High-Speed, High-Resolution, Multielemental LA-ICP-TOFMS Imaging

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Analytical Chemistry

High-speed, high-resolution, multi-elemental LA-ICP-TOFMS imaging: Part II. Critical evaluation of quantitative three-dimensional imaging of major, minor and trace elements in geological samples Marcel Burger,1 Alexander Gundlach-Graham,1* Steffen Allner,1 Gunnar Schwarz,1 Hao A.O. Wang,1 Luzia Gyr,1 Simon Burgener,1 Bodo Hattendorf,1 Daniel Grolimund,2 and Detlef Günther1* 1. Laboratory of Inorganic Chemistry, ETH Zurich, Vladimir-Prelog-Weg 1, CH-8093 Zurich, Switzerland 2. Swiss Light Source, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland

Abstract Here, we describe the capabilities of laser-ablation coupled to inductively coupled plasma time-of-flight

mass

spectrometry

(LA-ICP-TOFMS)

for

high-speed,

high-resolution,

quantitative three-dimensional (3D) multi-elemental imaging. The basic operating principles of this instrumental setup and a verification of 3D quantitative elemental imaging are provided. To demonstrate the potential of 3D LA-ICP-TOFMS imaging, high-resolution multielement images of a cesium-infiltrated Opalinus clay rock were recorded using LA with a laser-spot diameter of 5 µm coupled to ICP-TOFMS. Quantification of elements ablated from each individual laser pulse was carried out by 100% mass normalization, and the 3D elemental concentration images generated match well with the expected distribution of elements. After laser-ablation imaging, the sample surface morphology was investigated using confocal microscopy, which showed substantial surface roughness and evidence of matrix-dependent ablation yields. Depth assignment based on ablation yields from heterogeneous materials, such as Opalinus clay rock, will remain a challenge for 3D LAICPMS imaging. Nevertheless, this study demonstrates quantitative 3D multi-elemental imaging of geological samples at a considerably higher image-acquisition speed than

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previously reported, while also offering high spatial resolution and simultaneous multielemental detection.

Introduction Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) has become a widely applied technique for the determination of major, minor and trace elements in solid materials.1 It is routinely used for fast and sensitive elemental analyses in the fields of material sciences, geology, archaeology and forensics.2-3 LA-ICPMS is usually applied either at a fixed location to determine the elemental composition within an area of interest, or in a lateral scanning mode, in which the laser beam is scanned across a surface to generate a “map” of elemental concentrations. Hole-drilling LA-ICPMS experiments used to establish elemental depth profiles4-6 and elemental mapping with LA-ICPMS have been successfully demonstrated in the fields of geology, biology, and medicine.7-8 However, the chemical and physical processes that lead to element enrichment and heterogeneity in solid samples are not restricted to two dimensions; rather, these processes occur in all three spatial dimensions. The combination of the two aforementioned LA approaches has led to the development of in situ three-dimensional LA-ICPMS imaging of solid samples.9 Threedimensional imaging can provide information about physical and chemical processes that occur at boundary layers. For example, micro-heterogeneities and elemental diffusion patterns can be investigated. Furthermore, the analysis of impurities or layered materials would provide access to the spatial structure of inclusions. Unlike soft biological specimens like tissues, which are typically sectioned in an offline fashion to obtain three-dimensional information,10-11 the structure of minerals can be analyzed layer-by-layer through repetitive application of LA, provided that the ablation process does not alter the original composition 2 ACS Paragon Plus Environment

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of the sample.12-14 For example, 3D LA-ICPMS imaging has proven to be useful in the study of weathering processes and transport mechanisms within solid matrices.13,15 Compared to complementary elemental imaging techniques, such as X-ray fluorescence spectrometry (XRF), LA-ICPMS generally offers superior limits of detection and quantitation capabilities.1518

Synchrotron-based XRF techniques provide sensitivities similar to LA-ICPMS;19 however,

