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.

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Comparison of rapid scan (1.8 MG/s) and conventional CW EPR spectra of the low-field nitrogen hyperfine line

of 15N-mHCTPO.

Mitchell, D. G., et. al. J. Magn. Reson. 214, 221–226 (2012).

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.

• Power Saturation: Because of the rapid passage

through resonance, the rapid scan EPR signal is less easily saturated.

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Q res

BW resonator

Methods

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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).

Rapid-scan of Spin Trapped Adducts

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Hypoxanthine/Xanthine Oxidase

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Hypoxanthine Xanthine Oxidase

O2

Superoxide

Spin Trapping Scheme

Spin trapping of BMPO-OOH

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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.

Dr. Jerry Rosen

Mitchell, D. G., et al. (2013). Biophy.J. 105: 338-342.

BMPO-OOH: CW/RS comparison

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•BMPO-OOH with a O2•-

production rate of 0.1 µM/min.

•The concentration of BMPO-

OOH 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.

CW

Rapid-scan

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?

• 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.

12 Faster Scan Rate → Less Saturation for Same B1.

• 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.

Power dependence of Rapid-scan and CW

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S/N is enhanced with rapid scan EPR

0.2% N@C60 Rapid-scan is collected at 1.5 MG/s

a-Si:H

Mitchell, D. G., et. al. Mol. Phys. 111:2664 – 2673 (2013).

X-band EPR of Hydrogenated Silicon

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•Plasma Enhanced Chemical Vapor Deposition of Silicon. •EPR is used to quantify defects in sample.

•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 first-derivative 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*

Sample provided by Alexander Schnegg

Virginia Meyer

Mitchell, D. G., et. al. Mol. Phys. 111: 2664 – 2673 (2013).

RS/CW comparison of solid 0.2% N@C60

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• 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

Sample provided by Aharon Blank

Mitchell, D. G., et. al. Mol. Phys. 111:2664 - 2673 (2013).

E' defects in Irradiated Quartz

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•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.

Virginia Meyer

Mitchell, D. G., et al. (2011). Rad. Meas. 46:993-996.

Rapid scan to understand defects in Diamond

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• 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

Mitchell, D. G., et. al. Mol. Phys. 111:2664 - 2673 (2013).

Summary of Results

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S/N enhancement of Rapid-scan over CW increases

with increasing relaxation time

Relaxation Times Improvement of Rapid-

scan EPR over CW

Sample T1 (µs)a T2 (µs)a ΔBpp (G)a B1 for

CWb

(mG)

B1 for

rapid scanb

(mG)

Rapid scan

rate (MG/s)

S/N RS

relative to CW

a-Si:H 11 3.3 6 35 200 3.9 >250

40 µM OX63 14 5 0.16 ~12 96 0.6 11.5

N@C60 120 –

160

2.8 0.25 6 53 1.5 25

E' in

irradiated

fused quartz

200 20 ~1c 17 220 4.7 14.4

NS0 in

diamondd

2300 230 0.045 0.03 5.8 0.14 140

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 large.

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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.

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

Eaton Group

21 (Clockwise from Left): Hanan, Richard, George, Josh, Mark, Jason, Gareth, Debbie, Virginia, Sandy, Priyanka