a compact epr spectrometer for dnp-nmr spectroscopy …€¦ · · 2016-01-27a compact epr...

A Compact EPR Spectrometer for DNP-NMR Spectroscopy Thorsten Maly Bridge12 Technologies, Inc., Framingham, MA 01702, USA In recent years, Dynamic Nuclear Polarization has proven to be a robust method to increase signal intensities in NMR experiments in laboratories around the world and substantial progress has been made in adapting DNP for solid- and solution-state NMR spectroscopy. In a DNP experiment the large electron polarization is transferred to the nuclear spin reservoir by saturation of an allowed electron paramagnetic resonance (EPR) transition, providing a sensitivity boost in some cases close to the theoretical maximum of 660 (for 1 H). The efficiency of the DNP process depends on many different factors such as the strength of the magnetic field at which the experiment is performed, the strength of the microwave induced magnetic field (ω 1e ) the electron T 1e and T 2e relaxation times, or the strength of the electron dipolar coupling with respect to the Larmor frequency in the case of biradicals to just name a few. The shape or breadth of the EPR spectrum (Δ) is especially important since it determines the active DNP mechanism. Therefore, to understand and optimize the DNP process it is crucial to understand the EPR spectrum and relaxation properties of the polarizing agent at the same magnetic field at which the DNP experiment is performed. Figure 1: Pictures of the 263 GHz prototype EPR spectrometer and probe. A: Complete EPR bridge with diode detector. B: EPR bridge on top of NMR magnet. C: Characterization of the QO circulator using a Vector Network Analyzer. D: EPR probe with rooftop mirror and sample. Figure 2: 263 GHz EPR spectra. Left: BDPA in Polystyrene. Middle: TEMPO in Toluene. Right: DPPH in 3.2 mm MAS rotor. Unfortunately the number of spectrometers operating at frequencies at which DNP is currently performed (300 to > 800 MHz 1 H NMR, 197 to 526 GHz EPR) is very limited and researchers often have to extrapolate sample parameters from low-field measurements, or entirely speculate about the EPR characteristics of their DNP sample. Commercial HF- EPR spectrometers are available at 95 and 263 GHz, however, these instruments require large investments. We have developed a compact 263 GHz EPR spectrometer, which can easily be integrated into existing DNP infrastructure. The instrument is based on a 263 GHz solid-state microwave source, with a maximum output power of 60 mW and a quasi-optical duplexer, allowing reflection and induction mode detection (see Figure 1). Preliminary results are shown Figure 2. The BDPA/PS spectra were recorded in reflection mode by either sweeping the magnetic field (blue) or sweeping the frequency (red). Both spectra are almost identical. The DPPH spectrum is recorded in a DNP-MAS NMR probe using on-axis radiation of the sample. Here we will review the performance of the device and discuss its relevance to DNP-NMR spectroscopy. Magnetic Field [T] 9.381 9.382 9.383 9.384 9.385 9.386 9.387 9.388 9.389 Signal Amplitude [a.u.] 0 0.2 0.4 0.6 0.8 1 B Sweep f Sweep Magnetic Field [T] 9.396 9.398 9.4 9.402 9.404 9.406 9.408 Signal Amplitude [a.u.] -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 B Sweep f Sweep Microwave Frequency [GHz] 262.9 262.95 263 263.05 263.1 Signal Amplitude [a.u.] -0.2 0 0.2 0.4 0.6 0.8 1

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A Compact EPR Spectrometer for DNP-NMR Spectroscopy

Thorsten Maly

Bridge12 Technologies, Inc., Framingham, MA 01702, USA In recent years, Dynamic Nuclear Polarization has proven to be a robust method to increase signal intensities in NMR experiments in laboratories around the world and substantial progress has been made in adapting DNP for solid- and solution-state NMR spectroscopy. In a DNP experiment the large electron polarization is transferred to the nuclear spin reservoir by saturation of an allowed electron paramagnetic resonance (EPR) transition, providing a sensitivity boost in some cases close to the theoretical maximum of 660 (for 1H). The efficiency of the DNP process depends on many different factors such as the strength of the magnetic field at which the experiment is performed, the strength of the microwave induced magnetic field (ω1e) the electron T1e and T2e relaxation times, or the strength of the electron dipolar coupling with respect to the Larmor frequency in the case of biradicals to just name a few. The shape or breadth of the EPR spectrum (Δ) is especially important since it determines the active DNP mechanism. Therefore, to understand and optimize the DNP process it is crucial to understand the EPR spectrum and relaxation properties of the polarizing agent at the same magnetic field at which the DNP experiment is performed.

Figure 1: Pictures of the 263 GHz prototype EPR spectrometer and probe. A: Complete EPR bridge with diode detector. B: EPR bridge on top of NMR magnet. C: Characterization of the QO circulator using a Vector Network Analyzer. D: EPR probe with rooftop mirror and sample.

Figure 2: 263 GHz EPR spectra. Left: BDPA in Polystyrene. Middle: TEMPO in Toluene. Right: DPPH in 3.2 mm MAS rotor. Unfortunately the number of spectrometers operating at frequencies at which DNP is currently performed (300 to > 800 MHz 1H NMR, 197 to 526 GHz EPR) is very limited and researchers often have to extrapolate sample parameters from low-field measurements, or entirely speculate about the EPR characteristics of their DNP sample. Commercial HF-EPR spectrometers are available at 95 and 263 GHz, however, these instruments require large investments. We have developed a compact 263 GHz EPR spectrometer, which can easily be integrated into existing DNP infrastructure. The instrument is based on a 263 GHz solid-state microwave source, with a maximum output power of 60 mW and a quasi-optical duplexer, allowing reflection and induction mode detection (see Figure 1). Preliminary results are shown Figure 2. The BDPA/PS spectra were recorded in reflection mode by either sweeping the magnetic field (blue) or sweeping the frequency (red). Both spectra are almost identical. The DPPH spectrum is recorded in a DNP-MAS NMR probe using on-axis radiation of the sample. Here we will review the performance of the device and discuss its relevance to DNP-NMR spectroscopy.

Magnetic Field [T]9.381 9.382 9.383 9.384 9.385 9.386 9.387 9.388 9.389

Sign

al A

mpl

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[a.u

0.2

0.4

0.6

0.8

1 B Sweepf Sweep

Magnetic Field [T]9.396 9.398 9.4 9.402 9.404 9.406 9.408