Biomedical EPR Part-B Methodology Instrumentation and Dynamics - Sandra R. Eaton

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From its earliest days, the underlying technology was based on military developments in radar. In recent years, computers, cellular telephone technology and advances in digital devices have become increasingly important in contributing to the technological foundation of EPR spectrometers.

This reality – that our field is relatively small and dependent to a considerable degree on technology from larger scale development activities

– places constraints on future progress of the field. The fundamental technological event of our times is rapid increase in computing power and the performance of associated digital and mass storage devices. Our future, from a technological perspective, will be based on this fact. Advances in EPR digital detection, in data capture and storage as well as in use of advanced signal processing methods are discussed in the chapter on digital detection (see Ch. 7).

Even though EPR instrumentation is strongly dependent on technology developed in other fields, there are areas where the special constraints of EPR have led to significant technological advances. Some of these are discussed below.

2.RESONATORS

Basic contributions to EPR technology that have been developed from within the EPR discipline include resonator development. The requirements of sample access, variable microwave coupling, resonators free from impurities, wall penetrability by high frequency field modulation, temperature control, etc. place constraints on resonator design that we ourselves must face – there is little in the way of technology to borrow from other fields. It is appropriate for the EPR instrumental futurist to predict advances in those specific areas where we control the technology and an attempt is made here.

The Varian multipurpose cavity oscillating in the rectangular mode served as the default EPR resonator for many years. See Rempel et al. (1964) and Hyde (1995) for details on the design. This structure was designed to enable a number of specialized EPR experiments as follows: i) light irradiation, ii) dewar insert for flowing temperature controlled gas, iii) dewar insert for liquid nitrogen, iv) so-called “flat cells” for aqueous samples, v) flat cells for tissue samples, vi) a mixing chamber geometry for stopped and continuous flow EPR, and vii) an electrochemical cell. A collet system was developed to support the dewars, flat cells, and sample tubes of various sizes. Two cavity bodies were bolted together to form the dual sample cavity (Hyde, 1965a) oscillating in the mode in order to

examine a reference sample and a sample of interest simultaneously. A top coupled section was added (Piette et al., 1962) to form a cavity that permitted simultaneous optical absorption and EPR experiments on a sample. The demountable sidewall construction permitted disassembly and cleaning. Dr. Robert Rempel was the leader of this initiative with substantial contributions by Dr. Lawrence Piette, both of whom were Varian scientists. This outpouring of technology provided the basis for transfer of EPR spectroscopy from its original base in physics into chemistry and biology.

A number of special purpose X-band cavities were developed commercially. A list of some of these includes: cylindrical “wirewound” cavity (Hyde et al., 1965b, 1966) for enhanced sensitivity in certain classes of samples, ENDOR cavities, cavities for use at cryogenic temperatures, cavity, ELDOR cavities and the cavity for aqueous samples (Hyde, 1975).

This rich array of X-band resonators is the primary reason that this microwave frequency remains dominant in EPR spectroscopy. Nothing comparable exists at either higher or lower microwave frequencies. This is a significant opportunity for future technological development. Certainly all of these X-band capabilities could be replicated at Q-band (35 GHz) and S- band (ca 3 GHz), which would greatly strengthen the concept of “multifrequency EPR.”

In 1965, the author introduced a system of classification for EPR samples from the perspective of sensitivity and optimum microwave configuration: namely, eight classes of samples depending on yes or no answers to three questions: i) Does it saturate? ii) Is it limited in size or availability? iii) Does it exhibit substantial dielectric loss (Varian Associates, 1965)? Of course, real samples may not correspond to one of these classes, but would lie in some intermediate category. Nevertheless, this classification system has proven helpful in thinking about resonator design in EPR spectroscopy. To a certain degree, it lies at the heart of the development of loop-gap resonators (LGR) for use in EPR spectroscopy, largely by W. Froncisz and the author (See Froncisz and Hyde, 1982; Hyde and Froncisz, 1989; and ch. 2 in the present volume). This class of resonators fundamentally improves sensitivity for those four classes of samples that correspond to a YES answer concerning question ii): Is the sample limited in size or availability? The benefits with respect to sensitivity can be quite substantial. The X-band LGR with 1 mm diameter sample access hole is one of the enabling technologies for the development of site directed spin labelling. The LGR is also a central technology for in vivo small animal imaging and spectroscopy (see ch 9 and ch. 11 in volume 23.)

