Quantitative Distributions of Product Ions and Reaction Times with a Binary Mixture of VOCs in Ambient Pressure Chemical Ionization

Abstract

A model to quantitatively predict ion abundances from atmospheric pressure chemical ionization (APCI) between hydrated protons and a volatile organic compound (VOC) was extended to binary mixtures of VOCs. The model includes differences in vapor concentrations, rate coefficients, and reaction times and is enhanced with cross reactions between neutral vapors and protonated monomers. In this model, two specific VOCs were considered, a ketone, 6-methyl-5-hepten-2-one (M, and an amine, 2,6-di-tert-butyl-pyridine (N), with measured "conditional rate coefficients" (in cm 3 •s −1 ) of k M = 1.11 × 10 −9 and k N = 9.17 × 10 −10 , respectively. The cross reaction of MH + (H 2 O) x to NH + (H 2 O) y was measured as k cr = 1.31 × 10 −12 at 60 °C. Cross reactions showed an impact on ion abundances at t > 30 ms for equal vapor concentrations of 100 ppb for M and N. In contrast, this impact was negligible for vapor concentrations of 1 ppb and did not exceed 5% change in product ion abundance up to 1000 ms reaction times. The model was validated with laboratory measurements to within ∼10% using an ion mobility spectrometer and effective reaction time obtained from computational fitting of experimental findings. This was necessitated by complex flow patterns in the ion source volume and was determined as ∼10.5 ms. The model has interpretative and predictive value for quantitative analysis of responses with ambient pressure ion sources for mass spectrometry and ion mobility spectrometry.

Time-Resolved Ion Mobility Spectrometry with a Stop Flow Confined Volume Reaction Region

“…Two product ions for 2-butanone were seen at and can be attributed to octane-2,7-dione in sulcatone. 37 Resolving powers (ratio of the drift time to the full width at half-maximum) at ion shutter pulse widths of 250 and 500 μs were 34 and 15, respectively, for the Response with Ion Source in SFCV Mode. Response to samples of 2-butanone and sulcatone with the ion source in the SFCV mode is shown in Figure 3 and can be directly compared to findings in Figure 2 with identical sample amounts.…”

“…Chemicals were 6-methyl-5-hepten-2-one (sulcatone) (purity ≥97% PA 875 kJ mol –1 ) and 2-butanone (purity 99%, PA 827 kJ mol –1 ), isopropyl alcohol (purity ≥99.5% PA 793 kJ mol –1 ), and 1-heptanol (purity 98% PA 799 kJ mol –1 ), from Sigma-Aldrich Chemie GmbH (Schnelldorf, Germany), and pentyl acetate (GPR grade PA 840 kJ mol –1 ) from VWR Chemical BDH (Leuven, Belgium). 6-Methyl-5-hepten-2-one was identified to contain octane-2,7-dione (PA 892 kJ mol –1 ) with 2% by mass . Because of this, the sulcatone -2-butanone mixture may be referred to as a ternary mixture.…”

“…Reduced mobilities were obtained from peak positions using reduced mobility of sulcatone true( 1.72 normalc m 2 normalV .25em · normals true) , determined from data provided by Lattouf et al, as a reference for other reduced mobilities. The estimated uncertainty of determination of reduced mobilities is ± 0.04 normalc m 2 normalV · normals .…”

“…Residence times for sample neutrals, once introduced into drift gas flows of 100 to 300 mL min –1 , commonly range from 1 to 8 s as sample is carried through the reaction region volumes of 10 to 40 mL. Such times are relatively long compared to 1 to 100 ms for APCI where reaction times are governed by vapor concentration, reactant ion density, and rate coefficients. , Recently, in silico models showed that product ions were formed with characteristic dependencies for individual VOCs and a binary mixture of VOCs. Computational models were confirmed in laboratory measurements, and such dynamics have been observed in IMS, if not systematically described.…”

Detection of Triacetone Triperoxide by High Kinetic Energy Ion Mobility Spectrometry

“…In complex chemical environments, especially when operating IMS at ambient pressure, analytes with higher proton affinity can remove the proton from analytes that are already ionized. − As a result, substances with low proton affinity might not be detectable in IMS. To overcome this limitation, IMS is often coupled with gas chromatography (GC) to add a preceding separation dimension before sample introduction into the IMS. − Ideally, the substances elute sequentially from the GC, allowing for individual ionization in the IMS without the presence of other substances with higher proton affinity to avoid proton removal of the newly formed ions.…”