they are not easily accessible and less suited for routine applications. Secondary ion mass spectrometry (SIMS), which offers for some elements better sensitivities compared to LAICPMS, requires samples to be placed in vacuum, which limits certain applications. In addition, reference materials with matrix-matched composition are required to get accurate quantitative results for SIMS.20 Interest in 3D LA-ICPMS elemental imaging is apparent through a number of case studies; a selection of which are discussed below. In 2012, Peng et al.12 reported the 3D LAICPMS analysis of basaltic clast sediment contaminated by fabrication of nuclear fuel. In this study, the distribution of contaminants in a heterogeneous sediment sample was quantitatively determined by external calibration with the NIST standard reference material (SRM) 610 glass. Imaging was carried out in a layer-by-layer fashion by averaging the signals generated from repetitive laser pulses at each lateral position. A total voxel size of 100x100x14 µm was attained when 500 pulses were averaged. In another study, Van Elteren et al.13 used a single-laser-pulse resolved imaging approach to study corrosion processes in weathered glass with a depth resolution of ~7.5 µm. Specifically, across the sample surface, a grid of 80-µm-diameter craters was drilled with 50 consecutive laser pulses applied at each position at low-repetition rate of 1 Hz to avoid common problems such as re-ablation and pulse mixing. Quantification was based on the Corning Museum of Glass reference glasses B, C, and D. Finally, Chirinos et al.14 imaged a bastnaesite rare earth ore sample using a 3 ACS Paragon Plus Environment


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combination of LA-ICPMS and laser induced breakdown spectroscopy (LIBS), and employed layer-by-layer ablation with a depth resolution of 3 µm. A laser spot size of 35 µm was used while neighboring craters were separated by a gap of 50 µm. In part I of this paper, we described the combination of an ICP-TOFMS with a lowdispersion LA tube cell21 for fast, high-resolution 2D multi-elemental imaging. This combination takes advantage of benefits offered by time-of-flight mass spectrometry, such as fast and sensitive acquisition of multi-element mass spectra.22-23 As shown, the lowdispersion aerosol-transport system can deliver 99% of the total signal intensity within 10 ms. Moreover, this fast aerosol transport, combined with fast, simultaneous TOFMS detection, allows for single-pulse limits of detection (LODs) down to single digit part per million (µg g-1) levels for a 10-µm-diameter laser spot and a determined average ablation depth of 0.25 µm per laser shot on SRM NIST 610. The small amount of material ablated leads to absolute detection limits for uranium of around 100 attograms. Here, we investigate the capabilities of this new low-dispersion LA-ICP-TOFMS setup for quantitative 3D LA-ICPMS imaging. Importantly, compared to the state of the art in 3D LA-ICPMS imaging, the newly combined LA-ICP-TOFMS system was tested for speed of analysis, lateral resolution, and elemental coverage. To validate the quantification of single laser pulses in both lateral dimensions and in depth, 3D LA-ICP-TOFMS imaging was performed across the homogeneous glass SRM NIST 612 and results were compared with accepted reference values. Additionally, the capabilities of quantitative 3D LA-ICP-TOFMS imaging approach were investigated on a heterogeneous and well-characterized Opalinus clay rock sample.


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Experimental section Instrumentation. For all experiments, an argon-fluoride (ArF) excimer laser-ablation system (193 nm, GeoLas C, Lambda Physik, Goettingen, Germany) was coupled to a prototype ICPTOFMS instrument.23 The samples under investigation were placed inside the 3D-printed LA tube cell. Detailed information about the experimental setup and typical operating parameters thereof are reported in Part I of this paper. Investigations of crater shape and surface morphology were carried out using confocal microscopy (PLu neox, Sensofar, Terrassa, Spain). In these studies, a 50x magnification objective was used to deliver lateral and vertical resolutions of 0.33 µm and 3 nm, respectively. The X-ray tomography experiments were carried out at the TOMCAT beamline at the Swiss Light Source (Paul Scherrer Institute, Villigen, Switzerland). An X-ray energy of 35.8 keV was used, and the X-ray tomogram was acquired with a pixel size of 3.7 x 3.7 µm.24-25

Description of experiments. Three-dimensional multi-elemental LA-ICP-TOFMS imaging studies were carried out on NIST standard reference materials (SRMs) 610 and 612, and on a cesium-infiltrated Opalinus clay rock thin section. For all experiments, the laser repetition period used was longer than the LA signal duration, so that ICPMS signals from individual LA pulses were baseline separated. Adjacent LA spots were applied edge-to-edge so that the ICPMS signal from each laser shot originated from an adjacent position; the signals from a grid LA shots were assembled to generate an elemental image of the solid sample. In addition, 3D images were acquired by ablation of consecutive layers of the two-dimensional laser-ablation grid. For these studies, circular laser spots with diameters of 5 or 10 µm were



