Broadband Transmission EPR Spectroscopy

Abstract

EPR spectroscopy employs a resonator operating at a single microwave frequency and phase-sensitive detection using modulation of the magnetic field. The X-band spectrometer is the general standard with a frequency in the 9–10 GHz range. Most (bio)molecular EPR spectra are determined by a combination of the frequency-dependent electronic Zeeman interaction and a number of frequency-independent interactions, notably, electron spin – nuclear spin interactions and electron spin – electron spin interactions, and unambiguous analysis requires data collection at different frequencies. Extant and long-standing practice is to use a different spectrometer for each frequency. We explore the alternative of replacing the narrow-band source plus single-mode resonator with a continuously tunable microwave source plus a non-resonant coaxial transmission cell in an unmodulated external field. Our source is an arbitrary wave digital signal generator producing an amplitude-modulated sinusoidal microwave in combination with a broadband amplifier for 0.8–2.7 GHz. Theory is developed for coaxial transmission with EPR detection as a function of cell dimensions and materials. We explore examples of a doublet system, a high-spin system, and an integer-spin system. Long, straigth, helical, and helico-toroidal cells are developed and tested with dilute aqueous solutions of spin label hydroxy-tempo. A detection limit of circa 5 µM HO-tempo in water at 800 MHz is obtained for the present setup, and possibilities for future improvement are discussed.

Broadband Tunable Electron Paramagnetic Resonance Spectroscopy of Dilute Metal Complexes

“…Other studies have reported the use of analog or digital, scalar or vector network analyzers as source/detector of EPR 32−35 including my own work. 3 None of the cited papers provide examples of multifrequency data of high quality from randomly oriented, dilute transition ion complexes.…”

Nonresonant Transmission Line Probe for Sensitive Interferometric Electron Spin Resonance Detection

“…This result deviates from past studies on nonresonant ESR detection reports where sensitivity was a major hindrance. 12,13 A further examination of Figure 6a illustrates the transmission line probe approach has sufficient sensitivity to probe much lower microwave excitation powers than usually accessible in commercial systems. This is particularly advantageous when measuring a spin system with long relaxation times.…”

“…This design facilitates easy exchange of samples and eliminates contamination of the transmission line probe. A simple schematic of the design is shown in Figure 3 disadvantage, 12,13 the nonresonant nature (Q ∼ 1) suggests that the measurement will suffer from low sensitivity due to the lack of amplification in ESR excitation and detection. The excitation shortfall can be remedied by increasing the amplitude of the input signal (as discussed above).…”

“…Because resonant structures provide a sensitive means to excite and sense the sample, they are adopted almost universally; however, their purpose-built regions of B 1 uniformity limit applicability to a wider variety of sample composition and simpler sample holder/transfer experimental arrangements . Efforts to induce ESR transitions without resonant high-frequency microwave coupling are most often isolated to specialized surface interrogations or optically/electrically detected spin resonance measurements. − Proposals to employ nonresonant excitation and detection are often dismissed because of nonuniformity and sensitivity concerns, , which have largely precluded the approach from more commonplace spin resonance measurements. Our goal is to re-examine the utility of nonresonant transmission line structures combined with a modified ESR bridge as a means for the sensitive excitation and sensing of ESR transitions in a variety of simpler experimental arrangements and sample compositions.…”

Mapping of the Chromium and Iron Pyrazolate Landscape

“…54 Electron paramagnetic resonance (EPR) is a technique that probes transitions between electron-spin quantum states (m S ), rather than nuclear-spin quantum states (m I ) observed by NMR. 75 Electron paramagnetic resonance is used to study the spin-state in paramagnetic complexes. 76 The technique is used to characterize complexes is often the modulation of transient zero-field splitting (ZFS).…”