Sensitivity enhancement in structural measurements by solid state NMR through pulsed spin locking

Free induction decay (FID) signals in solid state NMR measurements performed with magic angle spinning can often be extended in time by factors on the order of 10 by a simple pulsed spin locking technique. The sensitivity of a structural measurement in which the structural information is contained in the dependence of the integrated FID amplitude on a preceding evolution period can therefore be enhanced substantially by pulsed spin locking in the signal detection period. We demonstrate sensitivity enhancements in a variety of solid state NMR techniques that are applicable to selectively isotopically labeled samples, including 13C-15N rotational echo double resonance (REDOR), 13C-13C dipolar recoupling measurements using the constant-time finite-pulse radio-frequency-driven recoupling (fpRFDR-CT) and constant-time double-quantum-filtered dipolar recoupling (CTDQFD) techniques, and torsion angle measurements using the double quantum chemical shift anisotropy (DQCSA) technique. Further, we demonstrate that the structural information in the solid state NMR data is not distorted by pulsed spin locking in the detection period.     

High power-efficiency terahertz quantum cascade laser

We demonstrate continuous-wave(CW) high power-efficiency terahertz quantum cascade laser based on semiinsulating surface-plasmon waveguide with epitaxial-side down(Epi-down) mounting process.The performance of the device is analyzed in detail.The laser emits at a frequency of ~3.27 THz and has a maximum CW operating temperature of ~ 70 K.The peak output powers are 177 mW in pulsed mode and 149 mW in CW mode at 10 K for 130-μm-wide Epi-down mounted lasers.The record wall-plug efficiencies in direct measurement are 2.26% and 2.05% in pulsed and CW mode,respectively.     

A new strategy for fast radiofrequency CW EPR imaging: direct detection with rapid scan and rotating gradients

Rapid field scan on the order of T/s using high frequency sinusoidal or triangular sweep fields superimposed on the main Zeeman field, was used for direct detection of signals without low-frequency field modulation. Simultaneous application of space-encoding rotating field gradients have been employed to perform fast CW EPR imaging using direct detection that could, in principle, approach the speed of pulsed FT EPR imaging. The method takes advantage of the well-known rapid-scan strategy in CW NMR and EPR that allows arbitrarily fast field sweep and the simultaneous application of spinning gradients that allows fast spatial encoding. This leads to fast functional EPR imaging and, depending on the spin concentration, spectrometer sensitivity and detection band width, can provide improved temporal resolution that is important to interrogate dynamics of spin perfusion, pharmacokinetics, spectral spatial imaging, dynamic oxymetry, etc.     

Comparing continuous wave progressive saturation EPR and time domain saturation recovery EPR over the entire motional range of nitroxide spin labels

The measurement of spin-lattice relaxation rates from spin labels, such as nitroxides, in the presence and absence of spin relaxants provides information that is useful for determining biomolecular properties such as nucleic acid dynamics and the interaction of proteins with membranes. We compare X-band continuous wave (CW) and pulsed or time domain (TD) EPR methods for obtaining spin-lattice relaxation rates of spin labels across the entire range of rotational motion to which relaxation rates are sensitive. Model nitroxides and spin-labeled biological species are used to illustrate the potential complications that arise in extracting relaxation data under conditions typical to biological experiments. The effect of super hyperfine (SHF) structure is investigated for both CW and TD spectra. First and second harmonic absorption and dispersion CW spectra of the nitroxide spin label, TEMPOL, are all fit simultaneously to a model of SHF structure over a range of microwave amplitudes. The CW spectra are novel because all harmonics and microwave phases were acquired simultaneously using our homebuilt CW/TD spectrometer. The effect of the SHF structure on the pulsed free induction decay (FID) and pulsed saturation recovery spectrum is shown for both protonated and deuterated TEMPOL. We present novel pulsed saturation recovery measurements on biological molecules, including spin-lattice relaxation rates of spin-labeled proteins and spin-labeled double-stranded DNA. The impact of structure and dynamics on relaxation rates are discussed in the context of each of these examples. Collisional relaxation rates with oxygen and transition metal paramagnetic relaxants are extracted using both continuous wave and time domain methods. The extent of the errors inherent in the CW method and the advantages of pulsed methods for unambiguously measuring collisional relaxation rates are discussed. Spin-lattice relaxation rates, determined by both CW and pulsed methods, are used to determine the electrostatic potential on the surface of a protein.     