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used with no refocusing of the laser beam between successive laser-ablation layers. A laser fluence of 12-15 J/cm2 was maintained throughout all the measurements. To determine effective spatial resolution and to investigate the response of the system to a concentration change of one order of magnitude, a 3D LA-ICP-TOFMS image was taken across the boundary of adjoined SRM NIST 610 and SRM NIST 612 samples. The artificial sample was prepared in house by embedding SRM NIST 610 and SRM NIST 612 edge-to-edge in epoxy resin. The gap between NIST glass pieces after epoxy hardening was 10-15 µm and therefore on the order of the crater diameter selected. The sample was ground to a suitable dimension and polished. A 200x400 µm wide and 20-layer-deep 3D multi-elemental intensity image was recorded across the boundary of this arrangement. The sample was scanned from high to low concentrations (NIST 610 to 612) using a laser spot diameter of 10 µm, a laser repetition rate of 20 Hz, and a scan speed of 200 µm/s. In another experiment, a 500x500 µm, 20-layer-deep quantitative image of a polished NIST 612 disk was recorded in order to compare measured 3D concentrations with expected concentration values of this homogeneous reference material. In particular, we selected the NIST SRM glasses to demonstrate proof-of-principle for high-lateral-resolution quantitative 3D-ICP-TOFMS imaging because they are well characterized and contain a broad selection of more than 65 elements at major, minor, and trace concentrations. Furthermore, for certain elements, compositional heterogeneities have been previously reported which would be interesting to study.26 A laser spot size of 10 µm, a laser repetition rate of 10 Hz, and a scan speed of 100 µm/s were applied. Quantification was based on the approach of Longerich et al.;27 NIST SRM 610 served as an external reference material and

27

Al was used as the

internal standard to correct for ablation-dependent ion yields, instrument instability, and signal drift. Sensitivities were calculated for each individual layer based on the signal 6 ACS Paragon Plus Environment

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intensities established from the LA of NIST SRM 610. This approach allowed to correct for changes in relative sensitivities observable during the first laser pulses in a LA experiment.28 Furthermore, a thin section (approximately 20-µm thick) of a cesium-chlorideinfiltrated Opalinus clay rock was imaged. Previous µ-XRF and 2D LA-ICPMS studies showed that the sample consists of clay-mineral and calcium-carbonate domains, and that cesium is localized in the clay domain.15 In addition, in small regions, the Opalinus clay rock sample also contained inclusions of micrometer-sized pyrite (FeS2) crystals. To investigate the performance of 3D LA-ICPMS imaging across heterogeneous media, a 250x500 µm, 20-layerdeep quantitative 3D multi-element image of the Opalinus clay rock was recorded across a region of the sample that included all domains. This image was acquired with a laser spot diameter of 5 µm, a laser repetition rate of 20 Hz, and a scan speed of 100 µm/s. Total image collection time for the 100,000-pixel 3D image, including delay times between line scans and between layer acquisitions, was 3 hours (the active ablation time was 83 minutes). To establish relative sensitivities, the USGS synthetic basalt glass reference material GSE-1G29 was imaged across a 25x50 µm area at a spot size of 5 µm and to a depth of 20 ablation layers. Relative sensitivities from ablation of GSE-1G were calculated for each ablation layer separately, and quantification was carried out based on a 100% mass normalization approach.30-32 For 100% mass normalization, all elements were assumed to be present in the form of their most stable oxides except for calcium and strontium which were normalized as carbonates because they were present in the carbonate domain of the Opalinus clay rock sample.

Data collection, processing, and evaluation. In order to efficiently process and handle the large datasets generated for the studies presented here, data-collection and data-treatment 7 ACS Paragon Plus Environment


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schemes had to be developed. We found that robust and consistent LA-ICPMS dataacquisition protocols are critical to the success of imaging experiments. Brief descriptions of the data-acquisition and data-processing strategies are provided below; Figure 1 provides a schematic representation of the data acquisition and processing approach. Three-dimensional LA-ICP-TOFMS data were obtained through the collection of signals from complete two-dimensional LA-ICPMS images across the region of interest, one after the other until the desired number of ablation layers was reached. These 2D LA-ICPMS images were collected by assembling a series of evenly spaced horizontal line scans. Each line scan was generated by operating the laser at a fixed pulse frequency while the sample was moved horizontally at a fixed scan rate with a high-precision translation stage (SmarAct GmbH, Oldenburg, Germany). The laser pulse frequency and stage scan speed were chosen to

