Single-pulse Experiments Research Articles

Water signal elimination in Vivo, using “suppression by mistimed echo and repetitive gradient episodes”

A number of techniques (I) are available for the elimination of an unwanted signal (usually the ‘H signal from water) from a spectrum. Unless the unwanted signal is removed before the preamplifier and ADC are encountered in the signal detection pathway, difficulties are experienced in the observation of very weak signals because of dynamic range problems. O f the available techniques the most efficient appears to be the 1331 method developed by Hore (I, 2); however, as Hore points out, this technique is only of limited value if the water resonance is broad. This is the situation often encountered in vivo where very broad water signals are observed, the broadness usually being the result of magnetic field inhomogeneity. In general, for signal elimination the most successful methods have in the past relied on overall nonexcitation of the unwanted resonance, the aim being to leave the unwanted spin coherence along the z axis and away from the detection plane. The alternative procedure would be to destroy selectively the unwanted spin coherence and it is this strategy which has led to the application of reverse polarization transfer methods (3) as probably the most efficient water signal elimination method yet devised. In this paper we develop a method for selectively destroying the water signal, applicable to in vivo NMR studies where broad water signals are often encountered. We call the technique SUBMERGE. The method eliminates transverse water spin coherence over a wide but controllable frequency band. The significant point is that the water spin coherence is destroyed, thus eliminating the signal from the preamplifier and ADC, allowing the efficient detection of weak signals. As a test, we attempt to observe the methyl signal from lactic acid at a concentration of 10 mM in neat HzO. The water signal is thus about 3700 times more intense than the signal of interest. A 250 ml round-bottom flask was used as the sample container. A water signal having a half-height width of 10 Hz was typically detected in a single pulse experiment. In Fig. 1 is shown the experimentally determined excitation profile from a 10 ms 7r/2 sine pulse. The excitation profile from a 10 ms 7r/2 gaussian pulse is also shown for comparison. At a ‘H resonance frequency of 100 MHz (field strength 2.4 T) the chemical-shift separation between the two resonances of interest is about 345 Hz. Thus, by use of such shaped pulses, it is possible to excite the water signal but not the

Dynamics of the intermediate state in nonequilibrium superconductors.

We report experimental measurements of the voltage response of current-biased superconducting thin films excited by 30-ps-FWHM (full width at half-maximum) pulsed laser radiation. Experiments performed with Pb and Sn films irradiated by a full train of laser pulses provide direct evidence for the existence of a nonstationary intermediate state. The state was observed during the superconducting-to-normal transition, as well as during the recovery of the film from the nonequilibrium normal state into the superconducting state. Single-pulse laser experiments enabled us to study the superconducting-to-normal transition in detail. Depending on the optical power, both triangular and quadruangular voltage pulses were observed. The triangular pulses had amplitudes dependent on the laser power and reflected the transition to the intermediate state, where superconducting and normal domains coexisted. The quadrangular pulses were associated with the full transition into the normal state. Very short triangular voltage pulses with full widths of the order of hundreds of picoseconds and rise times as short as about 100 ps were observed for thin Pb films illuminated by a low, near-threshold, laser power. The pulse rise time was almost constant and it only depended weakly on the film thickness, while the pulse width and the fall time were very sensitive to the optical input power as well as other experimental conditions (e.g., film thickness, Pb-substrate interface). The pulse decay was determined to be linear. The linear decay reflects a geometric relaxation of the intermediate state. An experiment performed with a stretched laser pulse (about 470 ps FWHM) demonstrated an initial thresholding effect and indicated that the subsequent voltage is proportional to the time integral of the laser pulse (optical energy). This confirms that the transition from the superconducting state to the normal state is a two-step transition. First, a sudden jump leads to the nonstationary, intermediate state followed by a gradual increase of the volume of the normal phase with a rate proportional to the rate of increase of the laser pulse energy. All of our observations are in good agreement with numerical simulations based on Elesin's theory of the intermediate state.

