Pulse frequency adjustment

Gaussian pulses are frequently applied as soft pulses in modern ID, 2D, and 3D NMR experiments. The power in such pulses is adjusted in milliwatts. Hard" pulses, on the other hand, are short-duration pulses (duration in microseconds), with their power adjusted in the 1-100 W range. Figures 1.15 and 1.16 illustrate schematically the excitation profiles of hard and soft pulses, respectively. Readers wishing to know more about the use of shaped pulses for frequency-selective excitation in modern NMR experiments are referred to an excellent review on the subject (Kessler et ai, 1991). 

A 90° Gaussian pulse is employed as an excitation pulse. In the case of a simple AX spin system, the delay t between the first, soft 90° excitation pulse and the final, hard 90° detection pulse is adjusted to correspond to the coupling constant JJ x (Fig- 7.2). If the excitation frequency corresponds to the chemical shift frequency of nucleus A, then the doublet of nucleus A will disappear and the total transfer of magnetization to nucleus X will produce an antiphase doublet (Fig. 7.3). The antiphase structure of the multiplets can be removed by employing a refocused ID COSY experiment (Hore, 1983). 

In order to determine the thermal time constant of the microhotplate in dynamic measurements, a square-shape voltage pulse was applied to the heater. The pulse frequency was 5 Hz for uncoated and 2.5 Hz for coated membranes. The amplitude of the pulse was adjusted to produce a temperature rise of 50 °C. The temperature sensor was fed from a constant-current source, and the voltage drop across the temperature sensor was amplified with an operational amplifier. The dynamic response of the temperature sensor was recorded by an oscilloscope. The thermal time constant was calculated from these data with a curve fit using Eq. (3.29). As already mentioned in the context of Eq. (3.37), self-heating occurs with a resistive heater, so that the thermal time constant has to be determined during the cooHng cycle. 

For routine PFT NMR measurements, the pulse frequency is adjusted by changing the frequency of the transmitter until the CW spectrum of a reference sample with signals at both ends of the spectral width (e.g. acetone, Fig. 1.10) is reproduced exactly by Fourier transformation of the FID signal. 

Fig. 2.11. Correct adjustment of the pulse frequency offset vq (a) and maladjustment (b). In (b) the signal at 144.1 ppm is folded to <5 < 125 ppm. Sample 90% ethylbenzene in hexadeuteriobenzene at 20 MHz single scan experiments with 90" pulses.

In the acquisition of a simple ID spectrum, our goal is to excite all of the spins of a certain type (e.g., H) in the sample, regardless of chemical shift, at the same time. This requires a radio frequency pulse of very high power and short duration. The frequency of the pulse is adjusted to correspond to the resonance frequency at the center of the spectral window, so that it will be close to the resonance frequency of all of the spins in the sample. 

Plasma polymer layers were deposited in the same reactor as described before. However, in this case, the pulsed plasma mode was applied. The duty cycle of pulsing was adjusted generally to 0.1 and the pulse frequency to 103Hz. The power input was varied between P 100 ()() V. Mass flow controllers for gases and vapours, a heated gas/vapour distribution in the chamber, and control of pressure and monomer flow by vaiying the speed of the turbomolecular pump were used. The gas flow was adjusted to 75-125 seem and the pressure was varied between 10 to 26 Pa depending on the respective polymerization or copolymerization process. The deposition rate was measured by a quartz microbalance. 

If the phase-sensitive detectors are adjusted to give a phase angle (Eq. 3.8) ( — 4>r ) = 0, the real part of the FT spectrum corresponds to pure absorption at the pulse frequency, but off-resonance lines display phase angles proportional to their off-resonance frequency as a consequence of limited rf power and nonzero pulse width (Eq. 2.55). However, acquisition of data as complex numbers from the two phase-sensitive detectors and subsequent processing with a complex Fourier transform permit us to obtain a spectrum that represents a pure absorption mode. 

During the "off period" the electrons re-establish equilibrium with the gas. The three operating variables are the pulse duration, pulse frequency and pulse amplitude. The relationship between the number of electrons collected and the collecting time (the pulse width) is shown in figure 13. It is seen that with no methane present electron collection takes nearly 3 psec to complete. However, with 5% or 10% of methane present in the argon all the electrons are collected in less than 1 psec. This reflects the increased diffusion rates of the electrons in argon-methane mixtures. By appropriate adjustment of the pulse characteristics, the current can be made to reflect the relative... 

Almost any on/off-mode timer or controller is suitable for regulating Valve 1. In this case, a process-control computer looks at the reactor temperature and, based on the controller error, adjusts the pulse frequency to maintain the setpoint. The loop volume is 0.12 cc, and about four pulses per minute are added to the reactor. 

