Helical resonators were introduced to EPR in the late-1950s as a
method to increase the magnetic field of a non-resonant shorted
waveguide. The helices that were described in the literature were
designed as slow-wave broadband focusing-resonators affixed to the
bottom of a waveguide short. The sample was placed within the
helix. Although this improved the filling factor of the shorted
waveguide, it had a Q-value essentially at unity but could be matched
with a slide-screw tuner. I propose re-evaluating the helix for
extremely limited samples in EPR, the first prototype tested is
shown in Fig. 1A, simulated fields in Fig. 1B. With the
self-resonant micro-helix described here, at least a factor of 10
in EPR signal intensity has been simulated and experimentally
confirmed in preliminary experiments. Simulation results are shown
in Table 1.
Recently, the micro-helix has gained some popularity in the NMR
literature for micro-fluidic environments. However, these coils
resonate well below 500 MHz, are typically not self-resonant, and
do not employ inductive coupling techniques. Extending these
geometries to X-band (9.5 GHz) and implementing a mutual
inductance coupling system are considered highly innovative.
Furthermore, the 0.17 mm micro-helix geometry cannot currently be
built by ordinary techniques. Therefore, such techniques as
three-dimensional printing are being investigated. Taking
advantage of this technology, a micro-helix with inner diameters
down to 0.17 mm can be reproducibly constructed.
Three characteristics differentiate the proposed micro-helix
configuration from those described in literature:
The helix is self-resonant, meaning that the self-inductance
per loop and self-capacitance between the loops resonate at a
frequency determined by ω√LC = 1, where is the resonant frequency ω in
radians/s. No external capacitors are used, minimizing losses.
The helix length is much smaller than a wavelength, which
increases the uniformity, and the diameter is less than 0.4 mm
at X-band, which increases the resonator efficiency.
That the helix is coupled by mutual inductance, which can be
designed to minimize noise and further increase the EPR signal
Table 1 presents simulated resonator characteristics of a few
geometries of interest with all EPR signals volume normalized. For
comparison, an X-band (9.5 GHz) dielectric cavity resonator (DR) is
presented. This is a standard cavity that can be purchased from
Bruker BioSpin, the leading EPR spectrometer manufacturer. EPR
Signal improvements and Λmax-Λave ratios are encouraging.
Simulations show the microwave field of the PMR
has at least a 21% inhomogeneity within a sample of dimensions 0.1
mm diameter by 0.1 mm height. In contrast, the micro-helix of Fig.
1B has 4.5% variation.
Links of Interest
Bio Hydrogen Group - Led by Dr. Edward J. Reijerse, the Bio Hydrogen Group looks to understand the chemical reaction of biological hydrogenase in order to create bio-inspired molecular catalysts.
Act-EPR project starts on 1st May, 2017.
2017.07 I was awarded the EPR2017 Award of Excellence for my talk.