High Frequency W-band Spectroscopy:
ADVANTAGES OF HIGH-FIELD EPR
- Greater dispersion in g-factor.
- Increased resolution.
- Determination of anisotropic g-factors for immobilized systems
- Enhanced sensitivity to motion due to larger dispersion in g.
- Enhanced sensitivity for samples of limited size.
- Multifrequency approach to solve for spin Hamiltonian parameters.
- Turn off a second or higher order effect, such as a Zero-field splitting (D).
- Turn on a second or higher order effect, such as a nuclear quadrupole coupling.
- Study of strain in disordered systems. Increase sensitivity to g-strain; Decrease sensitivity
to D-strain (line narrowing)
Sensitivity:
Sensitivity is an important design consideration for high-field/high-frequency spectrometers.
The performance of oscillators and detectors generally decreases with increasing frequency. To
some extent high sensitivity can be maintained by tuning components and reducing the
bandwidth of the spectrometer. At lower microwave frequencies, the use of low noise
microwave preamplifiers can greatly improve the noise figure of the receiver as the noise figure
of the LNA is much lower that of a microwave mixer diode. We have made modifications to our
W-band (94 GHz) spectrometer and have achieved increased sensitivity with a low noise
microwave preamplifier coupled with a cavity-stabilized oscillator (CSO). Shown here is a 50
nM solution of perdeuterated tempone in water obtained using the 94 GHz LNA. From this result
we calculate an aqueous concentration sensitivity of 1.2 nM sec Gauss-1. This is real achievable
sensitivity and does not include any correction for power saturation as often has been done. The
corresponding aqueous point sensitivity is 1.2 x 108 spins sec Gauss-1 and the sample volume is
0.4 microliters.
All images and text copyright 2002 M. J. Nilges
Last Updated on March 27, 2002 by M. Nilges