Pulse EPR Upgrade

CW EPR is often used for routine analysis to detect radicals and transition metals, obtain very limited structural information of biological and material samples, and only relatively slow dynamic processes can be studied. CW EPR also has limitations when studying complex systems that produce broad signals. There becomes a point where it is difficult to differentiate between the signal and baseline making interpretation challenging.

Pulsed EPR can overcome many of the drawbacks from CW EPR when trying to study complex systems. The highly resolved spectra and additional capabilities not possible with CW EPR can be easily used for measuring small hyperfine couplings in organic radicals and of ligand nuclei in transition metal complexes and to measure distances between electron spins in the nanometer range. At temperatures where pulse EPR signals can be obtained, measurement of relaxation times is also easier and more precise with pulsed EPR techniques.

The CW EPR instrument in the SIF was recently upgraded to enable Pulsed EPR experiments, through the funding obtained from an NSF MRI grant (Award # 2216355) headed by Prof. Avalos. To perform Pulse EPR experiments, additional components were added:

SpecJet-III

Combining high speed averaging and real time digital signal processing, SpecJet-III is the ideal digitizer for FT-EPR. In addition to optimizing EPR experiments in the time domain, SpecJet-III utilizes real time digital signal processing for optimizing and collecting experiments in the frequency domain.  The unit can achieve 0.5 ns time resolution and a 14 bit amplitude resolution.

PatternJet II

Extended memory on each channel for more pulses and more evolution times, with no reprogramming overhead.
– Maximum time resolution of 1 ns
– 1024 pulses per channel
– Direct phase cycling without reprogramming
– Direct 2D acquisition without reprogramming overhead

SpinJet Arbitrary Waveform Generator (AWG)

SpinJet-AWG is a leap forward in pulse EPR. Extensive MW pulse controls open up exciting new possibilities in experiment design and optimization.
– Frequency definition of each pulse
– Pulse shaping within the shot
– Frequency chirping of pulses
– Phase definition of each pulse
– Multiple channel architecture

AmpX Amplifier

This 600W X-band solid-state amplifier provides high power pulses for Pulse EPR experiments. With a fully overcoupled resonator, 90° pulses as short as 12 ns and saturation pulses a few hundred μs long can be routinely achieved. Paired with high phase stability, this amplifier is ideally suited for all X-band pulsed experiments such as ESEEM, HYSCORE, ENDOR, and DEER

Pulsed EPR has a vast library of pulsed experiments that can be used to characterize structures, study dynamics, and examine interactions of unpaired electrons in materials. Below is list of typical experiments and what it is used to study:

Hahn Echo Detect

• A Hahn echo to used to acquire standard FT EPR spectra. Usually good for narrow line widths.
• Situations where the spectra are broad, the magnetic field is swept and the echo intensity is monitored as a function of magnetic field.
• Very broad spectra can be easily observed with swept echo detect compared to CW because because of the nature of field modulated data acquisition.

Electron Spin Echo Envelope Modulation (ESEEM)

• ESEEM is often used to study the electronic environment of paramagnetic metal centers and metalloproteins and provide valuable information on metalloenzyme mechanisms, metal binding, substrate binding, and the ligand coordination sphere.
• This multi-pulse sequence modulates the echo intensity as a function of the delay between pulses generating a time domain ESEEM spectrum.
• The modulation is a result from the coupling between the electron of interest and nuclei within the system.
• The time domain spectrum can be Fourier transformed to yield a frequency domain spectrum.
• The information that can be obtained from these experiments, hyperfine interactions as well as nuclear quadrolpolar interactions for I ≥ 1, can give a measure of unpaired electron spin density on a given nucleus as well as the distance between it and the electron.

2D Hyperfine Sub-Level Correlation (2D HYSCORE)

• This 2 dimensional experiment is used to study the hyperfine interactions between unpaired electrons and nuclear spins in paramagnetic species
• You can obtain detailed information about the electronic and nuclear spin interactions.
• The HYSCORE technique is based on the correlation between the hyperfine sublevels of the electron spin manifold.
• The resulting two-dimensional spectrum correlates the hyperfine sublevels, providing information about the hyperfine couplings, nuclear quadrupole interactions, and the relative orientations of the hyperfine tensors.
• HYSCORE has high sensitivity and resolution, which enable the detection of small hyperfine couplings and the analysis of complex spin systems.

Double Electron-Electron Resonance (DEER)

• DEER (also known as PELDOR) is used to examine the coupling between two electron spins for distance measurements.
• A Hahn echo is generated then, after tau₂, a π pulse is applied to refocus the echo.
• Another π pulse at a separate microwave frequency (pump frequency) is applied to invert a separate set of spins. This π pulse is also incremented with time.
• The set of spins affected by the probe frequency experiences a different magnetic environment before and after the pump π pulse.
• The final refocused echo intensity is monitored resulting in a modulation of intensity. The periodicity of the modulation can be related to coupling strength and thus electron spin distances.