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Effect of Sample Concentration on Response

At low concentrations of the analyte, reactant ions are consumed proportionally to the density of the vapor neutrals per Equation 8.9  [Pg.177]

The rate constant k in Equation 8.9 is roughly the collision rate constant, which controls the formation of product ions. As shown in Eigure 8.10, the density of product ions, for a sample at a constant concentration, is dependent on both residence time of the sample in the source and its concentration. The depletion of reaction ions is naturally rooted in kinetics of Equation 8.9 and parallels inversely the formation of product ions. The relationship is virtually stoichiometric. Eor a fixed time of residence of sample molecules in the reaction region, the number of product ions and their peak intensity in the mobility spectrum will be quantitatively proportional to the concentration of sample molecules. Also, increases in residence time for [M] in the source region will lead to an increase in the number of product ions as long as the source is not saturated with sample vapor. Once the reservoir of reactant ion charge is totally consumed, no additional increase in product ion intensity will be seen even with additional increase in the concentration of sample molecules. In practice, this limits the upper end of the linear range associated with ion sources at ambient pressure. [Pg.177]

FIGU RE 8.10 Plots of ion number with reaction time or time of mixing for ions and neutrals at ambient temperature and pressure. The curves show response to two analytes of comparable proton affinity. The analytes differed by 10-fold in concentration, illustrating the kinetic basis of forming product ions. [Pg.178]

FIG U RE 8.11 Plots of ion intensities from vapor sampling of a moving plume of a chemical vapor. The numbers are time in seconds used to generate the plume. These plots show the dynamics in an IMS analyzer. [Pg.179]


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