Introduction
Analysis of environmental samples for contaminants is a necessity in order to restrict pollution and protect public health. Such analysis provides information regarding the nature and extent of contamination and helps specify and prioritize corrective actions based on potential risks to human health and the environment. ICP-MS (Inductively Coupled Plasma Mass Spectroscopy) has cemented its place in routine environmental analysis laboratories offering a wealth of benefits, including extended elemental coverage, high sensitivity, superior detection limits, increased dynamic range, unique isotopic ratioing capabilities and greater sample throughput.
In response to the unmatched sensitivity, precision and consistency of ICP-MS, global legislative authorities have regulated its use in environmental applications.
Regulatory outlook
In the USA, the Environmental Protection Agency (EPA) holds the legislative authority to develop standardized analytical methods for the measurement of various pollutants in environmental samples from known or suspected hazardous waste sites. Among the pollutants that are of concern to the US EPA is a series of inorganic analytes and cyanide that are analyzed using a range of techniques, including ICP-MS. Overall, the US EPA specifies the use of ICP-MS for the elemental analysis of a wide range of environmental samples.
In January 2007, the Office of Solid Waste and Emergency Response of the US EPA published the Multi-Media, Multi-Concentration, Inorganic Analytical Service for Superfund Method ILM05.4 for water and soil/sediment environmental analysis2. According to this method, ICP-MS is used to determine the concentration of dissolved and total recoverable elements in water/aqueous samples. The US EPA also mandates the use of ICP-MS for monitoring various elements in drinking water (Method 200.8), wastewater and solid waste (SW-846 Method 6020) and low level trace in drinking water (Method 1638). Additionally, ICP-MS has been approved by the US EPA as the sole multi-element method for monitoring arsenic and uranium levels in drinking water.
The International Organization for Standardization (ISO) has introduced Standard 17294-1:20043, which specifies the principles of ICP-MS and provides general directions for the use of this technique for determining elements in water. In general, measurements are carried out in water, however, gases, vapors or fine particulate matter may be introduced too.
Although ICP-MS offers a wealth of benefits and its use is mandated through legislation, it is also associated with a very important limitation: spectroscopic interferences. Combining ICP-MS with collision/reaction cell (CCT) technology has been found to address this shortcoming to a great extent.
Eliminating spectroscopic interferences
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. 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.
CCT technology represents a major step forward for ICP-MS. First introduced commercially in 1997 by Micromass (subsequently GVI), CCT is a technological method of removing the polyatomic ions that can form in the plasma and interfere with the analytes of interest. As a result, spectra interferences are reduced to negligible levels. CCT works by producing interactions, namely reactions or differential kinetic energy reductions, between the polyatomic ions and a reagent gas introduced into a cell between the mass spectrometer sampling interface and the mass analyzer. CCT-equipped instruments currently account for around 80% of ICP-MS units sold.
Collision-based analyzers use an inert collision gas, helium (He), to reduce the kinetic energy of the polyatomic interferent and prevent it from entering the quadrupole analyzer. The operation of reaction-based instruments is based on the use of a range of reactive gases such as hydrogen, methane and ammonia to chemically shift one member of the analyte-interferent pair to another mass.
The main difference between reaction and collision cells lies in their regime. Reactive chemistry is mostly suitable for polyatomic species that react with the gas, thus being either eliminated or modified. The method can also modify analyte ions and analyze them at masses different from their natural isotope mass. Collision technology, on the other hand, achieves separation of all kinds of overlapping molecular and polyatomic analyte ions from monoatomic ions when they have different kinetic energy. This procedure is called Kinetic Energy Discrimination (KED) and offers the important benefit of being able to reject all polyatomic interferences in any matrix. As a result, it is the preferred method for multi-elemental analysis in complex or unknown matrices. In order to achieve maximum flexibility and greatest detection power, several cell regimes should be implemented in the same application.
An experiment was performed to demonstrate the efficiency of CCT-based ICP-MS in analyzing a variety of common environmental sample matrices.
Application Example
For the purposes of this experiment, a Thermo Scientific XSERIES 2 ICP-MS analyzer equipped with third generation CCTED collision/reaction cell technology (Thermo Fisher Scientific, Bremen, Germany) was configured with an SC2 FAST system (Elemental Scientific Inc.). Immediate benefits provided by the FAST system include considerable reduction of sample uptake, washout times and matrix load that reaches the plasma. A universal gas mixture was used in the collision/reaction cell for the suppression of interferences.
Environmental Sample Analysis
The experimental methodology implemented to test the high throughput setup for environmental samples was based on conditions described in Method ILM05.4. Methane was used to improve the determination of analytes with a high ionization potential. This carbon-loading enhanced sensitivity for these analytes and improved long term stability by the reduction of matrix deposition on the ICP-MS interface.
A sequence of 500 samples, including a calibration and integrated QC, was performed for 23 analytes. A sample turnaround of approximately 83 seconds, including uptake, analysis and wash, provided an overall batch acquisition time of 12 hours for the entire 500 samples. Five samples were classed as unknown and looped continuously throughout the experiment.
Geological Sample Analysis
Rock samples weighing 0.5 g were digested using an Aqua Regia mix and diluted a further 10 times prior to analysis. A sequence of 478 samples, including a calibration and integrated QC, was performed for 30 analytes. A sample turnaround, including uptake, analysis and wash, of approximately 80 seconds resulted in an overall acquisition time of 11 hours for the 478 samples. Five samples were classed as unknown and looped continuously throughout the experiment.
Discussion
Experimental results have demonstrated that CCT-based ICP-MS is a powerful multi-elemental technique with high throughput capabilities. A prerequisite in order to achieve such superior results is to use a universal gas mixture for all analytes. The addition of methane significantly increases the analytical sensitivity of the method for analytes with a higher ionization potential. Such analytes often exist at lower concentrations in environmental samples. Methane addition also improves the long term stability of CCT-based ICP-MS. Throughput and stability are further improved and instrument maintenance is reduced thanks to the configured FAST system which cuts uptake and washout time and introduces less matrix into the plasma over time.
Conclusion
Environmental samples must be regularly monitored for contamination in order to limit pollution and protect public health. ICP-MS has long been established as a proficient technique for such types of analysis offering multi-elemental analytical capabilities at a fast rate. However, the method is associated with spectral interferences which limit its effectiveness. This can be easily addressed by combining ICP-MS with CCT technology. Spectral interferences are considerably reduced and sample throughput is increased leading to fast and accurate analyses of an extended range of common environmental sample matrices.
References
1. US Environmental Protection Agency, Trace Metals Analysis By ICP-MS, http://yosemite.epa.gov/r10/LAB.NSF/1887fc8b0c8f2aee8825648f00528583/9f18a1f3cf600033882565e2006d287d!OpenDocument
2. US Environmental Protection Agency, Multi-Media, Multi-Concentration, Inorganic Analytical Service for Superfund (ILM05.4), http://www.epa.gov/superfund/programs/clp/download/ilm/ilm54fs.pdf
3. International Organization for Standardization, ISO 17294-1:2004, Water quality -- Application of inductively coupled plasma mass spectrometry (ICP-MS) -- Part 1: General guidelines,
http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=32957