Introduction

Inductive Coupled Plasma Mass Spectrometry, better known as the ICP-MS, is a variation of mass spectrometer used to measure mass-to-charge ratio of ions (m/z). The resulting spectrum of the observation can be used to analyze the elemental composition and structure of a molecule.  The principle of ICP-MS follows a typical mass spectrometer, where the sample are turned into ions. The only difference is the use of inductively coupled plasma to turn the sample into ions. ^[1][2][3]

Components of ICP-MS

A simplified schematic of ICP-MS equipment is illustrated in Figure 1. It consists of 4 main components, which are the ICP torch, ion optics, mass analyzer and detectors.

Figure 1. Simplifed schematics of ICP-MS (Figure: O P Golim, adapted from^[2])

ICP Torch

The ICP torch is the component used to turns the sample into ionized form. Typically, the sample is introduced to the system in a liquid form, where it is then vaporized and turned into aerosol with the help of a nebulizer. The aerosol is then injected towards plasma torch where it is ionized and then passed into the chamber for detection. Figure 2 illustrates the simplified version of ICP torch used to ionize the samples. The plasma was generated in series of process known as the inductively coupled plasma (ICP) discharge as illustrated in Figure 3. It starts when the plasma forming gas (2), typically argon, is injected towards a strong electromagnetic field formed around the radio frequency (RF) powered coil (1). A free electron is then introduced through a spark to the electromagnetic field where it is accelerated (3) and hits the argon atoms. As a result the argon ionized and forms the plasma torch.^[1][4]

Figure 2. Simplified schematic of a ICP torch (Figure: O P Golim, adapted from^[4])

Figure 3. Plasma generation process (Figure: O P Golim, adapted from^[4])

The typical ion formation process from the liquid form can be described as desolvation, vaporization, atomization, and the ionization. An example of the reaction are as follow^[5]:

 M(H₂O)⁺x^- →  (MX)_n →  MX →  M → M⁺

Ion optics

The sample that has been ionized is then directed towards the mass detector through a component called the ion optics. The ion optics are composed of metal plates and cylinder with controllable electric charges. ^[3][6][7]

Mass analyzer / filter

Element that forms the sample are ionized altogether when it is introduced to the system through the ICP-Torch. Therefore, they have to be separated based on their mass to charge ratio before hitting the detector. The mass analyzer components is the core component of the mass spectrometer that serves the main function as a filter that allows a specific ions to be allowed into the detector.^[6][8] 

The two types of mass analyzer that is commonly used in an ICP-MS is the quadrupole mass filter and the magnetic sector mass analyzer. Their basic principle advantage and disadvantage are summarized in the following table:

Detector

The detector absorbs the ion that has been selected through the mass analyzer and converts it into electrical signal.^[6]

Advantage and Challenges

The advantage of using ICP-MS is their high precision to determine elements and their isotopes contained in a sample. It has a low detection capability that reaches to ppm or ppt resolution, dynamic range of detection, and covers most elements in the periodic table. However, the challenges of ICP-MS typically lie in the sample preparation and data analysis. ^[3][6]Conventional ICP-MS required the sample to be introduced in the liquid form, so that it can be turned into aerosol by the nebulizer. Solid state samples are usually dissolved in strong acid during the preparation. However, recent models of the ICP-MS allow the analysis of solid sample using laser ablation method. ^[10]

The data analysis is done by comparing the spectra obtained through the measurement with a known standard. However, the data obtained during the measurement often contains some interference when the solvents react with the argon plasma. Therefore, sometimes additional process is required to enable accurate data analysis. One method would be the desolvation process that reduces the amount of solvents that is introduced to the plasma. Another method is to replace the argon with helium, which is relatively more inert.^[5][6] However, the more widely used method to reduce the interferences is by using the system known as collision reaction cell (CRC) technology. The CRC is introduced between the ion optics and the mass analyzer as illustrated in Figure 6. It utilizes reaction gas such as hydrogen or helium to interacts with the ions that is generated by the plasma so that it will form noninterfering ions. Therefore, a more accurate analysis on the samples could be obtained. ^[1][6][11]

Figure 6. Simplified schematic of ICP-MS containing Collision Reaction Cell (Figure: O P Golim, adapted from^[11])

Applications

ICP-MS is a high precision equipment used in the quantification of isotope of environmental, geological, and biological samples ^[12]. For rapid measurement the ICP-MS can also be used in semi-quantitative application. Due to their precision and low detection capability, there are mostly used in analyzing trace elements. An example application for this would be the quality estimation of water and trace metals in wood samples. Figure 7 illustrates a spectra of ICP-MS for detecting mercury in wood samples, despite the low amount of mercury (0.56 mg/kg) it can still be observed in the spectra ^[13].

Figure 7. Example spectra of ICP-MS for detecting trace metal in wood samples (License: CC BY-NC-ND ^[13])

Reference