Method development experimental considerations

Before embarking on a method development experimental work, it is advisable to have a clear understanding of the purpose of the method, any prior knowledge of the analyte(s) of interest, and any identified requirements of the end user laboratory. Examples of such considerations include the following ... 

The mental images, no matter how well grounded scientifically, are individually and collectively biased, as they have been developed after considerable filtering. The filters result from the scientific training of individuals, available supporting information from other processes, existing theoretical methods, limitations of numerical simulation, and characteristics of experimental methods. 

Table 1 summarizes several of the experimental methods discussed in this chapter. A need exists for new or revised methods for transport experimentation, particularly for therapeutic proteins or peptides in polymeric systems. An important criterion for the new or revised methods includes in situ sampling using micro techniques which simultaneously sample, separate, and analyze the sample. For example, capillary zone electrophoresis provides a micro technique with high separation resolution and the potential to measure the mobilities and diffusion coefficients of the diffusant in the presence of a polymer. Combining the separation and analytical components adds considerable power and versatility to the method. In addition, up-to-date separation instrumentation is computer-driven, so that methods development is optimized, data are acquired according to a predetermined program, and data analysis is facilitated. 

In Table 5-2 a comparison between diffusivities obtained with the TSA method and experimental D is presented. From this table one can see that, in all cases computed D agree with experimental data to within an order of magnitude. Moreover most of these D are considerably smaller than the 5 10-7 cm2/s lower threshold assumed to be in reach of nowadays MD simulations Section 5.2.1. This is an encouraging sign that computer simulations of diffusional processes are already able to predict, with a reasonable accuracy and for small and simple penetrants, diffusion coefficients around 10-10 cm2/s. From the point of view the packaging sector it would be interesting to learn if and when further theoretical developments of the TSA method will be able to simulate (predict) such slow diffusional processes for organic penetrants with a much more complex structure, see Chapter 3 and Appendix I. 

There is considerable debate concerning the exact model that describes the paired-ion phenomenon and it will continue, no doubt, for some time. It is important, however, to emphasize that theory guides experimentation therefore, the importance of having a model is to understand the factors that control chromatographic retention, and thus to aid in the speedy and logical development of separations. Therefore, any of the three models discussed above can be useful in guiding your experimentation in methods development. 

Optimization or experimental design software packages for modeling the chromatographic response as a function of one or more method variables. These can also play a key role in data management of the considerable information that results from rigorous method development exercises. 

The method developed originally for microemulsion formulation (Section II above) has been adapted (Salager, 1983, 2000) to macroemulsion formation. In this method, the value of the left-hand side of equation 8.10 or 8.11 is called the hydrophilic-lipophilic deviation (HLD). When the value equals zero, as in Section II, a microemulsion is formed when the value is positive, a W/O macroemulsion is preferentially formed when it is negative, an O/W macroemulsion is preferentially formed. The HLD is similar in nature to the Winsor R ratio (equation 5.2) in that when the HLD is larger than, smaller than, or equal to 0, R is larger than, smaller than, or equal to 1. The value of the HLD method is that, on a qualitative basis, it takes into consideration the other components of the system (salinity, cosurfactant, alkane chain length, temperature, and hydrophilic and hydrophobic groups of the surfactant). On the other hand, on a quantitative basis, it requires the experimental evaluation of a number of empirical constants. 

Perhaps the most widely used kinetics method is that developed by Freeman and Carroll (83) in 1958. The advantage of this method is that considerably fewer experimental data are required than in the isothermal method and that the kinetics can be obtained over an entire temperature range in a continuous manner without any missing regions. In addition, where a sample undergoes considerable reaction in being raised to the temperature of interest, the results obtained by an isothermal method are often questionable. The order of reaction, nt and the activation energy of the reaction are calculated from the equation... 

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