Optimal electric fields for different sample shapes in high resolution NMR spectroscopy

For many applications, reducing sample resistance, rather than increasing probe Q or filling factor, is the only way to further improve the signal-to-noise ratio of cryogenically cooled NMR probes. In this paper, bounds are calculated for the minimum sample resistance that can be achieved for various sample geometries. The sample resistance of 100 mM NaCl in H(2)O in 5 mm sample tubes was measured on a 600 MHz cold probe to be within 14% of the optimum value. The minimum sample resistance can however be lowered by altering the tube cross section. Rectangular tubes oriented with the long axis along the RF magnetic field are particularly favourable.     

Performance of cryogenic probes as a function of ionic strength and sample tube geometry

The pursuit for more sensitive NMR probes culminated with development of the cryogenic cooled NMR probe. A key factor for the sensitivity is the overall resistance of RF circuitry and sample. Lowering the coil temperature to approximately 25 K and the use of superconducting coil material has greatly reduced the resistance contribution of the hardware. However, the resistance of a salty sample remains the same and evolves as the major factor determining the signal-to-noise ratio. Several approaches have been proposed to reduce the resistance contribution of the sample. These range from encapsulating proteins in a water cavity formed by reverse micelles in low viscosity fluids to the optimal selection of low mobility, low conductivity buffer ions. Here we demonstrate that changing the sample diameter has a pronounced effect on the sample resistance and this results in dramatic improvements of the signal-to-noise ratio and shorter pi/2 pulses. We determined these parameters for common 5 mm NMR tubes under different experimental conditions and compared them to the 2, 3 and 4 mm tubes, in addition, 5mm Shigemi tubes were included since these are widely used. We demonstrate benefits and applicability of studying NMR samples with up to 4M salt concentrations in cryogenic probes. Under high salt conditions, best results in terms of short pi/2 pulses and high signal-to-noise ratios are obtained using 2 or 3mm NMR tubes, especially when limited sample is available. The 4 mm tube is preferred when sample amounts are abundant at intermediate salt conditions.     

爆轰驱动固体套筒压缩磁场计算及准等熵过程分析

采用构形磁流体力学计算程序SSS/MHD对炸药爆轰驱动固体套筒压缩磁场实验进行了一维磁流体力学模拟计算, 得到空腔磁场以及样品管内壁速度随时间的变化历程, 分别与磁探针和激光干涉测量的实验结果符合. 由分幅照相结果阐述了套筒压缩空腔磁场过程中的屈曲失稳和Bell-Plesset不稳定性现象. 分析了样品管和套筒中的磁扩散、涡流和磁压力的变化规律. 结果表明, 由于聚心运动下样品管和套筒的运动速度不同、电磁力和内爆作用力平衡等原因, 样品管内靠近磁腔处的磁场、涡流和磁压力均高于套筒内距磁腔相同位置处的结果. 讨论了样品管内距磁腔0.05 mm处的熵增随该点压缩度的变化, 最大熵增与样品管材料定容比热的比值在10%左右, 爆炸磁压缩实验过程的等熵程度较高.     

内孔缝位置对嵌套腔体微波耦合特性的影响

 利用时域有限差分（FDTD）方法计算了不同内孔缝位置情况下微波脉冲与带缝嵌套圆柱形腔体系统的耦合过程；分别分析了内孔缝位置对外部带缝腔体和内部带缝腔体耦合特性的影响。结果表明:除了内孔缝附近区域以外，内孔缝位置对外腔耦合特性的影响很小，可以忽略不计；但内孔缝位置对内腔耦合特性的影响较大，而且内腔的屏蔽性能由外腔内的耦合场分布以及内孔缝的位置共同决定。     

Picoliter (1)H NMR spectroscopy

In this study, a 267-microm-diameter solenoid transceiver is used to acquire localized (1)H NMR spectra and the measured signal-to-noise ratio (SNR) at 500 MHz is shown to be within 20--30% of theoretical limits formulated by considering only its resistive losses. This is illustrated using a 100-microm-diameter globule of triacylglycerols (approximately 900mM) that may be an oocyte precursor in young Xenopus laevis frogs and a water sample containing choline at a concentration often found in live mammalian cells (approximately 33 mM). In chemical shift imaging (CSI) experiments performed using a few thousand total scans, the choline methyl line is shown to have an acceptable SNR in resolved volume elements containing only 50 pL of sample, and localized spectra are resolved from just 5 pL in the Xenopus globule. These findings demonstrate the feasibility of performing (1)H NMR on picoliter-scale sample volumes in biological cells and tissues and illustrate how the achieved SNR in spectroscopic images can be predicted with reasonable accuracy at microscopic spatial resolutions.     

