Meike Hamester

In reality, the technique is much more versatile and can offer more than just determinations of elemental concentrations. It offers high-precision isotope ratio determinations, and coupled to a chromatographic device, inductively coupled plasma–mass spectrometry (ICP-MS) provides information about the molecular composition and distribution of elements. Critical questions, such as "where from," "how old," and "what has happened to the sample," are answered accurately to ensure that the analytical results represent the truth as closely as possible. 

The challenge for users of ICP-MS is to maintain the method's multi-elemental capabilities during the analysis of a large variety of samples at low concentration levels while ensuring high throughput and the highest data quality. This is often hard to achieve because ICP-MS results can be affected by spectral interferences.  

Figure 1: Collisionâreaction cell technology.			

ICP-MS first entered the elemental analysis market as a revolutionary method able to deliver detection power similar to or better than an atomic absorption spectrometer while also being capable of multielemental analysis with high sample throughput like an optical emission spectrometry (OES) system. However, the accuracy of the results suffered from the lack of selectivity caused by spectral interferences.  

Spectral interferences are atomic (isobars) and molecular species that occur at the same nominal mass as the analyte of interest. They are formed in the plasma and interface region and originate from the plasma gases, solvent, and sample matrix components, resulting in inaccurate results.  

A variety of approaches, such as cold plasma or mathematical corrections, have been used to address the problem of spectral interferences, but these methods are limited to certain matrices. Because the matrix of a sample is a large contributor to spectral interferences, separation of the matrix on-line or off-line is also possible. However, off-line techniques are considered time-consuming and prone to contamination or loss of volatile elements. 

Two ICP-MS–based solutions emerged to overcome the limitations of conventional technologies and eliminate spectral interferences. The first one, high-resolution sector field ICP-MS (ICP-SFMS), ensures accurate results by spectral resolution of analyte signal from interfering species. The second one, quadrupole ICP-MS, employs collision cell or reaction cell technology. Quadrupole ICP-MS (ICP-QMS) is currently the most widely used method followed by ICP-SFMS. Choosing the most appropriate technology for a given application depends upon the sample matrix, the requirements for detection capabilities and precision, and of course, the available budget. This article details the use of ICP-MS in modern laboratory environments, discusses limitations associated with this technique, and suggests how users can overcome these limitations with the use of ICP-SFMS or ICP-QMS. 

Of all ICP-MS techniques, ICP-SFMS has been established as the most sophisticated technically. ICP-SFMS offers a wealth of benefits, including superior accuracy and transparency as well as the power to remove almost all spectral interferences.  

The core principle of the technique is the separation of ions in a magnetic field that has a high resolution capability of up to 10,000. ICP-SFMS and ICP-QMS have almost identical sample introduction systems and interface, whereas ions are accelerated into the spectrometer in a much higher voltage, as high as 8000 V, using ICP-SFMS. This enables the technique to deliver not only high resolution but also high sensitivity.  

After shaping by the ion optical system, the ion beam passes through an entrance slit and enters the magnetic field for mass separation according to the mass-to-charge ratio. The ions are then directed through an electrostatic analyzer before they finally reach the detection system. Three slits of different widths are used in ICP-SFMS to control the level of mass resolution. Switching between these slits takes less than one second. The narrower the slit, the higher the resolution, and the wider the slit, the higher the sensitivity. The slits also provide a constant relation, independent of mass and matrix, between the sensitivity of the signal and the level of mass resolution. 

The combination of the detector technology, the geometry, and high-performance optics incorporated into ICP-SFMS instruments provides an extremely low background of less than 0.2 cps in all three resolution modes. This feature, in combination with the high sensitivity, provides very high signal-to-noise ratio, which allows quantification at single digit parts-per-quadrillion levels. The technique allows reliable analyses at low concentrations in a few minutes. 

In ICP-QMS, the core principle of operation is based upon the separation of ions in an electrostatic field. Contrary to the operation of ICP-SFMS, the ions enter the mass spectrometer at low energy.  

