A pulsed and continuous wave 250 MHz electron paramagnetic resonance spectrometer

Abstract

ABSTRACT:A 250 MHz electron paramagnetic resonance (EPR) spectrometer was constructed to be an engineering test facility for in vivo EPR imaging of physiological samples and for protein structure determination. Innovations relative to prior low-frequency EPR spectrometers include a four-coil, air-core magnet and gradient coils, a crossed-loop resonator, dynamic Q-switching to decrease dead time in pulsed EPR, and a narrow-band bridge based on circulators. The automatic frequency control system uses a signal separate from the EPR signal to make the frequency control independent of the radiofrequency (RF) phase. The design incorporates multiple excitation and signal paths to facilitate testing of a variety of resonators, two magnets, and both a locally built console described here and a Bruker console. Plug-in cards in the bridge facilitate using reflection or crossed-loop resonators in continuous wave or pulsed EPR modes. In the locally built console there is a microprocessor-controlled interface unit to handle magnetic field modulation and scan, tuning display, and other functions.

Fast‐response VHF pulsed 2 KW power amplifiers

“…The 250-MHz bridge used was described in (5). Signals were digitized in a LeCroy 9354 500-MHz digital oscilloscope.…”

“…Consequently, one of the enabling technologies that needs to be developed for low-frequency pulsed EPR is pulsed RF amplifiers that amplify short pulses with good fidelity and then quickly blank the noise output after the pulse. Early development of our 250 MHz pulsed EPR spectrometer used a continuous-duty RF amplifier to achieve pulse fidelity, but this had the problem of continuous noise output that interfered with observing weak time-domain EPR signals (5). Subsequently, pulsed RF amplifiers were developed for this project.…”

Radio frequency continuous‐wave and time‐domain EPR imaging and Overhauser‐enhanced magnetic resonance imaging of small animals: instrumental developments and comparison of relative merits for functional imaging

“…However, when one uses special spin probes such as triarylmethyl derivatives (TAM, see below), which have line widths in the range 60-200 mGauss, much better homogeneity over the volume of interest is required. Air core and iron core magnets operating up to L-band frequencies with homogeneities better than 100 ppm have been constructed 26 and are also commercially available. There is no need for the incorporation of magnetic field shims that are of common use in NMR and MRI.…”

“…The isolation between the transmit arm and receive arm can be achieved by a transmit-receive switch such as a diplexer or a circulator. It is also possible to employ the so-called 'crossed-loop resonator', 26,38 in which the resonator consists of a transmit coil and a receiver coil with their RF field axes mutually orthogonal, which helps in isolating the transmitter and receiver and greatly reduces power leakage on to the detection arm. The diplexer provides the isolation by a set of crossed diodes which make the receiver arm non-conducting during the transmit cycle and prevents any reflections returning back to the transmitter arm during the receive cycle and is the standard method adopted in pulsed NMR.…”

“…20 Several groups are developing in vivo EPR spectroscopy and imaging methods. [21][22][23][24][25][26][27] Because of the relatively large line widths of most spin probes used in EPR, CW methods were preferred for EPRI. Such studies have provided valuable information on the pO 2 (partial pressure of oxygen) levels in tumors.…”