Several instrument configurations, especially operating at different ionization pressures, are typically needed to analyse the full range of compounds from non-functionalized parent compounds to their functionalized reaction products.
High-pressure adduct-forming CIMS is used for polar, functionalized compounds, such as Oxygenated Organic Molecules (OOMs) and Highly Oxygenated Organic Molecules (HOMs). These compounds often form strongly bound molecular clusters with other polar reagent ions. On the other hand, Volatile Organic Compounds (VOCs) such as benzene, toluene, and a-pinene are only weakly polar and interact weakly with ions, necessitating low-pressure methods with harder ionization.
Multi-Pressure Chemical Ionization Mass Spectrometry (MPCIMS, Figure 1) is demonstrated with a system combining a MION2 atmospheric-pressure chemical ionization inlet and a low-pressure ion source within an Orbitrap Exploris™ 120 mass spectrometer.
MION2 High-Pressure CI Inlet:
The MION2 inlet operates at ambient pressure, generating reagent ions through X-ray ionization. A series of electrodes guides the reagent ions into the Ion-Molecular Reactor (IMR), preventing contamination of the IMR with the neutral sample gas. For this study, reagent ions were chosen to detect a wide range of oxidation states of the a-pinene oxidation products: Bromide (Br− ) and protonated diethylamine (C4H12N+).
Orbitrap Exploris™ 120 (In-build low-pressure source):
The low-pressure ionization occurs within the Orbitrap at a pressure below one mbar. A positively charged fluoranthene (C16H10+ ) is used as a reagent ion. Fluoranthene is a novel reagent ion produced from crystalline fluoranthene (C16H10 ). From the crystalline form, it is evaporated in a temperature-controlled oven, and the supply is enough for one year of continuous operation. A DC plasma electron source generates free electrons, which ionize fluoranthene transported to the ionization volume. The ionization of a compound predominantly occurs through electron abstraction.
Calibration of the low-pressure source
The sensitivity and linearity of the low-pressure source were characterized using a set of VOCs seen in Figure 2. Fluoranthene (C16H₁₀+) served as the reagent ion for low-pressure ionization, successfully detecting VOCs in both dry (RH< 2 %) and humid (RH ≈30 %) conditions. The detection of the VOCs was linear over two orders of magnitude of volume-mixing ratio, from 5 ppb down to 50 ppt. The limits of detection in the demonstration setup range from single ppb (methylvinyl ketone, MVK) to a few ten ppt (e.g., xylene).
Alpha-pinene oxidation experiment
α-pinene oxidation was selected as a test case due to its relevance to atmospheric chemistry. The experiment involved dosing α-pineneinto an oxidative flow reactor where it reacted with ozone (generated by UV lamps), producing a spectrum of oxidation products.
The low-pressure ionization by C16 H10+ allows the monitoring of the non- and less-oxygenated precursors (N = 0, 1, 2), while high-pressure C4H12N+ ionization tracks moderately oxygenated compounds (N = 2, 3, 4 ), and finally Br− more oxygenated compounds (N = 4, 5, 6).
MPCIMS has the potential to be an all-in-one method for analysing complex gas mixtures by combining the strengths of high- and low-pressure ionization techniques. This allows for a broader detection range of compounds without compromising sensitivity.
Reference: Shcherbinin A, Finkenzeller H, Mikkilä J, Kontro J, Vinkvist N, Kangasluoma J, et al. From Hydrocarbons to Highly Functionalized Molecules in a Single Measurement: Comprehensive Analysis of Complex Gas Mixtures by Multi-Pressure Chemical Ionization Mass Spectrometry. ChemRxiv. 2024, doi: 10.26434/chemrxiv-2024-q1k6s. This publication is distributed under CC BY NC ND 4.0