Collision–reaction cell technology is currently the most accepted technique to effectively overcome spectral interferences in ICP-QMS. The cell is situated in the middle of the instrument, between the interface region and the quadrupole mass analyzer. A multipole is enclosed in a cylinder and a controlled flow of gas is introduced. Analyte and polyatomic ions entering the cell undergo collisions with the cell gas. Collisions with an inert gas will result in an energy drop of the ions; this energy drop is more significant for the polyatomic interfering species, which has a large cross-section and thus undergoes more collision. With a reactive gas, charge transfer or displacement of the analyte or polyatomic mass-to-charge ratio can occur, thus separating the interferences based upon the mass difference. In optimal situations, spectral interferences are removed and only analytes exit the cell. Of all the processes, collision retardation and energy filtering or kinetic energy discrimination (KED) is one of the most utilized within the collision–reaction cell.  

The combination of ICP-QMS and collision–reaction cell technology results in effective interference removal without a major effect on sensitivity. The high-performance ion optics results in a background of less than 0.2 cps, which in turn delivers an excellent signal-to-noise ratio. In addition, the system fulfills the requirement of laboratories for a detection system that is able to routinely acquire data from parts-per-quadrillion to parts-per-million concentration levels in various matrices with high accuracy and stability. The linear dynamic range of collision–reaction cell ICP-QMS is impressively large, over nine orders of magnitude. Together with the elimination of spectral interferences, all these benefits are essential to obtaining dependable results regardless of whether easy samples or the most challenging matrices are being analyzed.  

Comparing and Contrasting the Two Methods

In ICP-SFMS, the sensitivity factor between the resolutions is constant; in collision–reaction cell ICP-QMS, sensitivity will be enhanced or depressed depending upon the cell conditions employed. However, changing between the three different slits is much quicker in ICP-SFMS than changing gases in collision–reaction cell instruments.  

High sensitivity is definitely necessary in order for a system to generate accurate, trustworthy results. However, it is not enough. What matters even more is the signal-to-noise ratio, which depends upon the background of the instrument. Both ICP-MS methods under investigation deliver a low background of less than 0.2 cps, resulting in an excellent signal-to-noise ratio.  

In most cases, scientists do not want to analyze just traces of an element in a sample but a large number of elements at very different concentrations. Both ICP-SFMS and collision–reaction cell ICP-QMS offer the huge advantage of a large linear dynamic range of nine orders of magnitude, with simultaneous measurements using counting and analog detection modes. In ICP-SFMS, the optional addition of a Faraday cup in the detection system can increase the linear dynamic range from over 9 to over 12 orders of magnitude. 

The high acceleration voltage used in ICP-SFMS offers a significant advantage over collision–reaction cell ICP-QMS. Before the ions enter the detector, they hit a conversion dynode that is operated at 8 kV. The released electrons enter the detector and, as a consequence, cross-calibration becomes mass-independent. This represents an important benefit because a large dynamic range becomes very easily achievable and time-consuming cross-calibration procedures become redundant. Overall, up to 1000 times higher signal-to-noise ratios compared with collision–reaction cell ICP-QMS are generated, and even the lowest elemental concentrations down to single-digit parts-per-quadrillion levels are detected. 

Because both methods have their advantages and disadvantages, choosing which one to implement is highly dependent upon the specific application requirements. A few examples are presented to provide further clarification.  

Seawater is a very challenging matrix due to the high salt levels, which result not only in spectral interferences but also in physical matrix effects such as salt build-up on the interface and subsequent signal drift. To minimize the matrix introduction to the plasma, loop injection systems, very low sample flow rate, and a dilution, whether gas dilution or established liquid dilution, are required. Dilution helps prevent the build-up of matrix and improves long-term stability, but also compounds the fact that seawater already contains very low levels of trace elements. The main analytical challenge is to measure ultra trace concentrations in a complex matrix. Very carefully planned sample preparation and analytical procedures are also needed to avoid contamination issues, which are the major cause of inaccurate seawater analysis.  

ICP-QMS delivers typical limits of detection (LODs) at the parts-per-trillion or sub-part-per-trillion levels, while ICP-SFMS typically provides sub-part-per-trillion LODs across the analyte range. The difference in the LODs offered by the two methods is dependent on the mass response, isotope selection, and the degree of resolution that is required in each method. Both techniques are able to quantify elements in a seawater certified reference material (CRM) reliably, but ICP-SFMS tends to have a higher level of accuracy, demonstrating excellent agreement with certified values.  

