Laser ablation imaging is a relatively new analytical technique that can visualize the distribution of elements of interest within solid samples in two or even three dimensions
A laser ablation (LA) system introduces the sample to an elemental analyzer, which is typically an inductively coupled plasma mass spectrometer (ICP-MS). With proper synchronization of the laser ablation system and the analyzer, element signals can be recorded at discrete positions on the sample in order to generate elemental images. LA-ICP-MS imaging is increasingly applied in geological (e.g., Ubide et al. 2015), biological (e.g., Becker et al. 2010), and medical studies (e.g., Hare et al. 2017).
How does laser ablation ICP-MS imaging work?
In practice, laser ablation ICP-MS imaging is performed by rastering a pulsed laser beam across the surface of a sample. The laser beam is stationary and the sample stage is moved in x, y, and z directions by high-precision motors that typically have stage reproducibility that is better than 1 µm. Deep-UV lasers (e.g., 193 nm) are commonly used for the ablation of solid samples. The laser spot size can be adjusted according to the required spatial resolution. Modern LA systems offer spot sizes down to 1 µm.
A laser pulse will ablate a sample if the laser has an energy density (fluence) that is above a certain threshold. Because this ablation threshold is sample-specific the laser fluence needs to be optimized for each sample material. The ablation process occurs within a sealed, air-tight chamber – the ablation cell. The sample aerosol that is produced by ablation is washed out of the ablation cell in a continuous flow of carrier gas (usually helium) for transport to the ICP. The sample aerosol is then atomized and ionized as it passes through the high-energy ICP. The resulting ions are transferred from the ICP to the mass analyzer via interface cones and primary beam optics. Energy filters and reaction/collision cells can be used to remove interfering species from the ion beam. The mass analyzer (quadrupole, sector field, or time-of-flight) measures the intensity of elements of interest. These signal intensities correspond to the abundance of these elements in the ablated sample. Elemental maps are generated by repeating this ablation-analysis process at known x, y, z coordinates on the sample surface (2 dimensional) or within the sample volume (3 dimensional).
Spot-resolved laser ablation imaging
Recently, there has been a trend to develop laser ablation cells with fast washout capabilities (Wang et al. 2013, VanMalderen et al. 2015, Gundlach-Graham and Günther 2016). With these fast-washout systems, the signal is transported out of the ablation cell within milliseconds, as opposed to seconds with conventional systems. Faster washout can be achieved for example by employing so-called dual-volume cells, by reducing the inner diameter of the carrier tubing, or by introducing additional gases (such as argon) during the aerosol transport. Faster washout times reduce the time required for analysis. Additionally, faster washout implies less dispersion of the ablation signal before it enters the ICP, and thus results in higher signal-to-noise ratios in the recorded MS data.
Efficient detection of the short sample pulse that a fast-washout ablation cell transmits to the ICP-MS requires use of a fast mass analyzer, particularly if one aims to analyze multiple elements within the pulse. Whereas scanning mass analyzers (e.g., quadrupole or sector field) measure individual elements sequentially, time-of-flight (TOF) mass analyzers, such as that used in TOFWERK’s icpTOF, measure all elements simultaneously (Borovinskaya et al. 2013, Hendriks et al. 2017). The icpTOF is able to record a complete mass spectrum every 33 microseconds, so that short transient signals, such as the aerosol plume of a single laser shot, can be measured with sufficient time resolution.
The combination of the icpTOF and fast-washout (low dispersion) laser ablation cells enables fast, spot-resolved, multi-elemental imaging (Burger et al. 2017, Bussweiler et al. 2017). In this approach, the image is acquired as a raster of side-by-side laser spots. The repetition rate at which the laser can be fired is limited by the duration of the signal from a single laser shot, in order to avoid signal overlap of neighboring spots. Because the washout time is sample-specific the repetition rate needs to be adjusted prior to each imaging experiment (See, for example, TOFpilot white paper). The laser scan speed (um/s) then simply becomes the product of the spot size (µm) and the repetition rate (s-1). This approach has significant advantages over the continuous-scan mode of imaging in which a continuous signal enters the ICP-MS. In spot-resolved imaging, each pixel represents a “closed experiment” and each laser shot produces one multi-element pixel in the image with clearly-defined coordinates. Thus, the original geometry of the sample surface is preserved and the risk of artifacts, such as smearing, is greatly reduced.
Further Reading
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.
