Separation conditions prediction system

The method has been proposed for the prediction of retention data in isocratic systems from data measured in gradient elution and vice versa [84], Similar calculation methods may be very important in the analysis of natural extracts containing pigments with highly different chemical structure and retention characteristics. The calculations make possible the rational design of optimal separation conditions with a minimal number of experimental runs. 

Finally, structure-based predictive software is commercially available (such as CHROMDREAM, CHROMSWORD or ELUEX) for mobile phase optimisation in RPC. This software incorporates some features of the expert system, as it predicts the retention on the basis of the molecular structures of all sample components (which should be known) and the known behaviour of model compounds on various HPLC columns. No initial experimental runs are necessary as the retention data are calculated from the additive contributions of the individual structural elements to the retention, contained in the software databa.se and consequently optimum composition of the mobile phase is suggested. Such predictions are necessarily only approximate, do not take into account stereochemical and intramolecular interaction effects, and predicted separation conditions can be used rather as the recommendation for the initial experimental run in the subsequent optimisation procedure. 

The theory was found to predict complex behavior near the phase separation conditions. As the rate of shear increases, first, the system undergoes homogenization, then demixing, followed by another homogenization and demixing. At high rates of shear, the system should behave similarly as in a quiescent state. These predictions were found to be in qualitative agreement with experimental data, e.g., for blends of ethylene-vinylacetate copolymer with chlorinated polyethylene, EVAc/CPE, or polystyrene with poly(vinyl methyl ether), PS/PVME [Hindawi et al., 1992 Eernandez ef fl/., 1993, 1995]. 

Expert systems to predict best initial separation conditions on the basis of sample component molecular structures ELUEX, CHROMDREAM, HPLC- METABOLEXPERT, ProDigest-LC, LABEL 165, 166,172, 173... 

A computer-assisted system for predicting retention of aromatic compounds has been investigated in reversed-phase liquid chromatography. The basic retention descriptions have been derived from the studies on quantitative structure-retention relationships. The system was constructed on a 16-bit microcomputer and then evaluated by comparing the retention data between measured and predicted values. The excellent agreement between both values were observed on an octadecyl-silioa stationsu y phase with acetonitrile and methanol aqueous mobile phase systems. This system has been modified to give us the information for optimal separation conditions in reversed-phase separation mode. The approach could also work well for any other reveraed— phase stationsury phases such as octyl, phenyl and ethyl silicas. 

To demonstrate the potential of this function of RPS, an experiment was performed. The demonstration as shown in Figure 5 is the separation of a test-mixture of aniline derivatives. The chemical formulas of three aniline derivatives such as aniline,N-methylaniline and N-ethylaniline were input in RPS and the analysis time of 10 min and a resolution value of 1.2 for N-methyl- and N-ethylanilines were requested. RPS provided the predicted retention data of above three compounds both the conditions desired by the user and conditions attained in this system. The system showed that analysis time of 11 min and the resolution of 1.1 would be obtained at the 65 % acetonitrile in the mobile phase and the flow rate of 4 pL/roin. The synthesized chromatogram appeared on the CRT and the printer connected with the computer. The actual run was carried out to evaluate the performance of RPS at that optimized separation condition of 65 % acetonitrile in the mobile phase and the flow rate of 4 pL/min. The system s analysis time of 11 min compares favorably with measured analysis time of 11.5 min. 

Computer programs enable the user to use well-established relationships between separation, retention, and chromatographic conditions to allow the prediction of results of different experiments and optimization of separation conditions, working mainly with a PC rather than with an HPLC system. In this case, a user enters some information such as run data and conditions, and works with the software interactively. 

After these calculations, which are performed immediately after entering the structures, the system automatically commences wthe experimental runs. The results of the experimental runs are used to fine-tune the first theoretically set up retention model. Thus, the retention model becomes accurate enough to finally predict the optimum isocratic separation conditions. 

The separation performance of these systems (usually low-pressure, not close to critical conditions, and with similar components) can be predicted by Raoult s Law, applying to vapor and liquid in equilibrium. 

This distinction between the conditions in a chemical system at equilibrium and the rate at which these conditions are attained is very important in chemistry. By arguments that we shall consider a chemist can decide with confidence whether equilibrium favors reactants or products or neither. He cannot predict, however, how rapidly the system will approach the equilibrium conditions. That is a matter of reaction rates, and the chemist must perform separate experiments to learn whether a given rate is rapid or not. 

The Boltzmann equation, S = kXxiW, establishes the zero point for entropies. Because ln(l) = 0, this equation predicts that the entropy will be zero for a system with only one possible description. Figure 14-1 la shows a system with W =, nine identical marbles placed in nine separate compartments. The figure also shows two types of conditions where > 0. Figure 14-llZ) shows a condition when the marbles are not identical. If one marble is a different color than the other eight, there are nine different places to place the different marble. Figure I4-IIc shows a condition where there are more compartments than marbles. If there are more places to put the marbles than there are marbles, there are multiple ways to place the marbles in the compartments. 

Also in this case the calculated (predicted) retention values showed good agreement with the experimental results. It has been concluded that pH gradient elution may enhance the separation efficacy of RP-HPLC systems when one or more analyses contain dissociable molecular parts [81]. As numerous natural pigments and synthetic dyes contain ioniz-able groups, the calculations and theories presented in [80] and [81] and discussed above may facilitate the prediction of the effect of mobile phase pH on their retention, and consequently may promote the rapid selection of optimal chromatographic conditions for their separation. 

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