Continuous scanning mode

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. 

Figure 1.3 Modes of the mirror scanning in interferometry (a) continuous-scan mode and (b) step-scan mode.

The sensitivity and detection limits of an analytical technique are determined by the SNR of the measurement, an important metric for assessing both the instrumental performance and analytic limits of the spectral measurement. Following typical analytical practices, 3 and 10 times the noise have been suggested as limits of detection and of quantification for IR spectroscopy, respectively. The performance of interferometers in the continuous-scan mode, which is simpler compared with that of the step-scan mode, has been analyzed well. The SNR of a spectrum measured using a Michelson interferometer is given by12... 

Assume that the overnight experiment described in problem 14 is to be performed using the continuous scanning mode from 8 to 60° 29 with a sampling step A20 = 0.005°. Estimate both the scanning rate and the time it will take to finish the experiment. 

Figure 4.30. Powder diffraction pattern collected from an organo metallic compound on a Rigaku TTRAX powder diffractometer using Mo Ka radiation. The data were collected in a continuous scanning mode scan rate was 5 deg/min, sampling step 0.01°.

Figure 6.4. The powder diffraction pattern collected from a sample of LaNi4 35800.15 using Cu Ka radiation on a Rigaku TTRAX rotating anode diffractometer. The divergence slit was 0.75° and the receiving slit was 0.03°. The experiment was carried out in a continuous scanning mode with a rate 0.5 deg/min and with a sampling step 0.02°. The powder used in this experiment was prepared by gas atomization from the melt and therefore, particles were nearly spherical (see inset in Figure 3.32).

To account for the presumably statistical distribution of Ni and Sn atoms in the 2(c) and 3(g) sites in this crystal structure, the initial distribution of atoms in the unit cell has been assumed as listed in Table 7.2. The initial profile and structural parameters are found in the input file for LHPM-Rietica on the CD, the file name is Ch7Ex01a.inp. Experimental diffraction data, collected on a Rigaku TTRAX rotating anode powder diffractometer using Cu Ka radiation in a continuous scan mode, are located on the CD in the file Ch7Ex01 CuKa.dat. 

At lesser sensitivity requirements, or if a measurement of the peak shape is desired, a continuous scan of mass to record full peaks may be preferred. In modern instruments, control is really a digital process, so that the continuous scan mode is essentially peak hopping with a small mass interval—0.1 amu or less per step. Very high dynamic range instruments, in which the peak tails are to be measured, might use 50 or more steps per amu. 

In step-scan mode, the moving mirror of the interferometer is stopped at each data acquisition point and held for some time (seconds to minutes) during which data are acquired. In step-scan mode the collected interferograms contain the same information as in continuous-scan mode, only the time required for the complete experiment is much longer. Under stroboscopic measuring conditions, a time resolution of 100 ns can be achieved. This technique can be applied to processes which can repeatedly be started under highly reproducible conditions. The step-scan technique can also be applied for the acquisition of voluminous data. This... 

In the continuous-scan mode, a Michelson interferometer generates modulation of the radiation at each wavenumber v with a frequency F = 2Vv, where V is the mechanical velocity of the scanning mirror in centimeters per second. The frequency F is called the Fourier frequency. A typical speed of the scanning mirror is 0.1-10 cm s so that signals within the IR spectral region fall into the 10 -10" -Hz range. Since in most measurement schemes a low-pass filter is used to separate the Fourier and modulation frequencies, the modulation frequency should satisfy the sampling theorem. Specifically, the modulation rate... 

In routine FTIR spectroscopy the spectrometer is operated in continuous scan mode. In this mode of operation, the moving mirror is scanned at a constant velocity, v (cms ), with the light beam path difference at any time, t being given hy 6 = 2 1 (cm). An internal HeNe laser beam is also passed through the interferometer and, since it is essentially monochromatic (I5,798cm ), it is used to accurately calibrate the positions of Mm for data sampling. Continuous scan FTIR is most commonly used to monitor stable samples, but can also be used in rapid-scan mode to monitor time-dependent processes on timescales down to ca. 20 ms. 

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