Frequency resolved mass spectrometry (FRMS) is a new technique being developed in our laboratory to examine reaction kinetics and mechanisms of electrocatalytic and catalytic surface reactions. Typically, the use of mass spectrometry in studies of heterogeneous catalysis is either for a single mass, such as CO₂ produced by reaction of CO and O₂, or multiplexed to record multiple masses, such as CO₂ production, CO consumption, and O₂ consumption, as a function of time. In single-mass mode, the signal is recorded continuously as a function of time. In multiplexed-mass mode, the signals of multiple species are recorded at discrete points in time, where the time between each data point is very small compared to the time scale of the experiment.

Time dependence is a key aspect in studies of kinetics. Time dependence in studies of catalysis is invoked in a number of ways:

• Steady-state:
  The behavior of a one or more species species over long periods of time where all reaction variables are kept constant. These experiments examine long-term catalyst performance related to degradation, for example.
• Ramped:
  The effect of kinetic driving forces are examined for linear ramps of  electrode potential or temperature as a function of time. These studies provide first order estimates of kinetic parameters such as overpotential or activation energy. Since they are not steady state, the reaction environment continuously changes over the course of the measurement.
• Pulsed:
  Kinetic driving forces of electrode potential, temperature, or reactant concentrations are stepped between one or more values to examine the reacting systems response to the new value of the driving force. The pulsed method improves signal-to-noise ratio at the expense of probing all frequencies simultaneously, which cause a loss of frequency dependence.
• Sinusoidal (FRMS):
  The FRMS technique uses sinusoidal perturbations to examine reaction kinetics at one frequency. The method requires higher signal-to-noise to capture the signal, however. The mass spectrometer data are Fourier transformed to obtain harmonics and phase relationships of each species. Each individual reaction within a complex reaction network has a characteristic frequency that can be determined with FRMS. The phase relationships provide a direct means for determining reaction mechanisms.
  An especially powerful aspect is that the mass spectrometer data can be matched with the Nyquist plot obtained by electrochemical impedance spectroscopy (EIS). This enables a 1:1 mapping of electrochemical response with species response in the gas phase.