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NMR and EPR Technical Reports
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 to first see and 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.
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