Gas Chromatography Detectors - The Flame Ionization detector > The High Impedance Amplifier > Page 39


The ion current flows through the resistance to complete the circuit and the voltage developed across the resistance is applied to the input of a high impedance amplifier. This is the more stable of the configurations providing the jet insulator has a high enough resistance at the temperature of operation. The other alternative is to earth the jet (which can eliminate the need for a jet insulator) and insert the resistance between the power supply and ground. This means that the power supply circuit must be isolated from earth and thus will be more susceptible to electrical interference. It does, however, reduce the high impedance sensor connections to a single cable. The voltage applied across the jet and electrode ranges from 50 to 250 volts depending on the geometry of the sensor and, in particular, the proximity of the electrode to the jet.


The High Impedance Amplifier

Before the advent of field effect transistors (FET), high impedance amplifiers were constructed using specially selected thermionic valves with very low grid currents (for those familiar with thermionic valves, the EF87 was a specially selected 6J5G triode with a grid current of about 10-12 amp). Modern integrated circuits (e.g. an FET operational amplifier functioning in its non inverting mode) can have an input impedance of 1014 ohm. Consequently, there is no significant drain on the current source (such as the FID) and the voltage developed across the high resistance in figure 17 will be directly related to the ion current. The FID produces a base ion current when only hydrogen is being burned at the jet, and so in the early stages of the amplifier circuit an adjustable offset voltage is often provided to produce zero output when no solutes are being eluted. The FID can operate over a concentration range of more than six orders of magnitude and so the amplifier must also provide range switching.