Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is one of the most powerful and sensitive analytical tools for determining trace and ultra-trace metals in a wide array of complex chemical and biological matrixes. This technique combines the fundamental principles of an ICP instrument for the atomization of analytes, followed by spectroscopy (MS) for detection and quantification.
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Since its introduction in the 80s, ICP-MS has evolved from a single element detector instrument to one that is capable of detecting in parallel an entire mass spectrum following the introduction of more sophisticated MS detectors. The evolution is all-encompassing, including the method of sample preparation. Conventional acid digestion techniques have been replaced by more sophisticated options such as laser ablation and microwave-assisted digestions.
Sample preparation is a critical aspect of ICP-MS analysis, depending on its application area. Microwave digestion is a common approach that has taken over conventional acid dilutions owing to its numerous advantages. This preparation technique requires fewer solvents, has a lower risk of contamination, and allows for more reproducible results and recovery of volatile elements. This digestion method offers huge advantages at the ultra-levels, especially in the pharmaceutical industry. Meanwhile, applying more sophisticated preparation techniques like laser ablation is important for elemental mapping and spatial resolution of solid samples.
The technique relies on the bombardment of analytes from an ICP source at ambient pressure. Because vacuum pressure is not required as with other ionization techniques, this feature allows for the continuous monitoring of a sample providing more information about the variations in the sample's composition over the test period.
Recent trends in ICP-MS analysis involve combining other techniques like electrophoresis and chromatography to detect samples at the molecular and biomolecular levels. By applying pairing separation techniques to prevent the destruction of the molecules during the ionization process, sample selectivity can be achieved. This approach is considered important in medical and pharmaceutical applications where measurement at the molecular and biomolecular levels is required.
Instrumentation and Operational Principles of ICP-MS
The operational principle of ICP-MS is based on its individual instrument components, an inductively charged plasma (ICP) and mass spectrometry (MS). Based on this, the major parts of the instrument include:
- Sample introduction system
- An ion source (ICP)
- Ion lens
- Mass Spectrometry
The liquid sample is introduced through the sampling surface, where it is nebulized into a fine aerosol that is transported to the argon plasma pre-heated at 6000-10000K. The sample is desolvated, vaporized, atomized, and ionized in this chamber. The resulting ions are transported into a set of electrostatic lenses (ion lenses) through the interface region. The ion lens, through the processes of refraction, focuses the ions on the mass analyzer, where it is separated accordingly based on the mass to charge ratio (m/z) in line with the principles of mass spectroscopy. The separated metal ions are measured at the detector, usually an electron multiplier (EM).
The dynode of the detector is negatively charged and is first struck by the positively charged ions. This results in a cascade of emission of ions in a process known as secondary emissions. The detector measures the culmination of signals from these cascading secondary emissions. However, the detector can measure signals from every strike at the dynode, which explains the high sensitivity of ICP-MS and its continuing significance and preference over other spectrophotometric techniques.
Applications of ICP-MS
Trace metals naturally occur at various concentration levels in the environment and in different chemical and biological matrixes. While there are many other spectroscopic methods for analyzing trace metals, ICP-MC continues to gain significant advantage owing to its characteristic high sensitivity and low detection. The technique is widely used in environmental monitoring of water quality, ambient air, and soil. It is also a top choice in pharmaceutical and medical analysis of trace and ultra-trace metals in drugs and biological samples.
In a study published in the journal Minerals, the authors determined the concentrations of trace metals (Mn, Fe, Ni, Cu, Zn, Co, Cd, and Pb) in seawater using a single quadrupole ICP-MS device. The authors compared the results between offline and online preconcentration setups to capture the effect of signals from polyatomic interferences to conclude that both configurations offer comparable detection limits over four years, thereby offering optimal sensitivity. However, on short-term analysis, the online setup showed marginally lower blank levels for metals.
In another study published in the Journal of Agricultural and Food Chemistry, ICP-MS was applied to analyze dietary supplements for selected trace metals. Microwave-assisted digestion was used to prepare 95 dietary supplements, and recovery experiments were done using certified reference materials to demonstrate the level of precision and accuracy of the equipment. The results were used to carry out a risk assessment to estimate the level of exposure from the trace metals from the consumption of herbal supplements.
Challenges and Recent Practices
ICP-MS is one of the most powerful analytical tools that continues to receive undivided attention, especially in the analysis of trace and ultra-trace metal analysis across different matrixes. However, despite its numerous advantages, applying this technique comes with many challenges. In certain cases, when the ionized form of the analytes of interest has the same atomic mass, the possibility of signal interference is a huge limitation. While this is expected, numerous measures are in place to mathematically account for these interferences using mathematical models.
Based on this, there is a correlation between the skill and technical know-how of the user and the outcome of the analysis in terms of accuracy and precision. Understanding the chemistry of sample preparation techniques and the operational principles, as well as the possibility of interference in the analytes of interest, is key. While acid digestion is the most common sample preparation technique, microwave-assisted digestion offers huge advantages. Most current practices involve laser ablation techniques, which are particularly convenient for solid samples.
In a study published in the Journal Spectroscopy Supplements, the imaging of trace elements using laser ablation-inductively coupled plasma-mass spectrometry was explored to unravel current trends, benefits, and challenges. The authors concluded that using LA-ICP-MS removes the challenges that come with digestion in terms of contamination and solvent consumption. It also allows for the isotopic mapping of trace elements at isotopic levels and offers improved sensitivity and less interference. However, a major challenge is that it is a destructive approach, particularly when applied to biological samples.
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References and Further Readings
Daniel, K., et al. (2020). Imaging of Trace Elements Using Laser Ablation – Inductively Coupled Plasma-Mass Spectrometry: Emerging New Applications. Spectroscopy Supplements, 35(S4), pp. 16-26. https://www.spectroscopyonline.com/view/imaging-of-trace-elements-using-laser-ablation-inductively-coupled-plasma-mass-spectrometry-emerging-new-applications
Dolan, S. P., Nortrup, D. A., Bolger, P. M., & Capar, S. G. (2003). Analysis of dietary supplements for arsenic, cadmium, mercury, and lead using inductively coupled plasma mass spectrometry. Journal of Agricultural and Food Chemistry, 51(5), 1307–1312. https://pubmed.ncbi.nlm.nih.gov/12590474/
Nageswara Rao, R., & Kumar Talluri, M. V. N. (2007). An overview of recent applications of inductively coupled plasma-mass spectrometry (ICP-MS) in determination of inorganic impurities in drugs and pharmaceuticals. Journal of Pharmaceutical and Biomedical Analysis, 43(1),1–13. doi:10.1016/j.jpba.2006.07.004. https://pubmed.ncbi.nlm.nih.gov/16891084/
Pröfrock, D., & Prange, A. (2012). Inductively Coupled Plasma–Mass Spectrometry (ICP-MS) for Quantitative Analysis in Environmental and Life Sciences: A Review of Challenges, Solutions, and Trends. Applied Spectroscopy, 66(8), 843–868. doi:10.1366/12-06681.https://journals.sagepub.com/doi/10.1366/12-06681
Wilschefski, S. C., & Baxter, M. R. (2019). Inductively Coupled Plasma Mass Spectrometry: Introduction to Analytical Aspects. The Clinical biochemist. Reviews, 40(3), 115–133.
Saumik, S. et al. (2021). Determination of Trace Metal (Mn, Fe, Ni, Cu, Zn, Co, Cd, and Pb) Concentrations in Seawater Using Single Quadrupole ICP-MS: A comparison between offline and online preconcentration Setups. Minerals,11, 1289.