Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

ICP-MS

As a robust, versatile and high-precision analytic method, mass spectrometry with inductively coupled plasma (ICP-MS) has earned its place in inorganic elemental analysis. Characterized not only by its superior detection limits, this analytic method is also hard to resist thanks to its rapid measurement rate, unique diversity of applications and its robust performance.

The ICP-MS analytic process has become established in a wide array of industries as a powerful method for detecting inorganic elements such as mercury, lead and cadmium. The palette of applications for trace element analysis is broad, ranging from applications in environmental analysis such as potable water quality monitoring, analysis of clinical samples, measurement of isotope ratios in food analysis, to research topics such as measurement of single cells and nanoparticles.

Thanks to continual technical optimization of the ICP-MS analytic method, users today can benefit from ever-improving detection limits at the same as increased sample throughput and falling operational expenses. Meanwhile, utilization of the noble gas argon has been successfully reduced, benefiting sustainability.

A journey through the history of ICP mass spectrometry

The relationship between mass, velocity and curvature of a cathode ray can be described mathematically and used for both qualitative and quantitative analysis of elements. This was the finding of British physicist and Nobel prize winner J.J. Thomson as early as the end of the 19th century. Yet it would take more than 80 years from its date of discovery before mass spectrometry with inductively coupled plasma would be employed in analytic chemistry for the first time. The first commercial ICP-MS device only came to market in 1983.

The main impediment to a market-ready ICP-MS product was the extreme technical difficulty of ionizing the sample. Only at the end of the seventies was it possible for the first time to combine inductively coupled plasma with a mass spectrometer. Aiding this was the parallel development of commercial ICP-OES analytic methods that used inductively coupled plasma in combination with optical emission spectrometry for elemental analysis.

While the ICP-OES technique quickly established itself after its appearance on the market and became the new standard for elemental analysis in many laboratories, the ICP-MS method had to wrestle with teething issues for many years: Besides the susceptibility to interference, long-term stability and sample throughput also left much to be desired. With the introduction of quadrupole technology, however, ICP-MS finally achieved a breakthrough in the semiconductor industry. The technology later become of great interest for environmental analysis thanks to newly developed reaction cells. Today, thanks to its short analysis times, multi-element analysis capability, falling operational costs and impressive detection sensitivity, this method has proven itself in elemental analysis.

ICP-MS: Structure and principle of operation in detail

The principle of operation of the ICP-MS method is based on mass spectrometry analysis of samples ionized in plasma. Achieving the desired detection limits while maintaining high long-term stability and sufficient robustness against interference requires a very specific equipment layout. The following components are essential in an ICP-MS analytic device:

• High-frequency generator: The task of the generator is to (inductively) create an alternating magnetic field within the plasma coil in order to couple with the energy in a flow of gas. This process supplies the necessary plasma. The resonant circuit of the generator is matched to the induction coil and typically operated with a frequency of 27.12 or 40.68 MHz.
• Plasma torch: The torch is a multi-walled quartz tube whose outer channel contains the noble gas argon. The open end of the plasma torch is surrounded by the plasma coil or induction coil, which is responsible for transferring the energy to the argon stream.
• Plasma coil: The plasma coil is part of the high-frequency generator. It conducts an oscillating current that generates an electromagnetic field. This field accelerates charged particles, thereby transmitting energy to the argon plasma. The noble gas argon is ionized in the process and heated to a temperature between roughly 6,800 and 10,000 K.
• Sample insertion: A thin injector tube made of quartz, corundum or sapphire is used to insert the sample. It injects the sample into the plasma with the help of additional argon, destroying chemical bonds in the sample and ionizing free atoms. The practical implementation of sample injection differs depending on the state of matter of the sample. With liquid samples, the fluid is atomized along with argon into a fine aerosol, then fed into the injector tube. Gaseous samples can generally be connected directly to the injection tube, while solids are first turned into gas and inserted with a stream of carrier gas.
• Interface: The interface must transport the ionized sample from the plasma and into the high-vacuum region of the mass spectrometer. At the same time, the interface is responsible for directing the ion beam to the ion optics, where extraction lenses collimate the beam. Two conical pinhole apertures, called "cones", are used to separate the vacuum areas.
• Interference management: In ICP-MS, interference is handled through the use of collision gases and/or reaction gases. In collision mode, polyatomic interferences are removed through reduction of kinetic energy or by dissociation. In reaction mode, interfering ions and polyatomic ions are converted into new, non-interfering products.
• Ion optics: The ion optics are a lens system that serves to focus the ion beam and direct it into the mass spectrometer.
• Mass filter: The mass filter usually takes a quadrupole form consisting of four rods arranged in parallel. These generate an alternating magnetic field that guides the ions along spiral trajectories. A major performance parameter of ICP-MS devices is the quadrupole scan rate. It expresses the speed of the measurement process. High-performance devices achieve scan speeds of over 5,000 amu/s.
• Detection system: At the outlet of the mass filter there is a detection system that generates a measurement signal proportional to the frequency of the detected ions. In practice it is typical to use secondary electron multipliers and Faraday detectors.