Dielectric resonator-based side-access probe for muscle fiber EPR study

We present a novel dielectric resonator (DR)-based resonant structure that accommodates aqueous sample capillaries in orientations that are either parallel (i.e., side-access) or perpendicular to the direction of an external (Zeeman) magnetic field, B(0). The resonant structure consists of two commercially available X-band DRs that are separated by a Rexolite spacer and resonate in the fundamental TE(01delta) mode. The separator between the DRs is used to tune the resonator to the desired frequency and, by appropriately drilled sample holes, to provide access for longitudinal samples, notably capillaries containing oriented, spin-labeled muscle fibers. In contrast to the topologically similar cylindrical TE(011) cavity, the DR-based structure has distinct microwave properties that favor its use for parallel orientation of lossy aqueous samples. For perpendicular orientation of a dilute (6.25 microM) aqueous solution of IASL spin label, the S/N ratio was at least one order of magnitude better for the side-access DR-based structure than for a standard TE(102) cavity. EPR spectra acquired for maleimide spin-labeled myosin filaments also revealed ca. 10 times better S/N ratio than those obtained with a standard TE(102) cavity. For the side-access DR with sample capillaries oriented either parallel or perpendicular to the external magnetic field, the Q- and filling factors are in good agreement with the theoretical estimates derived from the distribution of magnetic (H(1)) and electric (E(1)) components.     

Influence of the movement of cylindrical samples with variable internal diameter and variable length along thex-axis of a double TE104 and a single TE102 rectangular cavity on the EPR signal intensity: A sample shape study

The response of the cavity to the movement of cylindrical samples with internal diameters from 0.7 to 4 mm and lengths from 5 to 50 mm along thex-axis of the Bruker double TE₁₀₄ and single TE₁₀₂ rectangular cavity has been analyzed. Independently of sample internal diameter, the experimentally observed dependences of the electron paramagnetic resonance (EPR) signal intensity versus sample position in the cavity showed the following: (i) a sharp maximum for sample lengths from 5 to 20 mm; (ii) a “plateau”, over which the signal intensity remained constant within experimental errors of 0.47–1.16%, for lengths from 30 to 40 mm; and (iii) a “sloping plateau” region, which could be approximated by the linear function (correlationr = 0.96–0.98) for the 50 mm sample. Theoretical predictions of the experimental dependences of the signal intensity versus sample position in the cavity were calculated with the “modified” and “revised” sine-squared function, and the correlation between observed and theoretically computed dependences is very good. Additionally, the experimental dependence of the signal intensity versus the sample internal diameter and length for cylindrical samples situated at the position in the cavity at which the signal intensity was a maximum was likewise numerically approximated by the surface fitting with the Lorentzian cumulative additive function (correlationr = 0.999). The experimental dependence of the signal intensity versus the sample internal diameter for the given sample length is nonlinear. The samples with internal diameters of 0.7 and 1 mm gave the total maximum of signal intensity for the 40 mm sample, however, the samples with internal diameters of 2, 3 and 4 mm gave the total maximal value of signal intensity, which was identical for both the 30 and 40 mm samples. The experimental dependence of the EPR signal intensity versus the sample volume clearly showed that the samples with identical volumes, however, with different shapes, can give significantly different signal intensities (with differences ca. 200–400%). Then, the comparison of cylindrical samples with identical volumes but different shapes may be a serious source of significant errors in quantitative EPR spectroscopy. Cylindrical samples to be compared should be of identical shape. Accurate and precise positioning of each sample in the microwave cavity is essential.     

