Pulse sampling

A wide variety of ID and wD NMR techniques are available. In many applications of ID NMR spectroscopy, the modification of the spin Hamiltonian plays an essential role. Standard techniques are double resonance for spin decoupling, multipulse techniques, pulsed-field gradients, selective pulsing, sample spinning, etc. Manipulation of the Hamiltonian requires an external perturbation of the system, which may either be time-independent or time-dependent. Time-independent... 

Okawa and Aono [50] demonstrated under ambient conditions a diacetylen chain polymerization induced in a self assembled monolayer of 10,12-nonacosadiyonic acid on graphite. First an artificial defect was created with the STM tip by applying a positively pulsed sample bias, the polymerization of a single diacetylene monolayer chain was initiated at another surface location with a negative voltage pulse. After progression of the chain reaction, the polymer chain was terminated at the artificial defect site. 

Equation 31 is plotted in Figure 5 for typical combustion conditions and with the combined uncertainty in k. > ft and V as a parameter. The sample time is 1 Usee, typical of flash-lamp pumped dye laser pulses, and thus remarkably low detectability limits are achievable with single pulse sampling. 

In the strobe or pulse sampling technique, the sample is excited with a pulsed light source. The intensity of the fluorescence emission is measured in a very narrow time window on each pulse and saved on the computer. The time window is moved after each pulse. When data have been sampled over the appropriate range of time, a decay curve of emission intensity vs. time can be constructed. 

The probability of a molecule absorbing a photon is directly proportional to the second power of the component of the transition moment in the same direction as the polarization of the exciting li t. Thus, for pulsed sample excitation at time t = 0... 

MALDI analysis was conducted on an aliquot of the original digest mixture. Mass measurements were made with a Bruker MALDI/TOF MS instrument (Billerica, MA) equipped with a nitrogen laser (337 nm). Spectra were averaged from 100 to 150 laser pulse samples, a-cyano-4-hydroxycinnamic acid was used as the MALDI matrix. Samples were prepared from the CytC digest solution, and acetonitrile and water (50 50) which was 0.1% in TFA. 

A pulsed sample introduction interface for LC-MS in a TOF instrument was described by Wang et al. [131-132]. Analyte ionization is performed by means of laser-induced multiphoton ionization. The interface is based TSP nebuhzation into a heated expansion chamber and a high-temperature pulsed nozzle. Experimental parameters in on-line LC-MS were evaluated [132]. 

A.P.L. Wang, L. Li, Pulsed sample introduction interface for combining flow injection analysis with multiphoton ionization TOF-MS, Anal. Chem., 64 (1992) 769. 

To test the ECD hypothesis, E. C. M. Chen measured the temperature dependence of the molar response. This entailed a detailed study of all parameters associated with the pulse sampling method. For these molecules the most important reactions were postulated to be electron generation and collection, electron and ion recombination, electron attachment and detachment. It was discovered that the simple thermodynamic model was not applicable and that a kinetic model was necessary to explain the change in temperature dependence. If we assume a steady state exists, an expression can be obtained that relates the ECD molar response to kinetic rate constants for the above reactions [24, 25],... 

Using a drop time (t) of 4 s and a pulse sampling time of SO ms, the enhancement factor of the currents resulting in chemical change alone is ca. 6. 

The apparent change in sign of the time axis required for formation of the echo is achieved in practice by sign inversion of the complete Hamilton operator, which can be manipulated by if pulses, sample reorientation, and magnetic field manipulations. But these manipulations can be carried out selectively for just some parts of the complete Hamilton operator. Then the evolution of the magnetization under the influence of the... 

PULSED SAMPLE INLET INTO THE FLOW CELL... 

Thermal Electron Attachment Studies by the Pulse-Sampling Technique... 

The pulse-sampling technique is quite simple both in principle and practice. The temperature dependence of the rate constant can be evaluated readily. For the two compounds investigated in detail, perfluoromethylcyclohexane had an Arrhenius activation energy of 1.7 kcal., whereas the value for SF6 was 0.9 kcal. 

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