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

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: Collision–reaction cell technology.

Spectral Interferences

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.

High-Resolution Sector Field ICP-MS

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.

Collision–Reaction Cell ICP-QMS

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

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.

Organic Solvents

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.

HPLC–ICP-MS for Speciation 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.

GC–ICP-MS

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.

Conclusion

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.

Meike Hamester and Shona McSheehy are with Thermo Fisher Scientific, Bremen, Germany. Ilia Rodushkin is with ALS Scandinavia, Luleå, Sweden.