Comparison of Continuous Wave, Spin Echo, and Rapid Scan EPR of Irradiated Fused Quartz

The E' defect in irradiated fused quartz has spin lattice relaxation times (T(1)) about 100 to 300 μs and spin-spin relaxation times (T(2)) up to about 200 μs, depending on the concentration of defects and other species in the sample. These long relaxation times make it difficult to record an unsaturated continuous wave (CW) electron paramagnetic resonance (EPR) signal that is free of passage effects. Signals measured at X-band (~9.5 GHz) by three EPR methods: conventional slow-scan field modulated EPR, rapid scan EPR, and pulsed EPR, were compared. To acquire spectra with comparable signal-to-noise, both pulsed and rapid scan EPR require less time than conventional CW EPR. Rapid scan spectroscopy does not require the high power amplifiers that are needed for pulsed EPR. The pulsed spectra, and rapid scan spectra obtained by deconvolution of the experimental data, are free of passage effects.     

Pulsed NMRON studies of insulating magnetic materials

The apparatus and instrumental techniques developed for pulsed NMRON studies of insulating ordered magnets are described. Experiments have been performed on two salts,⁵⁴Mn-MnCl₂·4H₂O and⁵⁴Mn-Mn(COOCH₃)₂·4H₂O, and rotation patterns, free induction decays and spin echoes have been obtained for both single quantum and double quantum transitions. The pulsed technique can produce signals larger than those obtained in CW NMRON, and is well suited for the measurement of the very short spin-lattice relaxation times that can occur in these systems.     

A CW and pulse EPR spectrometer operating at 122 and 244 GHz using a quasi-optical bridge and a cryogen-free 12 T superconducting magnet

A high-field continuous-wave (CW) and pulse electron paramagnetic resonance spectrometer operating at 122 and 244 GHz is described. The instrument is based on a millimeter-wave bridge built from quasi-optical components. To improve the sensitivity, a cryo-cooled detector/mixer is used. The magnetic field is generated using a cryogen-free superconducting 12 T magnet (warm bore, 88 mm) equipped with a helium-flow cryostat for sample cooling. The advantages of this spectrometer are described and first results (obtained in CW mode) on different types of samples at 122 and 244 GHz are presented. The extensions to pulse operation as well as double resonance techniques (electron-electron and electron-nuclear) are briefly discussed.     

Spin dynamics in continuous wave and pulsed ENDOR spectroscopy of coals

ENDOR experiments on coals recorded using continuous wave (CW) and pulsed techniques appear to give qualitatively different spectra. A matrix proton signal dominates the ENDOR spectrum of coals recorded in the CW ENDOR experiment while both a matrix and local proton ENDOR signals with huperfine couplings of up to 20 MHz are observed in spectra recorded using pulsed excitation techniques. Analysis of these spectra lead to different implications for the structure of the molecules that host the unpaired electron. Using a combination of pulsed EPR (Electron Spin Echo, FID detected hole burning) and pulsed Electron Nuclear Multiple Resonance (Sub-level relaxation, hyperfine selective ENDOR, EPR sub-spectra) experiments, we investigate the electron and nuclear spin dynamics in order to reconcile the different signal amplitudes observed in the CW and pulsed ENDOR spectra. In the CW ENDOR experiment, the results of the FID detected hole burning experiments prove that the low ENDOR signal intensity can not be attributed to spectral diffusion mechanisms competing with ENDOR mechanisms. Instead, we find that an unfavorable ratio of the electron and nuclear spin relaxation rates results in small local ENDOR signals. The matrix line dominates the spectrum because of the large number of matrix protons. In the pulsed ENDOR experiment, the hyperfine contrast selectivity mechanism suppresses the intensity of the matrix ENDOR signal and enhances the amplitudes of the local ENDOR signals. In addition, the ENDOR signal is not a function of the ratio of the electron and nuclear relaxation rates.     

