Methodology and Instrumentation

Resonator Development

Three papers on resonators have been written and published recently:

  1. JR Anderson, RR Mett, JS Hyde. Cavities with axially uniform fields for use in electron paramagnetic resonance.  II. Free space generalization. Rev. Sci. Instrum. 73:3027-3037 (2002)
  2. JS Hyde, RR Mett, JR Anderson. Cavities with axially uniform fields for use in electron paramagnetic resonance. III. Re-entrant geometries. Rev. Sci. Instrum. 73:4003-4009 (2002)
  3. RR Mett, JS Hyde. Aqueous flat cells perpendicular to the electric field for use in electron paramagnetic resonance spectroscopy. J. Magn. Reson. 165:137-152 (2003).

The first two of these describe alternative ways to construct the end sections of uniform field resonators. Resonators of this class consist of a central section that contains the sample and two end sections that terminate the central section. The central section is at cutoff, and the fields are strictly uniform along the sample, which is highly desirable in EPR spectroscopy. Each end section can be viewed as one-half of a cavity resonator that resonates at the cutoff condition. There are three ways to produce the end sections: fill them with dielectric, make them physically larger in cross-section than the central section, or make them re-entrant. Each has been thoroughly investigated making extensive use of the Ansoft High Frequency Structure Simulator finite-element modeling program, and the results published in the Review of Scientific Instruments.

The third paper is theoretical and develops a totally new way to design sample cells in EPR for aqueous samples. It is shown that if one has a large number of parallel lamina containing aqueous sample, separated by thin dielectric material and oriented such that the rf electric field is perpendicular to the lamellar surface, overall signal intensity can be improved by a factor between 5 and 10. This has enormous potential impact, although development of a practical cell configuration will be challenging.

Five resonators are in development for specific collaborations. Progress for four is described as follows:

  1. Three-loop—two-gap Q-band resonator for aqueous samples. Resonators have been constructed from both solid Al and solid Ag and used successfully to produce publishable data for pulse saturation recovery and multiquantum EPR experiments. Further refinement of this class of resonators is anticipated with goals of accommodating somewhat larger sample volumes and also being able to use with 100 kHz field modulation.
     
  2. Q-band large-sample-access TE011 cavity. Using HFSS, sample stack dimensions have been optimized to permit the use of X-band 3 mm ID frozen solution samples at Q-band. Field modulation is accomplished by cutting slots in the copper walls. The field modulation design was optimized using Ansoft 3D Maxwell. Considerable effort was made to suppress unwanted modes, since it was found that conventional methods result in unacceptable microwave leakage through the modulation slots.
     
  3. 1 ml frozen solution L-band loop-gap resonator for research on prions. Ansoft HFSS analysis has been completed and construction initiated. Detailed calculations indicate that a factor of 7 improvement can be obtained relative to the structure presently in use that accepts 70 μl samples.
     
  4. LGR for 10 to 20 μl aqueous sample at X-band. We will use use a central sample loop with four gaps and four surrounding "return-flux" loops. An innovative sample cell of complex cross-section has been designed, a quotation to produce this sample cell in long lengths of extruded Teflon has been obtained, and an order has been placed.

Broadband Digital Detection, Signal Analysis, and Archiving in EPR Spectroscopy

We have developed and implemented automatic frequency control (AFC) methods for MQ and SR spectroscopy. The AFC system used for MQ in which two incident powers are present uses the frequency difference component DF/2 present in the out-of-phase or dispersion channel of the detection system to derive the error signal. The pulse SR AFC uses a properly time-sampled signal from the dispersion channel, subsequently phase-detected at two times the repetition rate of the EPR experiment. These AFC systems do not modulate the incident microwave carrier. The AFC signal is the out-of-phase component of the carrier reflected by the sample resonator. The MQ AFC method gives reliable lock to the 35 GHz low Q-factor LGR at 1 μW incident power. Frequency modulation (FM) type AFC, because of its low index of modulation, would not lock reliably at power levels under 50 μW.

Effective sample resonator lock improved the baseline and stability of MQ spectra. Signal averaging of successive scans is now possible. We anticipate implementing the frequency difference AFC after analog-to-digital conversion in the computer, where the DF/2 component in the dispersion channel can be separated from the broad-band intermediate frequency signal from the signal mixer. This AFC system will be used in the proposed W-band spectrometer.

Field modulation EPR experiments will use FM of the carrier, or, alternatively, a Pound-type AFC. To meet the challenge presented to the AFC system by higher relative bandwidth millimeter wave LGRs, we rework the Q-band oscillator to allow a higher modulation frequency, thus improving AFC output level for a given modulation index. This involves redesign of the oscillator cavity endplate to lower the mass loading the piezoelectric element providing the FM. It is expected that this modification will raise the maximum frequency of modulation to at least 700 kHz. We intend to evaluate the Pound-type AFC implemented in software after analog-to-digital conversion operating on the carrier level in the dispersion channel, and compare this to the conventional FM type AFC.

The use of digital detection for saturation transfer spectroscopy is a straightforward application for broadband signal detection and processing. This technique is useful in motional studies of spin-labeled proteins, in which the second harmonic of the modulation frequency in the absorption signal, and the first harmonic in the dispersion are of particular interest. Spectra were obtained of 1 mM spin-labeled bovine albumin (BA) in water/glycerol, in a Varian Q-band cavity using 100 kHz field modulation. The BA was at room temperature, in a 0.075 mm glass capillary inside a 2 mm quartz sample tube. Digitized datasets were obtained at low and high powers using the ICS Daq PC. Detection of the harmonics, microwave phasing, and null-phasing of the first harmonic dispersion and second harmonic absorption signal at low power were performed after archiving of the broadband dataset on a desktop personal computer. Spectra were also obtained through conventional analog lock-in detection under identical conditions. Phasing for the null of the low power first harmonic dispersion and second harmonic absorption signal were accomplished by phase adjustment of the lock-in reference channel to achieve a signal minimum. The results show comparable sensitivity and displays between the two methods of detection.

Computer hardware has continued to evolve. Architectures are possible today that were too slow previously. RAID speed alone has risen by a factor of 6 from 50 to 300 MB/sec. We are exploring cost-effective ways to exploit the digital receivers we have today.

In terms of hardware, the analog front-end to the digital receiver has been finished and is waiting integration into the spectrometer and testing. It will be tested at Q-band while waiting for the Q/W translation bridge hardware.

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