EPR Everywhere

Abstract

This review is inspired by the contributions from the University of Denver group to low-field EPR, in honor of Professor Gareth Eaton's 80th birthday. The goal is to capture the spirit of innovation behind the body of work, especially as it pertains to development of new EPR techniques. The spirit of the DU EPR laboratory is one that never sought to limit what an EPR experiment could be, or how it could be applied. The most well-known example of this is the development and recent commercialization of rapid-scan EPR. Both of the Eatons have made it a point to remain knowledgeable on the newest developments in electronics and instrument design. To that end, our review touches on the use of miniaturized electronics and applications of single-board spectrometers based on software-defined radio (SDR) implementations and single-chip voltage-controlled oscillator (VCO) arrays. We also highlight several non-traditional approaches to the EPR experiment such as an EPR spectrometer with a "wand" form factor for analysis of the OxyChip, the EPR-MOUSE which enables non-destructive in situ analysis of many non-conforming samples, and interferometric EPR and frequency swept EPR as alternatives to classical high Q resonant structures.

Fig. 1

Simulation of the rigid-limit spectrum of natural abundance mixture of Cu⁶³/Cu⁶⁵ at frequencies from 34 GHz to 500 MHz using pepper in EasySpin. The spectral width narrows from 132.8 mT at 34 GHz to 32.2 mT at 500 MHz, g_xy = 2 and g_z = 2.2, A_xy = 50 MHz A_z = 400 MHz. The system linewidth is 1 mT

Fig. 2

Simulations of Cu (II) in different environments at 9.05 GHz (a, b) and 1.25 GHz (c, d). The example MATLAB simulation for Cu (II) with four imidazole ligands (nitrogen hyperfine coupling) is shown in a and c (g_xy = 2.05, g_z = 2.19, A(Cu)_xy = 56.96 MHzm A(Cu)_Z = 608.58 MHz, A(N)_xy = 43.47 MHz, A(N)_z = 50.96 MHz). The example for a simple Cu (II) with natural abundance ⁶³Cu and ⁶⁵Cu is shown in b and d (g_xy = 2 and g_z = 2.2, A_xy = 50 MHz A_z = 400 MHz). Note that these simulations do not explicitly include the effect of g- and A-strain

Fig. 3

Transient response observed in a rapid-scan EPR experiment. Data shown for a single particle of LiPc, recorded at X-band (ca. 9 GHz). a 11.8 kg/s, b 47.3 kg/s, c 80.2 kg/s

Fig. 4

The EPR MObile UniverSal Explorer (MOUSE) allows for samples to be placed directly on top of a unilateral B₀ assembly collocated with the B₁ resonator (solenoid). (Left) Schematic of the EPR-MOUSE, (right) picture of the actual unit. Reproduced with permission from Ref. [160]