make the spacing between laser pulses equal to the laser-spot diameter in order to minimize re-ablation of material and to preserve lateral resolution. All line scans were performed in the same direction and a waiting period between each line scan was employed to help distinguish ICPMS signals from each line. A vertical step size between each line scan equal to the laser-spot diameter was used to create a rectangular ablation grid of edge-to-edge ablation spots. Complete TOFMS spectra were collected at a spectral generation rate of 667 Hz (1.5 ms/spectrum) so that several data points were acquired across each single-shot LA signal. The position of each laser spot was then determined by correlation of time-resolved ICP-TOFMS signals to laser-pulse number. The time-resolved mass-spectral signals of all isotopes were stored in separate TOFMS data files for each 2D layer (Figure 1a). ICP-TOFMS data were processed into elemental concentration images with an inhouse-written MATLAB script (version R2014b, MathWorks, MA, USA). Data processing was


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semi-supervised. In this software, each 2D ICPMS image was treated independently and 3D images were created by stacking the individual 2D maps from each layer. To process the 3D LA-ICP-TOFMS data, integrated TOFMS-signal time traces were first background subtracted using the average gas-blank signal recorded prior to the ablation of each layer. Within each layer, signals from individual line scans were isolated by sectioning the total transient signal from an entire 2D scan into smaller segments of equal length; each of these segments corresponded to single line scans in a layer (Figure 1b). The total signal from each laser pulse was then obtained by summing the number of ion counts for each m/z across a constant time interval. This integration window was applied at regular spacing across each line scan according to the laser-repetition rate used in a given experiment, and was wide enough to capture at least 99% of the ion signal from each laser pulse. The short signal duration of each LA event (~10 ms), which is a property of the low-dispersion tube cell used in these studies, sufficiently isolates each laser-ablation signal and eliminates the effects of pulse-to-pulse mixing. With this approach, signal intensities of all isotopes were unequivocally assigned to each laser-ablation event, and thus to positions across the sample. The integrated single-pulse LA signals from each line scan were then aligned and assembled in MATLAB to create discrete 2D intensity matrices. To transform intensity matrices into concentration matrices, quantification by either an internal-standardization method or a 100%-mass-normalization approach was carried out on a pixel-by-pixel basis. For both quantification strategies, each sample layer was quantified using the average relative sensitivities established from the external reference material acquired at the same ablation layer. For instance, layer 5 of a sample would be quantified using the average relative sensitivity of the fifth layer of the 3D LA-ICPMS signals of the



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external reference material, and so on. The resulting 2D concentration maps were assembled in MATLAB to create 3D concentration matrices for all measured isotopes. To visualize 3D concentration matrices, data were plotted in MATLAB as a stack of 2D images with each pixel assigned to a square with a length equal to the ablation-spot diameter and color coded according to a false-color scale across the concentration range for a given element. No depth was assigned to the layers in the 3D concentration images because the depth of each layer cannot be measured online and is most likely not constant across a single layer due to matrix-dependent ablation rates in the different domains of the Opalinus clay rock. Iso-surface plots of isotopic intensities were used to assess the threedimensional structure of inclusions. These images were generated by only displaying voxels with an assigned isotopic intensity value above a user-defined threshold, which was set to allow for best discrimination between inclusions and surrounding material.


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Figure 1. Data processing and evaluation scheme. a) Background corrected ICP-TOFMS time trace for the LA signals of an individual layer with average TOFMS data acquired every 1.5 ms. The visible breaks between signals are due to waiting periods before the start of each LA line scan. b) ICP-TOFMS transient signal of a given layer is sectioned into individual line scans in order to register the starting edge of each ICPMS image. c) The total ion signal from each m/z from each laser shot is obtained through the integration of each LA-generated signal across a constant time width. The number of ions recorded across a single LA peak corresponds to the ion intensity in a single pixel of the 3D LA-ICPMS image. d) The 2D isotopic intensity maps are stacked to create a 3D single-isotope intensity image (left) or can be displayed as an iso-surface plot (right), in which only voxels with intensities above a threshold value are displayed.

