Robert A. Strangeway

Saturation recovery EPR and ELDOR at W-band for spin labels.

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, R.R. Mett, W. Froncisz, T.G. Camenisch, J.R. Anderson, J.S. Hyde, Multipurpose EPR loop-gap resonator and cylindrical TE(011) cavity for aqueous samples at 94 GHz, 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. Free induction decay (FID) signals for small nitroxides at W-band are also reported. Results are compared with multifrequency measurements of T(1e) previously reported for these molecules in the range of 2-35 GHz [J.S. Hyde, J.-J. Yin, W.K. Subczynski, T.G. Camenisch, J.J. Ratke, W. Froncisz, Spin label EPR T(1) values using saturation recovery from 2 to 35 GHz. J. Phys. Chem. B 108 (2004) 9524-9529]. The values of T(1e) decrease at 94 GHz relative to values at 35 GHz.

W-band frequency-swept EPR

This paper describes a novel experiment on nitroxide radical spin labels using a multiarm EPR W-band bridge with a loop-gap resonator (LGR). We demonstrate EPR spectroscopy of spin labels by linear sweep of the microwave frequency across the spectrum. The high bandwidth of the LGR, about 1 GHz between 3 dB points of the microwave resonance, makes this new experiment possible. A frequency-tunable yttrium iron garnet (YIG) oscillator provides sweep rates as high as 1.8x10(5) GHz/s, which corresponds to 6.3 kT/s in magnetic field-sweep units over a 44 MHz range. Two experimental domains were identified. In the first, linear frequency sweep rates were relatively slow, and pure absorption and pure dispersion spectra were obtained. This appears to be a practical mode of operation at the present level of technological development. The main advantage is the elimination of sinusoidal magnetic field modulation. In the second mode, the frequency is swept rapidly across a portion of the spectrum, and then the frequency sweep is stopped for a readout period; FID signals from a swept line oscillate at a frequency that is the difference between the spectral position of the line in frequency units and the readout position. If there is more than one line, oscillations are superimposed. The sweep rates using the YIG oscillator were too slow, and the portion of the spectrum too narrow to achieve the full EPR equivalent of Fourier transform (FT) NMR. The paper discusses technical advances required to reach this goal. The hypothesis that trapezoidal frequency sweep is an enabling technology for FT EPR is supported by this study.

Electron paramagnetic resonance Q‐band bridge with GaAs field‐effect transistor signal amplifier and low‐noise Gunn diode oscillator

A Varian Q‐band E‐110 microwave bridge for electron paramagnetic resonance (EPR) spectroscopy has been modified by addition of a low‐phase noise Gunn diode oscillator of our own design, a low‐noise GaAs field‐effect transistor microwave signal amplifier, and a balanced mixer requiring high input power (10 mW) at the local oscillator port. The oscillator has previously been found to have −129 dBc/Hz phase noise, 22 dB lower than for the original klystron. Noise measurements indicate that the microwave amplifier and mixer reduce the overall receiver noise figure by 24.6 dB, a very significant improvement. It is shown that reduction of both phase noise and receiver noise are required in order to achieve full improvement in signal‐to‐noise ratio over the full range of available microwave power. Spectra of 1.6×10−6 M 15N‐perdeutero TEMPONE (1‐oxyl‐2,2,6,6‐tetramethyl‐4‐piperidone) and of 10−6 M Mn2+ are shown in order to demonstrate sensitivity.

A general purpose multiquantum electron paramagnetic resonance spectrometer

We describe the design, construction, and characterization of an X‐band multiquantum electron paramagnetic resonance (MQEPR) microwave bridge, with MQ electron–electron double resonance and MQ electron–nuclear double resonance capabilities. The main feature of the bridge is the use of double‐balanced mixers as double sideband modulators to generate multiple irradiation fields with variable frequency separation. The microwave source is a low phase noise Gunn diode oscillator, the frequency of which is translated by a nominal 300±Δf MHz. This approach, called double sideband/fixed filter (DSB/FF), allows the use of fixed bandpass microwave filters to reduce incident spurious products to at least −70 dBc. Each frequency is amplified separately to avoid system‐generated intermodulation (IM) sidebands in the incident irradiation. As a result, the dominant source of system intermodulation is the nonlinearity in the receiver system, consisting of a low noise amplifier (LNA) and a double‐balanced signal mixer. A ...

Digital Detection by Time-Locked Sampling in EPR

All frequencies in a magnetic resonance spectrometer should be phase-locked to a single master oscillator. Departure from this principle leads to degraded instrument performance. The use of digital technology is making superheterodyne detection increasingly attractive, relative to homodyne detection, which has been used in most “modern” EPR spectrometers. The signal modulation frequency, the sampling, frequency, and the intermediate frequency (from the signal down-converter) are all locked to the same clock, so the method is called “time-locked.” The sampling of the analog signal to digitize it is done four times in an odd number of cycles, typically 3, 5, or 7, so this is “sub-sampling” relative to the Nyquist criterion. Hence, the name time-locked subsampling (TLSS). An essential feature of TLSS is broad-bandedness followed by digital filtering with internal consistency between the two quadrature detection channels. This type of broad-band acquisition followed by digital analysis permits, for example, study of multiple harmonics of the field modulated signal.

