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Atomic spectroscopy is the technique for determining the elemental composition of an analyte by its electromagnetic or mass spectrum. Several atomic spectroscopy analytical techniques are available and selecting the most appropriate one is the key to achieving accurate, reliable, real-world results. This article focuses on ICP-MS.

Overview

An inductively coupled plasma mass spectrometer is an analytical tool capable of measuring very low levels of about 72 different elements. Most of these elements have limits of detection below the part-per-trillion range with some even in the parts-per-quadrillion range. Samples are introduced into an argon plasma as an aerosol. The plasma dries this aerosol, dissociates the molecules, and then removes an electron from the components. This forms singly charged ions which are then directed into the mass spectrometer (MS). The MS is effectively a filtering device and only one mass-to-charge ratio will be allowed to pass through at any given time. After passing through the MS the ions strike an electron multiplier detector. The impact of the ions releases an electron cascade, which is amplified until it becomes a measurable signal. The software compares the intensities of the measured signals to those from known calibrated standards to determine the concentration of the element. Any ICP-MS consists of the following components:Sample introduction system for ICP-MS
  • Sample introduction system – the overwhelming majority of samples introduced into an ICP-MS system are liquids (it is possible to introduce solid particles by ablating a sample with a high-performance laser. Laser ablation ICP-MS has a small range of specialist application areas and is not discussed in detail here). The liquid sample is usually transported by a peristaltic pump to a nebulizer that creates an aerosol of fine droplets that are introduced into the argon plasma. The type of nebulizer used will depend upon the viscosity, cleanliness and the matrix of the sample to be analysed.
Each type of nebulizer will optimally improve the introduction of specific sample types leading to the best performance of the ICP-MS. Once the fine droplets are created, they will pass through a spray chamber before entering the plasma. Again, the spray chamber chosen will be the most suitable for the sample and application. The aim of the nebuliser and spray chamber choice and configuration is to ensure that consistently uniform droplets form the aerosol, which then enters the torch. ICP torch and RF coil
  • ICP torch and RF coil – these combine to generate the argon plasma, which serves as the ion source of the ICP-MS. The plasma generated in the ICP-MS torch creates a zone of approximately 6000 °C, which is similar to the surface of the Sun.
The plasma is generated by passing argon gas through a series of concentric quartz tubes (the torch) that are enclosed at one end within a radio frequency (RF) coil. Energy is supplied to the coil by the RF generator, usually at a frequency of 27 MHz, and this couples with the argon to produce the plasma. By controlling and altering the power of the plasma it is possible to handle a wide variety of sample matrices including organic solvents. The aerosol droplets, which still contain the sample matrix and the elements to be determined, pass through this 6000oC zone and are first dried to a solid and then heated to a gas. The atoms then absorb more energy and will eventually release one electron to form singly charged ions. The singly charged ions then pass through the plasma and enter the interface region and vacuum system.
  • Interface – there are large temperature and pressure differences between the plasma and both the ion lens and MS parts of the ICP-MS. The interface region links these two separate areas of the ICP-MS together, and enables singly charged ions to continue their journey to the detector. The interface typically consists of two cones that allow a two-stage reduction in pressure, and the cones are made from platinum or nickel.
  • Vacuum system – the singly charged ions need to reach the detector without striking with any other gas molecules. To achieve this it is necessary to remove as many potentially obstructive gas molecules as possible by creating a vacuum system; this usually requires both a roughing pump and a turbomolecular pump. Modern ICP-MS systems now have turbo pumps that require very little maintenance.
  • Collision/reaction cell – the cell is designed to remove interferences that can degrade the potential detection limits that can be achieved. Most modern ICP-MS’s have a cell that can be used both in the collision cell and reaction cell modes.
Interferences in ICP-MS are caused when ions generated have the same mass-to-charge ratio that is identical to that of the analyte ion. Common sources are the plasma, the sample, or a combination of the two. Some common interferences are
  1. Iron at mass 56 being interfered by ArO
  2. Arsenic at mass 75 being interfered by ArCl
  3. Selenium at mass 80 being interfered by the ArAr dimer
In the case of iron at mass 56 the iCRC (collision/reaction) cell can introduce He gas into the cell, which is physically bigger than the iron ion. The interfering ion will collide more often than the Fe with the helium atoms and loses kinetic energy with each collision. There is an energy barrier at the cell exit and it is set to allow only higher-energy ions can pass. So, in this case the ArO interferent is removed. This is known as Kinetic Energy Discrimination. We can also operate in Reaction mode; here we use hydrogen in the cell and the hydrogen will react with the interfering ion causing it to increase its mass by 1 amu. This mass will not be allowed through the MS. The ArCl interference when measuring Ar at mass 75 can be removed by this approach, for example. If there is no interference (some masses don’t have interfering ions or the matrix is very clean) then the cell is, effectively, empty and we can run in a ‘no-gas’ mode. Most ICP-MS’s have the collision/reaction cell as a specific unit between the ion source and the MS. More efficient and powerful ICP-MS’s have the cell at the interface region where more gas can be introduced much more efficiently and quickly.
  • Ion optics – Different manufactures have different approaches and designs for their ion optics but they are all try to achieve the same result; that only the desired ions-of-interest pass into the quadrupole mass spectrometer, while ensuring that any remaining neutral species and photons don’t reach the MS.
The ion beam is bent through 900 by the ion optics and the unwanted neutrals and photons pass straight through and are removed by the vacuum system. One problem is that as the ion beam moves through the ICP-MS from the interface region to the MS it is continually diverging and less ions will get through to the detector. All designs try to compensate for this but the more effective create a 3D electrostatic field that can be very finely tuned and focussed for the masses being analysed within each sample. Mass spectrometer - ICP-MS
  • Mass spectrometer – this is a mass filter to sort ions by their mass-to-charge ratio (m/z) There are several mass spectrometers used in commercial ICP-MS systems: quadrupole, time-of-flight, and magnetic sector. Most laboratories choose an ICP-MS with a quadrupole mass spectrometer due to overall performance and economic value.
The quadrupole works by setting voltages and radio frequencies to only allow ions of a specific mass-to-charge ratio to remain stable within the rods and therefore pass through to the detector. Any ions that have different mass-to-charge ratios are unstable in the cell and are ejected. To cover the full mass range, the electronics rapidly change the conditions of the quadrupole to allow different mass-to-charge ratio ions to pass through. So, although an ICP-MS is, in fact, measuring sequentially mass after mass, the quadrupole is capable of scanning at a rate more than 5000 atomic mass units (amu) per second. This enables the ICP-MS to measure many different elements very quickly even though only one mass passes through the quad at any one time.
  • Detector – When the ions exit the MS they strike the active surface of the detector and generate a measurable electronic signal. This active surface (dynode) releases an electron every time an ion strikes it and starts an amplification process. This cascading of electrons continues until a measurable signal is recorded.
Some detectors use a dual mode, which includes both digital and analog modes, but it is now accepted that digital-only detectors are more efficient, reliable and have a longer lifetime. Detectors used in commercial instruments are capable of a wide dynamic range so ICP-MS instruments can still measure at both high concentration levels as well as extremely low levels.

