Describe the principles of operation for the FID, TCD and ECD. Describe the types of analytes measured with each detector.

Sure, here is the information you requested:

1. Flame Ionization Detector (FID): This detector operates on the principle of ionization of the sample components by a flame, typically a hydrogen-air flame. The organic compounds in the sample are pyrolyzed in the flame, producing ions and electrons. These charged particles are collected at electrodes, producing an electrical signal proportional to the concentration of the compound. FID is most commonly used for detecting and measuring organic compounds, particularly hydrocarbons.

2. Thermal Conductivity Detector (TCD): This detector operates on the principle of changes in thermal conductivity of the gas mixture flowing through the detector. It consists of an electrically-heated filament in a temperature-controlled cell. When the thermal conductivity of the gas changes due to the presence of the sample component, the temperature of the filament changes, which in turn changes its resistance. This change in resistance is measured and related to the concentration of the component. TCD is a universal detector and can be used to detect any component that has a different thermal conductivity than the carrier gas.

3. Electron Capture Detector (ECD): This detector operates on the principle of electron capture by the sample components. It contains a radioactive source that emits beta particles (high energy electrons). When these electrons collide with the sample molecules, they can be captured if the molecule has a high electron affinity. This reduces the current flowing through the detector, and this change is measured and related to the concentration of the component. ECD is highly sensitive and selective for molecules containing electronegative functional groups, such as halogens, peroxides, quinones, etc.

I hope this helps! Let me know if you have any other questions.