Sweep time

In general, the longer the sweep time the better the sensitivity since the filter time constant parameter can be set longer with consequent improvement in signal-to-noise ratio. In practice, however, sweep times are usually set in accordance with the expected lifetime of the radical species, the stability of the instrument, and the patience of the operator. Decay of the radical or drift of the spectrometer during a scan is clearly undesirable. The sweep time is most commonly set in the range 4-10 min. 

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In order to minimize the probe heating effect (measure disturber), the number of the measure points will be 80x80 (the sweeping time negatively influences the probes ), for example ... 

The lesponsivity becomes independent of the bias voltage, V when the electric field-induced sweep time of the holes equals the hole lifetime. 

It is clear, that the various modes of LSV at the dme require an integrated and coherent regulation of sweep time, current sampling and drop knocking, preferably by an electronic device and on with computer guidance. A disadvantage of LSV. at the dme, in contrast to normal DC polarography, is that for mixtures of components the latter yields a simple evaluation by curve extrapolation on the basis of additivity [see Fig. 3.34(a)], whereas the former suffers from an uncertain evaluation [see Fig. 3.34(b)],... 

Sweep time Field offset Modulation amplitude Filter time constant... 

The data at 350 mp were further analyzed, and the decay curves are presented in Figures 6 and 7. In Figure 6 the decay curves of the shortest-lived species are shown, where a sweep time of 100 psec. per cm. was employed. For both the dry and the wet samples, the decay curves, plotted as first-order decays, exhibit complex shapes. A similar complex behavior is shown by a second-order plot of the data. Although there is no basis for choosing one plot over the other, we have chosen the former for convenience. Further investigation may show this arbitrary choice to have been wrong. We take this to indicate that there is... 

In Figure 7 the decay of the longer lived species is presented, again assuming first-order kinetics. Here the sweep time was 100 msec, per cm. There appears to be little difference, if any, in the behavior of the dry and the wet samples. The break in these two curves, yielding what appears to be a second component, is probably real. The slope of the line joining the points for the water- and air-saturated sample is essentially the same as that of the second region of the other curves. 

Thus, a sweep rate so slow that it will outrun the validity of 8, = (rcDt)in will be much less than the 55 mV s 1 calculated on the basis of a constant current density during the sweep. If one makes a gucstimatc that a typical sweep has a full limiting current for 5% (l/20th) of the sweep time, then the acceptable minimum sweep rate would be around 55/20 = 2.5 mV s I. 

The sampling time during which FID data points must be collected in order to obtain the true NMR spectrum after Fourier transformation depends on the spectral width A. According to information theory [18], the sweep time per data point, called the dwell time faw, must satisfy the minimum given by the Nyquist equation (2.12). 

U.K.), with a normal sweep width of 1000 Hz and sweep time of 500 s. Fourier transform PMR spectra were recorded using a Bruker WH 300 instrument (Bruker Instruments Inc., Manning Park, Billerica, MA 01821, USA) and methanol-d as solvent (NMR Ltd). 

The 1H NMR spectra of parbendazole was recorded with a JEOL-PS 100 NMR spectrometer operating at a frequency of 100 MHz and a magnetic field strength of 2.349 T. Spectra were determined over the region 10.8-0.0 parts per million (ppm), with a sweep time of 250 s. Chemical shifts were recorded as S (delta) ppm downfield from tetra-methylsilane (TMS). Proton noise and off-resonance decoupled 13C NMR spectra were measured on a JEOL FX 90Q Fourier Transform NMR spectrometer operating at 90 MHR and spectral width of 5000 Hz (220 ppm). All measurements were obtained with the compound being dissolved in deuterated dimethyl sulfoxide (DMSO-d6) for dT NMR and in deuterated trifluoroacetic acid (TFA-dx) for 13C NMR. 

Increased sweep times with state of the art EDS detectors may in some cases provide sufficient sensitivity to make this measurement. Consequently, the electron beam must be directed to a very small volume of catalyst by operating at high magnifications, i.e., 20 million times in a raster or spotted beam mode. Often count times of up to 100 seconds or more are required to determine if an area of high contrast contains a catalytic metal. This experiment is tedious, but in many instances is the only practical approach. 

At preselected time intervals the proton spectrum was scanned. By using a sweep time of 50 sec. and scanning only that spectrum portion of... 

Fig. 2. Electron paramagnetic resonance spectra of wild-type and TID and T2D mutants of FetSp as indicated. Spectra were obtained at a microwave frequency of 9.5 GHz and 120 K. The samples were prepared in 25% v/v ethylene glycol/50mM MES buffer, pH 6.0. The instrument settings were constant with values as follows microwave power, lOmw modulation frequency, 100 kHz modulation amplitude, 10 G time constant, 0.02 s sweep time, 60 s (Hassett et al., 1998).

In a vessel for electrolysis three parallel nickel plates were installed. The inner nickel plate was the working electrode (anode), and the two outer nickel plates were counter electrodes (cathode). A 1.2 liter mixture consisting of fluorene (0.01 mol) and LiPFs (0.1 mol) dissolved in propylene carbonate were then added to the vessel. The three nickel plates were immersed in the mixture to a depth of 90 mm. Two lithium metal sheets were used as reference electrodes, with each sheet placed between the anode and the cathode. The electrolysis was carried out by a potential-sweep method for 4 hours under a potential width of 4.5 to 6.7 V with a sweep time of 50 mV/s. The inner... 

The resulting radical can irreversibly dimerize to form a species which displays no electroactivity under the conditions examined. Figure 34 shows three voltammograms measured at scan rates between 75 and 250 kV s . The fastest scan rate shows the reoxidation of the radical on the return scan whereas with slower scan rates this is progressively lost as the sweep time becomes comparable with the time taken for the radical to dimerize. Interpretation of the current peak data in terms of an EC2 mechanism permitted the deduction that the dimerization rate constant was 2.5 X 10 M s corresponding to a half life of 20-50 ns under the conditions studied. 

We have obtained structural information from C NMR spectra (Fig. 2), taken in CS2 solvent, in which Cn has a solubility greater than 1 mg ml at room temperature. In con-strast to our previous report, we obtain an interpretable spectrum after only 1,0(X) sweeps (4 s sweep time with Cr(acac)3 as relaxant) because of the high solubility, and the spectrum shown... 

Detection of hydroxyl radical. For detection of hydroxyl radicals, a spin trapping agent, DMPO was obtained from Sigma-Aldrich.4 The water containing 500 pM DMPO was circulated for 30 min in exPCAW-l,and the water was sampled at 0, 1, 5, 10, 15, 20, 30 min of circulation and exposure to UV, USW and elevated 02. The samples in a flat-shaped quartz ESR cell were analyzed with an electron spin resonance spectrometer (JEOL-TE200, X-band). DMPO-OH signal were collected with a sweep width of 5 mM, a 100 kHz modulation frequency, 60 s sweep time, a time constant of 0.1 s and microwave power of 10 mW, at room temperature of 20-25°C. 

Figure 6-16 shows how the oscilloscope traces are used to interpret "percent metallic contact." The circuit is balanced so that the oscilloscope responds to only two extreme conditions full plate current for high resistance at the ball/ring conjunction or practically no current for low resistance at the conjunction. The "percent metallic contact" is calculated from the fraction of the sweep time that corresponds to low resistance (1 ohm or less). An integrating circuit (shown below the dashed line in Fig. 6-15) can be used to give direct readout of "percent metallic contact" on a recorder. It should be apparent that interpretation of the raw oscilloscope traces in terms of percent metallic contact is misleading. Since the response of the circuit is all or none, only... 

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