窄缝耦合的快速估算法

 基于小孔耦合理论和腔体格林函数,提出了长宽比大于10的窄缝耦合的快速算法,并与Micro-Stripes软件的计算结果进行比较,两者基本一致。研究了窄缝位置、数量及腔体尺寸对屏蔽效能的影响,结果表明：每增加一条相同尺寸、相同取向的窄缝可使腔体的腔体屏蔽效能下降约6 dB;窄缝长度不变的条件下,长宽比每增大1倍则屏蔽效能增加1 dB,并且增大腔体任一方向的尺寸都可以使屏蔽效能增大;除腔体的谐振频率与软件计算结果稍有差异之外,快速算法与软件的计算结果吻合很好,而快速算法的速度远大于软件计算速度,且适合于高频范围。     

估算双层屏蔽腔体窄缝耦合的混合方法

 用混合方法计算双层屏蔽腔体窄缝耦合时,外腔体的窄缝耦合用传输线模型计算,内腔体的用磁偶极子模型计算。混合法可以求出双层屏蔽腔体内的场分布和屏蔽系数的分布规律,避免了传输线模型只能计算腔体中心线上的屏蔽系数而不能分析腔内横截面上耦合场分布规律的缺点。将得到的窄缝耦合的传输线模型和磁偶极子模型的计算结果与实测值和Micro-Stripes软件仿真值进行对比,验证了传输线和磁偶极子模型的有效性。混合方法给出了窄缝数量及腔体内不同观测点对屏蔽系数的影响,结果表明：双层屏蔽腔体的屏蔽效能明显优于单层屏蔽腔体;随着相同尺寸的窄缝数量的递增,腔体内的屏蔽系数递减;在与双层屏蔽腔体中心线垂直的横截面上,观测点屏蔽系数以中心线上点为中心,沿窄缝方向向两边递增,也就是离中心线越远,腔体内的耦合电场越弱,且混合法的速度明显快于软件计算速度,适合于高频范围的分析。     

耐高压纳秒脉冲罗氏线圈的研制

针对ns级脉冲电流信号的测量，设计了一种带磁芯的新型自积分式罗氏线圈，具有信噪比高、动态范围广等优点。屏蔽盒开气隙防止涡流。屏蔽盒外层采用聚氨酯进行整体封装，聚氨酯层厚度大于1.5 mm，可耐受大于20 kV的冲击电压。采用高压方波发生器与Pearson4100线圈对罗氏线圈标定。罗氏线圈的参数为:灵敏度0.018 8 V/A，最高上升时间小于20 ns，方波脉宽300 ns，最大峰值电流300 A。     

The structure of the smectic phases of terephthal-bis-(butylaniline) studied by electron spin resonance spectroscopy

Electron spin resonance (E.S.R.) measurements at X-band (3·3 kG) of the spin probe 17β-hydroxy-4,4′-dimethylspiro[5α-androstane-3,2′-oxazolidin]-3′-yloxyl dissolved in the smectic phases (A, C and B) of the mesogen 4,4′-terephthal-bis(butylaniline) (TBBA) are reported. Two types of samples were studied in the dependence on temperature, sample orientation and strength of the magnetic field: (i) glass plate sandwiches with less than 0·1 mm of the mesogen, and (ii) 4 mm i.d. cylindrical glass tube. The sandwich samples were prepared as monodomains with the smectic layers parallel to the glass plates. In these samples the orientation of the layers and of the director is fixed and cannot be reoriented even in a magnetic field of 21 kG. From the angular dependence of the spectrum, the tilt angle, the order parameter and their temperature dependence were determined.

In the cylindrical samples the original orientation of the smectic layers is preserved, and up to about 3 kG the director's orientation with respect to the sample is almost unaffected in all smectic phases. On subjecting the sample to higher fields (～ 20 kG) the director in the smectic C phase will re-align so as to minimize the magnetic energy, subject to the fixed tilt angle. The new alignment is preserved when the strength of the magnetic field is reduced. No such re-alignment is observed in the smectic B phase even after subjecting the sample to a field of 21 kG, in contrast to previous deuterium N.M.R. measurements at 14 kG. It is suggested that in this phase the director is pulled by strong fields but relaxes to its original direction when the magnetic field is brought back to 3·3 kG.     

