Henning Finkenzeller and co-authors have published a modelling study in Atmospheric Measurement Techniques (AMT) looking at properties of MION2 and Eisele-type inlets. We had the pleasure of speaking with Henning about the key results and implications of this research.
Why is it important to do this kind of modelling work?
Chemical Ionization Mass Spectrometry is a powerful technique for analysing complex gas mixtures. However, some processes that occur within the instrument are poorly understood and studied. It is interesting to quantitatively understand the influence of individual control parameters,such as voltages and gas flow rates, on the processes, for example, to better tune these parameters, which needs to be done before the measurements.
Describe briefly the two inlets that were studied.
We have looked at two chemical ionization inlets: MION2 and Eisele-type inlets. For both inlets, ionisation happens at atmospheric pressure, and the reagent gas is ionised by the X-ray source.
MION2 is a multi-scheme chemical ionisation inlet consisting of an ion-molecule reactor (IMR) and usually two source towers. In each source tower, the formed reagent ions are transported to the IMR by the electric field created by electrodes on both sides of the source. In the IMR, reagent ions mostly form adducts with the selected molecules in the sample flow. Switching between source towers can be done in seconds due to the fast response of the electric field to voltage change by the deflector.
The Eisele-type inlet is a cylinder with a bigger diameter. The ionisation of the reagent gas takes place in the outer circle, while the sample flow is introduced to the middle of the inlet with the inner tube. The mixing of reagent ions and the sample flow happens due to the convective flow.
Which parameters were modelled in the study?
Using the 3D computational fluid dynamics physicochemical model, we looked at some of the key parameters that affect the performance of the inlets: (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.
What are the main differences in the physical quantities found between the inlets?
The MION2 inlet achieves a near-ideal ion delivery efficiency (maintains high ion concentrations during transport) to the pinhole due to its efficient charge separation and well-defined electroconvective fields. The Eisele inlet demonstrates lower ion delivery efficiency (10-20%), primarily due to its axis symmetry, which requires diffusion to get ions to the center line and ultimately the mass spectrometer. One big difference is that MION2 selectively introduces the reagent ion only – it keeps the reagent gas separate from the sample gas – while in the Eisele-type inlet the reagent gas is in direct contact with the sample gas, making it prone to contamination.
Reaction times within the IMR vary significantly between the two inlets. MION2 has a much shorter reaction time(~22 ms) than Eisele (~113 ms), making MION2 more suited for rapid analysis of short-lived compounds.
How did you validate modelled results?
We performed measurements of the ion currents that go to the electrodes, and we varied the control voltages in both the model and in the laboratory. The model is certainly not perfect, but based on the agreement we saw, we are confident that the model gets the fundamental processes right.
To conclude, what properties of the chemical ionization inlet affect performance the most and could be considered when designing the new inlets?
The modelling suggests that likely the initial ion production limits how much reagent ion is introduced into the IMR. Space charge or ion-ion recombination are not yet limiting, but we are not far away from that being the case. This means that it will be difficult to enhance the ion concentrations much. Considering that, I think that the well-controlled switching between different ions (and ionization schemes in general) is what will be most useful.
Read the full publication here: https://amt.copernicus.org/articles/17/5989/2024/amt-17-5989-2024.html