Results and discussion A three-dimensional, multi-element intensity image was recorded across the boundary of a NIST 610/612 composition (Figure 2). Individual line scans always started on SRM NIST 610, reaching the gap between the SRM glasses about half way to continue onto SRM NIST 612. Baseline separation of individual peaks allowed for the determination of lateral concentration changes without carryover effects. Deposition and remobilization of laser-generated aerosol particles was not observed with the 10-µm laser-spot size. The 10-15 µm wide gap between the two SRMs could be identified across all 20 layers and is visualized as a two-pixel wide, low-intensity region of all isotopic images. Figures 2b–f show distinct



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lateral separation resulting from the tenfold-lower trace-element concentrations of NIST 612 with respect to NIST 610, which could accurately be reproduced in the 3D intensity images. Due to similar concentrations of sodium in both SRMs, NIST 610 (13.4 ± 0.3 wt% Na2O) and NIST 612 (13.7 ± 0.3 wt% Na2O), sodium could not be distinguished (Figure 2a). In addition to significant intensity differences between signals obtained from NIST 610 and NIST 612, Figures 2a–f show homogeneous lateral intensity distributions for both SRMs within all layers. Pixel-to-pixel variation across each layer (up to 30% RSD) can partly be attributed to pulse-to-pulse variation in the ablation yield. Typically, ablation yield is correctable by application of an internal standard; however, internal standardization with

27

Al did not

completely compensate for lateral variation in isotopic signal levels across each layer. For example, pixel-to-pixel variations across the fifth layer of 23Na were shown to be reduced to 10% RSD (23% RSD without internal standardization).

23

Na was selected here because its

high signal intensity minimizes the variability introduced by counting statistics to only 0.8% RSD. Moreover, inhomogeneity of the elemental distribution in the sample cannot be excluded as a cause of lateral LA-signal deviations; however, further experiments are required to explore this possibility. Apart from signal variability introduced by counting statistics, fluctuations in pixel-to-pixel intensities could also be caused by single particles ionizing in the plasma, and therefore be related to the particle size distribution formed from a single laser pulse.33


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Figure 2. 3D intensity images recorded across the boundary of a NIST 610/612 composition. The dimensions of an individual layer are 20x40 pixels, with each pixel being 10x10 µm. 20 consecutive layers were ablated using a laser spot size of 10 µm and a laser-repetition rate of 20 Hz. Figures a–f show the intensity distribution given in ion counts per pixel for the isotopes 23Na, 133Cs, 139La, 140Ce, 208Pb and 238U, respectively. Non-contact optical 3D profiling of the ablation area was carried out using confocal microscopy. Figure 3a shows the surface morphology after the ablation of 20 consecutive layers on the NIST 610/612 sample. Figure 3b displays a depth profile across the NIST 612 sample. A uniform pattern of equally distributed ablation craters and remnant pillars between ablation spots can be observed. Importantly, the ablation craters exhibit a consistent depth of 4 to 5 µm, which is in reasonable agreement with a previously determined ablation rate of about 200-250 nm per laser pulse when using a fluence of 13.75 J/cm2 (Figure S-1). In addition, the remnant-material pillars are almost level with the surrounding bulk material, which indicates good precision in the lateral positioning of the laser pulses between layers and minimal ablated-aerosol deposition at crater rim. The gap between the two SRMs is represented by a grey barrier with no depth information assigned. 13 ACS Paragon Plus Environment


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The two plateau-like features located inside the ablated area that appear elevated from the sample surface are likely artifacts of the confocal microscopy caused by diffraction effects at extremely steep LA-crater edges.

Figure 3. a) Confocal microscopy image of the surface after the ablation of 20 consecutive layers on a NIST 610/612 composition and b) depth profile across the NIST 612 sample. Note that the reference point in (a) is about 10 µm above the sample surface. The black line in (a) marks the position at which the depth profile was extracted. To demonstrate and verify the possibility of single-laser-pulse quantitation at high lateral resolution and in depth, we performed 3D LA-ICP-TOFMS analysis of a NIST SRM 612 sample with a laser spot diameter of 10 µm. 20 consecutive layers were ablated, each of which was quantified individually using the according layer ablated for NIST SRM 610. Quantitative 3D elemental images of NIST SRM 612 are provided in Figure S-2 of the supplementary information and show good agreement to the expected concentration values. For most elements, laterally and vertically homogeneous profiles with pixel-to-pixel variation of at most 30% relative standard deviation (RSD) were observed, while for other elements, such as lanthanum and lead, the determined concentrations varied substantially across the first layer ablated. This may be due to surface contamination but could also 14 ACS Paragon Plus Environment

