MOOC: Instrumental analysis of cultural heritage objects
4.3. ICP-MS, LA-ICP-MS
In this lecture, an overview of the Inductively coupled plasma mass spectrometry (ICP-MS) and Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) will be made. These methods become the main workhorses for the analysis of trace element concentration and various isotopic ratios in a wide range of cultural heritage objects. Overview of the instruments’ working principle and limitations alongside main advantages and disadvantages of these methods will be discussed. Analysis of different materials will be explained, and practical tips and examples will be given.
1. General aspects of ICP-MS and LA-ICP-MS methods
In the following video general theoretical aspects of ICP-MS and LA-ICP-MS are discussed.
Inductively coupled plasma mass spectrometry (ICP-MS)
ICP-MS is an analytical technique that can be used for (ultra)trace element and isotopic ratio measurements. The method can be considered destructive because the sample must first be dissolved in an appropriate acid mixture before analysis. The choice of acids depends on the sample and the elements that need to be characterised but is usually HNO3 in combination with HCl and/ or HF.
A liquid sample is introduced into the spectrometer, where it is transformed into an aerosol and injected into an inductively coupled argon plasma. Inside the high-temperature plasma (10000 K), the aerosol dries, the solid residue evaporates, the molecules in the sample are broken apart, atomised, and the atoms are ionised. The formed ions are thereafter separated from one another by a mass spectrometer – a separation device that discriminates the ions based on their mass-to-charge ratio (m/z).
This analytical technique enables very low detection limits – for many elements a few ppt (parts-per-trillion, i.e. n 10−12 g of element per 1 g of sample) can be achieved. The determination of elemental composition by ICP-MS can however be complicated by numerous spectral interferences. These interferences can be corrected for or eliminated but the applicability of these correction/elimination methods can be instrument-specific. Although all types of ICP-MS spectrometers can be used for isotope ratio measurements, the accuracy of the obtained isotope ratios can range broadly, depending on the instrumentation. In some cases the results can be too inaccurate to produce usable data. As such, dedicated multi collector (MC-ICP-MS) spectrometers are commonly employed for isotope ratio measurements as they can provide adequate accuracy for most applications, in many case order or orders of magnitude more accurate than the ICP-MS designs used for routine analysis.
ICP-MS is increasingly used for the analysis of cultural heritage materials. Elemental composition and especially trace element concentrations, ratios and rare earth element (REE) patterns alongside isotope ratios (e.g. Pb and Sr isotope ratios) can be used to determine where the source material used for the production of cultural heritage study objects originates from and track human migration patterns. Trace elements and isotope ratios can be viewed as chemical fingerprints to determine where the metal to produce bronze artefacts was mined or where the humans from ancient burial sites were born.
In the following video, Dr Päärn Paiste introduces in the lab the ICP-MS technique and gives an overview of its principles.
Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)
LA-ICP-MS is a variety of ICP-MS analysis where a laser-based sample introduction system is coupled to an ICP-MS system. This enables in situ analysis of solid samples. A pulsed, high-energy laser beam with a diameter of a few tens to hundreds of micrometres is focused on the sample surface. Each laser impulse ablates a small portion of the sample, which is transported in a gas flow to the ICP-MS. By rastering the laser beam, it is possible to generate elemental and isotope ratio maps of the sample. The main limiting factor for the use of LA-ICP-MS is the availability of reference materials that have the same sample matrix as the samples. For accurate isotope ratio and quantitative elemental composition measurements, a calibration standard with matching composition to the sample must be used for the calibration of the system. If a suitable reference material is not available, it is still possible to produce relative elemental distribution maps. Although the LA-ICP-MS analysis results are less accurate than conventional ICP-MS results, LA-ICP-MS allows to characterize individual small areas in the sample compared to a bulk result from a dissolved sample.
More information on sample preparation and analysis can be found in the videos.
2. Analysis with ICP-MS method
In the following video, Dr Päärn Paiste introduces ICP-MS, sample preparation and how to perform measurement with this instrument.
3. Analysis with LA-ICP-MS method
In the following video, Dr Päärn Paiste introduces LA-ICP-MS, sample preparation and how to perform measurement with this method.
The slides used in the video can be downloaded from here: