Study On Retention Process And Theoretical Foundation Of The Programmed Development Of Analytical Method In Capillary Gas-Liquid Chromatography

A general review of recent publications in gas chromatography is included in this dissertation. It is the fact that most of the published papers are related to the establishment of analytical method for specified sample without question. Because of the complication of chromatographic process and variety of samples to be analyzed, it is impossible absolutely on theoretical basis to develop an optimal method to analyze specified sample, but it can still be established based on some information from some reasonable programmed-experiments and theoretical consideration. Based on these facts the idea of the programmed development of analytical method in capillary gas chromatography is proposed. This work does thus focus following two main aspects so as to promote realization of the programmed development of analytical method. The first is concerned with the investigation of retention process, and the second covers the establishment of some approaches for the correlation of chromatographic characteristic parameters under different operation conditions.1. Retention processFused silica capillary column was used to examine the retention mechanism by which solutes are retained in gas-liquid chromatography so as to eliminate the effect resulted from support used in packed column. An equation was derived to correlate capacity factor with equilibrium constants in process of partition between gas-liquid phases (K_L) and adsorption on gas-liquid interface (K_GL) and parameters of column from original equation by Martin in packed column. In this work PEG 20M and SE 54 were used to prepare columns with different thickness of liquid-film. No significant interfacial adsorption was observed on SE 54. On PEG 20M columns the capacity factors of 29 solutes belonging to various classes were determined at nine temperatures. The obvious interfacial adsorption was observed especially when low phase loading columns and low column temperature used. To examine the characteristics of interfacial adsorption the correlation of lnK_GL with carbon number for three homologous series' and with temperature were achieved. It was found that the enthalpy in interfacial adsorption is basically higher than that in partition, especially for n-alkanes;and lnK_GL can be linearly correlated with boiling point and lnK_L. Consequently, authors believed that the molecular interaction on gas-liquid interface is a mixed one from solute-solvent and solute-solute. Therefore, interfacial adsorption cannot be significantly decreased through the increase of column temperature when solute-solvent interaction is similar to solute-solute one. The data of lnK_GL and lnK_L at the temperature of solute's boiling point given by interpolation or extrapolation showed that the difference of them have nothing to do with the properties of solute.The retention behaviors of thirty-six solutes of different classes on poly(ethylene glycol) (PEG) stationary liquids of various weight-average molecular masses (Mw) such as 400, 600, 1000, 2000, 4000, 6000, 10000 and 20000 were examined. It was observed that the effect of Mw of PEG on retention values is not very obvious in totality, especially on those with high Mw solvent, but there is significant difference in hydrogen-bond basicity because retention behaviors of alcohols is more obviously different than other solutes. The specific retention volumes of almost all solutes but alcohols increase as PEG Mw rises and slightly decrease when PEG Mw goes up from 4000 to 20000. The thermodynamic functions of those solutes on the above stationary liquids were determined. And the results showed that both the difference of the partial molal enthalpy and entropy in retention process with these solvents affected on retention in the same degree. In addition, a detailed discussion of retention behavior on various ploy(methylphenylsiloxane) of different phenyl content is provided by using literature data, too.A quantitative relationship Eq.(l) is suggested to correlate retention parameters on different molecular mass PEG based on the idea of free energy used to disrupt solvent-solvent interaction when solute is transferred from gas phase to solvent one.AG, ^AGST =(ai-aST)-—-----+ (/?,- Psr)-MPEGMPEG+(AG;-AG'sr) (1)We think that the effect of solvent molecular mass on retention is from their different content of end-group, and if the end-group can not form a strong interaction this effect may be insignificant. At the same time, we obtained Eq.(2) to explain retention behavior on poly(methylphenylsiloxane) of different phenyl content,AG = Astst -m2Ph +(BsttSt+B^)mPh +(Cstsl +Cs,sl) (2)and accurate correlation of partition constant on these solvent was achieved with it. Some detailed explanations of their retention behavior is also described herein.The enthalpy-entropy relationship (HSR) is fully discussed in this work. No universal HSR can be found even for retention process of hydrocarbons on C87. Nevertheless, this discussion of HSR is of significance in examination of solute-solvent interaction mode.The above studies can not only to help in choice of solvent for analysis of specified sample but can also give some valuable illustration of retention process. In addition, based on the above analysis of retention process a retention rule was deduced.2. Correlation of chromatographic characteristic parameters under programmed operation conditionsAn approach has been established to predict retention time in operating parameters programmed gas-liquid chromatography with capillary column by using numerical calculation, which can be applied to multi-ramp programs for column temperature or/and carrier gas pressure at column inlet. With this approach the trajectory of solute band movement can be obtained as retention time is precalculated. Some results showed that a better or, at least the same accuracy for prediction of retention time in temperature-programmed gas chromatography with our approach than with Curvers' which has been widely utilized can be gained. Especially, our approach is suitable for those operating conditions containing inlet pressure program. At the same time, resolution under programmed-temperature conditions can be predicted from solute band moving trajectory in column, column efficiency and other operating parameters.hi this dissertation, we also developed an integrative method for the correlation of retention time among isothermal and/or multi-ramp programmed-temperature CGLC, which allows to calculate retention parameters including retention index and its temperature coefficient from data at any two or more temperature programs. We can thus optimize separation conditions from data acquired from initial experiments under programmed operation conditions, which frequently is available for complex unknown samples. An empirical equation is proposed to correlate isothermal data accurately in a wide range of temperature as Ink'= A* + B*JTX , with which retention times of different solutes tested on OV-101, SE-54 and PEG 20M were much more accurately correlated to temperature when A = 1.7 .Progressively, a numerical method for the calculation of retention index under various temperature conditions including single-ramping and multi-ramping programs and comparison of it with linear and cubic spline interpolation ones are described herein, hi addition, some equivalent temperatures for retention time and for retention index were also defined with numerical method based on retention parameters of n-alkanes. By comparison of various equivalent temperatures defined by some authors, it is shown that harmonic and arithmetic mean values of initial and retention temperatures are always smaller than that for retention index (TeqiRI). These equivalent temperatures but Te iRJ can not be almost applied to correlate isothermal and temperature-programmed retention indices accurately, especially for later-eluting and high temperature coefficient components. This study also showed that it was impossible accurately to calculate equivalent temperature for unknown component from retention time only acquired under one temperature condition, but the reliably approximate estimation of it can still be achieved.