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originate from single particles ionizing in the plasma and therefore be related to the particle size distribution formed from a single laser pulse.33 However, the fact that a more volatile element (Pb) and a more refractory element (La) shows this pronounced variation in concentration can only be explained by contamination. Furthermore, the concentration of sodium appears to be underestimated (17%) in the first layer only, which may be attributed to contamination on the surface of the external reference material. The elemental concentrations determined within each individual layer were averaged and relative standard deviations were determined (Table S-1). For many elements, the quantified concentrations were in agreement with the respective reference values (± 20%) and RSDs of ± 30% were obtained throughout all 20 layers. However, the precision and accuracy is not as good as reported for more bulk-type analysis carried out using larger crater diameters, which is directly related to improved statistics from an increased amount of mass ablated with large LA craters. For some transition metals including manganese, nickel, copper, zinc and arsenic, the limits of detection calculated according to the approach proposed by Longerich et al.27 were above the NIST SRM 612 reference concentrations. The average layer concentrations showed large relative standard deviations but were still in agreement with the NIST SRM 612 reference values. This demonstrates that a virtual increase of the spot size by summing up the intensities of several ablation events allows reliable quantification in cases when the sensitivity is not sufficient to quantify laser-ablation signals from small laser spot sizes.34 This is very promising for studies in which high resolution is desirable to determine the spatial distribution of major and minor components while bulk concentrations of low abundance elements are of interest as well. Following successful demonstration of quantitative high-resolution LA-ICP-TOFMS imaging of NIST SRM 612, this LA imaging approach was investigated for the quantitative 3D 15 ACS Paragon Plus Environment


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element imaging of a cesium-infiltrated Opalinus clay rock thin section. In a previous study,15 the sample was demonstrated to exhibit a complex mineralogical composition dominated by clay minerals (illite, smectite, kaolinite and chlorite). The non-clay-mineral fraction was found to be mainly composed of calcite and quartz, while trace amounts of dolomite, siderite, K-feldspars, albite and pyrite were observed. Here, quantification of the LA-ICPTOFMS signals was achieved by applying a 100% mass normalization approach using GSE-1G to establish relative sensitivities.30-31 Every element was assumed to be present in the form of its most stable oxide except for Ca and Sr, which were known to be present as carbonates. The crystal water potentially incorporated in the clay mineral structures was not known and thus not taken into account. In addition, potassium, which accounts for a substantial percentage of the bulk material,15 was not accessible with LA-ICP-TOFMS because the RFquadrupole notch filter used to remove the highly abundant

40

Ar+ from the ion beam also

eliminates ions of nearby m/z values, and completely suppressed 39K+ and 41K+ signals. The average potassium concentrations in the clay and carbonate domains were assessed in another experiment using an ICP-quadrupole mass spectrometer (ICP-QMS) and considered for the 100 % normalization procedure. Atypical to conventional ICPMS analyses, calcium concentration was determined on the basis of the

44

Ca+ intensity because

42

Ca+ and

43

Ca+

signals were significantly attenuated by the notch filter. However, the mass resolving power (1800 at full-width half maximum) of the ICP-TOFMS allowed separation of the 44Ca+ signal from the adjacent 12C16O2+ mass-spectral peak (Figure 4). Assuming Gaussian peak shape and a

12 16

C O2+ signal that is 1/5th as intense as the

44

Ca+ signal (c.f. Fig 4),

12 16

C O2+ would

contribute about 2% to the total 44Ca+ signal across integration range chosen. Moreover, an even smaller contribution of CO2+ on the

44

Ca+ signal is obtained for the ablation of the

CaCO3 domain, and allows for 44Ca+ signal to be used for 100% normalization quantification. 16 ACS Paragon Plus Environment

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The USGS standard reference material GSE-1G was chosen as an external reference material because the concentrations of major elements in this reference material more closely match those of the Opalinus clay rock than, for instance, NIST SRM 610. Reference values were taken from the GeoReM database.29 Different domains of the Opalinus clay rock thin section were separated in 3D element distribution images (Figure 5). High concentrations of aluminum and silicon are indicative for the clay mineral phase. High concentrations of calcium indicate the calcium carbonate domain. This study confirms that Cs preferentially partitioned into the clay mineral phase. Concentrations of up to 1.2 wt% of Cs2O were determined in the clay mineral domain, whereas the Cs2O concentrations in the calcium carbonate phase were typically below 0.05 wt%. Strontium was highly correlated with the calcium carbonate distribution but also accumulated in certain hot spots. Barium was associated with the clay minerals. The concentrations of titanium and vanadium amounted to 1.5 and 0.05 wt% of TiO2 and V2O5 respectively and were strongly correlated with the clay mineral fraction (Figure S-3).



