Optimization by isochronal analysis I. Changes in mobile phase composition and velocity

Summary The theory of isochronal analysis (time normalization) has been extended to reversed-phase liquid chromatography. Of the possible manipulation of experimental parameters, the present paper develops a framework for optimizing the resolution by the simultaneous change of mobile phase velocity and composition. For that purpose the resolution equation was written in terms of these two parameters and under the constraint of constant analysis time. This treatment allows plotting the resolution versus the mobile phase composition and velocity. The surfaces thus obtained show the changes that the analyst must perform in order to optimize the resolution. The treatment deals with three possible α dependencies on the amount of organic modifier: constant, increasing and decreasing. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Optimization of the mobile phase composition in gradient elution reversed-phase HPLC by stochastic prediction

Summary Quantitative analysis of more than ten compounds in a sample generally requires complex mobile phases to optimize the separation of the analytes by gradient elution reversed-phase HPLC. For this purpose, CHEOPS, a software package has been developed from the fully stochastic Computer Chromatogram Simulation Method. Calculation principles and optimization criteria are described. Experimental validation is presented with amino acids and steroids.     

Continuous TLC as a pilot technique for optimization of gradient HPLC. 1. Theoretical considerations for stepwise gradient TLC with binary mobile phase

Equations describing gradient elution in TLC, with a mobile phase of a constant composition in each step, have been derived. These equations are useful for calculating retention volumes from isocratic TLC data. Equations originally derived for gradient HPLC were adapted for TLC. The validity of the equations has been experimentally verified. The satisfactory agreement between experiment and theory for binary mobile phase composed of methyl ethyl ketone and n-heptane suggests that TLC can be used as a pilot technique for gradient HPLC.     

Advanced simplex optimization of two-factor selectivity by high performance thin-layer chromatography

Summary A computer-assisted mixture design simplex method is presented for optimization of the separation of a mixture of five benzoic acid derivatives in reversephase HPTLC and twelve PTH-amino acids in normal phase HPTLC. A two-factor selectivity rectangle concept was used. The method is based on a special polynomial estimated from nine experimental runs, using the R_F difference as the selection criterion, with connection to a general simplex method for optimization using a microcomputer. Excellent agreement is obtained between predicted data and experimental results, and more than half the number of experiments required in the general simplex method can be omitted.     

Thin-layer chromatography with high-boiling mobile phase. part 2: Effect of temperature on retention characteristics in TLC with high-boiling mobile phases

A study has been undertaken on the effect of temperature on retention characteristics in thin-layer chromatography (TLC) with low-volatility mobile phases (MP). It is shown that temperature variations in TLC in melts bring about variations in both the relative retention values and, in some cases, in the order of migration of the chromatographic zones across the layer. The variation in the capacity factor k' with temperature agrees well with Martin's equation. To explain the temperature dependence of R_f one must, in general, take into account the variations with temperature in both the partition coefficient and the phase ratio. To describe the variation in R_f with temperature in TLC with low-volatile MP one can use an approximate equation in which In R_f is a linear function of 1/T. The experiments indicate that temperature is a major factor in TLC in melts.     

Comparison of optimization methods in planar chromatography

Summary The optimisation of chromatographic parameters is studied. The Window Diagram and Overlapping Resolution Map (ORM) methods are compared. The latter is developed to take into account the spot diameter and the average plate height variations with the development distance, in order to improve the reliability of the method.     

Systematic and statistical errors in quantitative evaluation in TLC and HPTLC. 4. Linear calibration

Analytical results are often obtained from reflectance or fluorescence measurements in TLC or HPTLC with the aid of calibration lines. Curve fitting may not be done by conventional least squares methods for both the independent and the dependent variables are erroneous; errors occur in the volumes spotted on to the plate and in the reflectance/fluorescence measurements. Therefore the sum of the squares of the distances to the calibration line has to be minimized instead of the squared sum of the errors of the dependent variable. The algorithms used are derived and explained. Implications of error propagation for the analytical result are also given.     