Where are ICP-MS instruments used?

ICP-MS instruments are relatively expensive to purchase and run. Depending on the final configuration a typical system would be around £120,000 to buy and will cost a few thousand pounds a year to run. So, instruments are bought where there is either a regulatory requirement to measure very low levels of certain elements or a specific research interest for an element. Some of the larger markets for ICP-MS are
  • Environmental analysis of water or soil for compliance
  • Food or beverage analysis to protect consumers from harm
  • Pharmaceutical where it is required to measure both QC and contamination levels
  • Clinical analysis to determine elemental levels in the human body
Some of the more specific markets for ICP-MS are
  • Geological research for measuring very low elemental levels in rocks or ice cores
  • Semiconductor industry where the material being manufactured must be as pure as possible
  • Speciation of As, Se and Hg – key elements for human health
Since ICP-MS instruments measure specific isotopes of an element, the ratio of two or more isotopes can readily be determined. In isotope dilution, the sample is spiked with an enriched isotope of the element of interest. The enriched isotope acts as both a calibration standard and an internal standard. Because the enriched isotope has the same chemical and physical properties as the analyte element, it is the best possible internal standard. For this reason, isotope dilution is recognized as being the most accurate type of all analyses and is often used to certify standard reference materials. Isotope-ratio determinations are used in a variety of applications, including geological dating of rocks, nuclear applications, determining the source of a contaminant, and biological tracer studies. If very low levels of detection are needed or there is a large sample throughput required, then ICP-MS may be you best possible solution. For further information please use this link https://www.scimed.co.uk/product-category/icp-ms/