In direct determination of organic solvents such as naphtha and hexane, the challenges are the high volatility of the solvents that are loaded in the plasma and the introduction of the matrix, where chemically compatible peristaltic pump tubing often can be a source of contamination. The preferred type of sample introduction in this case is self-aspiration, which also is challenged by the different viscosities of the solvents. For this type of application, collision–reaction cell ICP-QMS is preferred. However, if low LODs are required, then the same sample introduction approach also can be implemented on ICP-SFMS. 

A vacuum-loaded sample loop is used with sample delivery controlled by a syringe pump. The system provides a constant sample flow rate without the use of peristaltic pump tubing for contamination-free determination of naphtha and hexane. Calibration standards also are used for calibration purposes. Sample introduction considerations include the use of a low flow nebulizer, the addition of oxygen into the nebulizer flow to prevent carbon deposition in the cone, and the reactive oxygen in the plasma. In the direct hexane analysis, the collision–reaction cell effectively deals with the carbon-based spectral interferences and demonstrates stable performance over more than 4 h of analysis.  

ICP-MS can be coupled with high performance liquid chromatography (HPLC) to facilitate the analysis of different elemental species. HPLC–ICP-SFMS offers a significant sensitivity advantage over collision–reaction cell HPLC–ICP-QMS as well as enabling lower LODs. Collision–reaction cell HPLC–ICP-QMS, however, achieves more than sufficient LODs for speciation analysis in environmental, clinical, and biological matrices.  

The most popular gas chromatography (GC)–ICP-MS application is the separation of ethylated mercury and tin species. The chromatography is identical with the two methods, but for the same injection, ICP-SFMS delivers around six times the peak height. Sensitivity of the technique is also approximately 8–10 times higher, resulting in two- to fivefold lower LODs. With collision–reaction cell ICP-QMS, LODs are at the low parts-per-trillion level, whereas ICP-SFMS provides LODs at the sub-part-per-trillion level. 

ICP-MS is a powerful method for investigating elemental concentrations, performing high-precision isotope ratio determinations, and identifying the molecular composition and distribution of elements. However, the technique is associated with spectral interferences, negatively affecting the quality of results. Two methods have emerged to address this shortcoming. ICP-SFMS is the most sophisticated ICP-MS technique, delivering superior accuracy, transparency, and sensitivity while eliminating spectral interferences due to the high mass resolution capability. Collision–reaction cell ICP-QMS, on the other hand, covers a large linear dynamic range of nine orders of magnitude and has the power to suppress spectral interferences based upon cell processes. Both methods achieve a low background, resulting in excellent signal-to-noise ratio. When choosing which method to use, users should consider the specific application requirements with regard to sensitivity levels and LODs. The two methods deliver equally accurate and reliable results. Nevertheless, ICP-SFMS is preferred when maximum sensitivity and the lowest possible LODs are required. ICP-QMS, however, is ideal in higher throughput laboratory environments and is fit for purpose for a number of applications, with parts-per-trillion LODs routinely achievable. 

 are with Thermo Fisher Scientific, Bremen, Germany. 

 is with ALS Scandinavia, Luleå, Sweden.

Articles

Inductively coupled plasma–mass spectrometry (ICP-MS) is a mature method that offers reliable measurements across almost the entire periodic table. It has been established as the key methodology for investigating elemental concentrations, which play a central role in biological, environmental, chemical, and industrial processes. ICP-MS is capable of performing multielemental analyses in a single analytical run, achieving an overall productivity of more than 50 samples/h. The method also delivers lower detection limits compared to graphite furnace atomic absorption (GFAA) or inductively coupled plasma–optical emission spectrometry (ICP-OES).