Figure 1 ICP-MS diagram (source: Analytik Jena)

Rapid technological advancement has successfully produced continual optimization of mass spectrometry with inductively coupled plasma. Today, high-performance devices like the PlasmaQuant MS Series are capable of analyzing extremely low element concentrations. Thanks to optimized plasma ionization sources, users also profit from significantly reduced plasma-related operating costs and a high matrix tolerance.

Today's modern and fully digital detection systems cover an extremely wide analytical range in pulse‑counting mode, allowing them to be used flexibly in multi-element analysis. All applications – from the ultratrace range all the way to high concentrations – can be covered in a single measurement.

Interference management as the key to long-term stability

One of the great technical challenges in ICP mass spectrometry is the presence of interference. Interference refers to disturbances that potentially distort the measurement result or even make the sample completely unreadable. The most common interference sources in ICP-MS include molecular (spectroscopic) noise that occur after the sample is atomized, for instance. One example is the formation of oxides. Interference management therefore focuses on targeted removal of these disturbances from the sample, thus ensuring the desired long-term stability even with challenging samples. One of the most important parameters is the degree of oxide formation, measured in % CeO+/Ce+.

Versatile applications of ICP-MS

The far-reaching advantages and flexible utilization of the ICP-MS analytic method have since caused many industries to put their trust in mass spectrometry with inductively coupled plasma. Here we wish to present one of the most important fields of application along with practical examples:

• Conservation: Thanks to the ICP-MS technique in environmental analysis, it is possible to detect even the slightest quantities of toxic elements in samples. Today, precise measurement of trace elements allows for early identification of hazardous environmental effects and targeted elimination of these. The ICP-MS technology helps preserve resources and prevent environmental contamination. Its range of applications includes everything from the measurement of toxic elements in filter dust to ultratrace measurement of mercury in natural waters.
• Food and agriculture: Production of agricultural commodities and foodstuffs is industrialized and automated in many industries today. All the more important, then, to constantly monitor the end products. ICP-MS makes an important contribution here, for example in supervising potable water quality or in measuring strontium isotope ratios in wine and grains.
• Geology, mining and metals: In the extraction of geological materials and in the processing of metals, the ICP-MS method reliably powers detection of even the slightest quantities of trace elements. For example, trace elements can be discovered in copper this way, or metal alloys analyzed with LA-ICP-MS.
• Pharma and life sciences:Through its use in the pharmaceutical industry, ICP-MS elemental analysis directly contributes to a high standard in the healthcare system and ultimately to a higher quality of life. For example, the method is used to detect traces of heavy metals in cannabis products, or measure iron isotope ratios in human blood.
• Oil and gas:The oil and gas industry sets stringent standards of purity and quality. One use of ICP-MS is to identify impurities in naphtha according to ASTM D8110-17.
• Chemistry and materials science: Testing toys for undesired trace elements and analyzing glass samples: These are only two of the numerous applications of the ICP-MS method in the chemical industry.
• The energy industry: One example of the use of ICP-MS analysis is in checking coolant quality to optimize the service life of high-efficiency power plants. Even low-level contamination of the coolant with sodium, calcium or magnesium can cause undesired side effects such as corrosion or deposits.

The areas of application of ICP mass spectrometry described here are just a few cases that exemplify the wide spectrum of applications in which this method is employed.