Ultrasonic particle concentration in a line-driven cylindrical tube

Acoustic particle manipulation has many potential uses in flow cytometry and microfluidic array applications. Currently, most ultrasonic particle positioning devices utilize a quasi-one-dimensional geometry to set up the positioning field. A transducer fit with a quarter-wave matching layer, locally drives a cavity of width one-half wavelength. Particles within the cavity experience a time-averaged drift force that transports them to a nodal position. Present research investigates an acoustic particle-positioning device where the acoustic excitation is generated by the entire structure, as opposed to a localized transducer. The lowest-order structural modes of a long cylindrical glass tube driven by a piezoceramic with a line contact are tuned, via material properties and aspect ratio, to match resonant modes of the fluid-filled cavity. The cylindrical geometry eliminates the need for accurate alignment of a transducer/reflector system, in contrast to the case of planar or confocal fields. Experiments show that the lower energy density in the cavity, brought about through excitation of the whole cylindrical tube, results in reduced cavitation, convection, and thermal gradients. The effects of excitation and material parameters on concentration quality are theoretically evaluated, using two-dimensional elastodynamic equations describing the fluid-filled cylindrical shell with a line excitation.     

Analysis of the Movement of Line-like Samples of Variable Length along theX-Axis of a Double TE104and a Single TE102Rectangular Cavity

Movement of line-like samples with lengths from 5 to 50 mm along thex-axis of the double TE₁₀₄rectangular cavity has been analyzed. The observed dependencies of the EPR signal intensity versus sample position showed: (i) a sharp maximum for sample lengths from 5 to 20 mm; (ii) a plateau, over which the EPR signal intensity remained constant within experimental errors of 0.26–1.07%, for lengths from 30 to 40 mm; and (iii) a “sloping plateau,” which could be approximated by the linear function (correlation,r= 0.98) for sample length 50 mm. Theoretical values of the experimentally observed dependencies of the intensity versus sample position were calculated using the modified sine-squared function and the correlation between observed and theoretically predicted dependencies is very good. The experimental dependence of the EPR signal intensity versus the sample length for samples situated at the same point in the cavity was nonlinear with a maximum for the 40-mm sample. The dependence of the EPR signal intensity upon the movement of a large cylindrical sample (o.d. 4 mm and length 100 mm) along thex-axis of the cavity was similar to that found for the 50-mm sample. However, an additional oscillating signal superimposed on the sloping plateau was observed. The presence of a large sample fixed in the complementary cavity of the double TE₁₀₄cavity caused an additional deformation of the signal intensity for a 30-mm sample which was moving in the first cavity. The primary effect was that the plateau was replaced by a region in which the intensity increased linearly with sample position,r= 0.99. Each of the above phenomena may be a source of significant errors in quantitative EPR spectroscopy. Cylindrical samples to be compared should be of identical length and internal diameter. Accurate and precise positioning of each sample in the microwave cavity is essential.     

石英管对平行WR-430波导谐振腔电场的影响

详细讨论了石英管对平行WR-430波导谐振腔内部电场强度的影响。在没有石英管时,电场强度在每个狭缝附近发生突变,其峰值沿着一个波导逐渐减小,而沿着另外一个波导逐渐增大。存在石英管时,内部电场变弱且沿着石英管内表面无规则振荡,而且电场沿着两个波导之间的中心轴线波动。当石英管壁厚度和离上下波导的距离分别为5和2 mm时,谐振腔内部的平均电场强度达到最大,而且电场强区面积较大。当上述两者分别超过5和2 mm时,内部电场的最大值会随着石英管壁厚度和距离逐渐减弱。低气压和大气压空气等离子体在谐振腔内部被激发,其形态比较接近各自的仿真的电场强度分布。     

Enhanced EPR sensitivity from a ferroelectric cavity insert.

We report the development of a simple ferroelectric cavity insert that increases the electron paramagnetic resonance (EPR) sensitivity by an order of magnitude when a sample is placed within it. The insert is a hollow cylinder (length 4.8 mm, outside diameter 1.7 mm, inside diameter 0.6 mm) made from a single crystal of KTaO(3), which has a dielectric constant of 230 at X-band (9.5 GHz). Its outside dimensions were chosen to produce a resonant frequency in the X-band range, based on electromagnetic field modeling calculations. The insert increases the microwave magnetic field (H(1)) at the center of the insert by a factor of 7.4 when placed in an X-band TM(110) cavity. This increases the EPR signal for a small (volume 0.13 microL) unsaturated nitroxide spin label sample by a factor of 64 at constant microwave power, and by a factor of 9.8 at constant H(1). The insert does not significantly affect the cavity quality factor Q, indicating that this device simply redistributes the microwave fields within the cavity, focusing H(1) onto the sample inside the insert, thus increasing the filling factor. A similar signal enhancement is obtained in the TM(110) and TE(102) cavities, and when the insert is oriented either vertically (parallel to the microwave field) or horizontally (parallel to the DC magnetic field) in the TM(110) cavity. This order-of-magnitude sensitivity enhancement allows EPR spectroscopy to be performed in conventional high-Q cavities on small EPR samples previously only measurable in loop-gap or dielectric resonators. This is of particular importance for small samples of spin-labeled biomolecules.     

