Shu-ying Han

Determination of n-octanol/water partition coefficients of weak ionizable solutes by RP-HPLC with neutral model compounds

The utilization of neutral compounds as model compounds is put forward for determination of the n-octanol/water partition coefficient (K(ow)) of highly hydrophobic, weak acidic compounds by reversed-phase high performance liquid chromatography (RP-HPLC). It is based on a linear relationship between the logarithm of apparent n-octanol/water partition coefficient (log K(ow)″), expressing hydrophobicity of acidic solutes more accurately, and the logarithm of RP-HPLC retention factor of the solutes corresponding to the neat aqueous fraction of mobile phase (log k(w)). The availability of neutral model compounds was theoretically tested for this novel protocol. Moreover, a high consistency of linear log K(ow)″-log k(w) correlations was demonstrated between a mixed training set of neutral and acidic model compounds, and a training set of neutral model compounds. It is proved in theory that for a certain set of compounds investigated, all derived linear relationships between log K(ow)″ and log k(w) have a unit slope and the same intercept, regardless of mobile phase pH. This model was applied to measure log K(ow) of lipophilic aristolochic acid I (AA I) and aristolochic acid II (AA II). Log K(ow) values for AA I and AA II are 4.45±0.07 and 3.99±0.06, respectively. To the best of our knowledge, this is the first report on experimental log K(ow) data for AAs. The proposed strategy solves the problem of lacking suitable acidic model compounds with reliable experimental K(ow) in determining K(ow) of lipophilic acidic solutes by RP-HPLC.

A novel evaluation method for extrapolated retention factor in determination of n-octanol/water partition coefficient of halogenated organic pollutants by reversed-phase high performance liquid chromatography.

The retention factor corresponding to pure water in reversed-phase high performance liquid chromatography (RP-HPLC), k(w), was commonly obtained by extrapolation of retention factor (k) in a mixture of organic modifier and water as mobile phase in tedious experiments. In this paper, a relationship between logk(w) and logk for directly determining k(w) has been proposed for the first time. With a satisfactory validation, the approach was confirmed to enable easy and accurate evaluation of k(w) for compounds in question with similar structure to model compounds. Eight PCB congeners with different degree of chlorination were selected as a training set for modeling the logk(w)-logk correlation on both silica-based C(8) and C(18) stationary phases to evaluate logk(w) of sample compounds including seven PCB, six PBB and eight PBDE congeners. These eight model PCBs were subsequently combined with seven structure-similar benzene derivatives possessing reliable experimental K(ow) values as a whole training set for logK(ow)-logk(w) regressions on the two stationary phases. Consequently, the evaluated logk(w) values of sample compounds were used to determine their logK(ow) by the derived logK(ow)-logk(w) models. The logK(ow) values obtained by these evaluated logk(w) were well comparable with those obtained by experimental-extrapolated logk(w), demonstrating that the proposed method for logk(w) evaluation in this present study could be an effective means in lipophilicity study of environmental contaminants with numerous congeners. As a result, logK(ow) data of many PCBs, PBBs and PBDEs could be offered. These contaminants are considered to widely exist in the environment, but there have been no reliable experimental K(ow) data available yet.

Influence of variation in mobile phase pH and solute pKa with the change of organic modifier fraction on QSRRs of hydrophobicity and RP-HPLC retention of weakly acidic compounds

The variation in mobile phase pH and ionizable solute dissociation constant (pK(a)) with the change of organic modifier fraction in hydroorganic mobile phase has seemingly been a troublesome problem in studies and applications of reversed phase high performance liquid chromatography (RP-HPLC). Most of the early studies regarding the RP-HPLC of acid-base compounds have to measure the actual pH of the mixed mobile phase rigorously, sometimes bringing difficulties in the practices of liquid chromatographic separation. In this paper, the effect of this variation on the apparent n-octanol/water partition coefficient (K(ow)″) and the related quantitative structure-retention relationship (QSRR) of logK(ow)″ vs. logk(w), the logarithm of retention factor of analytes in neat aqueous mobile phases, was investigated for weakly acidic compounds. This QSRR is commonly used as a classical method for K(ow) measurement by RP-HPLC. The theoretical and experimental derivation revealed that the variation in mobile phase pH and solute pK(a) will not affect the QSRRs of acidic compounds. This conclusion is proved to be suitable for various types of ion-suppressors, i.e., strong acid (perchloric acid), weak acid (acetic acid) and buffer salt (potassium dihydrogen phosphate/phosphoric acid, PBS). The QSRRs of logK(ow)″ vs. logk(w) were modeled by 11 substituted benzoic acids using different types of ion-suppressors in a binary methanol-water mobile phase to confirm our deduction. Although different types of ion-suppressor all can be used as mobile phase pH modifiers, the QSRR model obtained by using perchloric acid as the ion-suppressor was found to have the best result, and the slightly inferior QSRRs were obtained by using acetic acid or PBS as the ion-suppressor.

