Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results

Abstract

Dynamic Nuclear Polarization (DNP) experiments transfer polarization from electron spins to nuclear spins with microwave irradiation of the electron spins for enhanced sensitivity in nuclear magnetic resonance (NMR) spectroscopy. Design and testing of a spectrometer for magic angle spinning (MAS) DNP experiments at 263 GHz microwave frequency, 400 MHz (1)H frequency is described. Microwaves are generated by a novel continuous-wave gyrotron, transmitted to the NMR probe via a transmission line, and irradiated on a 3.2 mm rotor for MAS DNP experiments. DNP signal enhancements of up to 80 have been measured at 95 K on urea and proline in water-glycerol with the biradical polarizing agent TOTAPOL. We characterize the experimental parameters affecting the DNP efficiency: the magnetic field dependence, temperature dependence and polarization build-up times, microwave power dependence, sample heating effects, and spinning frequency dependence of the DNP signal enhancement. Stable system operation, including DNP performance, is also demonstrated over a 36 h period.

Fig. 1

Photograph and solid model cross section of 263 GHz gyrotron tube.

Fig. 2

Photograph of 263 GHz gyrotron showing from left to right: microwave transmission line, gyrotron magnet with tube installed, gyrotron control system console, and two chiller units.

Fig. 3

263 GHz gyrotron tuning curves: (a) output power (blue circles) and efficiency (red squares) as function of electron beam current at 12 kV cathode voltage, V₀. (b) Output power (blue circles) and frequency (red circles) versus beam voltage for fixed cavity inlet coolant temperature (20 °C), magnetic field, and filament power. The beam current, I₀, varied as indicated on the plot. (c) Frequency (blue circles) and output power (red circles) as a function of inlet cavity coolant temperature with all other parameters fixed. (d) Output power (blue circles) and frequency (red squares) versus beam voltage for fixed magnetic field and filament power. The cavity inlet coolant temperature is adjusted to maintain a constant frequency, 263.544 GHz.

Fig. 4

Infrared images of microwave beam exiting the 7.6-mm diameter probe waveguide after the final miter bend. The target was positioned perpendicular to the waveguide exit. The cross hairs indicate the center of the waveguide and the red circles denote a 7.6 cm diameter.

Fig. 5

(a) DNP CPMAS pulse sequence; (b) spectra of 0.5 M U- ¹³C, ¹⁵N-proline in glycerol-d₈–D₂O–H₂O (60 : 30 : 10 volume ratio) and 15 mM TOTAPOL biradical, 25 μl sample volume, with DNP (top trace) and without DNP (bottom traces). Spectra acquired at ω_r/2π = 8 kHz MAS, 95 K sample temperature, 4 scans, 2 s recycle delay, and one dummy scan. Higher numbers of scans were also acquired with microwaves off for more accurate DNP signal enhancement calculation.

Fig. 6

Field dependence curve on 2 M ¹³C urea in glycerol-d₈–D₂O–H₂O (60 : 30 : 10 volume ratio) with 15 mM TOTAPOL biradical. The NMR field was swept while the gyrotron remained at fixed frequency, 263.343 GHz. Each field position was measured with ¹³C CPMAS experiment with microwaves on and off for calculating the DNP signal enhancement. 4 scans, 10 s recycle delay, and 2 dummy scans per experiment, ω_r/2π = 5.1 kHz, 105 K sample temperature. The transmitter frequency of the ¹H and ¹³C channel was adjusted for each field point. Error bars on the DNP signal enhancement measurements are ±2%.

Fig. 7

Temperature dependence of the ¹H DNP signal enhancement (blue squares) and ¹H spin lattice relaxation times, T₁, (red triangles) measured on 2 M ¹³C urea in glycerol-d₈–D₂O–H₂O (60 : 30 : 10 volume ratio) with 15 mM TOTAPOL biradical. ¹³C CPMAS experiments with 4 scans, 10 s recycle delay, 2 dummy scans, ω_r/2π = 6.2 kHz. Each temperature point was measured with microwaves on and off for calculation of the DNP signal enhancement. ¹H T₁ was measured with ¹H saturation recovery with CP detection, 11 points and 2 scans per point. Standard deviation was less 1 × 10^–2 on the calculated T₁ values. Error bars on the DNP signal enhancements are ±2%. The sample temperature corresponds to calibrated temperature with the microwaves off.

Fig. 8

DNP polarization build-up and signal enhancement, ε, for 0.1 M U-¹³C-¹⁵N-proline in glycerol-d₈–D₂O–H₂O (60 : 30 : 10 volume ratio), red circles, and in glycerol–H₂O (60 : 40 volume ratios), blue circles, both with 10 mM TOTAPOL. Polarization build-up measured with ¹H saturation recovery with ¹³C CP detection and normalized for maximum value of ²H solvent. 4 scans per point, ω_r/2π = 8 kHz, 97 K sample temperature, microwaves on continuously. The dashed lines through the data points show the fit to a single exponential function.

Fig. 9

¹H DNP signal enhancement as a function of microwave power at the end of the probe waveguide measured on 0.1 M U-¹³C-¹⁵N-proline in glycerol-d₈–D₂O–H₂O (60 : 30 : 10 volume ratio). ¹³C CPMAS experiment with 8 scans, 10 s recycle delay, 1 dummy scan, ω_r/2π = 8 kHz, 97 K sample temperature, microwaves on continuously. 64 scans were acquired for the microwave off measurement and the error bar on the DNP signal enhancement is ±1%. The gyrotron electron beam current, cathode voltage, and cavity temperature were varied to cover the full power range with constant frequency. The microwave power was measured with the transmission line directional coupler and calorimeter, calibrated with the water load at the end of the probe waveguide.

Fig. 10

Spinning frequency dependence of the ¹H DNP signal enhancement of 0.1 M U-¹³C-¹⁵N-proline in glycerol-d₈–D₂O–H₂O (60 : 30 : 10 volume ratio), measured with ¹³C CPMAS experiment at 97 K. Spectra were measured with and without microwave irradiation at each spinning frequency for calculation of the DNP signal enhancement. 10 s recycle delay, 1 dummy scan, 16 to 64 scans per experiment depending on the spinning frequency. Error bars are indicated on the plot.

Fig. 11

NMR signal intensity for ¹³C_δ resonance of 0.1 M ¹³C-¹⁵N proline in glycerol-d₈–D₂O–H₂O (60 : 30 : 10 volume ratio) with 10 mM TOTAPOL during extended experiment run consisting of 36 h gyrotron-on, followed by 80 min gyrotron-off, 2 h gyrotron back on and off again. ¹³C CPMAS experiment with 1.5 ms 55 kHz CP, 33 ms acquisition time with 100 kHz SPINAL64 decoupling, ω_r/2π = 8 kHz, 97 K sample temperature, 16 scans, 1 dummy scan, 6 s recycle delay for each point.