Recombination Region:

Within the recombination region (Region I), as voltage increases to V1, the pulse height increases until it reaches a saturation value.  At V1, the field strength among the cathode and anode is sufficient for collection of all ions generated inside the detector. At voltages less than V1, ions move slowly toward the electrodes and the ions tend to recombine to form neutral atoms or molecules. In that case, the pulse height is less than it would have been if all the ions actually formed reached the electrodes. Gas ionization instruments are, thus, not operated within this region of response.