A recently proposed method for analyzing polydispersity with photon correlation spectroscopy is verified experimentally by directly estimating the molecular weight distributions (MWD’s) of two well characterized National Bureau of Standards (NBS) polystyrene samples in cyclohexane at 35.0 °C. The MWD’s were reproducible and in good agreement with the NBS molecular weight averages. The method finds the smoothest nonnegative MWD that is consistent with the data without requiring initial estimates or subjective judgements. It is shown to work at dust levels where the method of cumulants fails. Minimum necessary precautions against dust and other sources of error are discussed.
The occurrence of convective flows during crystal growth is believed to adversely affect crystal quality. Space-based crystal growth is therefore actively pursued, particularly for protein crystals, because buoyancy-driven convection is suppressed in microgravity. Here the authors demonstrate that magnetic fields can be used to tune the effective gravity from 1to−0.15g during the growth of diamagnetic lysozyme crystals and that convection can be damped, stopped, and even reversed. The growth velocity is strongly reduced in simulated microgravity. This method provides a versatile and accessible way to realize an earth-based tunable gravity environment for crystal growth in which protein crystal quality may be optimized.
A double focusing camera for synchrotron radiation is described. Focusing in the vertical plane is achieved with a segmented mirror whereas in the horizontal plane a bent triangular monochromator is used. The entire system is remotely controlled. A few examples of results obtained on biological samples illustrate the performance of the instrument.
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