Magnet or Cell? A Comparison of High-Resolution Sector Field ICP-MS and Collision–Reaction Cell Quadrupole ICP-MS

Inductively coupled plasma–mass spectrometry (ICP-MS) has been established as the most appropriate method for routine environmental and geological applications, owing to its extended elemental coverage, superior sensitivity, and rapid analytical capabilities. Nevertheless, the technique's association with spectroscopic interferences remains, despite the fact that in recent years the introduction of collision–reaction cell (CRC) instrumentation has solved this shortcoming. This article will discuss the ICP-MS method, its usability in environmental and geological analysis and relevant regulations, and will build the case for the CRC approach as the most efficient solution addressing the limitations of ICP-MS. An experiment illustrates the flexibility and power of collision cell technology–based ICP-MS to analyze a variety of common environmental and geological sample matrices.

The inductively coupled plasma–mass spectrometry (ICP-MS) technique is based upon 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 a plasma or gas consisting of ions, electrons, and neutral particles. Following dispersion into the stream of argon gas, samples are carried to the inductively coupled plasma, where their elements are atomized and ionized. 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 (

) and counted. The isotopes of the elements are identified by their 

. The intensity of a specific peak in the mass spectrum is proportional to the amount of that isotope in the original sample.  

ICP-MS in Environmental and Geological Analysis

ICP-MS is a popular method, routinely used in both standard and high-sensitivity modes for fast and accurate measurements of trace (parts-per-million to parts-per-billion) and ultratrace (parts-per-billion to parts-per-quadrillion) elemental concentrations in a wide range of geological and environmental applications. The increasing popularity of the technique is attributed to the important benefits that it offers, including its multielemental capability compared with conventional elemental techniques, high sensitivity, superior detection limits, increased dynamic range, and isotopic ratioing capabilities. ICP-MS can be used to analyze both solid and liquid samples, requiring only small sample quantities thanks to its analytical range from the parts-per-trillion to the parts-per-million regions.  

Used in conjunction with varying nebulization and calibration techniques, ICP-MS is applicable in both liquid and solid materials. This highly sensitive and powerful technique performs reliable trace element distribution analysis of geological materials; identification of ultratrace 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. Regardless of the sample type, it is important that it is free of particulate matter, as even the smallest particle can disrupt sample flow and block or clog the sample tips within the spray chamber. 

Having recognized 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. 

The U.S. Environmental Protection Agency (EPA) National Environmental Methods Index currently lists 11 approved ICP-MS methods by agencies within the United States. The most commonly used methods are those that have been developed by the EPA covering 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). In addition, the EPA has recognized ICP-MS as the only approved multielement method for determining arsenic and uranium in drinking water. The U.S. Department of Agriculture, Food Safety and Inspection Service, Office of Public Health Science has introduced Method CLG-TM3.01. This method specifies the use of ICP-MS for the analysis of cadmium (Cd) and lead (Pb) at parts-per-billion levels in beef, pork, and poultry muscle, liver, and kidney.  

Although very powerful and widely recognized by regulatory agencies, ICP-MS presents a few limitations, including spectroscopic interferences. 

Interferences are caused by polyatomic species that have similar mass to the target species and that stem from the sample matrix, Ar, or entrained gas. Solvents, reagents, glassware, and other sample processing hardware also can yield artifacts or interferences to sample analysis.  

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 

. 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 nebulization and transport processes as well as with ion-transmission efficiencies. Memory interferences or carryover occur when there are large concentration differences between samples or standards that are analyzed sequentially.  

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. 

The combination of ICP-MS and collision–reaction cell (CRC) instrumentation has demonstrated a unique capability of significantly reducing and even eliminating interferences.  

CRC instrumentation employs either passive collisional interactions in which no chemical reaction occurs, or reactive interactions in which the charge or structure of either the analyte or interferent are changed chemically. 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 analyzer. Reaction-based analyzers 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 consequently are either eliminated or modified. Additionally, reactive chemistry can modify analyte ions and analyze them at masses different from their natural isotope mass.  

Collision instruments can separate all kinds of overlapping molecular and polyatomic analyte ions from monoatomic ions if they differ in kinetic energy. This process is based upon collisions using inert gases like He and is called kinetic energy discrimination (KED). Collision instruments using KED have the advantage of being universally able to reject all polyatomic interferences in any matrix and are therefore well suited to multielemental analysis in complex or unknown matrices.  

Using ICP-MS in conjunction with state-of-the-art collision–reaction instrumentation can reduce spectral interferences efficiently and to negligible levels while 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 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. 

