X-Band Rapid Scan EPR - EPR Center

X-Band Rapid-scan EPR:A
Comparison of CW, Pulse, and
Rapid-scan EPR
Deborah G. Mitchell
2013 University of Denver Rapid-scan Workshop
Department of Chemistry and Biochemistry
University of Denver
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Applications of X-band Rapid-scan EPR
• Define rapid-scan EPR
• Rapid-scan EPR of transient species (spin
trapped radicals)
• Applications of rapid-scan EPR to samples with
long relaxation times
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Rapid-scan EPR
• Magnetic field is swept through
resonance in a time that is short
relative to the relaxation time T2.
– May cause oscillations on the
trailing edge of the signal.
• The decay of the oscillations is
dependent on T2 and
inhomogeneous broadening.
• Rapid-scan spectrum can be
deconvolved to obtain absorption
spectrum.
Mitchell, D. G., et. al. J. Magn. Reson. 214, 221–226 (2012).
Comparison of rapid scan (1.8 MG/s)
and conventional CW EPR spectra of
the low-field nitrogen hyperfine line
of 15N-mHCTPO.
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CW vs. Rapid-scan EPR
• Detection: In rapid-scan EPR, the signal is detected
directly. In CW, phase sensitive detection is used.
• Scan Rate: In rapid-scan EPR, the field is scanned
more rapidly than in normal CW.
• Resonator Q: If Q is high, linewidths may be
broadened at high scan rates. In CW EPR, high Q is
routinely used.
 res
Q
BW resonator
• Power Saturation: Because of the rapid passage
through resonance, the rapid scan EPR signal is less
easily saturated.

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Methods
A Bruker E500-T is used to
record rapid-scan signals with:
a.) Locally designed coil driver:
The Resonated Coil Driver (RCD)
can operate at 100% duty cycle
with 80 G sweep width and 50
kHz sweep frequency
b.) Rapid scan coils:
Relative to solid wire, Litz wire
decreases AC resistance (which
allows us to operate at higher
currents).
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Rapid-scan of Spin Trapped Adducts
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Hypoxanthine/Xanthine Oxidase
O2
Xanthine Oxidase
Hypoxanthine
Superoxide
Spin Trapping Scheme
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Spin trapping of BMPO-OOH
Dr. Jerry Rosen
6 µM/min O2•- production was produced by a
mixture of xypoxanthene/xanthine oxidase.
A) CW spectrum with 55 G sweep width, 0.75 G
modulation amplitude, single 42 s scan, and 20
mW (B1 = 170 mG) microwave power.
B) The first integral of spectrum in part A.
C) Deconvolved rapid-scan spectrum obtained
with 55 G scan width, 51 kHz scan frequency, 20
mW (B1 = 150 mG) microwave power, ~4 seconds
acquisition time.
Mitchell, D. G., et al. (2013). Biophy.J. 105: 338-342.
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BMPO-OOH: CW/RS comparison
CW
Rapid-scan
•BMPO-OOH with a O2•production rate of 0.1 µM/min.
•The concentration of BMPOOOH is about 0.3 µM.
•Both collected in 30 seconds.
A) CW spectrum obtained with
standard spin trapping conditions
20 mW (B1 = 170 mG) microwave
power.
B) Deconvolved rapid-scan
spectrum obtained with 51 kHz
scan frequency, 53 mW (B1 = 250
mG) microwave power.
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Mitchell, D. G., et al. (2013). Biophy.J. 105: 338-342.
Detection of BMPO-OOH with E. Faecalis
CW/RS comparison
BMPO-OOH in a suspension of
E. faecalis with 2x106 CFU/mL
and a O2•- production rate of 0.2
μM/min. The concentration of
BMPO-OOH is about 0.5 µM.
Both were collected in 30
seconds
A) CW spectrum obtained with
standard spin trapping
parameters.
B) Deconvolved rapid-scan
spectrum obtained with 51 kHz
scan frequency, 53 mW (B1 =
250 mG) microwave power.
Mitchell, D. G., et al. (2013). Biophys. J. 105: 338 – 342
Rapid-scan of samples with long
electron relaxation times (T1 and T2)
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Rapid-scan EPR: why is it useful
when T1 andT2 are very long?
•
Examples of easily saturated samples (long T1):
Diamond N@C60 OX63 a-Si:H E’ defects in quartz
• Collecting undistorted conventional CW spectra of
these samples is difficult.
•
Larger Linear Power Region:
– Magnetic field is on resonance for a short time
relative to CW.
– The power absorbed by the spins, for the same
microwave B1 and time, is less than in conventional
CW spectra.
Faster Scan Rate → Less Saturation for Same B1.
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Power dependence of Rapid-scan and CW
0.2% N@C60
a-Si:H
Rapid-scan is collected at 1.5 MG/s
S/N is enhanced with rapid scan EPR
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Mitchell, D. G., et. al. Mol. Phys. 111:2664 – 2673 (2013).
X-band EPR of Hydrogenated Silicon
•Plasma Enhanced Chemical Vapor
Deposition of Silicon.
