Characterization of the Axial Jet Separator with a CO₂/Helium Mixture: Toward GC-AMS Hyphenation

Abstract

Development of interfaces for sample introduction from high pressures is important for real-time online hyphenation of chromatographic and other separation devices with mass spectrometry (MS) or accelerator mass spectrometry (AMS). Momentum separators can reduce unwanted low-density gases and introduce the analyte into the vacuum. In this work, the axial jet separator, a new momentum interface, is characterized by theory and empirical optimization. The mathematical model describes the different axial penetration of the components of a jet-gas mixture and explains the empirical results for injections of CO2 in helium into MS and AMS instruments. We show that the performance of the new interface is sensitive to the nozzle size, showing good qualitative agreement with the mathematical model. Smaller nozzle sizes are more preferable due to their higher inflow capacity. The CO2 transmission efficiency of the interface into a MS instrument is ∼ 14% (CO2/helium separation factor of 2.7). The interface receives and delivers flows of ∼ 17.5 mL/min and ∼ 0.9 mL/min, respectively. For the interfaced AMS instrument, the ionization and overall efficiencies are 0.7-3% and 0.1-0.4%, respectively, for CO2 amounts of 4-0.6 μg C, which is only slightly lower compared to conventional systems using intermediate trapping. The ionization efficiency depends on to the carbon mass flow in the injected pulse and is suppressed at high CO2 flows. Relative to a conventional jet separator, the transmission efficiency of the axial jet separator is lower, but its performance is less sensitive to misalignments.

A Continuous-Flow Gas Interface of a Thermal/Optical Analyzer With14C AMS for Source Apportionment of Atmospheric Aerosols

“…It is known that the O – signal is much higher than the C – signal for direct gas ionization as shown by Fahrni et al (2013). At high carbon mass flows, the CO 2 concentration at the target is higher; therefore, it is possible that the oxygen capture most of the available electrons, suppressing the C – formation (Salazar et al 2016). The feeding of the CO 2 pulse into the ion source with a continuous-flow system cannot be kept as constant and controllable as performed by the gas-tight syringe of the GIS.…”