Additivity principle matching

In principle there are no differences between calibration procedures for flame and ETA methods although the latter case will take longer, as has already been pointed out. Calibration standards should match the samples as nearly as possible with respect to major components otherwise standard additions must be used. Indeed the method of standard additions will have to be used at some time to check accuracy, so this procedure will usually be tried first. If the standard additions graph is parallel to the direct calibration graph then freedom from matrix interferences would be indicated. The standard additions principle as applied to ETA is now described. 

As alluded to above, one approach to the application of surfactant HLB to formulation is to match that of the surfactant to the oil phase being employed. The HLB of the oil can be determined empirically or calculated using the procedures discussed previously. It is usually found that the additivity principle will hold for mixtures of oils in a way similar to that for surfactants, possibly even to the extent of nonlinearity in cases where oil structures differ significantly. Therefore, in formulating an emulsion, it is possible to determine the HLB of the oil phase and to vary the surfactant or mixture HLB to achieve the optimum performance. HLB numbers of some commonly used oil phases are given in Table 9.4. 

The pathway characterized with the lower energy barrier is expected to be the preferred reaction channel, especially when the addition leads to the same product. Following the local HSAB principle, one has to look at the softness matching criteria, and the minimum of sAi —. BJ and sAj —. B will determine the preferred site of attack. 

The ability to introduce several components into solution during the sol-gel step makes this approach especially attractive for the preparation of multicomponent oxides and bimetallic catalysts. Of course, the solution chemistry becomes more complex with additional components. But many of the concepts that we have discussed, such as matching relative precursor reactivity and changing the microstructure of the gel network, remain valid in principle. Furthermore, a promoter or an active species can be introduced the same way. There have been recent reports on the onc-step preparation of zirconia-sulfate aerogels [45] and Li/MgO catalysts [46], These samples arc active in the isomerization of n-butane and the oxidative coupling of methane, respectively. 

The continuous operation of LCVD could be controlled, in principle, by the strict controls of the flow rate, the system pressure, and the discharge power. The addition of a residual gas analyzer in each pumping system is advantageous in detecting abnormality in gas flow, occurrence of leakage, etc. The addition of more sophisticated diagnostic devices, such as an optical emission spectrometer, in-line XPS, etc., is often counterproductive because the stability of performance and the maintenance requirement cycle of these devices do not match those for the continuous mode operation of LCVD. The information obtainable by these devices... 

Photosensitive drugs in topical formulations can also be stabilized according to the principle of spectral overlay. Because the risk of the absorption of additives through the skin is reduced compared to oral drugs, a larger choice of suitable UV absorbers and colorants is available. Particularly, sunblockers, which are commonly used in sun creams, offer a variety of different UV spectra, which can even be combined to match the spectrum of a drug. 

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