Wavelength Tech Forum: ICP and ICP-MS

January 31, 2007

This month's Technology Forum looks at the topic of ICP and ICP-MS and the trends and issues surrounding it. Joining us for this discussion are Jerry Dulude, with Glass Expansion, Shane Elliott, with Varian, Inc., Steve Wilbur, with Agilent Technologies, and Ken Neubauer, with PerkinElmer.

This month’s Technology Forum looks at the topic of ICP and ICP-MS and the trends and issues surrounding it. Joining us for this discussion are Jerry Dulude, with Glass Expansion, Shane Elliott, with Varian, Inc., Steve Wilbur, with Agilent Technologies, and Ken Neubauer, with PerkinElmer.

What is the major appeal of the ICP and ICP-MS techniques?

(Dulude) As someone who has utilized a number of techniques for elemental quantification, I believe that it is the multielement capability of both ICP-OES and ICP-MS that is largely responsible for their surge in popularity. Until the advent of these techniques, flame atomic absorption spectrometry (FAAS) and, later, graphite furnace atomic absorption spectrometry (GFAAS) were the most popular techniques, both using one element at a time detection.

(Elliott) Both ICP-OES and ICP-MS offer multi-element capabilities, unattended operation and high sample throughput. As the techniques have become widely adopted and developed, the ease of use and reliability of both techniques has improved – it’s time to embrace and welcome the new workhorses of the modern trace element laboratory. The most obvious difference between the capabilities of these techniques is detection limits, with ICP-MS generally offering around 1000 times lower detection limits than ICP-OES.Having stated the obvious, it is also worth digging a little deeper… ICP-MS is a sequential technique, whereas modern ICP-OES instruments can be truly simultaneous instruments. This means that simultaneous ICP-OES is faster than ICP-MS, and with the aid of productivity enhancing sample introduction options many customers are routinely determining upwards of 30 elements in thousands of samples per day on their ICP-OES instruments – it is challenging to achieve even half that throughput using ICP-MS.The other major difference between these modern workhorses is in their ability to run ‘challenging’ samples. The rule of thumb for ICP-MS is that the dissolved solids levels in samples should be limited to 0.2% total dissolved solids (although some ‘easy’ matrices such as NaCl can be run at much higher levels). Above this limit, physical interferences such as coating of the interface cones and signal suppression in the ion beam begin to take hold, increasing maintenance requirements and reducing the reliability and quality of the data produced. In comparison, ICP-OES can routinely handle upwards of 30% total dissolved solids – more than 100 times higher than the limit in ICP-MS. If the quantification limits required are achievable using ICP-OES without having to dilute or perform further sample preparation, then ICP-OES will be the faster and more cost effective solution. If the sample is relatively ‘clean’ and does not require dilution or extensive sample preparation, ICP-MS offers much lower limits of quantification over ICP-OES.So who wins – ICP-OES or ICP-MS? I have to call it a tie – a dead heat. Each has their place depending on the required limits of detection, sample throughput, and sample preparation.

(Wilbur) ICP and ICP-MS share the common attributes of rapid, sensitive multi-element analysis in a variety of liquid (or gaseous) matrices. Coupled to Laser Ablation, they can be used to directly measure solid samples as well. Apart from these similarities, which result from the shared use of an argon plasma to atomize and excite or ionize the sample components, ICP and ICP-MS each have individual strengths as well. ICP (ICP optical emission spectroscopy) has a well-deserved reputation as a simple, robust technique capable of measuring many elements down to the parts per billion range in extremely dirty samples containing matrix components as high as several percent. ICP-MS, on the other hand, while possessing somewhat lower matrix tolerance, is noted for its extreme sensitivity - sub-ppt for many elements. Furthermore, since the spectra generated by ICP-MS are fairly simple, the potential for interferences is lower. As a mass spectrometric technique, ICP-MS is also capable of isotope dilution quantification, known for its unrivaled accuracy and ability to compensate sample preparation losses and for matrix effects. ICP-MS is also capable of semiquantitative analysis, where the concentration of any measurable analyte(s) can be estimated with fairly good accuracy from the response of one or a few internal standard elements.

(Neubauer) The major appeal of ICP-OES and ICP-MS is their ability to measure a wide variety of elements at very low and very high levels: ng/L to mg/L. ICP-OES is well-understood and best for samples which contain very high matrix levels or where ng/L measurements are not required. ICP-MS excels at measuring extremely low levels.

In which industries is the use of ICP and ICP-MS most popular? Why?