The surface-coil NMR receiver in the presence of homogeneous B 1 excitation

The signal intensity and inherent spatial localization in a single-pulse steady-state experiment using a circular surface receiver coil in the presence of a homogeneous rf excitation magnetic field ( B 1) is examined theoretically by computer simulation assuming zero interaction between the transmission-coil and the surface-coil radiofrequency circuits. The theoretical analysis is verified by NMR experiments employing appropriate “phantom” samples with the surface coil orthogonal to the transmitter coil to minimize interaction between the two. Under slow pulse repetition conditions the signal intensity of the homogeneous B 1 transmission-surface-coil reception experiment is essentially the same as that obtained in the single-coil configuration surface-coil experiment and is independent of the orientation of the homogeneous B 1 with respect to the surface coil. A significant consequence of this result is that the optimum performance of multiple-pulse sequences which require specific flip angles over the whole sample should be attained while retaining the spatially restrictive sensitivity profile of the surface coil. The homogeneous B 1 transmission-surface-coil reception sensitive volume penetrates deeper into the sample than the surface-coil single-coil configuration sensitive volume, and therefore may be a more appropriate shape for in vivo NMR spectroscopy. The homogeneous B 1 transmission actually induces more xy magnetization than surface-coil transmission does, but differences in signal phase from different regions of the sample result in an incoherent addition when the signal from all parts of the sample are summed together. In imaging experiments, each volume element of the sample is uniquely characterized by its Larmor frequency and phase, thus, signal reduction due to phase differences is absent.

15N NMR spectra of pyrimidine and [1,3‐15N2]pyrimidine

AbstractThe fully proton‐coupled 15N NMR spectra of pyrimidine with natural isotope abundance and of bis‐labelled [1,3‐15N2]pyrimidine, obtained using single pulse experiments, are described. The 15N and 1H spectra are analysed for 2J, 3J and 4J(NH) as well as N,N and H,H coupling constants.

Selenium and drug metabolism—II Independence of glutathione peroxidase and reversibility of hepatic enzyme modulations in deficient mice

Male mice were fed a diet containing less than 0.01 ppm selenium (Se −) for 6 months. A control group received the same diet containing 0.5 ppm selenium (Se +). In the livers of the Se − animals a drastic decrease in glutathione peroxidase (GSH-Px) activity was observed. It reached undetectable levels after 17 days of the Se − diet. At that time, GSH-transferase activity began to increase significantly, followed by changes in many other enzyme activities. After the 60th day, these enzyme modulations had reached a plateau with the following percentage changes compared to controls: GSH-transferases: 320% (1,2-dichloro-4-nitrobenzene), 218% (1-chloro-2,4-dinitrobenzene); glutathione reductase: 160%; ethoxycoumarin deethylase: 330%; cytochrome P-450-hydroperoxidase: 230%; heme oxygenase: 240%; UDP-glycuronyltransferase: 200%; GSH-thioltransferase: 64%; sulphotransferase: 62%; NADPH-cytochrome-P-450-reductase: 65%; flavin-containing mono-oxygenase: 57%. No significant changes were observed for GSH-transferase activity assayed with ethacrynic acid or for microsomal H 2O 2 formation and aniline hydroxylase activity. In single-pulse repletion experiments by injection of 250 μg selenium/kg body wt, different individual time constants for the recovery process of the enzymatic perturbations were observed. The half-times for the recovery ranged from 5.7 hr for the microsomal NADPH-cytochrome-P-450 reductase to over 29 hr for GSH-Px up to 44 hr for part of the GSH-transferase activity. 250 μg selenium/kg bodt wt were needed to restore 50% of GSH-Px activity in the long-term Se − mice compared to Se + controls. All other enzymatic changes in the Se − mice needed a dose of 7 μg selenium/kg body wt for 50% restorage. The results demonstrate that processes other than those related to GSH-Px take place in a later phase of selenium deficiency in mouse liver with a chronologically common beginning. The different repletion and depletion kinetics as well as the different need of these processes for the trace element are discussed with respect to the existence of two separate selenium pools.

The influence of experimental parameters in surface-coil NMR

The experimental conditions for surface-coil applications in biomedical NMR are considered in detail. It is shown that the sensitivity as well as the spatial discrimination of a surface coil depend on the coil geometry, the radiofrequency pulse length, the pulse repetition time, and the NMR relaxation times. To simulate practical situations the sensitivity per unit time obtained from selected volumes of interest within the sensitive area of a surface coil has been calculated. Although the single-pulse surface-coil experiment yields good signal-to-noise ratios, it cannot provide spatial selectivity. In cases where “focusing” on deep-lying organs is desired, the large unwanted signal contributions from surrounding tissues have to be reduced or separated using additional techniques.