An experiment was developed to illustrate the flexibility and power of collision cell technology (CCT)-based ICP-MS to analyze a variety of common environmental and geological sample matrices. 

A Thermo Scientific XSERIES 2 ICP-MS system equipped with third-generation CCTED CRC instrumentation (Thermo Fisher Scientific, Bremen, Germany) was configured with an SC2 FAST automated sample introduction system (Elemental Scientific Inc., Omaha, Nebraska). The schematic in Figure 1 outlines the principle of the sample introduction system, which relies on a six-port switching valve to load and inject samples.  

This system significantly reduces sample uptake and washout times and dramatically decreases matrix load that reaches the plasma. The ICP-MS instrumental parameters are shown in Table I. A universal H

 in He gas mixture was used in the collision–reaction cell for the suppression of interferences.  

The experimental methodology performed to test the high-throughput setup for environmental samples was based upon conditions outlined in the Multi-Media, Multi-Concentration, Inorganic Analytical Service for Superfund Method ILM05.4. This method was introduced by the EPA within the Contract Laboratory Program (CLP) for water and soil/sediment environmental analysis.  

To improve the determination of analytes with a high ionization potential (such as Be, As, and Se), a stream of methane was added to the nebulizer gas. This carbon-loading enhances sensitivity for these analytes (Figure 2) and improves 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 (Table II) was performed for 23 analytes.  

A sample turnaround of approximately 83 s, including uptake, analysis, and wash, provided an overall batch acquisition time of 12 h for the entire 500 samples. Five samples (an interference check solution [ICSA], the same solution with a 20-ppb spike [ICSAB], NRC SLRS-3 river water reference material with varying spike concentrations, NIST 1643c reference material, and a detection limit check solution [CRI]) were classed as unknown samples and looped continuously throughout the experiment. The mean results for periodic analysis (every 20 unknowns) of the laboratory control standards (LCS) are shown in Table III.  

The stability of the results for samples SLRS-3 + spike and NIST 1643c are shown in Figure 3.  

Rock samples weighing 0.5 g were digested using an Aqua Regia mix (12.5 mL 30%) and diluted an additional 10 times before 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 s resulted in an overall acquisition time of 11 h for the 478 samples. A standard operating procedure similar to the CLP method was followed, and a well-characterized rock was used as an LCS in the QC protocol along with a continuing calibration verification (CCV) and a continuing calibration blank (CCB). Five samples (an ICSA, the same solution with a 20-ppb spike, a rock sample [GEOMS], GEOMS spiked with varying analyte concentrations, and a CRI) were classed as unknown samples and looped continuously throughout the experiment. Results for the determination of analytes in the unknown samples over the 11-h period are shown in Figure 4.  

When used with a universal gas mixture for all analytes, CCT-based ICP-MS has been demonstrated to be a multielemental technique with high-throughput capabilities. With methane addition, the analysis can benefit from an increase in sensitivity for analytes with a higher ionization potential, such as Be and Se, which are often at lower concentrations in environmental samples, as well as benefiting from an improvement in long-term stability. Throughput and stability are further improved and necessary maintenance is reduced thanks to the configured sample introduction system, which cuts uptake and washout time and introduces less matrix into the plasma over time. 

CCT-based ICP-MS is a proven multielemental technique, capable of performing fast and accurate analyses of an extended range of common environmental and geological sample matrices. Through the use of a segmented flow sample introduction system, matrix deposition on the ICP-MS interface is reduced significantly, leading to increased first-time pass QC analyses and longer periods between necessary maintenance. Spectral interferences associated with the traditional ICP-MS method are reduced considerably, and sample throughput is increased. 

Julian Wills, Shona McSheehy, Tomoko Oki,

 is with Thermo Fisher Scientific, Winsford, UK. 

The authors discuss the ICP-MS method, its usability in environmental and geological analysis and relevant regulations, and how to address its limitations.

Meike Hamester

Articles

Magnet or Cell? A Comparison of High-Resolution Sector Field ICP-MS and Collision–Reaction Cell Quadrupole ICP-MS

Complete Multielemental Analysis of Environmental and Geological Samples Using CCT-Based ICP-MS