Simple acids are usually applied to suppress the ionization of weakly ionizable acidic analytes in reversed-phase liquid chromatography. The purpose of this study is to investigate the retention behavior of various weak acidic compounds (monoprotic, diprotic, triprotic, and tetraprotic acids) using acetic or perchloric acid as ion suppressor in a binary hydroorganic mobile phase. The apparent n-octanol-water partition coefficient (K(ow)") was proposed to calibrate the n-octanol-water partition coefficient (K(ow)) of weak acidic compound. LogK(ow)" was found to have a better linear correlation with logk(w), the logarithm of the retention factor obtained by extrapolating to neat aqueous fraction of the mobile phase, for all weakly ionizable acidic compounds. This straightforward relationship offers a potential medium for direct measurement of K(ow) data of weak acidic analytes and can be used to predict retention behavior of these compounds in the ion suppression reversed-phase liquid chromatographic mode.
A high performance liquid chromatography−photodiode array detection−electrospray ionization mass spectrometry (HPLC-PDA-ESIMS) method has been developed for separation and identification of relative substances in hydroquinone bis(2-hydroxyethyl) ether (HQEE), a kind of polyurethane chain extender synthesized by hydroquinone (HQ) and ethylene oxide (EO). HQEE and all relative substances were baseline separated on a reverse-phase C18 column with gradient elution of methanol and water. Of them, six relative substances were identified by ESIMS coupled with HPLC in positive ionization mode. The UV spectrum feature from online PDA and chromatographic retention behavior were adopted as evidence to conclude the rest relative substances. The results showed that EO combines with HQ in molar ratio of 1:1−5:1, and the tetra- (4:1) and penta-substitution (5:1) products are pairs of isomers, respectively. Therefore, the chromatographic fingerprint of industrial HQEE has been drawn on the basis of qualitative analysis of these products of different molar ratios.
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