Continuous scan

The two most important parameters in a continuous scan, which are defined by the user, are the sampling interval (step), s, and the angular velocity (scan rate), r. The sampling step is equivalent to the step size in the step scan mode. Everything said about the size of the step in the previous section, therefore, applies to the sampling step during the continuous scan. The two parameters, i.e. counting time, t, in the step scan and the scan rate, r, in the continuous scan are related to one another as follows 

t is in seconds, s is in degrees, and r is in degrees/min. Thus, a continuous scan with the rate r = 0.1 deg/min and with the sampling step 5 = 0.02° is equivalent to a step scan with the same step and counting time 12 s/step. When the sampling step is reduced at a constant scan rate, this is equivalent to the proportional reduction of counting time and vice versa. 

In modem diffractometers both scanning modes result in nearly identical quality of experimental data. A step scan is usually considered as the one with less significant positioning errors, which could be important in experiments where the maximum lattice parameter precision is essential. Continuous scans are used most often for fast experiments, whereas step scans are usually employed in overnight or weekend experiments. 

Many commercial powder diffractometers have physical limits on the lowest scan rate. For example in the Scintag XDS2000 system, the scan rate cannot be lowered below 0.1 deg/min. This scan rate is equivalent to the 

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Kung C Y, Chang B-Y, Kittrell C, Johnson B R and Kinsey J L 1993 Continuously scanned resonant Raman excitation profiles for iodobenzene excited in the B continuum J. Phys. Chem. 97 2228-35... 

The historical data is sampled at user-specified intervals. A typical process plant contains a large number of data points, but it is not feasible to store data for all points at all times. The user determines if a data point should be included in the list of archive points. Most systems provide archive-point menu displays. The operators are able to add or delete data points to the archive point hsts. The samphng periods are normally some multiples of their base scan frequencies. However, some systems allow historical data samphng of arbitraiy intei vals. This is necessaiy when intermediate virtual data points that do not have the scan frequency attribute are involved. The archive point lists are continuously scanned bv the historical database software. On-line databases are polled for data. The times of data retrieval are recorded with the data ootained. To consei ve storage space, different data compression techniques are employed by various manufacturers. 

X-ray Diffraction. Eliffiactograms were obtained with a ShiimdzuXD-Dl instrament with monochromator using CuKa, radiation. It was operated in continuous scan mode at 0.5° (20) min. ... 

A second data set (CU97, 1535 reflections of all parities, 0 < (sin )/ 1.3 h, k, l > 0 and h, k, l < 0) was recorded in continuous scan mode (i.e. the detector was read out during the co -moves). This scan mode accelerated the data acquisition and enhanced the accuracy of the derived integral intensities. Averaging of these data yielded 120 reflections with an internal consistency RJ[F2) = 0.0038. 

MS offers the opportunity for both qualitative and quantitative analysis in a single instrument and even in a single experiment. In full-scan mode, mass spectra are continuously scanned as the gas chromatographic analysis proceeds. As shown in Fig. 14.8, GC/MS provides a three-dimensional data set with axes of time, mass-to-charge ratio and... 

Nanocarbons can also be deposited onto surfaces via electrochemistry, such as electrophoretic deposition described earlier. A method for one-step electrochemical layer-by-layer deposition of GO and PANI has been reported by Chen et al. [199]. A solution of GO and aniline was prepared and deposited onto a working electrode via cyclic voltammetry. GO was reduced on the surface when a potential of approx. -1 V (vs. SCE) was applied compared to the polymerization of aniline which occurred at approx. 0.7 V (vs. SCE). Repeated continuous scans between -1.4 to 9 V (vs. SCE) resulted in layer by layer deposition [199]. A slightly modified method has been reported by Li et al. who demonstrated a general method for electrochemical RGO hybridization by first reducing GO onto glassy carbon, copper, Ni foam, or graphene paper to form a porous RGO coating [223]. The porous RGO coated electrode could then be transferred to another electrolyte solution for electrochemical deposition, PANI hybridization was shown as an example [223]. 

Hites, R.A. Biemann, K. Computer Evaluation of Continuously Scanned Mass Spectra of Gas Chromatographic Effluents. Ana/. Chem. 1970,42, 855-860. 

Both of these approaches are hence useful for a continuous stream of particles of similar composition where the mass spectrum can be continuously scanned, for example in laboratory environmental chamber studies. 

Unlike conventional incremental porosimeters, which produce a limited number of data points, the presentation of continuous scans eliminates the need to rerun an analysis to obtain information between data points for additional resolution in the regions of interest. 

Logarithmic signals from continuous-scan porosimetry... 

C. General Data-Acquisition Considerations D Acquisition with Continuous Scanning... 

The processes involved in data acquisition and the considerations relevant to obtaining appropriate data for deconvolution are nearly identical for both continuous scanning and step scanning instruments. Where no specific distinction is noted, no differences are significant at the current state of the deconvolution art. Sampling, aliasing, and other details of the data-acquisi-tion process can be analyzed using the methods discussed in Chapter 1. 

Blass (1976a) and Blass and Halsey (1981) discuss data acquisition for a continuous scanning spectrometer in detail. The principal concept is that as a system scans a spectral line at some rate, the resulting time-varying signal will have a distribution of frequency components in the Fourier domain. 

Preparation of data for deconvolution must begin prior to the acquisition of the first data point. Once the resolution of the system is set (in a dispersive spectrometer this is equivalent to setting the slit width), the density of data points per resolution element must be chosen as discussed in Sections III.D and III.E. There are some subtle factors that must be taken into account. For example, for continuous scanning, approximately 10 data points per resolution element are recommended (Blass, 1976a) to capture all of the information required by the data and the noise. On the other hand, the data-point density must be great enough to characterize the spectral lines after... 

Returning to the mainstream discussion of data preparation, we note that, for a 6-dB-per-octave-rolloff RC filter network in a lock-in amplifier, the continuous scan rate amounts to approximately one time constant per data point or 10 time constants per resolution element (Blass, 1976a). Some time is saved if only six data points are taken per resolution element. We have tried acquiring in this fashion, with no visible negative effects. 

In our operations, we often establish step-scan parameters as if we were continuously scanning. Because continuous scanning requirements are more demanding, no problems arise using this approach. 

Fig. 23 P6 of 28SiH4 recorded on a grating spectrometer. Trace (a) is from a continuous scan of natural-abundance silane at a resolution of 0.020 cm-1. Trace (b) is the same region recorded separately at a resolution of 0.012 cm-1 and is the average of 12 scans.

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