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Figure 4. The mass resolving power m/z 44 of the ICP-TOFMS was 1800 at full-width halfmaximum. This RP is sufficient to resolve isobaric species of 44Ca+ and 12C16O2+, which is apparent in the average mass spectrum (a) from a LA line scan recorded across a carbonaterich region of the Opalinus clay sample. While CO2+ signal is expected to be much lower than that of 44Ca+ for the ablation of CaCO3, narrow mass-integration windows were chosen to further reduce the interference of CO2 on 44Ca+. (b) In the time trace of integrated ion signals, it’s clear that the 44Ca+ signal is the most abundant species. The lower-intensity signal obtained for the CO2 integration window is due to contributions from the tail of the 44 Ca+ mass-spectral peak as well as CO2+ signal.


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Figure 5. Quantitative 3D element distributions recorded in an Opalinus clay rock thin section. GSE-1G was used as an external reference material to establish relative sensitivity factors and quantification was carried out applying a 100% mass normalization approach. The dimensions of an individual layer are 50x100 pixels or 250x500 µm. Twenty consecutive layers were ablated using a laser spot size of 5 µm and a laser repetition rate of 20 Hz. Figures a–f show the quantitative element distributions given in wt% of the most abundant oxide or carbonate species. Figure 6 shows iso-surface views of the 57Fe+ (a) and 32S+ (b) intensity distribution in a pyrite-rich region of the Opalinus clay sample. Only voxels with ion signals above 2000 counts for

57

Fe+ and 5000 counts for

32 +

S are displayed. The

32 +

S and the

57

Fe+ iso-surfaces

show good agreement, indicating that the features displayed were composed of iron disulfide (pyrite, FeS2). Due to the lack of an appropriate internal standard, as well as a suitable external reference material, these inclusions could not be quantified. However, to gain further insights into the quality of the qualitative imaging of the FeS2 inclusions, the area was investigated using X-rays. Figure 6c shows an iso-surface plot of an X-ray tomogram obtained for the same region of the Opalinus clay sample. The features displayed represent areas exhibiting electron densities above 110 arbitrary intensity units. The electron-dense 19 ACS Paragon Plus Environment


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parts of the sample closely match the inclusions determined based on 57Fe+ and 32S+ and can therefore be interpreted as pyrite crystals. However, in comparison to 3D LA-ICP-TOFMS imaging, the depth resolution achieved using X-ray tomography did not allow representation of the fine structure in vertical direction. Variations in intensity across the inclusions in the LA-ICP-TOFMS elemental image are due to fractional sampling of individual pyrite crystals by the 5-µm-diameter laser spot size employed. For example, the signal from a 10-µm-diameter pyrite crystal might be recorded across 15 µm of the elemental image because it is ablated three consecutive laser pulses; this results in blurring effects at the edges of the pyrite crystal with a maximum blur of just less than 5 µm. Here, resolution is limited by the laserspot diameter. In other work, Van Malderen et al.35 recently proposed an LA oversampling and image deconvolution approach that allows for lateral resolution better than the laser beam waist, and represents a promising method to improve image resolution. However, the accuracy of this deconvolution approach requires homogeneity of the sample to a depth of several laser shots, which is not the case for the heterogeneous Opalinus Clay sample studied here.


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Figure 6. Iso-surface images of 57Fe+ (a) and 32S+ (b) intensity distribution in the Opalinus clay rock thin section and iso-surface plot of the results obtained by X-ray tomography when the same area of the sample was investigated (c). Zoom-in views in (a) and (b) highlight the ability of the LA-ICP-TOFMS approach to resolve pyrite inclusion structure both in lateral dimensions and in depth. Note here that the depth is reported in number of layers ablated and corresponds to a depth of around 10 µm.

Similar to the NIST610/612 composition, the surface topography of the Opalinus clay rock sample was investigated after LA-ICPMS-image acquisition using confocal microscopy. As can be seen in Figure 7, distinct regions of the Opalinus clay sample were not ablated to the same depth. Differences in depth of up to 10 µm were measured. In fact, in heterogeneous materials like the Opalinus clay, constant ablation yields are often impossible to obtain, and pronounced variation of depth across the ablated area must be expected. The prominent low ablation yield area displayed in the depth profile (Figure 7b) correlates well with the low ICP-TOFMS signal intensities recorded at this location, which confirms that this feature is sample related and not an artifact of confocal microscopy.