•EPR is used to quantify defects in
sample.
Sample provided by Alexander Schnegg
•T1 ≈ 12 µs, T2 ≈ 3.3 µs
• ΔBpp ≈ 6G
•Field-swept echo detected spectrum
obtained with constant 500 ns spacing
between pulses, SRT = 100 s, 1024
shots/point, 10 scans
•Conventional field-modulated firstderivative CW EPR spectrum acquired
with 2 G modulation amplitude at 30
kHz, and B1 = 35 mG.
•FT EPR not an option with
hydrogenated silicon sample because
of short T2*
Virginia Meyer
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Mitchell, D. G., et. al. Mol. Phys. 111: 2664 – 2673 (2013).
RS/CW comparison of solid 0.2%
N@C60
Sample provided by Aharon Blank
• Endohedral N@C60 is an intriguing
sample because of its long electron
relaxation times due to the shielding
of its carbon cage.
• Quantum Computing: Each N@C60
would function as a qubit
• T1 ≈ 160 µs, T2 ≈ 2 µs, ΔBpp ~225 mG
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Mitchell, D. G., et. al. Mol. Phys. 111:2664 - 2673 (2013).
E' defects in Irradiated Quartz
•240 kGy (24 MRad) with 60Co 
rays
•T1= 100 µs, T2= 20 µs
•Because relaxation times are so
long, an unsaturated CW
spectrum that is free from
passage effects is difficult to
obtain.
•CW EPR obtained in 1 minute
using 0.02 mW power, 10 kHz
modulation frequency and 0.05
G modulation amplitude.
Mitchell, D. G., et al. (2011). Rad. Meas. 46:993-996.
Virginia Meyer
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Rapid scan to understand defects in
Diamond
• Diamond sample with 20 ppb nitrogen defects.
• T1 ≈ 2.3 ms, T2 ≈ 0.2 ms, ΔBpp ≈ 45 mG
•For CW, must operate at 70 dB (20 nW) to be
within the linear region.
•Data took 14 minutes to collect.
Sample provided
by Mark Newton
•CW spectrum acquired with 0.05 G
modulation amplitude at 6 kHz and
B1 = 0.25 mG, one scan.
•Field-swept echo detected
spectrum with a constant 600 ns
spacing between pulses, SRT = 3 ms,
64 shots/pt, 1 scan
•FT-EPR of data obtained with an
SRT of 200 s, 24o tip angle, and
40960 averages
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Mitchell, D. G., et. al. Mol. Phys. 111:2664 - 2673 (2013).
Summary of Results
Sample
T1 (µs)a
T2 (µs)a
ΔBpp (G)a
a-Si:H
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3.3
40 µM OX63
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N@C60
E' in
irradiated
fused quartz
NS0 in
diamondd
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B1 for
CWb
(mG)
35
B1 for
rapid scanb
(mG)
200
5
0.16
~12
120 –
160
200
2.8
0.25
20
2300
230
Relaxation Times
Rapid scan
S/N RS
rate (MG/s) relative to CW
3.9
>250
96
0.6
11.5
6
53
1.5
25
~1c
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220
4.7
14.4
0.045
0.03
5.8
0.14
140
Improvement of Rapidscan EPR over CW
S/N enhancement of Rapid-scan over CW increases
with increasing relaxation time
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Mitchell, D. G., et. al. Mol. Phys. 111: 2664 - 2673 (2013).
Summary of Results
• Benefits of Rapid-scan EPR :
– Larger linear response range
– Fast Data Acquisition
– Collect undistorted absorption spectra of samples with long
T1
– Enhanced S/N
– Transient Species (Spin Trapping)
• Other Applications of Rapid-scan EPR
– Absorption spectrum is good for imaging
– Frequency scans can be performed similarly, especially at
high EPR frequency, where the resonator bandwidth is
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large.
Acknowledgements
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University of Denver
Advisors: Drs. Gareth
and Sandra Eaton
Dr. Mark Tseitlin
Richard Quine
Eaton Group: George,
Mike, Virginia, Josh,
Jason, Priyanka, and
Hanan
Fellow DU graduate
students: Josh, Paul,
Jennifer, Brittany, and
many more.
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Collaborators
Spin Trapping: Gerald Rosen, University of
Maryland, School of Pharmacy
Bacteria: Barbee Lab: Breanna Symmes and
Katherine Nesler also Heather Wilkins and
Aimee Winter from Dr. Linsemann’s lab
Diamond: Dr. Mark Newton, University of
Warwick
Hydrogenated Silicon: Dr. Alexander Schegg,
Helmholtz-Zentrum Berlin für Materialien und
Energie GmbH
N@C60: Dr. Aharon Blank, School of
Chemistry
Bruker Biospin: Ralph Weber, Chuck Hanson,
Peter Hofer
$$$- NSF Graduate Research Fellowship
$$$- NSF IDBR 0753018
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Eaton Group
(Clockwise from Left): Hanan, Richard, George, Josh, Mark,
Jason, Gareth, Debbie, Virginia, Sandy, Priyanka
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