drift-cell experiments inherently involve a time element, making them
ideal for kinetics studies. There are two methods available to us for
determining rate constants:
Measurement: As shown in the Introduction,
we are able to control the amount of time ions spend in the drift cell
by changing the pressure in the cell and the voltage drop across it.
In addition, we can determine the relative concentrations of the various
ionic species present in the cell by obtaining a mass spectrum. Thus
by measuring concentration as a function of time, we can directly obtain
rate constants for reactions taking place in the drift cell.
Time Distribution Modeling: We can also obtain rate constants by
measuring ATDs. If all the ions pulsed into the drift cell remain intact
and maintain their conformation while they drift through the cell, the
resulting ATD will be composed of a symmetric peak for each conformer
present. If some of the ions dissociate or isomerize to a different
conformation, the shape of the ATD will change. The new shape will depend
on the rates at which the dissociation or isomerization reactions occur.
By modeling the ATD, we can obtain the rate constants for these processes.