Instrumentation

Instrumentation 

• An Electron Spin Resonance Spectrometer consists of 6 essential parts which are:
• The source of the microwaves
• The Detector
• The Phase Sensitive detector
• Electromagnet
• Sample Cavity 
• Modulation input

How is the technique implemented?

The E.S.R Spectrometer works by having a source which is referred to as a Klystron in old spectrometers and a Gunn diode in new ones. The source essentially generates the microwaves that will be absorbed by the sample. The microwaves produced, pass through an isolator which reduces any reflections back to the source which would result in fluctuations of the microwave frequency. The microwave power from the source is passed through a directional coupler which splits the microwave power into two paths, one directed towards the sample cavity and the other the reference arm. Both paths contain attenuators (modulation input) that allows for adjustment of the microwave power released from the source which interacts with the sample. On the reference arm, after the variable attenuator there is a phase shifter that sets a defined phase relationship (solid, liquid or gas) between the reference and reflected signal which permits phase sensitive detection.

Most EPR machines are reflection spectrometers, meaning that the detector should only be exposed to microwave radiation coming back from the cavity. This is achieved by the use of a device known as the circulator which directs the microwave radiation into the cavity. Reflected microwave radiation (after absorption by the sample) is then passed through the circulator towards the detector, ensuring it does not go back to the microwave source. The reference signal and reflected signal are combined and passed to the detector diode which converts the microwave power into an electrical current which interacts with the magnets.

If the electromagnetic field around the resonating cavity, while having the sample is changed to the value required for the resonance, the recorder will show an absorption peak.

If the magnetic field is swept slowly over a period of several minutes, the recorder will show the derivative of the microwave absorption spectrum against magnetic field as shown below: