Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry that uses a plasma source to ionize the sample. The plasma is supplied by electric currents that are produced by time-varying magnetic fields and atomize the sample to create ions, which are then detected.
The most common use of ICP-MS is for the detection of metals in liquid samples at, particularly low concentrations. Additionally, it can detect different isotopes of a given element, which makes it a powerful technique in isotopic labeling.
ICP-MS has greater sensitivity and speed when compared with atomic absorption spectroscopy. However, there are many interferences produced in ICP-MS when compared to other MS techniques such as the argon from the plasma, component gas leaks, and contamination from glassware. The ionized plasma, which is produced by heating the gas with an electromagnetic coil, results in a high concentration of ions and electrons to make the gas electrically conductive.
The advantage of using ICP-MS compared to other forms of mass spectrometry is that it can sample the analyte consecutively and continuously. In contrast, other methods such as Glow Discharge Mass Spectrometry (GDMS) and Thermal Ionization Mass Spectrometry (TIMS) require a two-step process involving vacuum chambers being sealed and pumped, and ions from the ionized sample sent to a mass analyzer.
In contrast, ICP-MS enables the sample to sit at atmospheric pressure. Ions created from argon plasma are transmitted through the mass analyzer for detection via the aid of electrostatic focusing techniques. This technique greatly improves sample throughput.
What is an inductively coupled plasma?
An inductively coupled plasma (ICP) produced for ICP-MS is contained in a torch that consists of three tubes. The end of this torch is placed inside an induction coil supplied with an electric current. A flow of argon gas is introduced to the torch and an electric spark is applied to introduce free electrons into the gas flow.
These electrons interact with the magnetic field produced by the electric current in the coil and the electrons are accelerated from one direction to another as the field changes. The accelerated electrons collide with argon atoms, which can cause some of the argon atoms to remove an electron.
The released electron is accelerated by the rapidly changing magnetic field. The process continues until the removal and recombination of electrons of the argon atoms are balanced. This produces a plasma of argon atoms at a very high temperature.
The ICP is retained in the torch because the flow of gas between the two outermost tubes keeps the plasma away from the walls of the torch. The second flow of argon gas keeps the plasma away from the end of the central tube.
The third flow of gas is introduced into the central tube of the torch. This gas flow passes through the center of the plasma, where samples are introduced as a mist of liquid to be analyzed.
For coupling to mass spectrometry, the ions from the plasma are extracted into the quadrupole of a mass spectrometer. The ions are separated based on their mass-to-charge ratio and a detector receives an ion signal relative to the concentration. The concentration of a sample can be determined through calibration with reference standards.
Improving Detection Limits in ICP-MS
Optimizing an ICP-MS to its highest sensitivity is not always going to provide the lowest detection limit. Minimizing and eliminating sources of potential contamination is key to achieving the lowest possible detection limits and accurate results.
Avoiding the use of certain glassware and flasks when preparing or storing solutions should be incorporated as good laboratory practice. This is because sample contamination could occur if some metals are leached out from the glass or adsorbed onto its surface.
Hence, all equipment before use must be thoroughly cleaned. Some cleaning techniques that are commonly practiced involves acid washing glassware for at least 24 hours to remove any potential contamination from metals. This is usually followed by a thorough cleaning with pure deionized water and leaving the clean containers filled with 1% v/v HNO3 until ready for use.
Assessing the standard deviation of a blank sample is a good method for monitoring sources of contamination and improving the detection limits. A lower standard deviation for the blank sample can be achieved by using a longer replicate measurement time.
Also, the lower the background equivalent concentration (BEC), the better the detection limits. This is usually achieved by using high-purity reagents and deionized water in all preparations of standards and samples in a dust-free clean room.
The counts for a blank solution should not exceed a few thousand. A higher blank count is often an indication of contamination. A low detection limit is typically attributed to low blank counts. Additionally, detection limits are also influenced by the sensitivity of a given isotope.
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- V. Thomsen, D. Schatzlein, and David Mercuro, “Limits of Detection in Spectroscopy”, Spectroscopy 18(12), 112 (2003).
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