Previous LA-ICP-MS Method Development on Garnet Mapping
October 28, 2022
Woodhead et al. (2007) proposed the use of LA-ICP-MS for isotopic and elemental imaging of geologic materials. Importantly, they noted that ablation cell designs must be optimized to promote rapid system response, in contrast to the signal-smoothing that is often preferred for simple spot analyses, and that experimental parameters must be carefully evaluated to avoid the blurring effects of re-sampling phenomena. In 2016, Petrelli et al. reported on their progress regarding trace element determinations and potential for 2D elemental imaging. Their trace element configuration at 40 microns spot size produced improved accuracy and precision. Paton et al. (2011) published the Iolite software for the visualization and processing of mass spectrometric data, which has been utilized as an initial step towards processing 2D elemental images after acquisition (Raimondo et al., 2017).
One of the first papers detailed method development of trace element mapping by LA-ICP-MS in garnets was written by Raimondo et al. (2017) and details analytical techniques and post-acquisition data processing of 33 elements in garnets from a shear zone in central Australia. In this study, they acquired LA-ICP-MS trace element maps using a Resonetics M-50-LR 193 nm excimer laser coupled to an Agilent 7700x Quadrupole ICP-MS housed at Adelaide Microscopy, University of Adelaide. Instrument conditions and mapping protocols were employed from previously described work (e.g. Lockington et al., 2014; George et al., 2015), and similar laser ablation imaging had been performed on other silicates in other studies (e.g., Petrelli et al., 2016), but they detailed a protocol specific to trace element analysis of garnet.
In their study on trace elements in sphalerite, Lockington et al. (2014) analyzed 23 trace and minor isotopes. They performed ablation using a consistent spot diameter of 30 B, a 5 Hz laser pulse rate and 80% power level. George et al. (2015) analyzed 24 trace and minor isotopes in galena. Imaging was performed by ablating sets of parallel line rasters in a grid across the sample with a consistent laser beam size (9 µm) and 10 µm/s scan speed. These analytical parameters were chosen to give the desired sensitivity of the elements of interest and adequate spatial resolution for their study. The spacing between the lines was kept at a constant 9 µm to match the size of the laser spot. A laser repetition of 10 Hz was selected at a constant energy output of 80 mJ, resulting in an energy density of ~4 J/cm2 at the target. The dwell time was 0.01 s for all elements. A 30 s background acquisition was acquired at the start of every raster, and a delay of 20 s followed each line to allow for cell wash-out, gas stabilization, and computer processing time
In the protocol of Raimondo et al. (2017), garnet mapping was performed on standard 30 µm -thick polished thin sections, and imaging was achieved by ablating a series of parallel rasters across the sample surface to form a square or rectangular grid. Pre-ablation of each raster was completed to minimize the effect of redeposition, and 30 s of background measurement was also acquired prior to each scan. Ablation was preformed in an atmosphere of UHP He (0.70 L min-1), mixed with Ar (0.93 L min-1) immediately after the ablation cell. A beam diameter of 16 µm coupled with a laser repetition rate of 10 Hz produced an energy density of ~7 J cm-2 at the target, and a scan speed of 22 µm s-1 and line spacing of 16 µms were employed. High fluence ensured that count rates were sufficient at the small spot sizes required for optimal spatial resolution, and trench depths were maintained at <5 µms given the scan speeds involved. Each analysis was run as a single continuous experiment and comprised a suite of 33 elements. Dwell times for major elements were 0.005 s (to reduce excessive count rates) and all other masses were 0.008 s, giving a total sweep time of 0.31 s. Standards were analyzed in duplicate at the start and end of each mapping run. The total run times were roughly comparable to the duration of typical EPMA X-ray maps of equivalent size. They performed post-acquisition processing using the software Iolite (Woodhead et al. 2007) and XMapTools 2.3.1 (Lanari et al., 2014).
Though their mapping efforts were largely successful, Raimondo et al. (2017) noted two main analytical challenges that can affect the quality of data acquired by their protocol. The first shortcoming is the lateral smearing of features in the direction of scanning. This can exaggerate the size of inclusions, grain boundaries, or intragrain domains and can blur interfaces characterized by sharp changes in abundance. Signal smearing is more pronounced in low abundance trace elements than in the major elements. The second problem is the compromise involved in pixel creation. There is competition between the increased mass range of the LA-ICP-MS technique and the time required for sequential acquisition of large element suites. Unlike EPMA X-ray maps, all elements are not measured at the same time. This means that successive masses do not share precisely the same spatial reference, and the extent of signal decoupling is determined by the cumulative dwell time. This means that if more masses are added to the sweep or dwell times are increased to improve total counts, the extent of decoupling becomes greater. This problem involves a balancing act between these parameters with respect to the spatial resolution required for the analysis.