Scan repetition rate

Solid state NMR data was acquired with a Chemagnetics M-100S solid state NMR at a frequency of 25.04 MHz for 13c and 99.58 MHz for h. A 9.5mm Kel-F rotor provided by Chemagnetics was used to spin samples sealed under vacuum at 3.0 to 3.3 KHz. Intensity ratios for all resonances were checked as a function of scan repetition rate on several identically prepared samples to insure that the same intensity ratios were obtained with larger repetition delays but fewer scans. 

Now reduce the scan repetition rate to 2/s and observe the electrode current closely. If the electrode is properly conditioned the electrode current (the background current ) will be stable, and will not fluctuate from scan to scan. If current fluctuation is observed, the cycle must be repeated, several times if necessary, until the scan current is stable (see Notes 11,12, and Fig, 3),... 

Perfluoroalkane-225 (PGR, Gainesville, FL) was admitted through a glass inlet system to provide reference peaks. Analytical and reference peaks for the nitrosamines studied are shown in Table I. Sample and reference peaks were scanned alternately at a repetition rate of approximately 1 sec and were monitored on an oscilloscope. When the nitrosamine peak appeared, the oscillographic recorder chart drive was engaged and remained on until the peak disappeared. Nitrosamine quantities were estimated by comparing the sum of sample peak heights measured from the chart (usually 10 to 20 values) with values derived from injection of standard solutions. 

It should be noted that the high repetition rate ( 80MHz) laser may give a large deviation of nonlinear parameters from the expected values since the thermal effect may be superimposed on the Z-scan signals [16]. 

Figure 5.18 Scanning electron microscopy image of a microcantilever, electromachined into a stainless steel sheet by ultrashort voltage pulses (100 ns, 2 V, 1 MHz repetition rate) in 3 M HCI + 6 M HF. The tool electrode was a tiny loop of a 10 pm thick Pt wire. (Reproduced with permission from Ref. [80].)...

Although it s possible to acquire many spectra in rapid succession using pulsed NMR methods, one needs to be aware that the speed with which multiple FIDs can be acquired is still subject to the fact that the nuclei in the sample need to relax between acquisitions (Section 5.1). If successive FIDs are acquired too rapidly, intensity information will be distorted because those nuclei which relax slowly will not be fully relaxed when subsequent scans are acquired and they will contribute less to the resulting signal. To ensure that the signal intensities are accurate, the repetition rate needs to be such that even any slowly relaxing nuclei in the sample are fully relaxed between scans. 

FIGURE 10.13 CARS images of the DNA network, (a) TE-CARS image at on-resonant frequency (1337 cm ). (h) TE-CARS image at the off-resonant frequency (1278 cm ). (c) One-dimensional line profiles of the row indicated hy the solid arrows. The scanned area is 1000 nm x 800 nm. The numher of photons counted in 100 ms was recorded for one pixel. The acquisition time was -12 minutes for the image. The average powers of the (0 and tOj beams were 45 pW and 23 pW at the 800 kHz repetition rate. 

The barriers to this approach have been technical in nature. Mode-locked Nd glass lasers remain a common light source for picosecond spectroscopic studies, but they suffer from poor reproducibility and very low repetition rates. These features combine to make wavelength scanning techniques unsuitable with such lasers. The alternative approach is to employ multichannel optical detection and thereby obtain full spectral coverage with each laser shot. It is also necessary to eliminate the effects of shot-to-shot variations of the laser output. 

After the advent of MR systems with powerful gradient hardware, dynamic scans with a repetition rate of at least one image or image stack per 2 s have become possible. By use of such sequences it has become feasible to visualize the transit of a bolus of paramagnetic or superparamagnetic contrast agent through the brain (Moseley et al. 1990 Rosen et al. 1990). For DSC, the bolus is injected intravenously. 

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