Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) is a major innovation in elemental microanalysis where unpleasant and rather contaminating wet-chemical digestion of solid samples is removed. In the laser pulse, aerosol particles of a small diameter are produced by ablating and vaporizing fine micro-volumes of the material using thermal energy, usually leaving craters with a diameter between sub-micrometers and hundreds of micrometers across.
Fundamental Principles and Instrumentation
The working system of LA-ICP-MS is established on the integrated work of the three fundamental elements, namely the laser ablation system, the ICP torch, and the mass spectrometer. A laser ablation system is composed of the laser source and an optical microscope to look at the sample and target it. Laser parameters (wavelength 193 nm ArF excimer is now standard to minimize fractionation), pulse duration (fs lasers are better than ns), fluence (energy density), and spot size are of critical interest to maximize ablation efficiency and minimize elemental fractionation, a phenomenon where the composition of the transported aerosol is not the same as the original solid sample.
Key Analytical Capacities and Performance
LA-ICP-MS can be characterized by outstanding capabilities of direct solid analysis. It has great detection limits, usually parts-per-billion (µg/g) to parts-per-trillion (ng/g) of many elements. comparable to solution-based ICP-MS, but without the dilution factor of digestion. Its greatest strength is its high spatial resolution, which can be as low as a micrometer with specialized installations, allowing analysis of individual mineral grains, inclusions, or cellular structures. The method has a maximum dynamic range of up to 9-10 orders of magnitude to allow one to determine major, minor, and trace elements.
Common Applications and Use Cases
The spatial resolution of elemental data has resulted in LA-ICP-MS becoming a foundation methodology in areas where spatially resolved data of an element are needed. In the petrogenetic studies of minerals and mapping of fluid inclusions, it is the most common in-situ U-Pb zircon geochronology method in the geological and mining sciences.
Advantages
The main benefit of LA-ICP-MS is that it can directly spatially resolve a solid sample with little or no sample preparation, and eliminates the risks of contamination, loss of analyte, and high acid blank of wet digestion. It involves minimal material and, therefore, is virtually non-destructive with larger samples and is well-suited to the analysis of valuable or unique objects. The method gives a quick analysis of high sensitivity throughout practically the entire periodic table.
Limitations
Quantitative accuracy can be a problem due to the absence of a universal, matrix-matched solid standard that can be used to calibrate. Polyatomic interferences of the sample matrix and gas species can also enter the technique and need high-resolution mass spectrometers to separate. Lastly, the very process of ablation is a microscopic-scale destruction, and analysis of data with complex, heterogeneous samples would demand both expertise and experience.
Conclusion
The ICP-MS laser ablation technique has essentially changed the bones of solid sample elemental analysis by offering a direct, sensitive, and spatially resolved analytical platform of high sensitivity. Its ability to conduct in-situ microanalysis of a wide variety of materials, including ancient rocks and modern biological tissues, with only a minimal preparation of the sample, has earned it an essential place in both basic research and industry.
