Multi-scheme chemical ionization inlet (MION) for fast switching of reagent ion chemistry in atmospheric pressure chemical ionization mass spectrometry (CIMS) applications

A novel chemical ionization inlet named the Multi-scheme chemical IONization inlet (MION), Karsa Ltd., Helsinki, Finland) capable of fast switching between multiple reagent ion schemes is presented, and its performance is demonstrated by measuring several known oxidation products from much-studied cyclohexene and α-pinene ozonolysis systems by applying consecutive bromide (Br) and nitrate (NO−3 ) chemical ionization. Experiments were performed in flow tube reactors under atmospheric pressure and room temperature (22 C) utilizing an atmospheric pressure interface time-of-flight mass spectrometer (APi-ToFMS, Tofwerk Ltd., Thun, Switzerland) as the detector. The application of complementary ion modes in probing the same steady-state reaction mixture enabled a far more complete picture of the detailed autoxidation process; the HO2 radical and the least-oxidized reaction products were retrieved with Br ionization, whereas the highest-oxidized reaction products were detected in the NO−3 mode, directly providing information on the first steps and on the ultimate endpoint of oxidation, respectively. While chemical ionization inlets with multiple reagent ion capabilities have been reported previously, an application in which the charging of the sample occurs at atmospheric pressure with practically no sample pretreatment, and with the potential to switch the reagent ion scheme within a second timescale, has not been introduced previously. Also, the ability of bromide ionization to detect highly oxygenated organic molecules (HOM) from atmospheric autoxidation reactions has not been demonstrated prior to this investigation.

Example spectra obtained from cyclohexene ozonolysis experiments in both ion modes shown with a common product mass axis, i.e., the Br-spectrum is displaced by 17 Th (=difference between reagent ion Br-and NO3masses) to overlap the same composition products horizontally. Upper panels show nitrate spectra (red) and lower panels bromide spectra (blue). a) Illustrates the reagent ion peaks, b) the monomer range (i.e., oxidation products which have the same number or less carbon atoms than cyclohexene), and c) the dimer range (i.e., oxidation products with about two times the carbon number of cyclohexene), respectively. For a few of the most prominent product peaks also the explicit compositions are shown.
Old Figure S4 and its caption: Figure S4 Example spectra obtained from cyclohexene ozonolysis experiments in both ion modes shown with a common product mass axis, i.e., the Br-spectrum is displaced by 17 Th (=difference between reagent ion Br-and NO3masses) to overlap the same composition products horizontally. Upper panels show nitrate spectra (red) and lower panels bromide spectra (blue). a) Illustrates the reagent ion peaks, b) the monomer range (i.e., oxidation products which have the same number or less carbon atoms than cyclohexene), and c) the dimer range (i.e., oxidation products with about two times the carbon number of cyclohexene), respectively.

Reviewer#2
Review Author reply: More physical details were added to the main text, based on the reviewer recommendation. In addition, an additional schematic figure of the inlet with marked dimensions was added to the supplementary material.
Addition to text (page 2, line 103): "The ion source connecting MION to the mass spectrometer entrance weights about 2.1 kg, with every additional stage weighting roughly 1.5 kg each. The distance of the first ion source to the mass spectrometer inlet orifice is approximately 3 cm, and the length of an additional ionization stage is about 10 cm, with a height of 16 cm. A schematic Figure  S1 with marked dimensions can be found in the supplementary material." New figure S1:

Figure S1
Schematic of the MION showing the approximate dimensions of the inlet design.

From figure 1a, it looks like that the directions of two ionization stages are 90degree different. Is this for experimental reason or for illustration?
Author reply: This is the schematic of the current design. The ion sources are 90degrees to each other to enable easier usage of the inlet connections. This should not matter to the measurement to any significant extent, as the flow inside the pipe has a circular symmetry.
3. Figure 2. It would be nice to include an insert figure with shorter time period, to better show the switch between two reagent ions.
Author reply: According to the reviewer recommendation we have added an insert figure to better demonstrate the rapid ion mode switch.
New figure 2 and its caption: Figure 2 An example of switching between multiple ion chemistries while sampling laboratory air. The ion modes utilized have been marked with separate colours and are further labelled above the figure with abbreviations: APi for ambient ion mode (no active ionization applied; black trace), Br-for bromide ion ionization mode (red trace), NO3-for nitrate ion ionization mode (blue trace), and TIC is used to indicate total ion count measured (black trace). The insert demonstrates the rapid ion mode switch, shown in a linear scale. All signals shown here were retrieved by high resolution peak fitting.
Old Figure 2 and its caption: Figure 2 An example of switching between multiple ion chemistries while sampling laboratory air. The ion modes utilized have been marked with separate colours and are further labelled above the figure with abbreviations: APi for ambient ion mode (no active ionization applied; black trace), Br-for bromide ion ionization mode (red trace), NO3-for nitrate ion ionization mode (blue trace), and TIC is used to indicate total ion count measured (black trace). All signals shown here were retrieved by high resolution peak fitting.