Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Use of resonant laser SNMS (secondary neutral mass spectrometry) to measure boron isotopic ratios of single foraminiferal cells

Resonant laser secondary neutral mass spectrometry (r-Laser-SNMS) is an emerging analytical technique for efficiently measuring the isotopic composition of trace elements. R-Laser-SNMS is similar to the well-known ToF-SIMS (time-of-flight secondary ion mass spectrometry) technique. Both techniques use a focused energetic ion beam, which can be focused down to 50nm in diameter, for bombarding a solid sample and a ToF mass spectrometer for analysing the sputtered particles. But instead of using only the small fraction of sputtered secondary ions, as SIMS does, r-laser-SNMS uses tuneable lasers to resonantly ionise the majority of the sputtered neutral atoms, which are less affected by the chemical composition of the surface, thus leading to much greater accuracy than SIMS.

The energy spectrum of discrete excited states is unique to each element, so that the selection of particular excited states for resonant multiphoton post-ionisation (RMPI) analysis provides extremely high selectivity, thus making the technique especially valuable for high precision isotope ratio measurements or detecting ultra-trace elements in complex samples. This post-ionisation technique also offers a high sensitivity: a useful yield, defined as the number of observed counts at the detector divided by the number of analyte atoms sputtered from the surface, of 3% to 8% has been demonstrated for many elements including boron and detection limits of sub-ppb was shown for boron in silicon.

Key Innovations
As the number of detectable neutrals is crucial for determining accurate isotope ratios it was important to investigate the sputtering process of atomic boron neutrals from different chemical environments under different analysis conditions. The experimental conditions have been optimised for a high and continuous flux of boron neutrals from calcite surface of single foraminiferal shells over the entire analysis time. For this purpose r-laser-SNMS analyses of different sample systems with various boron concentrations were performed under different analysis conditions.

In addition, the influence of the experimental set-up on the measured value of the boron isotope ratio was investigated and optimised for time-stable isotope ratio analysis of single foraminiferal shells resulting in a significantly improved accuracy of isotope ratio measurements of boron containing samples. Consecutive measurements on a test sample, a boron containing metallic glass alloy, were performed with a standard deviation smaller than two per mill. Although, this accuracy could not yet be reached on single foraminiferal shells, the values for the standard deviation correlate with the size of the counting statistical error. Thus, advancement in accuracy can be achieved by enhancing the number of sputtered boron atoms, by increasing the analysis time, by increasing the repetition rates of the analysis cycle and/or by increasing the primary ion current by using special designed high current ion guns.

Expected Benefits
The results of this project suggest that a microprobe instrument based upon the RMPI technology could significantly advance capabilities in earth sciences for isotope ratio measurements and nanoanalysis. The advantages of the r-laser-SNMS technique include excellent selectivity, sensitivity and efficiency, comprehensive elemental and isotopic applicability, reduced fractionation and matrix effects, nanoanalysis capabilities, and freedom of isobaric interferences. In addition, the integration of RMPI with TOF-SIMS will also allow performing imaging of nanostructure samples where ultra-trace elements can be detected and quantified by r-laser-SNMS, while quasi-simultaneous chemical images of other elements and molecules by ToF-SIMS will provide useful and complementary information of the samples.

Dissemination and use
The close collaboration between the University of Munster and the company ION-TOF GmbH guarantees that the RMPI technology becomes commercially available. Such an analytical tool would have widespread applications in the earth sciences, greatly extending the resolution in geochemistry, cosmo-chemistry, and paleoceanography brought about by the ion microprobe. It will also be significantly less expensive than other complex geoscience microprobe instruments and therefore very competitive. R-laser-SNMS will also find important applications in chemical and biomedical applications, geological exploration, materials and environmental sciences, and intelligence operations. The demand for analytical instrumentation with spatial resolution in the nanometre range is rapidly increasing in these research fields because reduction in sample sizes and structures and increases in materials purity and compositional fidelity are straining available characterization techniques. A substantial market for such instrumentation can be anticipated.

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