EPR saturation-recovery (SR) measurements of the electron spin-lattice relaxation time, T 1 , of nitroxideradical spin probes have been made from 2 to 35 GHz. T 1 values of small water-soluble spin probes increase linearly with microwave frequency throughout the full range of available frequencies. T 1 values of four commonly used hydrophobic probes in lipid bilayers also increase with frequency, but the dependence is weaker and complex. Contributions of dissolved molecular oxygen to relaxation rates were independent of microwave frequency. T 1 values of 15 N-containing labels are always somewhat longer than for 14 N labels. Details of the Q-band SR spectrometer, which is based on frequency translation technology, are provided. A new way to suppress free induction decay signals in SR experiments has been found: pump and observing frequencies time-locked and separated by about 1 kHz in frequency. A novel three-loop-two-gap resonator with a sample volume of 30 nl was used for the Q-band measurements. It is concluded that Q-band is a favorable frequency for SR spin-label oximetry studies.

A reference-arm W-band (94 GHz) microwave bridge with two sample-irradiation arms for saturation recovery (SR) EPR and ELDOR experiments is described. Frequencies in each arm are derived from 2 GHz synthesizers that have a common time-base and are translated to 94 GHz in steps of 33 and 59 GHz. Intended applications are to nitroxide radical spin labels and spin probes in the liquid phase. An enabling technology is the use of a W-band loop-gap resonator (LGR) (J.W. Sidabras et al., Rev. Sci. Instrum. 78 (2007) 034701). The high efficiency parameter (8.2 GW −1/2 with sample) permits the saturating pump pulse level to be just 5 mW or less. Applications of SR EPR and ELDOR to the hydrophilic spin labels 3-carbamoyl-2,2,5,5-tetra-methyl-3-pyrroline-1-yloxyl (CTPO) and 2,2,6,6,-tetramethyl-4-piperidone-1-oxyl (TEMPONE) are described in detail. In the SR ELDOR experiment, nitrogen nuclear relaxation as well as Heisenberg exchange transfer saturation from pumped to observed hyperfine transitions. SR ELDOR was found to be an essential method for measurements of saturation transfer rates for small molecules such as TEMPONE.