Workshop on Rapid Scan EPR

University of Denver EPR Center

and Bruker BioSpin

July 28, 2013

Direct detection • Direct detected magnetic resonance – that is, without modulation

and phase-sensitive detection at the modulation frequency – is old and now is new again.

• Our group is focused on – using relaxation times and – improving sensitivity to solve problems from materials science to medicine.

• Consequently, we have emphasized the rapid passage aspects of direct detection of CW EPR.

• However, the techniques we have developed can be applied to any rate of passage, including slow passage. – Slow scans have been emphasized in recent papers by the Hyde lab

• Since we have been emphasizing low EPR frequencies, magnetic field scans are preferred. – At high EPR frequencies, resonator bandwidth is large enough for

some frequency sweep spectroscopy of narrow spectra.

• The more general label might be “passage EPR” thus encompassing all rates of passage, but today we will call it rapid scan EPR.

BPP Phys. Rev. 73, 679 (1948)

Rapid-Scan NMR

Wiggles were observed in EPR in 1955 and interpreted in terms of the Jacobsohn and

Wangsness paper.

Sample was sodium in liquid ammonia

CW vs. Rapid Scan

CW

Rapid Scan

H

h

S. S. Eaton and G. R. Eaton, J. Magn. Reson. 223, 151 – 163 (2012).

Increasing scan rates for LiPc

J. P. Joshi, J. R. Ballard, G. A. Rinard, R. W. Quine, S. S. Eaton, and G. R. Eaton J. Magn. Res. 175, 44 - 51 (2005).

• LiPc has T1 = 3.5 ms T2 = 2.5 ms • Oscillations on the trailing edge

of the signal increase as scan rate increases.

• The slow-scan signal (- - -) can be found by deconvolution.

rapid-scan EPR

• Scan through resonance in a time that is short relative to electron spin relaxation times.

• Signal is averaged for many transients • e.g. 10,000 times per second

• Select the scan rate to match the spectrum width and resonator Q.

• Direct detection captures the periodic EPR signal. • Detect in quadrature

• Transient effects are mathematically deconvolved to give the absorption and dispersion spectra.

• Spectra are free from the broadening that occurs when CW spectra are recorded with over-modulation.

Rapid scan of nitroxide radical at VHF

50 Gauss scans of 0.5 mM tempone-d16 obtained in 41 s. ( ___ ) First integral of CW spectrum. ( ___ ) Deconvolution of 1 kHz rapid scan. Amplitudes of signals were scaled to match.

Deconvolving The Rapid Scan Signal How to get the slow-scan spectrum.

The examples of rapid scan presented today

In all cases presented today, we will • encompass the spectrum fully with the linear or sinusoidal field

scan,

• obtain the slow-scan spectrum by deconvolution, which will be discussed by Mark Tseitlin

The post-acquisition mathematical treatment will recover the slow-scan spectrum from the transient response, and have no effect on spectra that are already in the slow-scan limit.

Why rapid scan?

• Experiments to be shown shortly demonstrate dramatic improvements in S/N per unit time • relative to CW,

• and also relative to pulse in many cases

• Improve signal-to-noise for signals with T2* too short for Fourier transform EPR

• Acquire rapidly changing signals with good signal-to-noise

• Improve in vivo imaging of multi-line spectra, such as nitroxides and pH-sensitive trityls

Enabling Technologies

• Resonator with minimal eddy currents

• Magnet – standard EPR magnet

• air-core magnet for low RF frequencies

• Magnetic field scan coils - match to sample size, spectral

width, relaxation times

• Coil driver – linear, sinusoidal, arbitrary shape

• Quadrature detection

• Transient response bridge or digital EPR

• Rapid coherent data acquisition and averaging

• 2D EPR

• Post-acquisition data analysis

• Recover slow scan spectrum

• Remove background

• Filter

• Image reconstruction algorithms that use the absorption

signal

George Rinard will describe scan coil and resonator design.

Design criteria are different for X-band studies of small samples VHF in vivo imaging

Magnetic field scan coils

• The larger the sample, the larger the scan coils.

• Helmholtz coils provide a field that is uniform to 0.1% over nearly 20% the diameter of the coils.

• 4-coil systems can provide greater uniformity.

• Power required increases as the square of the scan width.

• Available power amplifiers limit the frequency and amplitude (=rate) of the scans.

Using ENDOR coils

• If the sample is small enough that the field of standard ENDOR coils is sufficiently uniform over the sample,

• Then, rotate the ENDOR resonator 90o to make the “B2” field parallel with the B0 field, and very rapid scans of the order of 109 G/s can be achieved.

• Caution – for very fast scans, resonator Q and eddy currents may distort the EPR response.

Other Rapid Scan Experiments

• Bruker has long had a rapid scan accessory

– 50 or 200 G sweep coils

– ca. 5 kHz repetition, up to 106 G/s

– uses field modulation and phase-sensitive detection

• Many experiments in which the field was swept faster than “normal” have been called rapid scan.

– Sohma et al., Jpn. J. Appl. Phys. 1968 4x104 G/s

– Rengen et al. JMR 1979 106 G/s

– These did not exhibit passage effects.

Rapid frequency scan

W-band, Q = 100, bandwidth = 1 GHz 38 MG/s equivalent scan rate Hyde et al., JMR 2010

Frequency chirp; polyphase excitation

Rapid frequency sweep, LiPc sample, Digital phase-sensitive detection. Tseitlin et al., JMR 2011

Mx, My response

NARS

X-band spectrum obtained in segments; 10 G steps of center field, linear scans. The scan repetition rate (5.2 kHz) is fast enough to suppress 1/f noise. Dashed line is field-modulated spectrum.

A spectrum of immobilized Cu(II) has recently been obtained in segments.

EPR spectra of 200 mM pdMTSL in degassed sec-butylbenzene

Kittell et al., JMR 2011

Range of applications

• From spin trapping to quantum computing

• Spectra that change rapidly – many potential kinetic applications of 2D rapid scan

• Direct detection rapid scan is a sufficiently general EPR method that it can take its place as the third general EPR method, along with CW and pulse.

What is next?

• New generations of scan drivers, resonators, coils, and data analysis.

• Wider scans for broader spectra.

• Hardware and software improvements to reduce background and facilitate removing the background.

• Fully digital implementation.

• Smaller, cheaper, better. Ideally, a Bruker bench-top spectrometer.

Learn more about rapid scan

• Demonstrations in the lab this evening

• Members of the Center are available to answer your questions

• A list of papers is provided

• PowerPoint presentations will be on our web site

• Some additional notes will be on our web site