(Dulude) We see a distinction between these two techniques in terms of which industries use them the most. For ICP-OES, in-house quality assurance labs represent a large share. However, these labs are in very diverse industries ranging from metals, chemicals, and petrochemicals to hardware, pharmaceuticals, and foods. The reasons for this are clear; ICP-OES provides ample detection limits and high productivity. Of course, ICP-OES remains in integral tool in the environmental service laboratory as well. For ICP-MS, we see the major share divided between academic and environmental labs. The reasons for this are quite disparate. Academic labs capitalize on the capability of the ICP-MS to achieve ultra low detection limits and to distinguish between isotopes. Environmental labs enjoy the ultra trace capability and are motivated for regulatory reasons. The semiconductor industry and clinical laboratories are other areas where ICP-MS is enjoying growth.

(Elliott) The largest user of both ICP-OES and ICP-MS remains the environmental industry. The growth of the environmental industry around the world, and the move towards consolidating smaller laboratories into larger more efficient laboratories has favored the growth of these high throughput multi-element techniques. The technique of choice for this industry is now ICP-MS – spurred by the ever-decreasing regulated trace element limits around the world in samples such as drinking water. In many regions around the world, the regulated limits for trace elements in drinking water (a relatively clean sample) have favored ICP-MS over ICP-OES. But not all environmental samples are as clean and easy as drinking water… for many laboratories, digested soils, sludges and waste waters still find their way to the ICP-OES lab.The Chemical and Petrochemical industries are also large users of these techniques – in this case, due to the challenging nature of the samples, ICP-OES remains the technique of choice. In most cases an ICP-OES can run the samples that are common to this industry neat (without any sample preparation) or with a simple dilution. Other industries of note that use ICP-OES and ICP-MS include the food and agriculture industry, the semiconductor industry, and the metals and mining industry.

(Wilbur) Both techniques are widely used in many industries where the rapid simultaneous measurement of metals and other elements is required. These industries include the environmental testing, clinical, geological and chemical industries to name a few. ICP-MS, due to its superior sensitivity is widely used for ultra-trace analysis of reagents and devices in the semiconductor manufacturing industry as well. The general trend in most industries is a shift towards the sensitivity of ICP-MS as regulatory levels decrease.

(Neubauer) ICP-OES and ICP-MS are both used in a wide variety of industries, including environmental, clinical, semiconductor, geological, food, metallurgical, and hydrocarbon processing. ICP-MS is usually the preferred technique in the clinical and semiconductor industries because of the low measurement levels required (ng/L) and is moving into the environmental industry as regulations push analytical requirements lower. Because of its simplicity, sample handling ability, and good detection limits, ICP-OES remains the technique of choice in the environmental, clinical, geological, food, metals, and hydrocarbon processing industries. However, in several of these industries, the techniques are complementary: ICP-OES serves as a screening tool for samples and only those which require ultra-low level measurements are analyzed with ICP-MS.

What are some recent significant developments in ICP and ICP-MS?

(Dulude) ICP-OES is a mature technique as evidenced by the historical decline in price and the emphasis on productivity and ease of use (software interface). Probably the most recent significant advance has been the solid state detector, an advance in which nearly every manufacturer participates. The advantages of this development all stem from the simultaneity of detection. Simultaneous detection of analytes increases productivity. Simultaneous detection of signal and background emission enhances precision. Simultaneous measurement of analyte and internal standard enhances accuracy. ICP-MS is less mature but many have said that its infancy is past. Today’s instruments are rugged and have a sophisticated user interface. The development of reaction and collision cells on the front end of the detector has led to a dramatic reduction in isobaric interferences in quadropole systems. Although they command a higher price, multi-collector and magnetic sector systems have achieved ultra low detection limits and resolution high enough to obviate the need for reaction cells. Developments on the front end of these systems include sample concentration and desolvation and interfaces to LC and GC systems.

(Elliott) Recent developments in the more mature ICP-OES field have focused on one of the true strengths of the technique – speed and sample throughput. By using more efficient means of sample introduction such as on-line switching valves, sample uptake and washout times have been significantly reduced. Coupled with a truly simultaneous ICP-OES, the throughput of these systems make them invaluable to high throughput contract labs and industries requiring fast sample turn around alike.Although improvements in sample throughput in ICP-MS have also been achieved, development in ICP-MS has recently focused on improving some of the limitations of the technique. In particular, in reducing or eliminating spectral interferences. Through the development of collision/reaction interfaces, collision cells and reaction cells, some of the troublesome interferences that have plagued ICP-MS for many years are being managed in such a way as to improve the accuracy and detection capabilities of the technique.

