Abstract
ABSTRACTA fully automatic continuous-flow gas injection interface was built to couple an elemental analyzer with a MICADAS accelerator mass spectrometer (AMS) as a low-cost option that does not require an absorber trap for CO2 injection. The complication of the variable ion current during gas injection can be overcome by understanding and controlling the mass flow-dependent ionization yield. The time-varying CO2 concentrations and carbon mass flows are estimated with a mathematical model in order to investigate their relationship with the abundant isotope (12C–) signal. This model is based on a complete CO2 diffusion equation and instantaneous mass flow. It shows a good agreement between model calculations and the measurements. A reversible suppression of the formation of ions occurs, if the carbon mass flow exceeds 2.0–2.3 µg C/min. This result repeats for different injection capillaries and for different carrier volumetric flow rates.
Highlights
Direct CO2 analysis allows one to process small radiocarbon (14C) samples more efficiently, when CO2-producing devices are coupled with accelerator mass spectrometers (AMS) (Ruff et al 2007)
We present a low-cost CF interface with large dead volume for hyphenating an elemental analyzer with the gas ion source of a MICADAS (Szidat et al 2014); and develop a theoretical-mathematical model to predict carbon mass flow during CF injection
The MICADAS is operated at 135oC Cs temperature, 128 W power for the Cs reservoir heater and at a background pressure in the ionizer chamber of 2 × 10–6 mbar during gas injection
Summary
Direct CO2 analysis allows one to process small (tens of μg C) radiocarbon (14C) samples more efficiently, when CO2-producing devices are coupled with accelerator mass spectrometers (AMS) (Ruff et al 2007). We present a low-cost CF interface with large dead volume for hyphenating an elemental analyzer with the gas ion source of a MICADAS (Szidat et al 2014); and develop a theoretical-mathematical model to predict carbon mass flow during CF injection. Our hypothesis is that it is possible to determine a real-time or instantaneous carbon mass flow by calculating the concentration profile of the injected CO2 peak This mass flow should govern the intensity and efficiency of the ionization in a similar fashion as non-CF interfaces. The purge valve is closed and the split valve is opened This new arrangement directs some of the carrier flow to the split vent at a rate controlled by the needle valve, keeping the dead volume pressurized around a desired value. The MICADAS is operated at 135oC Cs temperature, 128 W power for the Cs reservoir heater and at a background pressure in the ionizer chamber of 2 × 10–6 mbar during gas injection
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