Errors absorption

In atomic absorption, errors of accuracy can occur due to poor calibration (particularly if the element does not have the same behaviour in the sample and the reference), inadequate correction of interference, contamination at any step in the analysis or partial elimination of the element during preparation. Much of the loss of accuracy may thus occur during the preparation of the sample. 

X-Ray data have been collected for KHY, which is isomorphous with RbHY but less affected by absorption errors. Refinement has led to a more precise analysis (705), some interim results of which have been included in Table 13. 

RAE = reduced absorption error use of a short wavelength. FAST is a tradename of the Enraf-Nonius television area detector. 

Experience with the television detector system (Enraf-Nonius FAST) on the focussed wiggler beam line at Daresbury using a A of 0.9 A suggests that the intensity of the diffraction patterns is often somewhat too strong for the detector. Use of even shorter wavelengths than 0.9 A will reduce the strength of the pattern whilst giving further reduction in absorption errors and enhanced sample lifetime in the beam, etc, outlined above. 

High resolution data collection has been given a strong emphasis because of the central role that correct, accurate structural models play in all uses of protein crystal structure results. The trend is clearly to true atomic resolution, the use of shorter wavelengths to get rid of systematic (absorption) errors in the data and measurement of many equivalents each with many counts to cut down random errors. The IP detector developments (section 5.4.2.6) are a key step forward (figure 10.4 shows an example). The X-ray undulator should provide new impetus to this area (section 6.7). 

The high brilliance at ultra-short wavelengths (0.15 A) of the higher harmonics from X-ray undulators would allow X-ray diffraction from macromolecular crystals totally free of absorption errors whilst stimulating the K edge anomalous dispersion of high atomic number elements (e.g. mercury, platinum, etc). 

Alternatively, native protein crystal data sets measured at wavelengths as short as 0.33 A (i.e. utilising the K edge of the barium in an image plate as detector) would also be free of absorption errors and with greatly reduced random errors. This latter arises due to the ability to have prolonged exposure times and repeated measurements before the protein crystal suffers radiation damage. Hence, unprecedented data... 

Direct measurements may be complicated by the phenomenon of self-absorption, especially in the case of low-energy jS- or a-radiation. Two approaches can be used to minimize self-absorption errors ... 

Figure 14.12 Effective diffusion coefficient of Irganox 1076 as a function of maximum solvent absorption. Error bars are the SD for the determination of solvent absorption obtained for at least five measurements at equilibrium. Reproduced with permission from Helmroth and co-workers. Food Additives and Contaminants, 2002, 19, 2, 176 [38], 2002, Taylor Francis)...

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