Ultimate detection limits by accelerator-based mass spectrometry

Accelerator-based analytical methods, mainly accelerator mass spectrometry (AMS) and ion beam analysis (IBA), have been applied to numerous research projects in recent decades. The key element of both methods is a high-energy particle accelerator running at a terminal voltage of 0.2-14 MV.

For AMS negative ions (molecules or elements) are extracted from samples containing long-lived radionuclides (t_1/2 >100 a) in a Cs-sputter ion source. By inserting these ions in a tandem accelerator, they gain MeV-energies, and by passing through matter (gas or foil) at the positively charged terminal in the middle of the accelerator, the negative ions lose outer electrons and convert into multiple-positively charged ions being then further accelerated towards the exit. Effectively all molecules are destroyed by this stripping process. Generally, AMS is measuring isotope ratios, i.e., stable isotopes are usually detected in Faraday-cups and radionuclides in ionization chambers. Such a set-up of two mass spectrometers in one, namely the first with negative ions of keV-energy, the second with high-energy positive ions of MeV-energy, and combined with several magnetic and electrostatic analyzers, allows analyzing isotopic ratios as low as 10^-16, thus, providing the ultimate detection limit of all mass spectrometry methods. Very recently, a new AMS facility has been installed at the Ion Beam Centre of the Helmholtz-Zentrum Dresden-Rossendorf: DREsden AMS (DREAMS) [1-3].

Probably in 2014, AMS will be also used for the detection of stable element ratios at DREAMS. The most common terms for this are Trace Element AMS (TEAMS, e.g. [4]) or if the Cs-beam is focused and spatial resolution kept, Accelerator-SIMS or Super-SIMS [5]. However, mainly due to the background e.g. from the ion source, detection limits are expected to be not as low as for “standard” AMS, but still some orders of magnitude better than is the case for traditional dynamic SIMS, i.e. around 10^-9-10^-12.

References: [1] S. Merchel et al., GIT Labor-Fachzeitschrift 2012, (2) 56, 88. [2] S. Akhmadaliev et al., Nucl. Instr. Meth. B 2012, doi:10.1016/j.nimb.2012.01.053. [3] www.dresden-ams.de. [4] S. Merchel et al., Geochim. Cosmochim. Acta 2003, 67, 4949. [5] S. Matteson, Mass Spectrom. Rev. 2008, 27, 470.