Go Direct® Electrode Amplifier (GDX-EA) is the amplifier that comes as part of:
Go Direct® pH Sensor (GDX-PH)
Go Direct® Tris-Compatible Flat pH (GDX-FPH)
Go Direct® Glass-Body pH Sensor (GDX-GPH)
Go Direct® ORP Sensor (GDX-ORP)
It has two measurement channels: pH and Potential (mV) and is shipped with the appropriate channel set as the default. It can also be purchased separately, in which case it is set to Potential (mV).
Go Direct® Ion-Selective Electrode Amplifier (GDX-ISEA) is the amplifier that comes as part of:
Go Direct® Chloride Ion-Selective Electrode (GDX-CL)
Go Direct® Calcium Ion-Selective Electrode (GDX-CA)
Go Direct® Ammonium Ion-Selective Electrode (GDX-NH4)
Go Direct® Nitrate Ion-Selective Electrode (GDX-NO3)
Go Direct® Potassium Ion-Selective Electrode (GDX-K)
It has six measurement channels: Chloride (mg/L), Calcium (mg/L), Ammonium (mg/L), Nitrate (mg/L), Potassium (mg/L), and Potential (mV), and is shipped with the appropriate channel set as the default. It can also be purchased separately, in which case it is set to Potential (mV).
Note that for both of these amplifiers, the selected channel must match the attached electrode. For example, changing the ISE Amplifier to the Nitrate (mg/L) channel will not actually measure nitrate concentration unless there is a Go Direct® Nitrate Ion-Selective Electrode BNC (GDX-NO3-BNC) attached.
If you wish to purchase other electrodes and change the channels of your amplifier to match, see How do I change the displayed measurement channel on my Go Direct Electrode Amplifier or ISE Amplifier?.