The theory of nuclear orientation via electron spin locking (NOVEL)

Nuclear orientation via electron spin locking (NOVEL) is a technique to orient nuclear spins embedded in a solid. Like other methods of dynamic nuclear polarization (DNP) it employs a small amount of unpaired electron spins and uses a microwave field to transfer the polarization of these unpaired electron spins to the nuclear spins. Traditional DNP uses CW microwave fields, but NOVEL uses pulsed electron spin resonance (ESR) techniques: a 90 degree pulse–90 degree phase shift–locking pulse sequence is applied and during the locking pulse the polarization transfer is assured by satisfying the Hartmann–Hahn condition. The transfer is coherent and similar to coherence transfer between nuclear spins. However, NOVEL requires an extension of the existing theory to many, inequivalent nuclear spins and to arbitrary, i.e. high electron and nuclear spin polarization. In this paper both extensions are presented. The theory is applied to the system naphthalene doped with pentacene, where the proton spins are polarized using the photo-excited triplet states of the pentacene molecules and found to show excellent agreement with the experimentally observed evolution of the polarization transfer during the locking pulse.     

Nonlinear NMR in a superfluid B phase of <Superscript>3</Superscript>He in aerogel

The properties of liquid ³He in a low-density aerogel preliminarily covered with a few monolayers of ⁴He were studied by pulsed and nonlinear CW NMR techniques. It was found that an NMR frequency shift from the Larmor value exhibits a sharp increase at a magnetization tilting angle exceeding 104°. Nonlinear CW NMR signals related to the formation of a macroscopic region featuring homogeneous precession of the magnetization (homogeneous precession domain) were observed. The experimental results confirm that the low-temperature superfluid ³He phase in the aerogel is analogous to the B-phase in bulk ³He and indicate that the spin supercurrents play an important role in the spin dynamics of superfluid ³He in aerogel.     

微波脉宽对检波器性能影响的实验研究

 采用脉宽20~120 ns、步长10 ns的微波信号，对WB-CD01型、2087-6001-00型同轴检波器的灵敏度和检波波形进行了实验研究。实验结果表明：脉宽为20~120 ns时，在2.7 GHz频率下，WB-CD01型检波器灵敏度比连续波时的降低0.5~0.7 dB，而2087-6001-00型检波器灵敏度比连续波时的降低近1 dB。在4.1 GHz频率下，WB-CD01型检波器在微波脉宽为20 ns时，检波灵敏度略高于连续波时的近0.2 dB，其它给定脉宽下，检波灵敏度与连续波时的基本一致；2087-6001-00型检波器不同脉冲宽度条件下与连续波时的灵敏度基本一致。WB-CD01型检波器的检波波形前沿出现过脉冲，而2087-6001-00型检波器无此现象。     

Investigating hyperpolarized Xe and CPMAS for spin polarization transfer to surface nuclei: a model study

Several experimental techniques have been developed to utilize spin-polarized xenon gas for sensitivity and selectivity enhancement in surface studies using solid-state NMR. Although previously reported as a viable spin polarization transfer mechanism, the details of high-field cross-polarization (CP) have not been thoroughly investigated. We recently reported observations of CP from an adsorbed layer of hyperpolarized xenon (HP Xe) to a variety of surface nuclei at temperatures as high as 323 K [J. Am. Chem. Soc. 105 (2001) 1412]. In this paper, we investigate many of the issues associated with HP Xe surface CP studies, including polarization transfer kinetics and the effects of temperature on the dynamics. Protonated and methylated silica samples are used as model systems for comparison. A comparison of the rate analysis data from CP and SPINOE (Spin Polarization-Induced Nuclear Overhauser Effect) experiments provides information on the origin of the difference in polarization transfer efficiencies between the two techniques. Lineshape analysis of ¹H spectra for CP and SPINOE experiments demonstrates the difference in selectivity of methods due to longer SPINOE evolution times that lead to greater spin diffusion. The results of this work help to assess the viability of HP Xe CP as a surface analysis technique.     

