Advantages of digital phase-sensitive detection for upgrading an obsolete CW EPR spectrometer

Multiharmonic electron paramagnetic resonance spectroscopy was demonstrated for two samples with both narrow and broad lines: (i) α,γ-Bisdiphenylene-β-phenylallyl (BDPA) with ΔBpp of 0.85 G plus ultramarine blue with ΔBpp of 17 G, and (ii) a nitroxide radical immobilized in sucrose octaacetate. Modulation amplitudes up to 17 G at 41 kHz were generated with a rapid scan coil driver and Litz wire coils that provide uniform magnetic field sweeps over samples with heights of 5 mm. Data were acquired with a 2-D experiment in the Xepr software through the transient signal path of a Bruker E500T and digitized in quadrature with a Bruker SpecJet II. Signals at the modulation frequency and its harmonics were calculated by digital phase-sensitive detection. The number of harmonics with signal intensity greater than noise increases as the ratio of the modulation amplitude to the narrowest peak increases. Spectra reconstructed by the multiharmonic method from data obtained with modulation amplitudes up to five times the peak-to-peak linewidths of the narrowest features have linewidths that are broadened by up to only about 10% relative to linewidths in spectra obtained at low modulation amplitudes. The signal-to-noise improves with increasing modulation amplitude up to the point where the modulation amplitude is slightly larger than the linewidth of the narrowest features. If this high a modulation amplitude had been used in conventional methodology the linewidth of the narrowest features would have been severely broadened. The multiharmonic reconstruction methodology means that the selection of the modulation amplitude that can be used without spectral distortion is no longer tightly tied to the linewidth of the narrowest line.

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“…The data analysis for multiharmonic EPR is described in [2,3,7]. This method does not require prior knowledge of the signal lineshape.…”

Section: Methodsmentioning

confidence: 99%

Multiharmonic electron paramagnetic resonance for extended samples with both narrow and broad lines

Tseytlin

Eaton

2015

Journal of Magnetic Resonance

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“…Data from the LeCroy were transferred to the PC and the harmonics of the spectra were calculated using a Matlab program. The numerical demodulation method described in [18] was implemented. The 1 st harmonic spectra measured with the XEPR software and by digital demodulation of the digitized modulated signal were in good agreement, which confirmed that digital phase sensitive detection is equivalent to the analog data processing.…”

Section: Methodsmentioning

confidence: 99%

“…The spectrum s n (B) can be calculated by multiplication of the digitized signal by the n th harmonic of the sinusoidal reference followed by digital low-pass filtering. A few labs have implemented this method [15–18]. Bruker recently announced a Signal Processing Unit, SPU, with the capability to simultaneously digitally detect up to five harmonics [http://www.bruker-biospin.com].…”

Section: Introductionmentioning

confidence: 99%

Reconstruction of the first-derivative EPR spectrum from multiple harmonics of the field-modulated continuous wave signal

Tseitlin

Eaton

2011

Journal of Magnetic Resonance

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Selection of the amplitude of magnetic field modulation for continuous wave electron paramagnetic resonance (EPR) often is a trade-off between sensitivity and resolution. Increasing the modulation amplitude improves the signal-to-noise ratio, S/N, at the expense of broadening the signal. Combining information from multiple harmonics of the field-modulated signal is proposed as a method to obtain the first derivative spectrum with minimal broadening and improved signal-to-noise. The harmonics are obtained by digital phase-sensitive detection of the signal at the modulation frequency and its integer multiples. Reconstruction of the first derivative EPR line is done in the Fourier conjugate domain where each harmonic can be represented as the product of the Fourier transform of the 1st derivative signal with an analytical function. The analytical function for each harmonic can be viewed as a filter. The Fourier transform of the 1st derivative spectrum can be calculated from all available harmonics by solving an optimization problem with the goal of maximizing the S/N. Inverse Fourier transformation of the result produces the 1st derivative EPR line in the magnetic field domain. The use of modulation amplitude greater than linewidth improves the S/N, but does not broaden the reconstructed spectrum. The method works for an arbitrary EPR line shape, but is limited to the case when magnetization instantaneously follows the modulation field, which is known as the adiabatic approximation.

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“…Recent efforts to accelerate EPR data collection include both hardware and algorithm developments. For example, overmodulation [10], fast scan [11], rapid scan [12], pulsed EPR [13], parametric modeling [14], adaptive and uniform data sampling [15], digital detection [16, 17, 18] and multisite oximetry [19, 20] have shown potential to accelerate the acquisition process.…”

Section: Introductionmentioning

confidence: 99%

Multisite EPR oximetry from multiple quadrature harmonics

Ahmad¹,

Som²,

Johnson³

et al. 2012

Journal of Magnetic Resonance

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Multisite continuous wave (CW) electron paramagnetic resonance (EPR) oximetry using multiple quadrature field modulation harmonics is presented. First, a recently developed digital receiver is used to extract multiple harmonics of field modulated projection data. Second, a forward model is presented that relates the projection data to unknown parameters, including linewidth at each site. Third, a maximum likelihood estimator of unknown parameters is reported using an iterative algorithm capable of jointly processing multiple quadrature harmonics. The data modeling and processing are applicable for parametric lineshapes under nonsaturating conditions. Joint processing of multiple harmonics leads to 2-3 fold acceleration of EPR data acquisition. For demonstration in two spatial dimensions, both simulations and phantom studies on an L-band system are reported.

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Advantages of digital phase-sensitive detection for upgrading an obsolete CW EPR spectrometer

Cited by 16 publications

References 6 publications

Multiharmonic electron paramagnetic resonance for extended samples with both narrow and broad lines

Multiharmonic electron paramagnetic resonance for extended samples with both narrow and broad lines

Reconstruction of the first-derivative EPR spectrum from multiple harmonics of the field-modulated continuous wave signal

Multisite EPR oximetry from multiple quadrature harmonics

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