Installation of hybrid ion source on the 1-MV LLNL BioAMS spectrometer

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Abstract

A second ion source was recently installed onto the LLNL 1-MV AMS spectrometer, which is dedicated to the quantification of 14C and 3H within biochemical samples. This source is unique among the other LLNL cesium sputter ion sources in that it can ionize both gaseous and solid samples. Also, the injection beam line has been designed to directly measure 14C/12C isotope ratios without the need for electrostatic bouncing. Preliminary tests show that this source can ionize transient CO2 gas pulses containing less than 1 μg carbon with approximately 1.5% efficiency. We demonstrate that the measured 14C/12C isotope ratio is largely unaffected by small drifts in the argon stripper gas density. We also determine that a tandem accelerating voltage of 670 kV enables the highest 14C transmission through the system. Finally, we describe a series of performance tests using solid graphite targets spanning nearly 3 orders in magnitude dynamic range and compare the results to our other ion source.

Introduction

The 1-MV spectrometer at the Center for Accelerator Mass Spectrometry, located at Lawrence Livermore National Laboratory, is dedicated to the quantification of 14C [1] and, recently, 3H [2] within biochemical samples. Over 50,000 samples have been analyzed since operations began in May, 2001. High measurement throughput is enabled by the use of the LLNL high-output Cs-sputter solid sample ion source [3].

The use of solid targets necessitates the off-line conversion of biochemical samples to graphite [4] or TiH2 [5]. Ion sources that are compatible with the direct input of biochemical separatory instrumentation, such as liquid chromatography, gas chromatography, capillary electrophoresis or other instruments would allow for real-time, automated sample preparation, potentially leading to increased resolution, improved sensitivity through reduced sample handling and the ability to do molecule-specific tracing of very small samples, but with a cost in precision. One such approach would involve the direct introduction of carbon, as CO2, into the ion source. As the LLNL-designed ion source will only accept solid samples, a gas-accepting ion source was installed. This source is a heavily-modified version of the NEC MCGSNICS ion source and is designed to accept both solid and gaseous samples [6].

The injection beam line of this ion source has been designed to allow for the simultaneous measurement of 14C+ and 12C ions without the need for electrostatic or magnetic isotope switching. In this case, the 12C ions are measured in an off-axis Faraday cup after a magnet located immediately after the ion source. High accuracy and precision require that the transmission of the 14C ions through the entire beam line remain constant. In our system, the largest source of transmission variations is from changes in the stripper gas density, which can drift 5–8% during the course of a day’s operation. We measured the extent of this effect, as well as changes in the tandem accelerating voltage on 14C ion transmission.

In order to have confidence in the results obtained using this new ion source, a series of performance tests were conducted using solid graphitic targets with 14C/C isotope ratios spanning 3 orders in dynamic range, which encompasses the majority of bioAMS samples. The use of solid graphitic targets allowed for the direct comparison of the performance of this source to that of the other ion source which can only accept solid samples.

Section snippets

Description

Fig. 1 is a schematic layout of the 1-MV AMS system. The new ion source and its associated injection beam line is attached through an existing port of a 45° electrostatic analyzer which can rotate to enable the operation of either ion source. The new injection beam line has been designed to allow for the direct quantification of either 14C/12C or 3H/1H isotopic ratios [7]. This configuration increases our 3H-AMS measurement throughput as it eliminates slow magnetic field switching of the

Transmission

The optimal transmission through the stripper canal depends on the careful matching of the accelerator entrance lens with the injected ions’ energy and mass. Again, since our system measures the 12C ions after the first injector magnet and we do not bounce the ions through the accelerator, we wanted to select the optimal tandem accelerating voltage to maximize the 14C transmission. Maximizing the 14C transmission has a direct impact on the precision that can be obtained from a transient CO2

Conclusions

These results from our newly installed ion source demonstrate its effectiveness in reliably measuring 14C/12C isotope ratios from solid graphite targets. However, much work remains to be completed before the source and interface are brought into service for routine analysis of transient CO2 pulses. In particular, the effects of sample-to-sample cross contamination on ion source and interface operation need to be fully understood. Before we can begin tests with CO2 containing elevated levels of

Acknowledgements

We wish to acknowledge the efforts of the following people who assisted in the construction and installation of this ion source: Tom Brown, Skip Fields, Lee Kruse, Sean Watson and Joe Ruth. Kurt Haack prepared all the graphite samples for analysis. Avi Thomas and Paul Daley assisted in operation of the moving wire combustion interface. Larry Cohbra assisted in the measurement and analysis of data. Work performed (partially) at the Research Resource for Biomedical AMS, which is operated at LLNL

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