Abstrakt
Ion chemistry that takes place inside a flow or a drift tube reactor of a soft chemical ionization mass spectrometer, SCIMS, is highly sensitive to presence of water vapour in carrier gas. This effect is very prominent especially in a proton transfer reaction system initiated by the H3O+ reagent ion.
Presence of water molecules inside the flow/drift tube leads to a series of association reactions and to formation of (H2O)N*H3O+ clusters [1]. Effective evaluation of absolute concentrations of investigated analytes strongly depends on accuracy of the evaluation of rate constants and on information about relative ratio of the reagent ions.
This evaluation can be done in a controlled laboratory environment, but operation of instruments in regular environment as for a clinical or environmental application can't satisfy this condition. It is thus important to understand the dynamic influence of humidity on results of analyses using SCIMS techniques.
Here we present a comparison of experimentally obtained data and theoretical calculation of expected concentrations of reacting ions with respect to concentration of neutral water molecules inside the flow/drift tube reactor containing He carrier gas. Experimental data were provided by investigation of acetone using SIFT-MS [2] and investigation of several alcohols using SIFDT-MS [3] systems.
The experimental results are compared with a reaction system kinetics model based on numerical solution of a complete set of differential equations describing the ion chemistry using the classical Runge-Kutta method. Based on a number of free parameters, only approximate analysis was obtained and the uncertainty of the rate coefficients