(Wilbur) ICP technology is more mature than ICP-MS and there has been little recent technical development though one manufacturer has introduced a small footprint instrument. In ICP-MS, the development and refinement of collision/reaction cell (CRC) technology has broadened the application range of ICP-MS. CRC technology uses interactions between the analyte ion beam and a gas or mixture of gases within a small collision cell located between the ICP interface and the mass analyzer to reduce or eliminate polyatomic interferences. Eliminating polyatomic interferences provides numerous advantages including; significantly better detection limits for interfered analytes, analyte confirmation by isotope ratio measurement, and generally improved analytical confidence in complex matrices. 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. Passive devices use an inert collision gas (helium) to preferentially reduce the kinetic energy of the polyatomic interferent which is then prevented from entering the quadrupole analyzer due to its low kinetic energy. Reaction based devices use a range of reactive gases including hydrogen, methane, ammonia and others to chemically shift one member of the analyte-interferent pair to another mass. Passive cells using kinetic energy discrimination have the advantage of being universally able to reject any polyatomic interference in any matrix and are therefore well suited to multi-element analysis in complex or unknown matrices.

(Neubauer) Productivity and workflow improvements are driving developments in both ICP-OES and ICP-MS: users want their instruments to efficiently produce reliable results. These techniques are also benefiting from developments in the sample introduction area, such as new nebulizers and autosamplers. Cell technology in ICP-MS is leading to further reduction of interferences and lower detection limit boundaries through fundamental understanding of reaction chemistry.

What further advancements do you see in ICP and ICP-MS?

(Dulude) I believe that many of the future advancements will improve instrument utility. For example, we have already begun to see commercial instruments with smaller footprints. Self-optimization and built-in diagnostics are just a few of the software developments that will facilitate successful operation. Productivity enhancement is another area of interest.

(Elliott) There is always room for improvement – and for both of these techniques (ICP-MS in particular) there is room for improvement in the ease of use and robustness of the instruments and software. The interference management techniques mentioned above have clear benefits, but some also come at the expense of speed and ease of use. This is an area where we will see advancements in the future.

(Wilbur) The current hot topics in elemental analysis are related to the use of hyphenated techniques for elemental speciation. The techniques employ the use of a separation technique such as gas or liquid chromatography, or capillary electrophoresis coupled to ICP-MS to resolve the elemental components of a sample into their species. As this technology continues to be refined, it is finding applicability in life sciences applications, giving birth to the new science of metallomics. Metallomics is the science of understanding the role of metals in biological and ecological systems. Metals in these systems are almost always associated with more complex molecules and it is this association that defines their characteristics and roles.

(Neubauer) The sensitivity of ICP-MS has lead to a new role: use as a metal-specific detector for chromatography separation techniques, such as HPLC and GC. This capability offers significant new opportunities in both the clinical and environmental fields by detection of molecular metal-containing species. There is an opportunity for real-time on-line measurements to support industry production and process stream monitoring. ICP-OES is particularly well suited for this and will benefit from continued developments in the sample introduction area.

What application areas for ICP and ICP-MS remain unexplored?

(Dulude) Much work is currently being done in speciation using the interface of a separation instrument (LC or GC) with an ICP-MS. This work enhances the utility of the technology to industries such as those involved in pharmaceuticals, health research, and environmental hazard assessment. As the cost of ICP-OES instrumentation continues to fall and the operation becomes more fool proof, I would expect this technology to replace more flame AAS systems which are still widely used particularly in less developed countries. Agricultural and minerals are two applications where this evolution is more likely to occur. ICP-OES is already routinely used to monitor used engine oil for large engines as a service diagnostic. I would expect this to extend into consumer automobile diagnostics within the next decade.

(Elliott) I think that there has been some ‘exploration’ in a vast number of fields using these techniques. Improvements in efficiency by taking the lab to the sample may be one area that is largely unexplored, along with expansions into the biotech and life science research fields.

(Wilbur) This is a difficult question, since, if we knew what they were, we would be exploring them. However, there are some areas, where current limitations in the technology restrict their applicability. One such area that is just being investigated is the area of nano sampling, such as the analysis of the contents of a single cell for metal containing components. Additionally, ICP and ICP-MS instruments, due to their relatively large size and significant power and argon requirements have also not found their way into space or extremely remote locations on earth.

(Neubauer) Speciation is an application area that will be a huge opportunity once governmental regulations specify these types of results. These instruments also have a role in real-time on-line measurements in process and waste streams, although they currently have limited applications. These restrictions mostly result from limitations in software design: enabling on-line measurements will require software that allows a greater degree of customization than currently available. The advent of powerful, next generation computers will open possibilities of solving analytical questions in new ways that will further strengthen the analytical capabilities of ICP-OES and ICP-MS.

What do you think?

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