Retention prediction and hydrophobicity estimation of weak acidic compounds by reversed-phase liquid chromatography using acetic and perchloric acids as ion suppressors

Insight into the hydrophilic interaction liquid chromatographic retention behaviors of hydrophilic compounds on different stationary phases

In this work, the correlations between retention behavior and lipophilicity of a large set of hydrophilic neutral and ionic analytes were studied based on three hydrophilic interaction liquid chromatography (HILIC) stationary phases, including zwitterionic, crosslinked diol and triazole stationary phases. It was found that HILIC, due to the diversity of retention mechanism, is a more complex chromatography separation mode than reversed-phase liquid chromatography (RPLC) which has been widely accepted for lipophilicity assessment. Because electrostatic interactions contributed to the overall retention of the charged solutes on all three stationary phases, ion-strength of the mobile phase kept the same during the whole experiment. After the correlations between retention factor log k and water volume fraction Φ were investigated, the mixed retention model was revealed to be more suitable for HILIC retention behavior than other single models including partitioning and adsorption model. Moreover, in order to bridge the relationship between HILIC log k and lipophilicity parameter log D, net charge n_e and Abraham solvation parameter were introduced in the quantitative structure-retention relationship (QSRR) model. Although the correlation coefficients between log D and log k were still moderate, the significant improvement in correlation has made HILIC a potential choice as the complement of RPLC for log D measurement.

Monolithic alkylsilane column: A promising separation medium for oligonucleotides by ion-pair reversed-phase liquid chromatography

In this paper, a monolithic octadecylsilane column and particle-packed octadecylsilane columns were used to investigate the retention behaviors of oligonucleotides by ion-pair reversed-phase liquid chromatography (IP-RPLC). Results showed that, with same base composition, hairpin oligonucleotides always had weaker retention than corresponding random coil oligonucleotides on the monolithic column, but not on the particle-packed columns. In addition, the linear correlation between the retention factor k of oligonucleotides and the reciprocal of temperature (1/T), especially for hairpins, was relatively weaker on the particle-packed columns, as compared to the correlation on the monolithic column. The correlation between k and 1/T became weaker with decreasing particle size of the particle-packed columns. Moreover, results revealed that the overall retention order on the particle-packed column with small particles (3 μm) differed greatly from that on the monolithic column. In contrast, the retention order on the 10 μm particle-packed column was very close to that on the monolithic column. From the above, we inferred that oligonucleotides could keep their primary conformations unchanged when passing through the monolithic column, attributed to the special pore structures of the monolith. However, the conformations of oligonucleotides were suppressed or even destroyed when oligonucleotides passed through the particle-packed columns. This because the narrow and tortuous channels created by the stacked stationary phase particles could lead to more complex and unequable retention behaviors. Therefore, the monolithic column exhibited better retention regularity for oligonucleotides of secondary structure especially for hairpins than the particle-packed columns. It is noteworthy that the monolith-based IP-RPLC opens an intriguing prospect in accurately elucidating the retention behaviors of oligonucleotides.

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

Effect ofn-octanol on retention behavior, mechanism and QSRRs of ionizable compounds in RP-HPLC.

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.