A Review of the On-line Suppression of Sulfur by Charge Transfer with Nitrogen Dioxide in the Analysis of Chlorine-36 by Accelerator Mass Spectrometry – Hints from Liquid Chromatography Mass Spectrometry
Dr Jean-François Alary1, Dr. Lisa M. Cousins1, Erin L. Flannigan2, Dr. Gholamreza Javahery1, Prof. William E. Kieser1,2
1Isobarex Corp., Vaughan, Canada, 2University of Ottawa, Ottawa, Canada
The range of isotopes that can be measured in Accelerator Mass Spectrometry (AMS) is in large part determined by the ability of the technique to eliminate interfering stable isobars either in the ion source or by dE/dx separation in foils or gases, using a gas ionization detector or a gas-filled magnet. In the case of the detection of chlorine-36 by small AMS systems, attempts at selectively suppressing sulfur via ion-molecule reactions have been made using the electron transfer reaction S- + NO₂ -> S⁰ + NO₂-, exothermic by 0.2 eV. The equivalent reaction involving a chlorine anion is endothermic by 1.34 eV [Dunkin et al, Chem. Phys. Letters 15 (1972) 257-259]. Unfortunately, energy conditions found in classical AMS systems do not allow this suppression reaction to occur with appropriate efficiency.
We discuss the various approaches that were tried to enable this specific suppression reaction in the low energy line of an AMS system, where it would be most useful. We review results obtained initially with a charge exchange canal, then an early Radio Frequency Quadrupole (RFQ) system, the Isobar Separator for Anions (ISA), both tested at the IsoTrace laboratory in Toronto, and a more advanced ISA currently operated at the A. E. Lalonde AMS Laboratory, University of Ottawa. Comparing these three systems, which operate over an energy range from ~1 eV to 1 KeV, illustrates the different phenomena governing the suppression reaction of sulfur, as well as the transmission of chlorine anions towards the tandem accelerator. A comparison is also made with conditions found in Liquid Chromatography Mass Spectrometry (LC-MS), another RFQ-based technique [Douglas, J. Am. Soc. Mass Spectrom. 9 (1998) 101–113].
These results are reinterpreted considering the most recent results obtained at A. E. Lalonde. Notably, the degree of thermalization of anions on their passage through the device is extensively discussed. The transition from non-thermal to near-thermal conditions appears to be essential to accomplish the sulfur suppression reaction to the degree required for AMS determination but entails a cost in the transmission of chlorine anions unless appropriate precautions are taken. Lessons learned over the multi-decade development of LC-MS systems, in which ions are transferred efficiently between thermal and non-thermal zones (albeit at generally lower energy levels), can be applied to improve transmission of chlorine anions through the ISA. The approach also improves markedly the transmission of molecular anions such as SrF₃-, of great interest in the determination of other rare isotopes, by reducing their fragmentation during their passage through the ISA.
Dr. Alary has expertise in radio-frequency quadrupole technologies used for trace chemical analysis and ion-molecule chemistry. His knowledge ranges from inductively coupled plasma and liquid chromatography mass spectrometry, to the isobar separator for anions used in accelerator mass spectrometry. His academic background includes physical and analytical chemistry, and he has spent the last two decades in the scientific instrumentation industry. Dr. Alary is currently the president of Isobarex Corp.