AMT: Multiphysical description of chemical ionisation in MION2

This article provides a detailed comparison of two atmospheric pressure interface chemical ionization inlets used in mass spectrometry: the MION2 inlet and the Eisele type inlet. The performance of these inlets was evaluated and quantified with 3D computational fluid dynamics physicochemical models and confirmed with laboratory experiments.  

Key results:

• The MION2 inlet achieves a near-ideal ion delivery efficiency (over 90%) due to its efficient charge separation and well-defined electroconvective fields. It is capable of maintaining high ion concentrations during transportation to the Ion-Molecule Region (IMR), which is critical for sensitive measurements.
• The Eisele inlet demonstrates lower ion delivery efficiency (10-20%), primarily due to its reliance on convective transport in a electric field-free ionization volume, leading to more significant ion losses during transport.
• Reaction times within the IMR vary significantly between the two inlets, with MION2 having a much shorter reaction time (~22 ms) compared to Eisele (~113 ms), making MION2 more suited for rapid analysis of short-lived compounds.

‍Modelled characteristics of the inlets

Using the 3D computational fluid dynamics physicochemical model, some of the key parameters that affect the performance of the inlets were considered: (1) the ion generation from the reagent gas, (2) electro-convective transport of ions to the pinhole, (3) gas flows through the inlet and (4) interactions of gas-phase species with the surfaces.

MION2 inlet

Figure 1 shows the geometry (a) and selected physical quantities in the MION2 inlet, using nitric acid as a reagent gas. The reagent gas is ionised by the X-Ray and the ions are transported to the ion–molecule mixing region (IMR) by the electric field, created by 20 electrodes on both sides of the source. A purge flow ensures that neutral molecules are not transported to the IMR.

‍Eisele type inlet

Figure 2 shows the geometry and modelled parameters of the Eisele type inlet. The ionisation volume of the Eisele inlet is essentially electric field free; reagent ions and their complements are transported out of the ionisation volume by convection only.

‍Conclusions

This study elucidates the inlet-internal processes, explains observed sensitivities, and highlights the design differences between the MION2 and Eisele type inlet. While the Eisele type inlet performs well for a relatively simple setup, MION2 type inlets extract ions more efficiently because of the electric field within the ion volume and are near-ideal in delivering the ions to the IMR. MION2 type inlets also allow ion switching or the sampling of ambient ions.
‍

‍Reference: Finkenzeller, H., Mikkilä, J., Righi, C., Juuti, P., Sipilä, M., Rissanen, M., Worsnop, D., Shcherbinin, A., Sarnela, N. and Kangasluoma, J., 2024. Multiphysical description of atmospheric pressure interface chemical ionisation in MION2 and Eisele type inlets. Atmospheric Measurement Techniques, 2024. https://doi.org/10.5194/amt-17-5989-2024. This article is distributed under the Creative Commons Attribution 4.0 License.

https://amt.copernicus.org/articles/17/5989/2024

‍

‍