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High-Speed Laser Ablation ICP-TOFMS Mapping at Mpx/h and Beyond

Martin Rittner & Lyndsey Hendriks
TOFWERK, Switzerland

Laser ablation imaging, or more precisely mapping, is moving towards ever faster acquisition speeds to allow larger coverage of samples at higher spatial resolution. Compared to already established technology, the challenges going forward include improving the laser repetition rate, ensuring minimal dispersion in sample aerosol transport, enhancing measurement sensitivity, and achieving high-speed data acquisition and storage. Modern laser ablation systems with low-dispersion ablation cells are able to deliver the clearly separated aerosol plumes from individual laser shots at repetition rates up to several thousand shots per second (kHz imaging). Measuring true multi-element information in such short transient signals is currently only possible with time-of-flight (TOF) instruments like the TOFWERK icpTOF.

Challenges of High-Speed Laser Ablation ICP-TOFMS Mapping

With small spot sizes, sensitivity for trace elements becomes a major challenge. On the other hand, very large laser spots will result in too much sample aerosol being introduced, leading to detector saturation. All system parameters need to be carefully balanced to achieve a linear signal response over the whole dynamic range of the detector, allowing for accurate quantification. The wide linear dynamic range of the detector used in the icpTOF allows the measurement of major down to trace elements in a single ablation pass.

A clean separation between individual laser shots is necessary to obtain crisp images without any signal carry-over into consecutive pixels. Laser parameters and gas flows tuning determine the minimum wash-out time after each laser shot.

The data acquisition system needs to be able to handle acquisition of full-elemental information for each laser shot (pixel). At the high repetition rates modern laser ablations systems are able to deliver.

Experimental

The integrated software TOFpilot seamlessly interfaces with laser ablation systems most commonly applied in LA-ICP-MS, providing running of the set-up ablation patterns with per-shot synchronization, integration of the laser metadata, automatic tuning for sensitivity, and a live preview of the ablated image.

For the analyses depicted in Figure 1 and 2, an ESL imageGEO with TV3 ablation cell was coupled via the DCI to a TOFWERK icpTOF 2R. The laser repetition rate was 500 Hz (maximum repetition rate of the specific laser model employed in the given setup). Each pixel results from one separate laser shot. Sharp grain boundaries testify to the fast signal wash out between laser shots. Reference materials and gas blanks were acquired at regular intervals throughout the ablation of the images. Iolite 4 was used to directly import the data files and produce the presented maps.

Figure 1.  High-speed False-color laser ablation ICP-TOFMS image of a volcanic rock sample, Cu, Ti, and Mn signal in red, green, and blue, respectively. Laser spot size 2 µm, repetition rate 500 Hz. Image dimensions 6353 x 1600 pixel (10.8 Mpx, area: 12.7 x 3.2 mm), acquired at 1.1 Mpx/h.
Figure 1.  False-color LA-ICP-TOFMS image of a volcanic rock sample, Cu, Ti, and Mn signal in red, green, and blue, respectively. Laser spot size 2 µm, repetition rate 500 Hz. Image dimensions 6353 x 1600 pixel (10.8 Mpx, area: 12.7 x 3.2 mm), acquired at 1.1 Mpx/h. Note signal stability over 9 hours of run time, intensity variations result from sample topography only.

Figure 2.  Petrographic thin section, Amphibolite from the Pfitschtal area, Austria. In the image, different sections highlighted, specifically (1) Photomosaic of the thin section, transmitted light, (2) multi-elemental maps acquired by LA-ICP-TOFMS. Spot size 5 µm, repetition rate 500 Hz. Image dimensions are 2563 x 1600 pixel (4.2 Mpx; area: 12.8 x 8.0 mm), (3) False-color map obtained by combining three element maps and (4) Pixel classification determined by k-means clustering of pixels based on a selection of element concentration.
Figure 2.  Petrographic thin section, Amphibolite from the Pfitschtal area, Austria. In the image, different sections highlighted, specifically (1) Photomosaic of the thin section, transmitted light, (2) multi-elemental maps acquired by LA-ICP-TOFMS. Spot size 5 µm, repetition rate 500 Hz. Image dimensions are 2563 x 1600 pixel (4.2 Mpx; area: 12.8 x 8.0 mm), (3) False-color map obtained by combining three element maps and (4) Pixel classification determined by k-means clustering of pixels based on a selection of element concentration.

The icpTOF was fitted with the dedicated dry plasma sampler cone, boosting sensitivity for laser ablation analyses by a factor of 2 – 4 compared to the standard interface (Figure 3).

Figure 3.  LA-ICP-TOFMS sensitivity improvements using the dry plasma cone.

Summary

The icpTOF allows for full-elemental, quantitative LA-ICP-TOFMS imaging from major to trace elements in a single ablation run. Ablation speeds of over 1 Mpx/h have been demonstrated here, with imaging speed mainly limited by the laser repetition rate and the sample transport from the laser ablation system. The maximum time resolution for studying ultra-fast transient signals, such as nanoparticles, is 12 µs (icpTOF S2).

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