Chemical Ionization Mass Spectrometry is a powerful technique for the rapid and sensitive detection of trace gases relevant to atmospheric chemistry. Usually,an inlet with a specific reagent ion chemistry is coupled to a mass spectrometer. However, this setup is not able to cover the whole range of compounds. Therefore, the Multi-Scheme Chemical Ionization Inlet, MION, was developed to switch rapidly between reagent ion chemistries at atmospheric pressure.
The same ionization time is possible for three chemical ionization methods by positioning them at the same distance from the pinhole.
Enhanced ion optics is more efficient in focusing reagent ions, which helps to achieve lower limits of detection.
Additional purge flow prevents sample flow from entering the ion source and the other way around, preventing neutral reagent from entering the sample flow (Fig. 1).
Calibration experiments for H2SO4, HOI and HO2 were performed using MION2, APi-TOF and a flow reactor section, which consisted of a calibration source and several gas feeds. Calibration coefficients for H2SO4, HOI, HO2 and SO2 were derived for Br- and NO3- chemical ionization methods with 35 ms and 300 ms ionization time. A lower calibration factor 9.8x108 was obtained for H2SO4 with longer ionization time as more molecules had time to convert to H2SO4 •Br-.
One of the important characteristics of the chemical ionization inlet system is the limit of detection. Limits of detection were determined for Br--MION2 to be at the level of 105 cm-3 for H2SO4, HIO3 and I2, comparable to commonly used Eisele-type inlet, and even lower for longer ionization time. During 300 ms ionization time the detection limit of H2SO4 was measured to be as low as 2.9x10-4 molec cm-3 when using Br--MION2.
The sensitivity of the ambient measurements with a Br- ion source can be affected by higher relative humidity, therefore, its performance under a wide range of humidity conditions was explored. The detection humidity effect of H2SO4, HOI, HO2, SO2 and I2 with RH from below 1 % to 60 % at 25◦C were examined. Generally, it was found that an excess amount of water content results in lower detection sensitivity for all considered species. Tolerance to humidity changes depends on the binding strengths with Br-: the stronger the binding of the compounds with Br-, the higher the tolerance to humidity changes, and they can be ordered as I2 > HOI > HO2 > SO2. Contrary, H2SO4 is observed to have a non-linear effect: the detection sensitivity first increases with higher RH until around 33% when it has a sharp drop. Moreover, shorter ionization time was found to have a lower detection humidity effect for all considered species.
Two methods were tested to reduce the detection humidity effects of a Br- ion source: dilution and the use of an additional core-sampling device for adjusting the sheath and sample flows. Both methods were efficient for detecting species that are weakly bonded with Br- (for example, SO2), but care should be taken when utilizing them.
Additionally, the study also confirmed the existence of gaseous iodic acid (HIO3), which was previously a subject of discussion. Experiments were done with both Br- and NO3- ion sources by keeping constant iodine concentrations (I2) and light intensity while only varying the ozone concentrations. A linear relationship between signals of gaseous HIO3 and IO3- was obtained, which validated the existence of gaseous HIO3.
Reference: He, X.-C., Shen, J., Iyer, S., Juuti, P., Zhang, J., Koirala, M., Kytökari, M. M., Worsnop, D. R., Rissanen, M., Kulmala, M., Maier, N. M., Mikkilä, J., Sipilä, M., and Kangasluoma, J.: Characterisation of gaseous iodine species detection using the multi-scheme chemical ionisation inlet 2 with bromide and nitrate chemical ionisation methods, Atmos. Meas. Tech., 16, 4461–4487,2023. https://doi.org/10.5194/amt-16-4461-2023
This work is distributed under the Creative Commons Attribution 4.0 License. CC BY 4.0 Deed | Attribution 4.0 International | Creative Commons