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In comparison to the craters produced on the glass SRMs (Figure 4), the ablation craters on the Opalinus clay rock sample are about twice as deep and no pillars are observed between adjacent spots. The material that remained intact around each laser spot in the case of NIST SRM 612 was, in the case of ablation of the Opalinus clay sample, most likely removed by the laser-induced shockwave.36-37 The large distribution of crater depths across the sample indicate that the depth from which the ion signal is retrieved from is not necessarily uniform across the entire section for a heterogeneous sample. It is currently not possible to assign uniform depth information to every LA signal of a given layer when dealing with heterogeneous specimens. Stacking of 2D elemental maps is thus a simplified approach that inadequately describes the real depth sampled at each pixel by LA. A combination of laser ablation with a three-dimensional profiling technique such as confocal microscopy or interferometry would be desirable in order to determine the ablation yield for each individual spot after the ablation of a layer. An assignment of ablated volume on a pixel-by pixel basis could be enabled this way.


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Figure 7. Confocal microscopy image of the surface of the Opalinus clay rock thin section after the ablation of 20 consecutive layers (a) and a depth profile across the sample (b). The black line in (a) marks the position from which the depth profile was extracted.

Conclusions The combination of a fast-washout laser ablation tube cell with ICP-TOFMS was demonstrated to allow for quantitative 3D imaging of geological samples. Multi-element images were acquired using laser spot sizes of 5 and 10 µm. The laser was fired at repetition rates of 20 Hz while still maintaining baseline separation of individual laser ablation signals. Quantification of single laser-ablation events at a laser spot size of 10 µm was possible down to the single digit µg/g level for the most sensitive isotopes. On the other hand, mid-massrange transition metals were not detectable from single 10-µm laser pulses at trace concentrations of ~50 µg/g. In this regard, the application of small laser-spot sizes enables high lateral resolution but comes at the expense of sensitivity. Furthermore, we report sucessful 3D LA-ICP-TOFMS imaging of a hetereogeous Opalinus clay rock thin section. In particular, the LA-ICP-TOFMS imaging approach clearly resolved clay-mineral and calcium carbonate domains within the Opalinus sample.

In

addition, mass fractions of major, minor and trace elements were in agreement with the 23 ACS Paragon Plus Environment


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expected values acquired at much lower spatial resolution. High-resolution LA-ICP-TOFMS imaging was further demonstrated through the resolution of pyrite-rich regions of the Opalinus clay rock. In particualr, iso-surface images of the 32S+ and 57Fe+ intensities illustrate the ability of this LA-ICPTOFMS approach to deliver three-dimensional structural information. Confocal microscopic investigations demonstrated that assigning depth information to a certain laser-ablation signal is only possible when dealing with homogeneous materials and constant ablation rates. Ablating heterogeneous specimens, such as an Opalinus clay rock, results in different ablation yields depending on the target phase. Laser ablation rastering creates a complex surface morphology in those cases, which poses a problem for adequate three-dimensional representation of the data. A combination of LA with a 3D profiling technique such as confocal microscopy or interferometry on a spot-by-spot basis could be one route to address this problem. LA-ICP-TOFMS equipped with a low-dispersion aerosol transport system provides a unique advantage by enabling quasi-simultaneous detection of multi-element mass spectra of the material sampled from each LA event with sufficient speed to characterize LA aerosol peak shape. The combination of high speed LA and resepesentative multi-isotope detection from each LA event allows for rapid, quantitative image acquisition. In addition, the LA-ICPTOFMS imaging setup presented here offers LA-ICPMS imaging with very low sample consumption and without the deleterious effects of pulse-to-pulse mixing and spectralintensity skew, both of which are major challenges to overcome when scanning-type mass spectrometers are employed.


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Associated content Supporting information Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Author information Corresponding authors *E-mail: [email protected]. Fax: +41 44 633 10 71 *E-mail: [email protected]. Fax: +41 44 633 10 71

Acknowledgements The X-ray tomography work was performed at the TOMCAT beamline at the Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland. Support and contributions of F. Marone are gratefully acknowledged. Confocal microscopy was performed at the laboratory for Surface Science and Technology at ETH Zürich, Switzerland. Contributions and support of C. Cremmel are gratefully acknowledged. The authors are grateful to Roland Mäder and Philippe Trüssel of the mechanical workshop at ETH Zürich for the manufacturing of the LA cell. Financial support for the development of the ICP-TOFMS was provided by the KTI (project number: 10804.2 PFNMNM) and support for the research presented here was provided by the SNF under grant agreement n° 200020_141292. A. Gundlach-Graham also acknowledges financial support through the Marie-Curie International Incoming Fellowship from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 624280. 25 ACS Paragon Plus Environment


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