Influence of n-octanol in mobile phase on QSRRs of lipophilicity and retention mechanism of acidic and basic compounds in RP-HPLC

The influence of n-octanol additive agent on the retention behavior, the uniformity of the retention mechanism, as well as the quantitative structure–retention relationships (QSRRs) of weak acidic and basic compounds on reversed-phase high performance liquid chromatography (RP-HPLC) was systematically discussed in this paper, especially for the QSRRs of logarithm of apparent n-octanol/water partition coefficient (log K′′ow) and logarithm of retention factor extrapolated to neat aqueous mobile phase (log kw(o)), which have not been discussed in other studies to date. For this purpose, the aqueous fraction of mobile phase was saturated with n-octanol and 0.25% (v/v) n-octanol was added into organic modifier. Eleven substituted benzoic acids, as well as fifteen anilines or pyridines were selected to establish QSRR models by using different types of ion-suppressors. The results indicated that the roles of n-octanol were different in various systems. For acids compounds, if perchloric acid (strong acid) acts as an ion-pair agent, the silanophilic interaction between solutes and residual silanol groups of alkyl-silica stationary phase can be ignored, and n-octanol/water partition and chromatographic process are homo-energetic. In this case, n-octanol acts only as organic modifier. However, if acetic acid (weak acid) or phosphoric acid/potassium dihydrogen phosphate (buffer salt) were used as the ion-suppressor, n-octanol is not only an organic modifier, but also a masking agent of free silanols. For weak bases, if ammonium chloride–ammonia was employed as the ion-suppressor, the addition of n-octanol will make QSRRs correlation significantly worse. Therefore, for studying on QSRRs of lipophilicity and retention behavior of acidic and basic compounds, n-octanol is only recommended for acidic system, and strong monoprotic acids, e.g., perchloric acid, are recommended as the ion-suppressors.

RELATIONSHIP BETWEEN HYDROPHOBICITY AND RPLC RETENTION BEHAVIOR OF AMPHOTERIC COMPOUNDS

In this paper, we systematically investigate the regulation of retention behavior of amphoteric compounds, amino acids, and small peptides in the whole pH range in reversed-phase liquid chromatography (RPLC) using methanol-ammonium acetate buffer solution as a binary mobile phase. According to Snyder-Soczewinski equation, logarithm of hypothetical retention factor corresponding to neat aqueous fraction of hydroorganic mobile phase (logk w) is obtained by extrapolation from different compositions of the mobile phase. Then, we establish the relationship between octanol-water partition coefficient (P) and k w of these amphoteric compounds and discuss the dependence of retention behavior on mobile phase pH. The experimental results reveal that the linear correlation between logarithm of apparent octanol-water coefficient (logD) and logk w is better than that between logP and logk w, as well as that between logD and logk, the logarithm of retention factor at a specific organic modifier fraction for common amphoteric compounds. The linear relationships of logD–logk w approximate those of logP–logk w, and they are superior to those of logD–logk for amino acids and small peptides. In addition, we provide an insight into the fact that isoelectric point (pI), dissociation constants (p K a1 and p K a2 ) of amphoteric compounds affect their RPLC retention behavior significantly.

Determination of Polymerization Retarder, 4-Hydroxyanisole, in Triallyl Cyanurate for Industrial Use by HPLC

Abstract A reversed phase high performance liquid chromatographic method has been developed for the determination of 4-hydroxyanisole in industrial triallyl cyanurate. The separation was accomplished on a C18 column by linear gradient elution consisting of methanol, water, and 0.1% perchloric acid, at a flow rate of 1.0 mL/min. The detection was performed by UV absorption at wavelength of 254 nm and 288 nm simultaneously. The response behavior of sample determined was compared between two detection wavelengths. It is found that the use of 288 nm makes the chromatogram simple, so that isocratic elution can be employed instead of gradient elution. The method has been successfully applied for the quantitative analysis of 4-hydroxyanisole as the retarding agent in triallyl cyanurate for industrial purposes in the laboratories of some chemical plants.

Aerobic microbial degradation of aromatic sulfur-containing compounds and effect of chemical structures

Batch data of aerobic microbial degradation rate constants Kb of phenylthio, phenylsulfinyl and phenylsulfonyl acetates have been determined, and the qualitative relationships between their Kb and chemical structures were analyzed. The phenylthio acetates were most subject to microbial transformation, followed by the phenylsulfonyl acetates. The compounds with a methoxy group were easier degraded than those with a isopropoxy group. The nitro-group and chloro-group on benzene were shown to lower the biodegradability, while the nitro-group at the para-position had stronger side effect on degradation than at the ortho-position.