X‐band low phase noise Gunn diode oscillator for EPR spectroscopy

Two low phase‐noise Gunn diode X‐band oscillators intended for use in electron paramagnetic resonance (EPR) spectroscopy are described. In the first, a 250‐mW MA49159 Gunn diode oscillator (M/A‐COM, Burlington, MA) is mounted in a coaxial transmission line that is closely coupled to a TE011 transmission cavity that in turn is loosely coupled to the output transmission line. The output power is 50 mW and the phase noise is −145 dBc/Hz at 100 kHz offset. In the second, two such coaxial assemblies are used with 500‐mW MA49110 diodes for increased power. The output power is 150 mW and the phase noise is −150 dBc/Hz at 100‐kHz offset. These phase noise values are in the range of 24–29 dB better than the specification for a normal high quality klystron used in commercial spectrometers.

Broadband W-band Rapid Frequency Sweep Considerations for Fourier Transform EPR

A multi-arm W-band (94 GHz) electron paramagnetic resonance spectrometer that incorporates a loop-gap resonator with high bandwidth is described. A goal of the instrumental development is detection of free induction decay following rapid sweep of the microwave frequency across the spectrum of a nitroxide radical at physiological temperature, which is expected to lead to a capability for Fourier transform electron paramagnetic resonance. Progress toward this goal is a theme of the paper. Because of the low Q-value of the loop-gap resonator, it was found necessary to develop a new type of automatic frequency control, which is described in an appendix. Path-length equalization, which is accomplished at the intermediate frequency of 59 GHz, is analyzed. A directional coupler is favored for separation of incident and reflected power between the bridge and the loop-gap resonator. Microwave leakage of this coupler is analyzed. An oversize waveguide with hyperbolic-cosine tapers couples the bridge to the loop-gap resonator, which results in reduced microwave power and signal loss. Benchmark sensitivity data are provided. The most extensive application of the instrument to date has been the measurement of T1 values using pulse saturation recovery. An overview of that work is provided.

Hyperbolic-cosine waveguide tapers and oversize rectangular waveguide for reduced broadband insertion loss in W-band electron paramagnetic resonance spectroscopy. II. Broadband characterization

Experimental results have been reported on an oversize rectangular waveguide assembly operating nominally at 94 GHz. It was formed using commercially available WR28 waveguide as well as a pair of specially designed tapers with a hyperbolic-cosine shape from WR28 to WR10 waveguide [R. R. Mett et al., Rev. Sci. Instrum. 82, 074704 (2011)]. The oversize section reduces broadband insertion loss for an Electron Paramagnetic Resonance (EPR) probe placed in a 3.36 T magnet. Hyperbolic-cosine tapers minimize reflection of the main mode and the excitation of unwanted propagating waveguide modes. Oversize waveguide is distinguished from corrugated waveguide, overmoded waveguide, or quasi-optic techniques by minimal coupling to higher-order modes. Only the TE10 mode of the parent WR10 waveguide is propagated. In the present work, a new oversize assembly with a gradual 90° twist was implemented. Microwave power measurements show that the twisted oversize waveguide assembly reduces the power loss in the observe and pump arms of a W-band bridge by an average of 2.35 dB and 2.41 dB, respectively, over a measured 1.25 GHz bandwidth relative to a straight length of WR10 waveguide. Network analyzer measurements confirm a decrease in insertion loss of 2.37 dB over a 4 GHz bandwidth and show minimal amplitude distortion of approximately 0.15 dB. Continuous wave EPR experiments confirm these results. The measured phase variations of the twisted oversize waveguide assembly, relative to an ideal distortionless transmission line, are reduced by a factor of two compared to a straight length of WR10 waveguide. Oversize waveguide with proper transitions is demonstrated as an effective way to increase incident power and the return signal for broadband EPR experiments. Detailed performance characteristics, including continuous wave experiment using 1 μM 2,2,6,6-tetramethylpiperidine-1-oxyl in aqueous solution, provided here serve as a benchmark for other broadband low-loss probes in millimeter-wave EPR bridges.

A versatile Q-band electron paramagnetic resonance spectrometer

The design, construction, and operation of a Q-band (34.5-35.5 GHz) electron paramagnetic resonance (EPR) spectrometer system that is versatile to accommodate several sample irradiation and signal detection configurations is described. The spectrometer system that was developed includes a microwave bridge, synthesizer array, receiver, and signal processing unit. The main feature of the microwave bridge is the use of frequency translation technology to translate the fundamental microwave frequency, nominally 33 GHz, to spectrally pure multiple time-locked closely-spaced frequencies, nominally centered about 35 GHz, from an array of time-locked microwave synthesizers. A spectral purity of all incident spurious products < -65 dBc has been achieved. The design strategies to this end are described. The main features of the receiver are low-noise, low intermodulation products, and synchronous down- conversion to a nominal 1 GHz first intermediate frequency (IF). This first IF can then be downconverted either to baseband or to a nominal second IF of 12.8 MHz. The noise figure from the sample port of the circulator to the output of the IF preamplifier is approximately 7.5 dB. The main feature of the signal processing is direct analog-to-digital (A/D) conversion of the 12.8 MHz IF using a time-locked sub-sampling (TLSS) approach for subsequent analysis on a computer. EPR information at multiple microwave frequencies of interest can now be collected in a single sweep of the DC magnetic field. The automatic frequency control (AFC) system in both single-frequency and multiple-frequency environments is described.