Kinetics

The 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:

• Direct 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.
  
  
• Arrival 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.