A recent article by Runlong Cai and co-authors, published in the Analytical Chemistry, describes the impressive synergy between the MION2 chemical ionization ion source and the Orbitrap Mass Spectrometer and their ability to enhance the detection of a broad range of oxygenated organic species with fast polarity switching.
MION Inlet: MION is a multi-scheme chemical ionization inlet that allows for rapid switching of reagents. Its design enables efficient ionization with positive and negative ions, crucial for detecting a broader range of trace compounds.
Orbitrap Mass Spectrometer: The Orbitrap mass spectrometer stands out with its high mass resolution (>100 000) and accuracy (<1 ppm). When combined with the MION inlet, their ability to quickly switch polarities makes them a powerful tool capable of detecting and analyzing even the most complex organic mixtures in the air.
Polarity switching chamber experiments: The MION-Orbitrap system was used to measure Oxygenated Organic Molecules (OOMs) produced from the alpha-pinene ozonolysis experiment. The system switched the measurement cycle between negative nitrate ions (NO3-) and positive diethylammonium reagent ions (C4H12N+) within 1 min, analyzing trace species with high resolving power ( Fig. 2). When the polarity was switched, the signal increased from 0 to a plateau within 1−2 scans (∼1−2 s), and the OOM-clusters signals appeared at the same time as the reagent ion signal. The plateaus of the reagent ions (Fig. 2a) and the OOMs (Fig. 2b) were stable over time, showing that polarity switching had negligible interference with a precision of signals. The faster time of 12 seconds for each polarity was also tested, and stable signals were observed.
The results from the alpha-pinene ozonolysis chamber experiment show that the combination of negative and positive modes allows to cover a much wider range of detectable compounds, comprising oxygenated and highly oxygenated organic molecules (Fig. 3). In this experiment, the switching was done every 7.5 min, with the experiment lasting for 48 hours. Nitrate and diethylammonium are selective towards different oxidation states of the compounds: nitrate - towards highly oxygenated organic species, while diethylammonium towards OOMs with low oxidation states. The figure shows that only a minor fraction of OOMs were measured with the widely used NO3− ionization compared to the combined results from NO3− and C4H12N+ ionization. Using these reagent ions in the same system allows to detect species covering all oxidation states.
Pesticide detection was also tested with this system: more than 80% of compounds from the standard pesticide mixture were detected using switching capabilities,compared to around 30% of compounds detected with only nitrate reagent ion. The polarity switching capability of the MION-Orbitrap allowed it to detect more pesticides than using only one ionization mode, increasing the total number of detected pesticides from 23 to 59 in the standard pesticide mixture.
Conclusions:
The polarity switching MION-Orbitrap is shown to be a versatile and powerful technique for the online analysis of a wide range of reactive trace species, offering significant improvements in non-targeted chemical analysis of complex samples.
Reference: Cai, R., Mikkilä, J., Bengs,A., Koirala, M., Mikkilä, J., Holm, S., ... & Kangasluoma, J. (2024).Extending the Range of Detectable Trace Species with the Fast PolaritySwitching of Chemical Ionization Orbitrap Mass Spectrometry. AnalyticalChemistry, 96(21), 8604-8612. This publication is licensed under CC-BY 4.0 .
https://pubs.acs.org/doi/abs/10.1021/acs.analchem.4c00650
