EPR 101: A New Standard for Time-Domain EPR
Standard samples have played a critical role in EPR spectroscopy for years. Who hasn't set-up an EPR experiment without using a strong pitch sample first then DPPH and weak pitch to calibrate a spectrometer's performance? While these standards are useful in CW experiments, more laboratories today are using pulse spectrometers in Saturation Recovery (SR) and Electron Spin Echo (ESE) experiments. Traditional standards aren't acceptable for time-domain applications, so a new standard has been proposed.1 The standard consists of an irradiated piece of quartz rod. WILMAD, in cooperation with the authors of this proposal, has recently made this reference standard available, sealed in a WILMAD precision 4mm OD EPR Sample tube. This technical bulletin describes the new standard, assigned Product No. WGSR-01-4, in detail.Irradiated Clear Fused Quartz (CFQ) is known to exhibit a variety of EPR signals.2-5 The predominant signal, though, is an easily saturated, narrow single EPR signal very near g=2.00 known as an E'1-Center. The authors of the proposal arranged for irradiation of 10mm lengths of 3mm OD CFQ rod at Cobe Labs (Lakewood, CO). A dose of 24.4 MRad (244 kGy) from a 60Co source traceable to NIST standards was employed. The mass of each sample was determined using a Mettler Model H54AR Analytical Balance recently serviced and calibrated by the manufacturer before being sealed into a precision WILMAD 4mm OD CFQ EPR sample tube (715-PQ-250M). Each standard is marked with the mass of the irradiated sample contained within.
Signal strength makes obtaining an unsaturated CW spectrum difficult on commercial spectrometers. More important, though, the sample can be used to display passage phenomena at room temperature, including changes in lineshape due to changes in microwave power, modulation amplitude, and scan rate.
Figure 1 shows the Fourier Transform of a Hahn Echo (90-t-180-t-echo) using a t of 200ns. Increasing t produces a nearly single exponential decay of echo intensity with a decay constant, Tm, of 2.1µs. Concentration comparison to pitch samples is unreliable since saturating power levels are used. Further analysis of Tm data provides an estimate of spin concentration of 5 X 1017 spins/cm3 or 0.8mM.
The SR signal of this irradiated quartz sample is also observable without signal averaging. The T1 is ca. 200µs, but a better fit is achieved using the sum of two exponentials with constants of ca. 100 and 200µs, respectively. The longer time constant is dependent on the field position used to prepare the plot.
Because this standard provides a strong signal at room temperature, there is no need to assemble variable temperature apparatus to calibrate your spectrometer. Because the sample is contained in a standard 4mm EPR tube, special resonators aren't needed. Most Loop Gap Resonators (LGRs) not specifically designed for microsampling can accept a 4mm sample tube.
In addition to determining spectrometer signal-to-noise (S/N), this new EPR standard can also be used to compare time-domain performance of spectrometers, such as relaxation times. Reference to the literature about precautions in using standards is recommended to avoid pitfalls associated with standardizing EPR spectrometer performance.