High spatial resolution trace element analysis by LA-ICP-MS using a novel ablation cell for multiple or large samples

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Abstract

An ablation cell was developed to host large or several small samples (up to dimensions 230 mm × 34 mm × 16 mm, L × W × D) for high spatial resolution analysis. The performance of the cell was tested using silicate glass NIST SRM 610 and steel CRM JK-2D. To obtain high resolution profiles of trace element distributions, the ablation cell must offer fast washout of the aerosol to decrease mixing of the particles from different laser pulses. The washout of the new ablation cell is 70% faster when compared to the standard ablation cell previously applied in our laboratory and provides washout times within 2.6 s for 99.9% of the signal. Gas flow patterns in the cell were modeled by computational fluid dynamics (CFD) and validated by measurements on two reference materials. To demonstrate the potential of the ablation cell, p-(hydroxymercuri)benzoic acid (pHMB) derivatised ovalbumin was ablated from polyacrylamide gels after electrophoretic separation.

The evaluation of the ablation cell demonstrates high resolution capabilities on large solid samples. It promises to have a significant impact on numerous applications, such as imaging of biological samples or trace element determinations on climate archives (e.g., stalagmites).

Research highlights

► Ablation cell design for fast washout of laser generated aerosol. ► High spatial resolution LA-ICP-MS for analysis of large heterogeneous samples. ► Detection of Hg-derivatised ovalbumin in electrophoresis gel with LA-ICP-MS.

Introduction

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a widespread technique for analysis of major, minor and trace elements in solid samples with high dynamic range and low limits of detection [1]. Quantitative analysis of elements and isotopes are required in a wide variety of applications (bulk analysis, micro analysis, fingerprinting [2], depth profiling [3], line scans [4], [5], [6], [7], [8], isotope ratio analysis [9], and mapping [10], [11], [12] in many research fields [13] (earth sciences, forensics, gemology, material sciences, life sciences and in industry). In recent years, the detection of metal containing proteins after separation on polyacrylamide gels has become a frequent application for LA-ICP-MS [14]. Especially electrophoresis in two dimensions provides high separation efficiency for a given sample. However, sensitive and spatially resolved detection is a prerequisite for the successful interpretation [15] of these gels.

The performance of LA-ICP-MS is crucially dependent on the characteristics of the ablation cell. Various investigations on ablation cell design have been carried out on the volume of the cell and transport tube material [9], [16], transport efficiency in dependence on geometry [17], [18], [19], as well as on the applicability of cooled cells for hosting tissues [20], open-contact cell for large, flat samples [21] or double cell configurations with fast washout [22]. However, so far the different designs have not led to the development of an ablation cell, which can host large samples enclosed in an airtight Argon or Helium atmosphere. Since fast washout requires a small volume, a number of small cell volumes or double cells have been reported in the literature. Fast washout and transport of the laser generated aerosol to the ICP have been extensively studied for different ablation cell designs and approaches such as a laminar flow cell [23], cell in a cell [17], [18], [22], [24], using the venturi effect [25], rotating nozzle [26], cyclonic gas flow [27], and removing the cell by in-torch ablation [28]. Numeric modeling of gas flows [29], [30] and particle trajectories [19] in the ablation cell are becoming a valuable source of information for designing and optimizing of new ablation cells for specialized purposes. Some of these small volume cells would be ideal to be placed on large sample holders. However, as indicated by Lindner et al. [31], this requires the design of a specific interface.

The requirement of large ablation cells evolved from a number of applications, where the samples cannot be cut into pieces for analysis, such as historic artifacts, precious objects, corals or stalagmites. For the tightly grown stalagmites, a cutting process would lead to significant loss of information recorded in the sample. For bulk analysis, a short washout of the aerosol is not critical. However, it is essential for analysis with high spatial resolution for scanning and mapping of solid sample surfaces. While in scanning mode, laser ablation runs along a heterogeneous sample (such as sediments [32], corals [5], stalagmites [10], [33], biological tissue [11], [34], etc.) the composition of the aerosols changes over time and therefore needs to be recorded as a transient signal without mixing of information [3], [35], [36].

Based on the literature and the variety of samples analyzed in our laboratory, we focused on the development of a routinely applicable ablation cell design for high resolution analyses of large, heterogeneous samples. The parameters of interest included signal intensities (transport efficiency) and stability in dependence of the position of ablation. These studies were carried out using prolonged ablation of profiles across two standard materials. The empirical investigations were furthermore validated by gas flow velocity simulations using computational fluid dynamics (CFD). Finally, we applied the ablation cell to perform imaging of the distribution of a pHMB-derivatised protein after electrophoretic separation using the intensity profile of mercury. As natural ovalbumin contains only S and P (but no metals) for detection with Q-ICP-MS, the artificial introduction of Hg was necessary to improve detection capabilities.

Section snippets

Instrumentation

All analyses were carried out using an ArF excimer laser at 193 nm (GeoLasC, Lambda Physik, Göttingen, Germany), the ablation cell was mounted on a computer-controlled xyz-stage of an Olympus BX51 microscope. The prototype ablation cell was built in-house to host samples with max. dimensions of 230 mm × 34 mm × 16 mm (L × W × D). The total volume of the cell is around 1 L, whereas the volume, into which the laser aerosol expansion takes place, is only around 13 cm3. The entire corpus was made from

Modeling

Modeling of the gas flow velocities in the cell suggested non-symmetric gas velocities in the ablation area, when ablating at different positions of samples and sample holder. Symmetric gas flows – which are favorable for analysis – were observed only in the center position of the sled. All measurements on NIST 610 were performed using a single gas inlet in one corner of the cell (Fig. 2C). Surprisingly, simulations with a single or a second He gas inlet, respectively, revealed only minor

Conclusion

A large ablation cell capable of holding large objects was designed, built and tested successfully with three different sample types (glass, metal, organic matrix). It is shown that this ablation cell is able to provide high spatial resolution data of main and trace elements as well as hosting of large samples. A fast washout is achieved by restricting the effective volume for the expansion of the laser-generated aerosol at the ablation site with a geometry favoring laminar gas flows.

The source

Acknowledgements

The authors would like to dedicate the manuscript to Prof. Dr. Dietze's 75th birthday and would like to thank him for many fruitful discussions on LA-ICP-MS and science in general.

ETH Zurich is acknowledged for funding of the project. Furthermore, the authors would like to thank Roland Mäder (ETH Zurich, machine shop, LAC) for building the ablation cell. The group of Prof. Hilvert at ETH Zurich is kindly acknowledged for the possibility of using the electrophoresis unit. Two anonymous reviewers

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