AMS technique development 
 

Fig 1. Illustration of ICE-MS technique.

 Key contact:

Mike Hotchkis
ANSTO Institute for Environmental Research
PMB 1, Menai NSW 2234, Australia
Phone: +61 2 9717 3148
Email: Michael.hotchkis@ansto.gov.au

 New methods of measuring isotopic ratios are under development to expand ANSTO's capabilities.

ECR ion sources

Conventional ion sources produce beams of singly-charged positive or negative ions of molecular or atomic species. Examples are positive molecular ions such as CO+ ions, used in isotope ratio mass spectrometers, and negative C‑ ions, which are injected into accelerators for accelerator mass spectrometry.

Electron Cyclotron Resonance (ECR) ion sources, on the other hand, are used to generate beams of multiply-charged atomic ions. The use of low charge state multiply-charged ions has benefits for mass spectrometry, as a way of eliminating potential mass interferences caused by molecular ions. In most cases, small molecules become unstable if two or more electrons are removed. We are using a compact ECR ion source to provide multiply-charged ions for experiments with two new methods of mass spectrometry for measurement of isotope ratios.

ICE-MS

In one method, which we call Ion Charge Exchange Mass Spectrometry, we have demonstrated the detection of radiocarbon at low levels and determination of 14C/12C ratios. This is achieved in a two stage mass spectrometer, shown in Figure 1 below. In the first stage, 3+ ions of carbon are selected, thereby eliminating interferences at mass 14 due to molecular species such as 13CH and 12CH2. The 3+ ions are then injected into a charge exchange cell, converting them to negative ions with efficiency of the order of 10 per cent. This step eliminates 14N interference, as nitrogen negative ions are too short-lived to be detected in a mass spectrometer.

IRMS++

In another method, the first stage of the above system is used alone, to measure the isotopic ratios of stable isotopes of light elements such as carbon, nitrogen and oxygen. Selection of the 2+ charge state eliminates hydride and other interferences which would affect measurements using singly-charged ions. For example, this method enables direct oxygen isotope ratio measurements of water vapour samples, for both 18O/16O and 17O/16O ratios. Natural variations in such isotopic ratios are of great interest in environmental sciences.