Combination of a pulsed bias and noncontact AFM has been found to improve the control of the writing process [78]. This method reduces the tip-substrate interaction time and thus improves the reliability and lithographic resolution. The frequency of oscillation and the field pulsing frequencies need to be adjusted to create a definite phase relation between the two and it was found that the minimum line width is obtained when the applied field is on during the time the cantilever tip is furthest from the substrate. The process also needs adjustment of the duty cycle. 

The pump-probe pulses are obtained by splitting a femtosecond pulse into two equal pulses for one-color experiments, or by frequency converting a part of the output to the ultraviolet region for bichromatic measurements. The relative time delay of the two pulses is adjusted by a computer-controlled stepping motor. Petek and coworkers have developed interferometric time-resolved 2PPE spectroscopy in which the delay time of the pulses is controlled by a piezo stage with a resolution of 50 attoseconds [14]. This set-up made it possible to probe decoherence times of electronic excitations at solid surfaces. 

In the early stages, ECD used a constant and relatively low (10 - 20 V) voltage apphed to the detector cell and the variations of the current during the analyte elution were recorded. A recent design apphes pulse frequency (which is constantly adjusted) to maintain the cell current at a constant value. This method avoid reactions of analyte molecules with high energy electrons, and offer a much wider dynamic linear range than the constant frequency ECD. 

The major difference between soft shaped pulses and DANTE methods is the occurrence of strong sideband excitation windows either side of the principal window with DANTE. These occur at offsets from the transmitter at multiples of the hard-pulse frequency, 1/x. They arise from magnetisation vectors that are far from resonance and which process full circle during the x period. Since this behaviour is precisely equivalent to no precession, they are excited as if on-resonance. Further sidebands at 2/x, 3/x and so on also occur by virtue of trajectories completing multiple full circles during x. Such multisite excitation can at times be desirable [50,51] but if only a single excitation window is required, the hard pulse repetition frequency must be adjusted by varying x to ensure the sideband excitations do not coincide with other resonances. 

Many VOCs absorb infrared (IR) light at a wavelength of 3.4 pm, and in instruments based on PAS the air sample is exposed to IR light pulses. If IR radiation is absorbed (i.e. hydrocarbons are present in the air sample), the light pulses will generate changes of pressure in the measurement cell, and the frequency of the light pulses is adjusted to... 

This effect can also be seen in Fig. 8, in which the resulting bed pulse frequency is plotted as a function of the adjusted liquid feed frequency for three different base flow times. 

Adjust the microdispenser parameters to ensure droplet ejection. There are a number of settings, such as the voltage, pulse duration, and pulse frequency that affect the output of the microdispenser. The values of each of these settings must be tailored to ensure proper droplet ejection. A stroboscope is available from Gesim that enables visualization of ejected droplets, and the fine tuning of these settings to ensure a reliable droplet ejection. 

In addition, since ions are analyzed in packets, TOF MS has a natural compatibility with the pulsed ion release of MALDI interfaces. If there is interest in a given m/z value, the pulsing frequencies can be adjusted to increase the duty cycle and the sensitivity by about one order of magnitude (lower micromolar range). 

Operative conditions can be adjusted so that only metals with rate of dissociation of their complexes, within a desired range, are included in the electroactive fraction. Conditions that can be adjusted to achieve selectivity are deposition potential, electrode rotation rate, solution stirring, pulse frequency, potential scan rate, temperature, pH, etc. As electrochemical techniques require much less sample handling than other speciation methods, such as solvent extraction, dialysis or ultrafiltration, the potential sources of contamination are highly reduced. An in depth discussion of the pro and cons of electrochemical speciation is far beyond this article. Theoretical aspects and applications have been covered in great detail by Niirnberg, Florence et al., cf. ° and references therein. 

Images were acquired in straddle mode by a frame grabber and stored in the hard disk of a PC. Image acquisition was synchronized with laser pulses in such a way that image pairs were captured at a maximum frequency of 7 Hz, while the lime lag between the two images of a pair (i.e. dt, the time lag between two laser pulses) was adjusted according to the expected velocity inside the channel. For the present experiments, dt was varied between 20 and 1000 ps, depending on the combination of flow rates inside the channel. 

The time constant of these processes must be adjusted to meet the time constant of reaction. This can be achieved by altering the pulse frequency. Pulse fiequency solely modifies the period in between the pulses because the pulse duration is almost unaffected by gas and liquid flow rates. Some experimental results on pulse fiequency are reported in Fig. 4. At the present, a publication on hydrodynamics of pulsing flow for different packing materials and column diameters is being prepared. 

---