Low Field (10 mT) Pulsed Dynamic Nuclear Polarization

EPR irradiation by a train of inverting pulses has potential advantages over continuous-wave EPR irradiation in DNP applications; however, it has previously been used only at high field (5 T). This paper presents the design and testing of an apparatus for performing pulsed DNP experiments at 10 mT with large samples (17 ml). Experimental results using pulsed DNP with an aqueous solution of a narrow-linewidth paramagnetic probe are presented. A maximum DNP enhancement of about -36 with a train of inverting pulses (width 500 ns, repetition time 4 micros) was measured. A preliminary comparison showed that, when the same enhancement value is considered, the pulsed DNP technique requires an average power that is about three times higher than that required with the CW irradiation. However, for in vivo DNP applications it is very important to minimize the average power deposited in the sample. From the experimental results reported in this work, when considering the maximum enhancement, the pulsed technique requires only 2% of the average power necessary with the CW DNP technique. We believe that this reduction in the average power can be important for future DNP studies with large biological samples.     

Effects of processing parameters on laser cutting of aluminium-copper alloys using off-axial supersonic nozzles

Conventional laser cutting involves the utilization of converging coaxial nozzles to inject the assist gas used to remove the molten material. This processing system prevents the utilization of this technique to cut aluminium alloys for aerospace applications. The inefficient removal of molten material by the assist gas produces cuts with poor quality; very rough cuts, with a large amount of dross, and a large heat affected zone (HAZ) are obtained. An alternative to increase the assist gas performance is the utilization of off-axial supersonic nozzles. Removal of molten material is substantially increased and cuts with high quality are obtained. On the other hand, pulsed laser cutting offers superior results during the processing of high reflectivity materials as aluminium alloys. However, there are no experimental studies which explore the pulsed laser cutting of aluminium alloys by means of a cutting head assisted by an off-axis supersonic nozzle.The present work constitutes a quantitative experimental study to determine the influence of processing parameters on the cutting speed and quality criteria during processing by means of off-axial supersonic nozzles. Cutting experiments were performed in pulsed mode and the results explained under the basis of the molten material removal mechanisms. Performed experiments indicate a reduction in cutting speed as compared to continuous wave (CW) mode processing and the existence of two processing regimes as a function of the pulse frequency. Best results are obtained under the high pulse frequency one (f > 100 Hz) because the superior capabilities of molten material removal of the supersonic jets are completely exploited in this processing regime.     

Comparison of CW-EPR and ESEEM technique for determination of water permeability profile in liposome membranes

The hydrophobicity profile across a membrane is an important parameter of biological membranes as it defines the barrier for permeation of polar molecules into the cell interior and of the stability of transmembrane proteins. In this work, the hydrophobicity profile was measured by conventional continuos-wave electron paramagnetic resonance (CW-EPR) and by electron spin echo envelope modulation (ESEEM) in two liposome suspensions with different ionic strengths in order to compare the sensitivity of both methods for the determination of the water penetration depth into the liposome membrane. Multilamellar liposomes from egg-phosphatidylcholine, cholesterol and dicetyl phosphate were prepared in deuterium oxide, buffered with phosphate buffer with or without NaCl at the phosphate concentrations 0.1 or 0.01 M. Liposomes were spin labeled with stearic acids spin probes with the nitroxide group at different positions on the acyl chain. The deuterium penetration depth was measured by the ESEEM technique and the signal was analyzed in the frequency domain (Fourier transform [FT]-ESEEM). The ratio of the intensities of FT-ESEEM lines at deuterium and hydrogen Larmor frequencies was used as a measures of the water penetration depth. It was also indirectly determined from the hyperfine splitting constants in frozen and in fluid state, measured by CW-EPR, as they depend on the polarity of the nitroxide group environment. The comparison of the two techniques demonstrated that FT-ESEEM is more sensitive to the changes on the surface of the membrane and thus more appropriate for the determination of the hydrophobic barrier than the conventional CW-EPR method.     

