Laser-Ablation ICP-MS Imaging of Geological Samples

Laser-ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS) is well suited for highly sensitive elemental and isotopic analysis of solid samples. In this technique, a laser beam ablates the sample and generates fine particles that are then transported to the ICP-MS system for rapid elemental analysis. Detlef Günther is Professor for Trace Element and Micro Analysis and Vice President Research and Corporate Relations for ETH Zurich, and he and his group use LA-ICP-MS for two- and three-dimensional imaging of geological samples such as rocks and meteorites. He recently spoke to us about this research.

A recent paper describes your group’s use of LA-ICP-time-of-flight (TOF)-MS for two-dimensional imaging of rock and meteorite samples (1). Several methods have been used for elemental imaging, including X-ray methods such as microsynchrotron X-ray fluorescence (µ-SR-XRF) spectroscopy, micro proton-induced X-ray emission (µ-PIXE) spectroscopy, energy-dispersive X-ray spectrometry (EDXS), and electron-probe X-ray microanalysis (EPMA), and MS-based methods such as secondary-ion mass spectrometry (SIMS). In general, how does LA-ICP-MS compare with these alternate imaging methods with respect to detection limits, range of elements investigated, isotopic information, lateral resolution, measurement time, and ease of use?

LA-ICP-MS has been developed over the past 30 years into a routinely applicable technique for direct solid analysis, and many methods have been developed to access element concentrations and isotope ratios in a wide variety of solids. The development of the laser-ablation technique in combination with TOF-MS started in the mid 1990s and was pioneered in the Hieftje group at Indiana University. However, the sensitivity of the first commercially available ICP-TOF-MS instruments prevented some applications. The potential of the technique has remained interesting, especially in combination with laser ablation. Based on our experience from the early days we built a new prototype together with Tofwerk in Thun.

The comparison of LA-ICP-TOF-MS to all the other techniques has been carried out to validate our results and to demonstrate the advantages of laser ablation in terms of fast sample throughput, high sensitivity, and high spatial resolution. But in general the direct solid techniques are very complementary, and any serious validation requires more than one technique. But it needs to be mentioned that secondary ion mass spectrometry (SIMS) is currently the most advanced technique when it comes to spatial resolution and sensitivity. 

Your instrumental setup for the study includes a low-dispersion laser-ablation cell with a piezo-electrically driven xyz translational stage and a tube that carries ablated material to the ICP. What are the advantages of this cell over previous cell systems? What spot diameters were you able to analyze using this setup?

The quasi-simultaneous measurement of all isotopes by TOF-MS allows minute amounts of materials to be sampled. To take full advantage of the TOF system, the sample has to enter the plasma as dense as possible. That means, the lower the dispersion of the material, the better the detection capabilities. Based on our experience with an in-torch ablation approach (dispersion of 2 ms) we constructed a low-dispersion cell (10-ms washout time) that is especially advantageous for imaging. If the sample dispersion is very low, the sampling frequency can be increased without aerosol mixing. Also, the lower the sample mixing the better the image, because it depends only on the crater diameter. The highest spatial resolution is currently 1 µm or a little bit below.

What are the benefits of TOF-MS in this system, compared with the scanning mass analyzers used in earlier LA-ICP-MS imaging studies? How do the image acquisition times compare with those of conventional LA-ICP-MS systems?

TOF-MS allows all isotopes to be measured within a period of 33 µs. The latest versions of sequential mass spectrometers allow measurement of one isotope within 20 µs. So, multielement analyses with high spatial resolution require discrete sampling and simultaneous detection of all isotopes. And here it really counts how fast the ablation cell can be washed out. A 10-ms washout time allows a sampling rate of 100 Hz, which is a significant reduction in time (5 times) when compared to a washout time and sampling rate of 50 ms and 20 Hz, respectively.

Your group has also used this system for quantitative three-dimensional imaging of geological samples (2). Was the approach able to resolve the various mineral domains within the samples? What were the approximate dimensions of the sample volumes studied?

Two- or three-dimensional sampling has gained a lot of attention and is currently applied widely. However, the crater diameter and the simultaneous detection capabilities are crucial for constructing a quantitative image of high quality. We tried it on various meteorite samples and received a lot of information about various phases. The remaining problems exist at the boundary layers between two phases. There we still have problems in quantification, especially between metallic and oxide phases.

In the laser-ablation process, are heterogeneous materials such as rock samples ablated uniformly, or is ablation yield dependent on the matrix? Does it pose a challenge for three-dimensional imaging studies? What steps could be taken to address that potential problem?

Heterogenenous samples are a challenge for LA-ICP-MS, because there is no homogeneously distributed internal standard, which is typically required for quantitative analysis. To overcome this limitation, measuring the entire matrix and using a 100 wt% normalization procedure has been successfully applied to many matrices. However, this approach is only valid if you can access all the elements within a matrix. And here we have some severe limitations when highly interfering elements-for example, sulfur or phosphorus-are present at high concentrations. The solution would be a high-resolution TOF-MS system or a reaction cell in combination with a TOF-MS system. Both solutions are currently being investigated, and I am confident that this type of system will become commercially available.

What are the next steps in your research with laser-ablation techniques?

Currently we are exploring the capabilities of ICP-TOF-MS for laser ablation and for nanoparticle analysis. There are still issues with the background and of course with interferences and with the sensitivity of the ICP-TOF-MS system. Furthermore, the non-matrix matched calibration in laser ablation remains one of the most important challenges, and we are currently investigating the ablation behavior of nano-powdered minerals versus the use of glass standards. This very interesting grinding procedure, developed by Dieter Garbe-Schönberg, seems to be very promising to be used for quantification of geological samples (3). Quantification procedures for biological samples such as tissues are also required, but we are lacking a routinely applicable calibration technique. But I am sure that these problems will be solved and then I think LA-ICP-TOF-MS will have a very bright future and will make significant contributions to geology, biology, medicine, and of course material science.

References

• A. Gundlach-Graham, M. Burger, S. Allner, G. Schwarz, H.A.O. Wang, L. Gyr, D. Grolimund, B. Hattendorf, and D. Günther, Anal. Chem.87, 8250–8258 (2015). DOI: 10.1021/acs.analchem.5b01196

• M. Burger, A. Gundlach-Graham, S. Allner, G. Schwarz, H.A.O. Wang, L. Gyr, S. Burgener, B. Hattendorf, D. Grolimund, and D. Günther, Anal. Chem.87, 8259–8267 (2015). DOI: 10.1021/acs.analchem.5b01977

• D. Garbe-Schönberg, and S. Müller, J. Anal. At. Spectrom.29(6), 990 (2014). http://doi.org/10.1039/c4ja00007b