High-frequency resonance methods

A review has been given on paramagnetic resonance spectroscopy of metal centers. It comprises both paramagnetic NMR as well as high frequency EPR methods applied mostly to copper proteins of the blue type but also to iron and manganese containing proteins.42... 

The semiquinone biradical produced in the photocycle of bacterial photosynthetic reaction centres was trapped at 77 K and examined at 9.6, 35, and 94 GHz.16 Simulations of spectra at the multiple resonant frequencies using the simulated annealing method revealed the spatial and electronic structure of the biradical. The value of r was found to be 17.2 + 0.2 A, which is in good agreement with the value of 17.4 0.2 A obtained in an X-ray crystal structure. This study shows the power of high-frequency EPR combined with data obtained at lower frequencies. 

Electromagnetic vibrators can cover a very wide frequency range, in particular being capable of very high frequencies up to at least 104 Hz, although at this level considerable care is needed. Quite high power is obtainable, but electromagnetic drive is more commonly applied to relatively small machines and low strains. It is also used in forced oscillation methods at resonance. 

In passive methods, the quartz crystal is perturbed with a sinusoidal high frequency ac voltage in a few kHz frequency range around resonance. The most commonly used instrument is the Frequency Response Analyser (FRA) at I-lOMHz. Several commercial instruments can measure automatically a range of frequencies and evaluate the BVD circuit parameters. 

At high frequencies, bridges can still be used, provided that special precautions are taken to eliminate the effects of stray inductances which become very large, but it is usually better to adopt a resonance method above 106Hz. 

In this report we present results of a study of a titanium - ethylene complex formed in an ultra high vacuum chamber and collected on a quartz sensor for mass analysis. The technique we use for mass analysis is unique. It is based on the application of a frequency modulation method to unambiguously track the resonant frequency of a quartz sensor to better than 0.1 ppm. A downward shift in the resonant frequency of the sensor, either due to the initial deposition of the complex or due to the absorption of hydrogen gas, indicates a mass accumulation on its face. This shift can be measured with a 0.1 ppm resolution. 

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