES


Interferences are caused by polyatomic species that have similar mass to the target analytes and stem from the sample matrix, Ar or entrained gas. Solvents, reagents,


glassware and other sample processing hardware may also yield artifacts and/or interferences to sample analysis. Interferences limit the ability of ICP-MS to determine certain elements of interest while also increasing maintenance requirements and reducing the reliability and quality of the data produced.


There are other types of interferences including isobaric elemental interferences that are caused by isotopes of different elements forming atomic ions with the same nominal mass-to-charge ratio (m/e). Ions consisting of more than one atom or charge trigger the formation of isobaric molecular and doubly-charged ion interferences respectively. Physical interferences are associated with the sample nebulisation and transport processes as well as with ion-transmission efficiencies. Memory interferences or carry-over occur when there are large concentration differences between samples or standards, which are analysed sequentially.


Eliminating interferences provides numerous advantages including significantly improved detection limits for interfered analytes, analyte confirmation by isotope ratio measurement and superior analytical confidence in complex matrices. The combination of ICP-MS and collision/reaction cell (CRC) technology has demonstrated a unique capability of significantly reducing and even eliminating interferences.


COLLISION/REACTION CELL (CRC) TECHNOLOGY


CRC technology employs either passive collisional interactions in which no chemical reaction occurs, or reactive interactions where the charge or structure of either the analyte or interferent, are chemically changed. Collision-based instruments use an inert collision gas, helium (He), to reduce the kinetic energy of the polyatomic interferent, thus preventing it from entering the quadrupole analyser. Reaction-based analysers use a range of reactive gases, including hydrogen, methane and ammonia to chemically shift one member of the analyte-interferent pair to another mass.


Reaction and collision cells share the same technology but where they differ is the regime established inside. Reactive chemistry can be applied to polyatomic species that react with the gas and are consequently either eliminated or modified. Additionally, reactive chemistry can modify analyte ions and analyse them at masses different from their natural isotope mass.


Collision technology can separate all kinds of overlapping molecular and polyatomic analyte ions from monoatomic ions if they differ in kinetic energy. This process is based on collisions using inert gases like He and is called Kinetic Energy Discrimination (KED). Collision technology using KED has the advantage of being universally able to reject all polyatomic interferences in any matrix and is therefore well suited to multi-elemental analysis in complex or unknown matrices.


Using ICP-MS in conjunction with state-of-the-art collision/reaction technology can efficiently reduce spectral interferences to negligible levels whilst the analytes remain relatively unaffected. Accurate analysis at low concentration levels can be performed even in difficult matrices. There are a number of spectral interferences that can be reduced ideally with reactive chemistry while others are better treated under KED conditions. For highest flexibility and best detection power in sample analysis, experiments should switch between several cell regimes.


APPLICATION EXAMPLE


An experiment was developed to illustrate the flexibility and power of CRC-based ICP-MS to analyse a variety of common environmental and geological sample matrices.


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