Microdensitometer

Comparison of the measurements with the microdensitometers and the algorithms of calculation inclusive the filter function and the accuracy of measurement of all project partners. 

The distance of each reflection from the center of the pattern is a function of the fiber-to-film distance, as well as the unit-cell dimensions. Therefore, by measuring the positions of the reflections, it is possible to determine the unit-cell dimensions and, subsequently, index (or assign Miller indices to) all the reflections. Their intensities are measured with a microdensitometer or digitized with a scanner and then processed.8-10 After applying appropriate geometrical corrections for Lorentz and polarization effects, the observed structure amplitudes are computed. This experimental X-ray data set is crucial for the determination and refinement of molecular and packing models, and also for the adjudication of alternatives. 

Reflection intensity in the SAED negatives was measured with a microdensitometer. The refinement of the structure analysis was performed by the least square method over the intensity data (25 reflections) thus obtained. A PPX single-crystal is a mosaic crystal which gives an "N-pattem". Therefore we used the 1/d hko as the Lorentz correction factor [28], where d hko is the (hkO) spacing of the crystal. In this case, the reliability factor R was 31%, and the isotropic temperature factor B was 0.076nm. The molecular conformation of the P-form took after that of the P-form since R was minimized with this conformation benzene rings are perpendicular to the trans-zigzag plane of -CH2-CH2-. 

The photographs were microdensitometered using a Joyce-Loebl double beam micro densitometer. 

So far, we have seen that if we measure the Bragg angle of the reflections and successfully index them, then we get information on the size of the unit cell and, if it possesses any translational symmetry elements, also on the symmetry. In addition, we have seen that the intensity of each reflection is different and this too can be measured. In early photographic work, the relative intensities of the spots on the film were assessed by eye with reference to a standard, and later a scanning microdensitometer was used. In modern diffractometers, the beam is intercepted by a detector, either a charge coupled device (CCD) plate or a scintillation counter, and the intensity of each reflection is recorded electronically. 

At its best, the counter diffractometer can give more accurate intensity measurements than photographs measured by a microdensitometer, and can give results more rapidly but the equipment is more complex arid requires more effort iit maintenance. The accuracy of the results depends on the number of quanta counted at any angle of diffraction ... 

An internal aluminum standard was used to calibrate the spacings, which were measured with a photo-microdensitometer. The peak intensity of the main arcs corresponds to an interlamellar spacing of 3.47 A. 0.02 A. (compare graphite, 3.35 A., with second and third orders at appropriate spacings). Other peaks present were at 2.07 A. 0.02 A. and 1.20 A. 0.02 A. These appear to represent (10) and (11) spacings, respectively. Only an extremely diffuse pattern, without evidence of preferred orientation, was obtained from the pitch surrounding the spheres. 

Microdensitometers -use m atomic emission spectroscopy [SPECTROSCOPY, OPTICAL] (Vol 22)... 

Figure 7.30 (a) Microdensitometer records of a photographic plate showing the absorption of the triplet state of anthracene-9-carboxylic acid. Horizontal axis, wavelength in nm vertical axis, absorbance, (b) Oscilloscope trace of the absorption of the ketyl radical of benzophenone in ethanol, (c) Second-order plot of the decay kinetics against time t... 

The accuracy of most current scanning microdensitometers is thought to be in the range of 3%, but enough error sources exist to cast serious doubt upon this estimate. 

The scanning microdensitometer was a modified Jarrell-Ash model 23-500. Figure 2. is a block diagram. 

In some instances, intensities are still measured by visual estimation, although in general this practice has now been superseded by the use of one-dimensional microdensitometers. Typically, a radial scan is taken through the centre of each spot, the shape of the background is estimated and sketched in, and overlapping reflections are apportioned. The area under each reflection profile is then measured, and an empirical "arcing factor" is applied. In correcting for the Lorentz and... 

The standard deviation of each scanner reading has been estimated to be 0. 7 when using a scale of 0-255 units, and the positional accuracy of the instrument is claimed to be 2pm. Data from the microdensitometer are stored in binary format on magnetic tape as an array, typical dimensions being 600 x 600. Prior to analysis the data are reconverted to integer format and copied into a random-access disc file. 

Figure 2. Calibration curve for the Daresbury Optronics P-1000 microdensitometer by using Ilford Industrial G x-ray film. The figures above the data points indicate the nominal optical density of each measurement, assuming that a scanner reading of 255 corresponds to three optical density units.

Figure 3. Example of a radial scan. The microdensitometer readings Ds are plotted against a radial coordinate. The signal-to-noise ratio is poor because of...

When the microdensitometer data have been corrected for oblique incidence, they are in a form suitable for substitution into equation 18 for the specimen intensity transform. In practice, the correction is made as the integration is performed (see section 10.2). 

It is very difficult to mount a specimen in an X-ray fibre camera such that it is precisely normal to the beam. Indeed, frequently the fibre must be tilted deliberately by a nominal amount to observe specific meridional reflections. Because the tilt angle p features in the Lorentz factor and other expressions given in section 7, it is necessary to obtain a value for it using the microdensitometer data. AXIS provides two methods for calculating /3 both are similar to procedures outlined by Fraser et al. (4) and, superficially, resemble those described in section 6 for calculating 5. The first method again requires the user to specify the positions of pairs of equivalent spots from a data file. This time, however, both members of a pair must be... 

Werner (21) has made a detailed study of the errors associated with the measurement of intensities from a protein single-crystal X-ray diffraction pattern. He has demonstrated that when a two-dimensional scanning microdensitometer is used, an individual intensity measurement differs by approximately 4% from the mean value of the four symmetry-related reflections. In general, it will not be possible to measure a fibre pattern to this degree of precision because of the various factors which complicate the analysis. If Werner s criteria are applied to a series of intensity measurements made from fibre patterns using AXIS, the precision attained lies between 7% and 10%. While we do not have enough data to draw any firm conclusions at present, this result is sufficiently close to 4% to be encouraging. 

Joyce-Loebel MK III C flat-bed microdensitometer. The structure factor magnitudes were derived from the integrated intensities according to F(hk ) 2 = I(hk ). The largest errors in the intensity measurements were for the darkest spots. The scattering factors of C, 0 H for the diffraction of electrons were taken from the International Tables for X-Ray Crystallography, vol. IV (7). No correction for multiple scattering and n-beam dynamical interactions were applied because the thickness of the crystals was of the order of 100 X (6). 

Quantitative Processing. Plates or film with the diffraction patterns were scanned with a Joyce-Loebl microdensitometer. Radial (20) densitometric plots of the crystalline pattern (eventually three successive exposures of the crystalline pattern are analyzed) and of the corresponding amorphous pattern were recorded on the same curve. In this way, the plot of the amorphous pattern was used as a reference standard. The densitometric recording began with the optical density of the non-irradiated emulsion this allowed the evaluation and normalization of the optical density of the diffraction pattern. When the analytical slit passed through the image of the border of the 75 pm objective aperture, the densitometric curve showed a sudden density raise "A d". (Fig. 6) The plots of the amorphous and crystalline patterns were thus normalized to the same reference " A d". Crystallinity was determined on the normalized curves by measuring the areas "C + A" and "A" under the crystalline and amorphous plots respectively. 

Figure 2. Microdensitometer traces of RIKES spectra of two solutions of cyclohexane in CClt in the C-H stretching region. The peak ruby pump power was ISO MW cm 2 and the pump beam was crossed with probe beam at about 10°.

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