Experimental observations of periodic and chaotic regimes in a forced chemical oscillator

Abstract Experiments showing periodic and chaotic behavior in a Belousov-Zhabotinski (BZ) reaction mixture in a stirred flow reactor with periodic addition of bromide ions are presented. An alternating sequence of periodic and aperiodic regimes as a function of the period of Br - pulses is observed in experiments. This behavior is well described by a simple model based on the results of single-pulse experiments.

A stochastic approach for the interpretation of single pulse experiments in morphological multicompartments of renewing and exponentially growing cell populations

A general approach for the interpretation of single pulse experiments in multicompartment systems is presented. It allows an extension of the classical single compartment Barret's methodology, and has been detailed in the pipeline model case to show its capabilities. FLM, FLC, grain count curves, labelling indices and compartment size ratios of each compartment, are fitted into a coherent scheme that fully describes the statistical aspects of the phase durations including possible losses. It is shown that if the compartments are identical, irrespective of the feedback coupling between them, the system may be treated as a single compartment. It is also shown that the FLC information is necessary to identify the existence of losses in the system, and how to correct the “apparent” transit times if losses are present. The pipeline model is treated and a suggestion is made to reconcile the “British” and “American” interpretations of the erythroid system. As a corollary, simple formulae are derived in the deterministic case through a coupling matrix describing the interaction between compartments. Computer codes are described and have been implemented in the J.E.N. Thermoecology Laboratory.

Picosecond dephasing of coherently excited vibrations in liquid N 2/ar mixtures

The dephasing time of the coherently excited molecular vibration of N 2 has been measured by a single high power laser pulse experiment via stimulated Raman scattering. In mixtures of liquid N 2 with Ar the surroundings of the excited N 2 molecules can be changed by varying the mole fraction of Ar in the mixture. In this way the influence of intermolecular interactions on the dephasing of vibrationally excited N 2 molecules can be studied. It turns out that the dephasing time is strongly dependent upon the mole fraction of Ar and varies from 90 ps in pure N 2 to 35 ps in almost pure Ar. The measured dephasing times are in good agreement with Raman linewidth measurements. Resonant V-V transfer between N 2 molecules does not contribute to the dephasing, but the dephasing is initiated by stochastic perturbations of the surrounding molecules.

Charge pulse studies of transport phenomena in bilayer membranes. I. Steady-state measurements of actin- and valinomycin-mediated transport in glycerol monooleate bilayers.

A charge pulse technique has been applied to studies of transport phenomena in bilayer membranes. The membrane capacitance can be rapidly charged (in less than a microsecond). The charge then decays through the membrane's conductive mechanism-no current flows through the solution or external circuitry. The resulting voltage decay is thus a manifestation of membrane and boundary layer phenomena only. There are a number of advantages to this approach over conventional voltage or current-clamp techniques: the rise-time of the voltage perturbation is not limited by the time constant deriving from the membrane capacitance and solution resistance, thus permitting study of extremely rapid rate processes; the membrane is exposed to high voltage for relatively short times and thus can be subjected to higher voltages without breakdown; the steady-state current-voltage behavior of the membrane can be deduced from a single charge pulse experiment; the charge (and therefore the integral of the ion flux through the membrane) is monitored allowing detection of rate processes too rapid to follow directly. In this paper we present what is primarily a steady-state analysis of actin (non-, mon-, din-, trin-)-mediated transport of ammonium ion and valinomycin-mediated transport of cesium and potassium ions through glycerol monooleate bilayers. We introduce the concept of the "intercept discrepancy", a method for measuring charge lost through extremely rapid rate processes. Directly observable pre-steady-state phenomena are also discussed but will be the main subject of part II.

