Drying sample composition dependence

Some types of chemical analysis require that the sample be in dry powder form, either because of restrictions imposed by the analytical system to be used or in order to concentrate the sample and boost the sensitivity of the analytical method applied. The necessary condition for freeze-drying to be applicable with this purpose is that it should not alter the sample composition, which will depend exclusively on the particular freeze-drying conditions used. 

The temperature dependence of the density of dry samples of native and purified elastin is shovm in Figure 1. In the temperature range explored, the native protein shows a higher density than the purified one, typical values being 1.245 g/ml and 1.232 g/ml, respectively, at 25 C. In a first approximation, this difference in density can be accounted for by the different composition of the native and purified protein. If native elastin is considered a two phase composite material (approximately 80% elastin and 20% collagen), disregarding other minor components whose density data are not available, the density of the composite can be calculated from the equation ... 

The extent of the reduction may depend on many parameters, including the nature of the support. The size of the gold particles is also influenced by the thermal treatment, the flow rate and composition of gas through the catalyst (sometimes hydrogen is a component of the gas), and the size of the catalyst sample [1,2], It is recommended that as prepared samples are stored in a refrigerator at <273 K and that calcined catalysts should also be kept cold, and that after drying samples should be kept in a vacuum desiccator in the dark, reduction being performed immediately before use. 

The reliability of the results depends in large measure on how well deviations from the (ideal) linear relationship between log / and dry weight per unit area can be eliminated or allowed for. As is well known, this can be accomplished by the comparative method (3.10), provided that standard (reference system) and unknown, identical in mass, shape, and elementary composition, are exposed to the same x-ray beam. In the cytological investigations, these conditions are difficult to meet, not only because the samples are complex in composition, but also because they are very small, as is clear from the units employed (micromicrograms per square micron or 10 12 gram per 10 8 sq cm). 

Correct determination of the sample mass is critical. Usually drying conditions are specified by the producer. The user has to be aware that there is an amazing variety of recommended drying conditions presented in the various certificates. Since the optimum conditions are very dependent on matrix and composition, it is of utmost... 

The results of stress-strain measurements can be summarized as follows (1) the reduced stress S (A- A ) (Ais the extension ratio) is practically independent of strain so that the Mooney-Rivlin constant C2 is practically zero for dry as well as swollen samples (C2/C1=0 0.05) (2) the values of G are practically the same whether obtained on dry or swollen samples (3) assuming that Gee=0, the data are compatible with the chemical contribution and A 1 (4) the difference between the phantom network dependence with the value of A given by Eq.(4) and the experimental moduli fits well the theoretical dependence of G e in Eq.(2) or (3). The proportionality constant in G for series of networks with s equal to 0, 0.2, 0.33, and 0. Ewas practically the same -(8.2, 6.3, 8.8, and 8.5)x10-4 mol/cm with the average value 7.95x10 mol/cm. Results (1) and (2) suggest that phantom network behavior has been reached, but the result(3) is contrary to that. Either the constraints do survive also in the swollen and stressed states, or we have to consider an extra contribution due to the incrossability of "phantom" chains. The latter explanation is somewhat supported by the constancy of in Eq.(2) for a series of samples of different composition. 

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