Experimental study of the effects of pulsed Doppler sample volume size and position on the Doppler spectrum

With a pulsed Doppler system, the recorded Doppler spectrum is expected to vary depending upon the sample volume size relative to the diameter of the vessel, the position of the sample volume in the vessel and the velocity profile. In the in vitro experiments described in this paper, the velocity profile was kept constant by using steady parabolic flow in a flow model. As the Doppler sample volume size and position were changed, the maximum variations of quantitative measurements from the Doppler spectrum were determined. The maximum, mean and mode frequencies and spectral broadening index (SBI) were affected by the position of the sample volume but to a lesser degree by its length (1.5-5.0 mm) relative to the 9.5 mm beam path length across the tube. When the centre of the Doppler sample volume was moved within the central 25% of the tube, the maximum variations were as follows: maximum frequency 3-5%, mean frequency 8-9%, mode frequency 8-9% and SBI 16-18%, where the range indicates the effect of increasing the sample volume size. Based on these results obtained under steady flow conditions in vitro, it is concluded that quantification of pulsed Doppler spectra may be feasible if the sample volume is positioned within the central 25% of the vessel.     

Electro-Magnetic Eigenmodes of the Toroidal Cavity

The eigenmodes of the empty toroidal cavity, relevant for H. F. heating of toroidal plasmas, are calculated using a development in the inverse aspect ratio. The eigensolutions are classified as quasi-TE and quasi-TH modes according to their cylindrical limit. Two exceptional modes are found for which both the electric and the magnetic field in the axial direction are nonzero already to the lowest order. The existence of these modes is connected with the accidental degeneracy of the cylindrical TH1 and TE0 modes.     

Conductance asymmetry of a slot gate Si‐MOSFET in a strong parallel magnetic field

We report measurements on a Si‐MOSFET sample with a slot in the upper gate, allowing for different electron densities n_1,2 across the slot. The dynamic longitudinal resistance was measured by the standard lock‐in technique, while maintaining a large DC current through the source‐drain channel. We find that the conductance of the sample in a strong parallel magnetic field is asymmetric with respect to the DC current direction. This asymmetry increases with magnetic field. The results are interpreted in terms of electron spin accumulation or depletion near the slot.     

Continuous wave, 30 W laser-diode bar with 10 GHz linewidth for Rb laser pumping

A laser-diode bar incorporated into an external cavity with a volume Bragg mirror produced 30 W of cw output power within a 20 pm (10 GHz) spectral linewidth (FWHM) centered at 780 nm. The device output power exceeded 90% of that for the free-running laser-diode bar. The emission wavelength was tuned over a 400 pm range without broadening laser spectrum width. Absorption of 90% of the laser radiation by a 25 mm vapor cell containing Rb that has been pressure broadened with 300 torr of ethane was demonstrated.     

Aqueous sample in an EPR cavity: sensitivity considerations

The radial mode matching (RMM) method has been used to calculate accurately the microwave field distribution of the TE(011) mode in a spherical EPR cavity containing a linear aqueous sample, in order to understand in detail the factors affecting sensitivity in EPR measurements at X band. Specific details of the experiment were included in the calculations, such as the cavity geometry, the presence of a quartz dewar, the size of the aqueous sample, and the sample's dielectric properties. From the field distribution, several key physical parameters were calculated, including cavity Q, filling factor, mean microwave magnetic field at the sample, and cavity efficiency parameter Lambda. The dependence of EPR signal intensity on sample diameter for a cylindrical aqueous sample was calculated and measured experimentally for non-saturated and half-saturated samples. The optimal aqueous sample diameter was determined for both cases. The impact of sample temperature, conductivity, and cavity Q on sensitivity of EPR is discussed.