Simulation of electromagnetic fields in microwave cavity resonators using finite-element computer-driven solutions of Maxwell’s equations holds great

412 JAMES S. HYDE

promise for future development of EPR resonators. The author is aware of six papers in the literature where finite element modeling of electromagnetic fields was employed in a context of microwave resonators for EPR use. The earliest papers were from A. Schweiger’s group in Zurich (Pfenninger et al., 1988; Forrer et al., 1996). In Pfenninger et al. (1988) the program known as MAFIA (Solution of MAxwell’s equation by the Finite Integration Algorithm) was used as an aid to development of bridged loop gap resonators. The work of this group in this area was updated in Forrer et al., 1996. Also in that year, the MCW group used the Hewlett-Packard version of High Frequency Structure Simulator, HFSS, to develop a bimodal loop gap resonator (Piasecki et al., 1996).

Recently, the author and his colleagues wrote a series of three papers on a new class of microwave resonators that we chose to call uniform field (UF) cavities (Mett et al., 2001; Anderson et al., 2002; Hyde et al., 2002). Transverse magnetic field (TM) modes exist, that exhibit uniformity of the fields along the axis that is perpendicular to the transverse field. This is because Maxwell’s equations allow magnetic fields that are tangential to conducting end walls to exist. The familiar cylindrical cavity widely used in EPR for aqueous samples is an example of such a structure. However, previous to these three papers there were no known transverse electric field (TE) modes that exhibited this property of uniformity of the fields along the perpendicular axis. This is because components of the microwave electric field tangential to conducting end walls cannot exist. The UF modes described in these three papers consisted of a central section where the fields were precisely uniform, with end sections designed to satisfy the boundary conditions at the end walls.

These three papers made extensive use of high frequency structure simulation (HFSS) finite element software. UF resonators were, in fact, discovered using HFSS. We were working on an unrelated problem and noticed some unusual microwave field patterns that we did not understand. When the structures were built, they worked exactly as designed. Although they were largely theoretical, experimental “proof-of-principle” data were presented. Design of microwave coupling structures and sample entrance structures (so-called “stacks”) for these prototypical resonators was also carried out using HFSS.

The HP HFSS software, which is no longer available, operated in the socalled driven mode. We found it very difficult to use for microwave cavity development because every change in the structure required a tedious hunt for the slightly shifted resonant frequency. When Hewlett-Packard dropped out of this business, we shifted to Ansoft HFSS (Ansoft Corp., Pittsburgh, PA). This software incorporates both the driven mode and the 3D modal frequency or “eigenmode” solution method (Brauer, 1997). This method is

also available in MAFIA. We find that this method is extremely useful for EPR resonator development (Mett et al., 2001; Anderson et al., 2002; Hyde et al., 2002). Since resonators are characterized by a single resonant mode of interest, use of the eigenmode method permits focus only on that mode; a frequency sweep is not needed. Changes to the structure can be made and the resulting effect seen on the fields, resonance frequency and Q-value without having to track the changes through a frequency scan. Second, using the eigenmode method, we can focus on the design of the resonator without introducing a coupling structure. Mechanical drawing time and computation time are each reduced by one to two orders of magnitude due to the increased symmetry of the structure. Introduction of a coupling structure as well as sample access stacks is straightforward in the final stages of a design. We have also used the software to calculate the effects of sample support structures, dewar inserts, and the effect of the dielectric properties of the sample on the electromagnetic field distribution. The resonator efficiency parameter (Hyde and Froncisz, 1989) can be calculated, which permits an estimate of EPR sensitivity. Currently we run this code on a Compaq W8000 workstation with dual Xeon 1.7 GHz processors with 2 GB of RAM.

In another cavity development initiative, we gave ourselves the goal of finding a way to examine a normal X-band sample in a 3 mm i.d., 4 mm o.d. quartz sample tube at Q-band. The problem was to determine whether the microwave sample access stacks would be beyond cutoff. As the sample diameter increases, the stack, viewed as a cylindrical waveguide propagating in either the cylindrical or cylindrical modes will first become evanescent and eventually radiate power into space. It was found that practical structures for this purpose can be constructed providing that the stack is sufficiently long.

Ansoft offers another software package known as Maxwell 3D that is well configured for computation of the 100 kHz field modulation patterns and eddy currents in surrounding metallic structures. Four approaches to the design of field modulation assemblies for use in EPR can be identified from the existing literature: i) Use of electrically thin walls – less than one skin depth at 100 kHz but many skin depths at the microwave frequency. ii) The wirewound technique introduced by the author for use with the cylindrical

mode (Hyde, 1965, 1966, and Fig. 1). All microwave currents must be substantially parallel to the wires. iii) Partially cut-through slots for use with loop gap resonators (Fig. 2). iv) Direct insertion into the resonator of wires or rods that carry field modulation current. In addition to modeling of fields produced by modulation coils, Maxwell 3D is well suited for modeling of fields in a context of ENDOR and RF coils. It is apparent that these various