We studied the dissociation of CO and N-2 gases on a W(110)-oriented tip by an atom probe (AP) technique. We paid attention to the effect of electric field induced dissociation as well as the temperature dependence of dissociation probabilities. We first show that the AP can discriminate between physisorbed and trapped states based on the difference in desorption field strength. Then we show that field-induced dissociation can be properly measured. Finally, we conclude the activation energy required for the conversion from the trapped state to the dissociated state to be 30 meV for CO gas and 22 meV for N-2 gas.