Mixture designs effect, variation

As a result of using mixtures of solvents, small variations in the composition might occur, that may have significant effects on the variability of the partition coefficients of a compound or on the selectivity a j of two compounds. For mixture designs these effects were previously noticed and investigated by de Boer et al. [30-32]. 

Nevertheless the investigations presented here show that if the holdup effects are understood for the range of separations and mixtures to be handled by a column, it may also be possible to use this information (optimum holdup) at the design stage, and balance the design requirements against additional dynamic effects and column performance variations due to holdup. For further details see Mujtaba and Macchietto (1998). 

When preparing these zeolites, this dual effect must be considered. Any variation from the desired batch composition can present problems if the test is designed to react for a fixed period of time. Variations which slow down the reaction could produce a zeolite which has poor properties simply because it has not completely crystallized. If one has variations which speed up the reaction, one could produce a mixture of zeolite phases. 

The excessive number of animals per test group is also pointed out and tackled with recommendations on the reduction of animal numbers. In particular, a better use of statistics in the design of experiments is recommended in order to reach an optimal compromise between animal number and variability of the results. The use of homogenous populations is advocated as a means to minimize interindividual variability if physiological variation between individual animals can be controlled, and statistical methods used to exploit this control to the full, the number of animals necessary for assay purposes can be dramatically reduced. This results, for instance, in the use of only one breed of rats for one set of tests, rather than a mixture of different breeds with the objective of mimicking the phenotypic variability of humans. This latter approach would result mainly on the study of the differences between breeds rather than the actual effects of the compound. 

An experiment is designed to determine the effect of sulfur dioxide, one of the EPA criterion pollutants, on plants. Among the variations used is a mixture that has mole fractions given in the table. 

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