Laser ablation imaging, which is a comparatively new analytical method, has the capability to visualize the distribution of elements of interest inside solid samples in two or even three dimensions.
The sample is introduced into an elemental analyzer by a laser ablation (LA) system, where the analyzer is usually an inductively coupled plasma mass spectrometer (ICP-MS). Elemental images can be generated by recording element signals at discrete positions on the sample by ensuring proper synchronization of the analyzer and the laser ablation system. LA-ICP-MS imaging is largely employed in biological (for example, Becker et al. 2010), geological (for example, Ubide et al. 2015), and medical studies (for example, Hare et al. 2017).
How Does Laser Ablation ICP-MS Imaging Work?
On the practical level, laser ablation ICP-MS imaging is carried out by rastering a pulsed laser beam across a sample’s surface. High-precision motors with a typical stage reproducibility better than 1 µm are used for moving the sample stage in x, y, and z directions, and the laser beam is stationary. Ablation of solid samples is commonly performed using deep-UV lasers (for instance, 193 nm). The laser spot size can be adjusted based on the needed spatial resolution. The spot sizes offered by modern LA systems are down to 1 µm.
A sample will be ablated by a laser pulse if the energy density (fluence) of the laser is above a specific threshold. Since the ablation threshold is sample-specific, it is necessary for the laser fluence to be optimized for each sample material. The ablation process takes place inside an air-tight, sealed chamber — the ablation cell. A continuous flow of carrier gas (usually helium) washes out the sample aerosol generated by ablation out of the ablation cell, for subsequent transport to the ICP.
Then, the sample aerosol is atomized and ionized when it passes through the high-energy ICP. The ensuing ions are transferred from the ICP to the mass analyzer through primary beam optics and interface cones. Interfering species can be removed from the ion beam by using reaction/collision cells and energy filters. The intensity of the elements of interest is measured by the mass analyzer (sector field, quadrupole, or time-of-flight). These signal intensities are dependent on the abundance of these elements in the ablated sample. Elemental maps are produced by repeating this ablation-analysis process at familiar x, y, z coordinates on the sample surface (two-dimensional) or inside the sample volume (three-dimensional).
Spot-Resolved Laser Ablation Imaging
In recent times, there has been a trend to create laser ablation cells with fast washout capabilities (Wang et al. 2013, VanMalderen et al. 2015, Gundlach-Graham and Günther 2016). These fast-washout systems ensure that the signal is transported out of the ablation cell in a few milliseconds, in contrast to a few seconds taken by traditional systems. Faster washout can be realized, for instance, by using the well-known dual-volume cells, by minimizing the carrier tubing’s inner diameter, or by introducing additional gases (like argon) at the time of the aerosol transport. Faster washout times minimize the time needed for analysis. Moreover, faster washout relates to lesser dispersion of the ablation signal before it reaches the ICP, thereby leading to higher signal-to-noise ratios in the recorded MS data.
In order to efficiently detect the short sample pulse transmitted by a fast-washout ablation cell to the ICP-MS, it is necessary to use a fast mass analyzer, specifically if one intends to analyze multiple elements in a single pulse. In contrast to, scanning mass analyzers (for instance, quadrupole or sector field) that sequentially measure individual elements, time-of-flight (TOF) mass analyzers, for example, those used in TOFWERK’s icpTOF, simultaneously measure all elements (Borovinskaya et al. 2013, Hendriks et al. 2017).
The icpTOF has the capability to record a comprehensive mass spectrum every 33 μs, such that short transient signals, like the aerosol plume of a single laser shot, can be measured with adequate time resolution.
The combination of the fast-washout (low dispersion) and icpTOF laser ablation cells allows spot-resolved, fast, multi-elemental imaging (Burger et al. 2017, Bussweiler et al. 2017). This method involves acquiring the image as a raster of side-by-side laser spots. The duration of the signal from a single laser shot limits the repetition rate at which the laser can be fired, thereby preventing signal overlap of neighboring spots. Since the washout time is sample-specific, the repetition rate must be adjusted before each imaging experiment.
Then, the laser scan speed (µm/second) simply turns out to be the product of the spot size (µm) and the repetition rate (s-1). This method offers considerable benefits over the continuous-scan mode of imaging where a continuous signal enters the ICP-MS. In the case of spot-resolved imaging, a “closed experiment” is represented by each pixel and each laser shot generated a multi-element pixel in the image with clearly defined coordinates. This preserves the sample surface’s original geometry and substantially reduces the risk of artifacts, for example, smearing.
Becker, J.S., Zoriy, M., Matusch, A., Wu, B., Salber, D., Palm, C. and Becker, J.S., 2010. Bioimaging of metals by laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). Mass spectrometry reviews, 29(1), pp.156-175.
Borovinskaya, O., Hattendorf, B., Tanner, M., Gschwind, S. and Günther, D., 2013. A prototype of a new inductively coupled plasma time-of-flight mass spectrometer providing temporally resolved, multi-element detection of short signals generated by single particles and droplets. Journal of Analytical Atomic Spectrometry, 28(2), pp.226-233.
Burger, M., Schwarz, G., Gundlach-Graham, A., Käser, D., Hattendorf, B. and Günther, D., 2017. Capabilities of laser ablation inductively coupled plasma time-of-flight mass spectrometry. Journal of Analytical Atomic Spectrometry, 32(10), pp.1946-1959.
Bussweiler, Y., Borovinskaya, O. and Tanner, M., 2017. Laser Ablation and inductively coupled plasma-time-of-flight mass spectrometry-A powerful combination for high-speed multielemental imaging on the micrometer scale. Spectroscopy (Santa Monica), 32(5), pp.14-20.
Bussweiler, Y., Spetzler, T., Tanner, M. and Borovinskaya, O., 2017. TOFpilot–An Integrated Control Software for the icpTOF that Enables High-Speed, High-Resolution, Multi-Element Laser Ablation Imaging in Real Time.
Gundlach-Graham, A. and Günther, D., 2016. Toward faster and higher resolution LA–ICPMS imaging: on the co-evolution of LA cell design and ICPMS instrumentation. Analytical and bioanalytical chemistry, 408(11), pp.2687-2695.
Hare, D.J., Kysenius, K., Paul, B., Knauer, B., Hutchinson, R.W., O’Connor, C., Fryer, F., Hennessey, T.P., Bush, A.I., Crouch, P.J. and Doble, P.A., 2017. Imaging Metals in Brain Tissue by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). Journal of visualized experiments: JoVE, (119).
Hendriks, L., Gundlach-Graham, A., Hattendorf, B. and Günther, D., 2017. Characterization of a new ICP-TOFMS instrument with continuous and discrete introduction of solutions. Journal of Analytical Atomic Spectrometry, 32(3), pp.548-561.
Ubide, T., McKenna, C.A., Chew, D.M. and Kamber, B.S., 2015. High-resolution LA-ICP-MS trace element mapping of igneous minerals: In search of magma histories. Chemical Geology, 409, pp.157-168.
Van Malderen, S.J., van Elteren, J.T. and Vanhaecke, F., 2015. Development of a fast laser ablation-inductively coupled plasma-mass spectrometry cell for sub-μm scanning of layered materials. Journal of Analytical Atomic Spectrometry, 30(1), pp.119-125.
Wang, H.A., Grolimund, D., Giesen, C., Borca, C.N., Shaw-Stewart, J.R., Bodenmiller, B. and Günther, D., 2013. Fast chemical imaging at high spatial resolution by laser ablation inductively coupled plasma mass spectrometry. Analytical chemistry, 85(21), pp.10107-10116.
This information has been sourced, reviewed and adapted from materials provided by TOFWERK.
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