Chaksawat Sangawitayakorn ^a, Prapin Wilairat ^b,∗, Rattikan Chantiwas ^a,∗
a Department of Chemistry and Center of Excellence for Innovation in Chemistry and Flow
Innovation-Research for Science and Technology Laboratories (FIRST Labs), Faculty of Science,
Mahidol University, Rama VI Rd., Bangkok 10400, Thailand
^b Analytical Sciences and National Doping Test Institute, Mahidol University, Rama VI Rd.,
Bangkok 10400, Thailand
*Corresponding author: prapin.wil@mahidol.ac.th (P. Wilairat), rattikan.cha@mahidol.ac.th,
rattikan.cha@mahidol.edu (R. Chantiwas).

This work proposes an experimental method for the estimation of the phase ratio of reversed-phase C8 columns by employing the equation log(??) = ?? log(??????) + log(Φ), where k is the retention factor, Kom is the octane-mobile phase partition coefficient, a is a proportionality constant and is the phase ratio (defined as volume ratio of the stationary phase to the mobile phase). The immiscible liquid octane and mobile phase are chosen as the surrogate model for the C8 stationary phase and mobile phase of the chromatographic system. The octane-mobile phase is used for measuring the partition coefficient Kom of six compounds of the homologous series of linear alkylbenzenes, viz. benzene, toluene, ethylbenzene, propylbenzene, butylbenzene and pentylbenzene. The distribution of a compound between the octane and mobile phase is proposed to simulate the partitioning process in the chromatography. The retention factor k of each compound is measured using the same mobile phase for two C8 columns (Zorbax Eclipse XDBC8 and Symmetry C8). The set of data of k and Kom is fitted to the above linear equation to give the best-fit values of ?? and log(Φ) for each column and various mobile phase compositions (methanol-water or acetonitrile-water). The regression analyses have coefficients of determination r 2 > 0.992. This observed linear relationship can therefore be expressed as k = ?????? ?? Φ. The experimental values of for Φ the C8 columns are in the range of 0.206 to 0.842, with a from 0.544 to 0.811, respectively