Chemometric characterization of TLC systems of the type silica-binary non-aqueous mobile phase in the analysis of flavonoids

Summary Retention-mobile phase composition relationships have been determined for eighteen flavonoids chromatographed with non-aqueous systems of the type silicadiluent+polar modifier (S). The slopes of the plots depended on the molecular structures of the solutes, primarily on the number of hydroxyl groups. The selectivities and sequences depended on the diluent and modifier applied; plots ofR _M against log [S] (where [S] is the percentage concentration of S) were approximately linear, although deviations from linearity were observed occasionally. The plots enable selection of mobile phases of optimum composition for qualitative and quantitative analysis of given sets of flavonoids.     

用双波长薄层色谱扫描法测定糖脂双降茶中菝契皂甙元的含量

应用双波长薄层色谱扫描法糖脂双降茶中菝契皂甙元的含量,回收率为98.18％,相对标准偏差为2.54％,相关系数为0 9977.该法糖脂双降茶中菝契皂甙元的含量,操作简便、结果稳定、重复性好,可作为质量检验的一个定量方法。     

Parameters influencing theoretical determination of RM-values of naphthalene derivatives in thin-layer chromatography by using binary mobile phase

The purpose of the studies being carried out is to find some regularities of a general character which determine the relationship between the structures of the chromatographed substances and the composition of the mobile phase, i.e., between the log of the partition coefficient of the sample (A_z), the K₁-values which characterize the adsorption equilibrium of component “1” in a given chromatographic system, the structures of the mobile phase components and the structures of the substances being chromatographed. This paper deals with the relationship between the A_z and K₁ parameters and the structure of naphthalene and a number of its derivatives. It has been shown that there is a close relationship between the kind, size, frequency and site of the substituent on the one hand and the A_z and K₁ values on the other. In order to eliminate the possibility of unwanted side effects, the investigations were carried exclusively using non-active binary mobile phases of the type N + N or N + /B/. From these investigations, a comparison between the parameter A_z and the surface of adsorbed molecules (A_s) was also possible.     

Optimization of binary solvents in thin layer chromatography

ΔR_f, the separation between a pair of compounds on a thin layer chromatographic plate, can be predicted as a function of solvent composition for certain binary systems. This allows the prediction of optimum solvent composition for separating a mixture of compounds by thin layer chromatography. A new solvent polarity ranking, based on calculation of ΔR_f is described.     

Ion-pair chromatography with UV absorbing counterions on reversed-phase layers. Part 1: Chromatography with hydroxynaphthalenesulfonic acids

The results of ion-pair chromatography with UV-absorbing counterions are described. Hydroxynaphthalenesulfonic acids are used as ion pair formers, and alkaloids as test samples. The separations are performed on silanized silica gel from Merck and KC₁₈F plates from Whatman. The influence of the counterion on the R_f-value is demonstrated. Some alkaloids exhibit characteristic fluorescence as ion pairs.     

Thin-layer adsorption chromatography with mixed mobile phases. 2. Effects of solute-solvent interactions and energetic heterogeneity of surfaces with regard to admolecules in TLC

In the Part 1 of this series a new equation for determining the R_m values in TLC with mixed mobile phases was proposed and examined by using the parameter m, which is a measure of energetic heterogeneity of the adsorbent surface with regard to the adsorbing molecules (admolecules). The numerical values of m, evaluated for 13 chromatographic systems, have been compared with that of the O?cik's parameter A, which characterizes solute-solvent interactions.     

A new procedure for separation of complex mixtures of pesticides by multidimensional planar chromatography

The objective of this communication is to report a new procedure for the separation of complex mixtures of pesticides by a combination of different modes of multidimensional planar chromatography. Complete separation of a mixture of pesticides was carried out by multidimensional planar chromatography on silica gel plates. The plates were videoscanned to show a real picture of the experimental results. Complete separation of all compounds was not possible by a single development on silica gel. The separation can be characterized as [PC x (nPC + PC + PC + PC)]. With the aid of this new procedure a complex mixture of fourteen compounds was separated on a 10 x 10 cm TLC plate.