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Electron Paramagnetic Resonance (EPR)

Related products: MFLI

EPR Application Setup using the Zurich Instruments MFLI Lock-in Amplifier

Application Description

Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) is one of the most informative techniques on the electronic structure of paramagnetic species. EPR spectroscopy is particularly suitable for the investigation of (bio)chemical systems with strongly localized spin density and their interaction with the environment. For these systems EPR provides information on the structure and dynamics and is widely used in chemistry, physics and biology.

EPR measurements are typically done either in continuous wave (cw) or pulsed mode. The cwEPR spectrometer - depicted in the image to the left - applies a magnetic field of about 3500 G (0.35 T) and measures the absorption of microwaves in the 9-10 GHz regime (X-Band). Usually, the microwave is kept at a fixed frequency and the magnetic field is swept (for X-Band from 0 mT - 700 mT).

Graph of an EPR

The left panel of this figure shows a typical signal detected with the microwave detector. The application of a small additional oscillating magnetic field at a typical frequency of 100 kHz improves the sensitivity using lock-in detection and adds the possibility of extracting phase information. The resulting signal is the first derivative of the absorption, as illustrated in the right panel of the figure.

Measurement Strategies

To achieve high resolution in a short acquisition time a trade off regarding three parameters has to be made: modulation frequency, modulation amplitude, and lock-in filter bandwidth.

First, the spectral resolution depends on the signal to noise ratio (SNR) and the spectral distortion which are both influenced by the amplitude of the magnetic field modulation. Large modulation amplitudes increase the SNR due to an increase in signal intensity. But at large amplitudes the detected EPR signal broadens and becomes distorted, hence decreasing the resolution because close lines can not be resolved. A similar distortion effect applies when high modulation frequencies are used and the spin relaxation is too slow to follow fast changes in the magnetic field.

On top of this, the SNR and vice versa the spectral resolution also directly depend on the modulation frequency. This is a consequence of lock-in detection, which is explained in detail in the White Paper Principles of Lock-in Detection. A high modulation frequency will result in high SNR but will also cause spectral distortion as described above.

And last, the filter bandwidth used in the lock-in detection also influences the SNR but also the acquisition time. A low filter bandwidth will cause a high SNR but leads to a slow acquisition time at each step of the sweep of the magnetic field, because low filter bandwidths need slow settling times. Another way to achieve high SNR is by averaging - remember that SNR is proportional to averaging time - and using large filter bandwidth with fast settling times and fast acquisition times. In a stable laboratory environment and stable spectrometer, a lot of signal averaging with large filter bandwidth and acquiring a spectrum with few averages and low filter bandwidth are equivalent. In reality, there are always signal drifts that have to be taken into account. This requires finding suitable trade offs between the filter bandwidths and averaging time.

The following table summarizes the parameters and their effect on resolution and acquisition time.

  SNR spectral distortion time constant acquisition time
modulation amplitude small low small X X
modulation amplitude large high large X X
modulation frequency low low small X X
modulation frequency high high large X X
filter bandwidth low high X large slow
filter bandwidth high low X small fast

X = no effect

An ideal cwEPR measurement needs a cautious tuning of the modulation amplitude and frequency as well as the filter bandwidth and number of averages. For this crucial tuning EPR users need instruments that give full control over these parameters and also provide tools to analyze the signals in the time and frequency domain to judge on SNR and spectral resolution.

The benefits of Choosing Zurich Instruments

Considering the requirements for an ideal cwEPR measurement the Zurich Instruments MFLI 500 kHz Lock-in Amplifier is a perfect match for cwEPR:

  • Very fast acquisition times are feasible because of a fast time constant and a low input voltage noise.
  • Get quickly up to speed and in full control of your measurements with an easy to use web interface that can be accessed via any browser.
  • Observe and record all relevant time-domain and frequency domain signals conveniently through the the LabOne Plotter and Spectrum-analyzer.
  • Simplify your setup and enjoy the high level of integration. No additional digitizer card is required to record your measurements. The MFLI comes with a fast digital data transfer through USB or 1 GbE connections.
  • The auxiliary outputs of the instrument can be used to sweep the magnetic field or microwave. Together with the large number of APIs (Python, C, MATLAB®, LabVIEW and .NET) it is easy to integrate the MFLI in existing cwEPR spectrometer setups.

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