Field Modulated and Direct Detected Continuous Wave EPR
CW Field modulated and direct detected EPR measurements can be conducted on all three available instruments and involve continuous exposure of the sample to the microwaves.
Field Modulated EPR
However, the magnetic field is not simply swept. In addition to the bulk magnetic field of the electromagnet being moved at each data point, an additional field is present from a set of coils within the cavity. The magnetic field created by these coils is modulated, typically at a frequency of 100 KHz, meaning that it oscillates from a positive to a negative field (with respect to the bulk magnetic field). The amplitude of the modulated field is set on the instrument by the Modulation Amplitude parameter and is set based on the linewidth of the EPR line. For example, if the bulk field is set to 100 G and the modulation amplitude is set to 10 G, the actual field that the sample feels oscillated sinusoidally from 95 G to 105 G. In the electronics of the instrument, the signal is registered as a comparison between the signal at the high field and the signal at the low field. This gives a positive value for signals that are sloping up in the direction of the field sweep, a negative value for signals sloping down and zero for signals which are the same. This gives rise to the derivative lineshapes observed.
Field Modulated EPR Spectrum of a Nitroxide Spin Label
The reason for this detection method is not to give lineshapes that are easier to examine. In fact, these methods give a greatly enhanced signal to noise ratio as only the signal that has a periodicity of 100 KHz is amplified, leaving out much of the random noise.
Direct Detected CW EPR
For conditions under which detection of a transient signal that decays much more rapidly that the time scale of 100 KHz field modulation, direct detection is required. This leads to much lower sensitivity, but allows one to monitor the kinetics of short lived radicals. Typically, this is used in conjunction with a laser to monitor light induced radicals.