High−vacuum trapping experiments on the astron

Experimental results of high−vacuum (10−7 to 10−6 Torr) E−layer trapping experiments on the astron are presented. Both single and multiple pulse experiments are reported. The precessional mode is stabilized with a toroidal field. At sufficiently low pressures (≲5×10−6 Torr), E layers were observed to be stable for up to 200 msec. For greater times, the applied toroidal field had decayed sufficiently so that the layer was unstable to precessions. Axial compression of the layer was observed as it was neutralized by ionization of the background gas. Compressions by a factor of two were observed. For layers whose self−well is the dominant focusing force, this compression agrees with adiabatic theory. A small (≈20%) E−layer loss was observed during the compression. It is felt that this loss arises from scattering in electric field inhomogeneities. The effect of resistive walls on trapping efficiency was studied and it was shown that the efficiency scales with the theoretically calculated energy loss in the resistive walls. Finally, the results of the multiple pulse experiments are presented. Failure to achieve buildup of the E layer is attributed to a combination of losses during compression and poor trapping efficiency on later pulses.

The Effect of Silencers of the Helmholtz-Resonator Type on Pressure Waves of Finite Amplitude: Single Pressure Pulses

The attenuation of large-amplitude waves effected by silencers of the so-called Helmholtz-resonator type is envisaged as being due to the finite efflux of gas through the holes of the silencer with resulting partial reflection, and hence reduced transmission, of the incident wave. Quasi-steady, one-dimensional flow arguments are used to predict the attenuation, the flow conditions being assumed reversible and adiabatic, that is, isentropic. This latter assumption is avoided in an alternative method by assuming a knowledge of the relationship between pipe Mach numbers and the pressure difference in the pipe across the holes. Indicator diagrams resulting from single pulse experiments are, in general, in good agreement with those predicted.

Radiolysis of Hydrogen Sulfide at Very High Dose Rates. The Effect of SF61

Gaseous H2S has been irradiated with electron pulses from a Febetron 705 at a dose rate of ~2 × 1027 eV g−1 s−1. For single pulse experiments, the yield of hydrogen is G(H2) = 12.0 ± 0.5, independent of pressure from at least 350 to 1600 Torr. Addition of SF6 reduces the yield to G(H2) = 7.9 ± 0.3 which is fairly close to that observed for pure H2S at low dose rates. The reduction, ΔG(H2) = 4.1 ± 0.3, agrees very well with the ion pair yield based on a W value of 25.3 eV.In multi-pulse irradiations, for pure H2S, the yield falls off with dose giving a limiting yield close to G(H2) = 8.0. No similar fall-off is observed for H2S–SF6 mixtures. It is proposed that at high absorbed doses and at low dose rates, there is no contribution to the hydrogen yield from neutralization processes; and that this is due to neutralization of H3S+ by an ion of the type Sn− rather than a free electron.

Metal/Metal Ion Electrodes with Two Charge‐Transfer Steps: Mex ‐ Alloy / Mez + and Mez + / Me ( z + 1 ) + Potentiostatic Transients under Charge‐Transfer and Diffusion Control

A theoretical treatment of the transients obtained in potentiostatic single pulse experiments on metal alloy/metal ion electrodes coupled to a redox reaction is presented. Formulas have been derived for the concentration of the electrochemically active component in the alloy and of the corresponding metal ions of different valent state which form the redox couple in the electrolyte, respectively, as a function of the distance from the interface and the duration of the voltage pulse. Expressions for the partial current densities carried by the two consecutive charge‐transfer reactions, and , and for the total current density flowing through the cell have been worked out. Analysis of the current density transients on the basis of these expressions allows interpretation of experimental curves in terms of the characteristic kinetic parameters of the electrochemical system.

Shock-tube study of high-temperature pyrolysis of benzene

High-temperature pyrolysis of benzene was studied by the shock-tube method. Single-pulse experiments were carried out at 1400°–1900°K, 2–8 atm total pressure (benzene was diluted to 10%–20% with argon) and with a reaction time of about 400 μsec. From the experimental results it was found that the pyrolysis reaction of benzene was second order with respect to the benzene concentration and the rate constant was k = 10 9.5 exp ⁡ ( − 30 ± 5 kcal / RT ) liter mole − 1 sec ⁡ − 1 . It was also found that the pyrolysis reaction was slowed down considerably by the addition of hydrogen and promoted by the addition of methane Absorption spectro-photoelectronic experiments were carried out under almost the same conditions as the single-pulse experiments. There were two absorption maxima in the wave-length range from 3000 to 6500 . Monochromatric experiments at 3000 and 6500,showed induction periods for the appearance, the peak, and the decay of absorption in oscillograms. A new scheme for the chain mechanism, involving several elementary reactions, was developed and compared with the experimental results.