Evaluation of sub-microsecond recovery resonators for in vivo electron paramagnetic resonance imaging

Time-domain (TD) electron paramagnetic resonance (EPR) imaging at 300MHz for in vivo applications requires resonators with recovery times less than 1 micros after pulsed excitation to reliably capture the rapidly decaying free induction decay (FID). In this study, we tested the suitability of the Litz foil coil resonator (LCR), commonly used in MRI, for in vivo EPR/EPRI applications in the TD mode and compared with parallel coil resonator (PCR). In TD mode, the sensitivity of LCR was lower than that of the PCR. However, in continuous wave (CW) mode, the LCR showed better sensitivity. The RF homogeneity was similar in both the resonators. The axis of the RF magnetic field is transverse to the cylindrical axis of the LCR, making the resonator and the magnet co-axial. Therefore, the loading of animals, and placing of the anesthesia nose cone and temperature monitors was more convenient in the LCR compared to the PCR whose axis is perpendicular to the magnet axis.     

A design of resonant cavity with an improved coupling-adjusting mechanism for the W-band EPR spectrometer

We report a new design of resonant cavity for a W-band electron paramagnetic resonance (EPR) spectrometer. An improved coupling-adjusting mechanism, which is robust, compact, and suits with both solenoid-type and split-pair magnets, is utilized on the cavity, and thus enables both continuous-wave (CW) and pulsed EPR experiments. It is achieved by a tiny metal cylinder in the iris. The coupling coefficient can be varied from 0.2 to 17.9. Furthermore, two pistons at each end of the cavity allow for adjustment of the resonant frequency. A horizontal TE₀₁₁ geometry also makes the cavity compatible with the two frequently used types of magnets. The coupling-varying ability has been demonstrated by reflection coefficient (S₁₁) measurement. CW and pulsed EPR experiments have been conducted. The performance data indicates a prospect of wide applications of the cavity in fields of physics, chemistry and biology.     

Purpose-designed probes and their applications for dynamic NMR microscopy in an electromagnet.

The electromagnet provides a favorable environment for certain applications of NMR microscopy. These include plant imaging experiments and measurements of slow molecular diffusion, where high magnetic field gradients for the pulsed gradient spin echo (PGSE) technique are required. In this paper, two probes designed specifically for these two applications are described. In the first case, the open space within the probe has been maximized in order to incorporate environmental support systems for the plant, while in the second the smallest possible PGSE gradient coil former has been used to maximize the gradient strength. Examples are given of Dynamic NMR Microscopy experiments on a castor bean stem and on poly(ethylene oxide)/water solutions under shear thinning conditions.     

Flame dynamics of azimuthal forced spinning and standing modes in an annular combustor

Azimuthal forcing has been applied to flames in a laboratory scale annular combustor in order to accurately control the azimuthal mode of excitation. A new forcing configuration permitted not only the pressure amplitude, but also the spin ratio and mode orientation to be accurately controlled, in order to generate standing modes and for the first time strong spinning modes in both a clockwise (CW) and anti-clockwise (ACW) direction. The phase averaged heat release dynamics of these modes was compared and a number of differences observed depending on the direction of pressure wave propagation, demonstrating characteristic ACW and CW heat release patterns. A new spin compensating averaging method was then introduced to analyse the flame dynamics, and it was shown that through the application of this method the dynamics of standing wave oscillations could be decomposed to recover the characteristic ACW and CW heat release responses. The global heat release response was also assessed during strongly spinning modes, and the magnitude of the response was shown to depend strongly on the direction of propagation, demonstrating the importance of the local swirl direction on the global heat release response, with important implications for the modelling of such flows.     

Distance determination in human ubiquitin by pulsed double electron-electron resonance and double quantum coherence ESR methods

Recently, distance measurements by pulsed ESR (electron spin resonance) have been obtained using pulsed DEER (double electron-electron resonance) and DQC (double quantum coherence) in SDSL (site directed spin labeling) proteins. These methods can observe long range dipole interactions (15-80A). We applied these methods to human ubiquitin proteins. The distance between the 20th and the 35th cysteine was estimated in doubly spin labeled human ubiquitin. Pulsed DEER requires two microwave sources. However, a phase cycle is not usually required in this method. On the other hand, DQC-ESR at X-band ( approximately 9GHz) can acquire a large echo signal by using pulses of short duration and high power, but this method has an ESEEM (electron spin echo envelope modulation) problem. We used a commercial pulsed ESR spectrometer and compared these two methods.