Properties of the Adsorption Equilibrium Isotherms Used and Measured in RPLC F. Gritti 1 , A. Felinger 2 , G. Guiochon 1,& 1 Department of Chemistry, The University of Tennessee, Knoxville, TN, 37996-1600, USA and Division of Chemical Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, USA; E-Mail address: guiochon@utk.edu 2 Department of Analytical Chemistry, University of Veszpre ´m, Veszpre ´m, Egyetem utca 10. 8200 Hungary Received: 16 December 2003 / Accepted: 9 February 2004 Online publication: 9 July 2004 Abstract The variety of isotherm models needed to account for the high-concentration adsorption behavior of small molecular weight compounds is surprisingly rich. For the sake of convenience, isotherms are divided into groups depending first on the sign of their curvature at very low concentrations and then on the number of their inflection points. Conventional models are easily found in this arrangement. The methods of determination of isotherms are also divided into two groups, those that give adsorption data points and those that allow only the derivation of best estimates of the coefficients of a preselected model. Finally, examples are given of the wide diversity of isotherm behavior found in RPLC and of the surprisingly large influence of the mobile phase composition on the isotherm parameters. Keywords Column liquid chromatography Frontal analysis Adsorption equilibrium Langmuir isotherms Monolithic columns Introduction Considerable progress has been made in the recent years regarding the measure- ment, determination and modeling of equilibrium isotherms [1]. A relatively large amount of data is now available on the reproducibility of these measurements and on their variations from brand to brand of conventional RPLC phases. The importance of the bank of data already acquired is still limited but comparisons between the results obtained for a num- ber of compounds on several phases, with different solvents suffice to elucidate general trends for a certain variety of compounds. This topic has a great importance in preparative liquid chromatography be- cause the initial slope of the isotherm determines the retention time of a com- pound and the saturation capacity con- trols the production rate that can be achieved. The saturation capacity is the scale used to measure the degree of overloading corresponding to a given sample size. Obviously, the solubility of the compound in the mobile phase is also of critical importance to assess the max- imum production rate that can be ex- pected. Thus, the determination of the equilibrium isotherms of the important components of a feed is critical to per- form a computer assisted optimization of a separation by preparative HPLC. This is essential in the applications of simu- lated moving bed separation because the time required for the achievement of steady state operation with this process is very long (much longer than in over- loaded elution) and the interactions be- tween the different design and operation parameters may be most complex under nonlinear conditions. Thus, the conven- tional trial-and-error approach is long and costly. The purpose of this report is to review the recent results obtained in the acqui- sition of equilibrium isotherm data, in the investigation of isotherm behavior, and in the modeling of equilibrium isotherm data in systems of interest in preparative chromatography. The investigation of phase equilibria in reversed phase liquid chromatogra- phy is limited to a particular situation of liquid–solid systems. The liquid phase is most frequently an aqueous solution of an organic solvent, the modifier. The most frequent modifiers used are methanol and acetonitrile. Both mix with water in all proportions. Both exhibit strong interactions with DOI: 10.1365/s10337-004-0273-1 Presented at: 5 th Balaton Symposium on High- Performance Separation Methods, Sio´fok, Hungary, September 3–5, 2003 2004, 60, S3–S12 Ó 2004 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH Review Chromatographia Supplement Vol. 60, 2004 S3