Pulsed-cycles

A thermal pulse cycle is a means of conserving thermal energy in... 

Two-dimensional NMR spectroscopy may be defined as a spectral method in which the data are collected in two different time domains acquisition of the FID tz), and a successively incremented delay (tj). The resulting FID (data matrix) is accordingly subjected to two successive sets of Fourier transformations to furnish a two-dimensional NMR spectrum in the two frequency axes. The time sequence of a typical 2D NMR experiment is given in Fig. 3.1. The major difference between one- and two-dimensional NMR methods is therefore the insertion of an evolution time, t, that is systematically incremented within a sequence of pulse cycles. Many experiments are generally performed with variable /], which is incremented by a constant Atj. The resulting signals (FIDs) from this experiment depend... 

Pulsatile drug delivery systems, 9 57-61 Pulsating heat pipes (PHP), 13 235-236 Pulse combustion heat sources, 9 104-105 Pulse cycles, 9 778 Pulsed baffle reactors, 15 709-710 Pulsed discharge detector (PDD) gas chromatography, 4 614 Pulsed dye lasers, 23 144 Pulsed electrochemical machining (PECM), 9 604-605... 

In the TRAPI experiment, the new ions that are formed immediately after each X-ray pulse are often of greatest interest. Later in time, these relatively energetic ions will be converted to more stable terminal ions by unavoidable reactions with common buffer gas impurities (primarily water). Because ions are lost relatively slowly at atmospheric pressure, these terminal ions can persist in time over many pulse cycles (25 to 50 X-ray pulses are applied per second) while the fast decay curves of the more energetic ions are monitored after each pulse. In this way, for example, the rate of the reaction of NJ with added Oj was determined from the observed time dependence of the Nj ion in nitrogen buffer gas at atmospheric pressure during the first 300 (is after each X-ray pulse. 

For the multiple pulse studies, an eight-pulse cycle that has been discussed in detail previously (5,6,16,17,18) was used. Cycle time, tc, the time required for a single eight-pulse cycle, was 42-48 /isec for measurements reported here. 

For line shift measurements with the eight-pulse cycle between 180°K and room temperature, the reference was acetyl chloride. Its frequency was measured relative to a spherical tetramethylsilane (TMS) sample at room temperature, and all results are reported relative to this TMS on the r scale, (r = a + 10 ppm, where a is the signed chemical shift used in solid state NMR.) At lower temperatures, the reference was a single crystal of Ca(OH)2, oriented in the magnetic field such that the major axis of its proton chemical shift tensor was parallel to the external field (19). Thus, it is assumed that the proton chemical snift of the Ca(OH)2 remained unchanged as the temperature was varied. 

Figure 6 is a plot of the proton NMR spectrum obtained from H2Os3(CO)io when using an eight-pulse cycle (5, 6,16,17,18) to suppress the effects of proton-proton dipolar interactions. The curve results from a computer fit that assumes the lineshape is caused by the chemical shift tensor. The center of the spectrum is near r = 19 ppm, and thus it agrees reasonably with that expected from the solution NMR results (r = 21.7 ppm (37)). The three principal values of the tensor, according to this fit, are at r values 5.6 ppm, 19.9 ppm, and 31.6 ppm. Since approximately one-third of the proton pairs interact with a near... 

The entire loading and pulsing cycle as described takes approx 30 min Using an increased flow rate of 1 mL/min and a single elution pulse, at least 20 mg of anti-body-toxin conjugate can be purified within a day on this size of HPLC column. 

All these pulse-cycle techniques are in practice complicated and there are many pitfalls. The potential experimentalist is therefore recommended to refer to the literature4 25 28 and to the summary in ref. 1, p. 196. 

For this second experiment we route data from detector A to memory location II, change sign, and coadd them to the data already there. Likewise, the data from detector B are routed to location I and coadded to data already there. Thus we obtain in memory after the two-pulse cycle... 

A DC offset between the two channels is not eliminated by this cycle but is eliminated by a two-pulse cycle in which the phase of the transmitter is altered by 180° on successive acquisitions and the resulting signals are alternately added and subtracted. Because the desired Fourier-transformed signal changes sign while the DC offset does not, subtraction cancels the offset but causes all real signals to add. To cancel both channel imbalance and DC offsets simultaneously, we must nest our original two-pulse cycle in a two-pulse phase-alternated cycle to produce the four-step CYCLOPS cycle ... 

FIGURE 7.7 Pictorial representation of the effect of the WAHUHA pulse cycle. A series of 90° pulses applied as indicated causes M to spend a period 2r along each of the Cartesian axes in the rotating frame. The average orientation of M during the period 6r is thus along the diagonal at 54.7° from the z axis, as shown. 

A rigorous quantum mechanical treatment shows that the process is more complex, particularly when pulse imperfections are taken into account. The basic four-pulse cycle just given (called WAHUHA after its inventors)85 has been supplanted in practice by cycles that use 8-24 pulses (e.g., MREV-8, MREV-16, BLEW-24). The cycle is repeated many times during the period T2, and observation of the magnetization is made after each cycle during one of the t periods. These multiple pulse cycles are difficult to apply but are quite effective in narrowing lines. 

WALTZ-16 is based on an element R = 90J180° x-270°. WALTZ-16 is more effective than MLEV-16, primarily because it includes only 180° phase shifts, not the 90° phase shift inherent in MLEV. To simplify the notation, these pulse cycles are usually abbreviated in terms of multiples of 90° pulses, with a phase inversion denoted by a bar, as in the R,R notation. In these terms the basic WALTZ element R becomes 123 (hence the acronym WALTZ). Permutations of R and R lead to WALTZ-16. Further computer-optimized improvements have been devised and are not restricted to integral multiples of 90° pulses. 

In solid state cross polarization the spin lock is obtained with a long, high power pulse, but for HOHAHA such a single, unmodulated pulse is not effective. Instead, the pulse cycle MLEV-16, as described in Section 9.6, or a variant with an additional pulse, MLEV-17, covers a sufficiently broad frequency range, much as in the broadband decoupling methods we discussed in Section 9.6. 

To suppress both types of artifacts, one phase cycle must be nested within the other to create a four-pulse cycle. In addition, many spectrometers also require... 

A pulsed depolarizing iontophoretic system has been developed by Advance Co. [26] that delivers a current of frequency 40 Hz and an on-off duty of 30% to deliver a significant amount of metoprolol into the blood without any observed skin irritation or erythema at the site of application. Okabe et al. [26] hypothesized that the high-frequency pulses provided low skin impedance in addition, the capacitance of the skin was restored to its initial state at the start of each pulse cycle. Chien et al. [89] reported that a sine waveform induced a faster hypoglycemic effect with insulin, with the peak at approximately 2 hours, than either a trapezoidal (7 hours) or a square waveform (12 hours) however, the duration of the hypoglycemia was also shorter (11 hours) compared with the other two waveforms [89]. 

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