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  • Title: [Effects of buffer salt types and non-counter ions of ion-pair reagents on the retention behavior of strongly ionized acid compounds in ion-pair reversed-phase liquid chromatography].
    Author: Liu X, Gao W, Liang C, Qiao J, Wang K, Lian H.
    Journal: Se Pu; 2021 Sep; 39(9):1021-1029. PubMed ID: 34486842.
    Abstract:
    Ion-pair reversed-phase liquid chromatography (IP-RPLC) enhances separation by adding ion-pair reagents to the mobile phase, thereby improving the retention of oppositely charged solutes. IP-RPLC is primarily used for the separation and analysis of strongly ionized compounds. In IP-RPLC, researchers often focus more on the influence of the counter-ion type and concentration, buffer salt concentration and pH, and column temperature, on the retention behavior of solutes. However, the effects of the buffer salt type and non-counter ions in ion-pair reagents on the retention behavior of solutes have rarely been investigated. Accordingly, in this work, the effects of buffer salt types and non-counter ions on the retention behavior of strongly ionized compounds were investigated by IP-RPLC using 14 sulfonic acid compounds as model compounds. Experiments were performed using a silica-based C18 column with methanol as the organic modifier. In the first type of experiment, tetrabutylammonium bromide was kept unchanged as the ion-pair reagent in the mobile phase, and ammonium dihydrogen phosphate, ammonium chloride, and ammonium acetate were used as buffer salts, respectively. The retention factor (k) was obtained at different methanol ratios, and linear solvent strength (LSS) models were established to determine the log kw (logarithm of retention factors of solutes when 100% aqueous phases were used as the mobile phase) and S (intercept of the LSS model) values of each solute. All solutes exhibited the highest log kw with the ammonium chloride buffer system; most compounds also exhibited the highest S values with this system, except for 1,5-naphthalenedisulfonic acid, 4-methylbenzenesulfonic acid, 5-amino-2-nanphthalenesulfonic acid, and 4-hydroxybenzenesulfonic acid. However, the chromatographic hydrophobic indices (CHIs, log kw/S) of the solutes with different buffer salts were approximately equal. In the second type of experiment, ammonium dihydrogen phosphate was kept unchanged as the buffer salt in the mobile phase, and tetrabutylammonium bromide, tetrabutylammonium dihydrogen phosphate, tetrabutylammonium hydrogen sulfate, tetrabutylammonium nitrate, and tetrabutylammonium acetate were used as ion-pair reagents, respectively. Almost all solutes exhibited the highest S with the tetrabutylammonium acetate system, indicating that weakly ionized anions (such as acetate ions) in ion-pair reagents will improve the S values of sulfonic acid compounds. Interestingly, the CHIs of the solutes were almost the same for solutes with different non-counter ions. These results suggest that both, the buffer salt types and non-counter ions, influence the log kw and S values of sulfonic acid compounds. Comparison of the retention behavior of solutes with different mobile phases suggested ion-pair mechanisms as well as dynamic ion-exchange mechanisms plays role in the IP-RPLC retention of sulfonic acid compounds. In addition, with all the experimental mobile phases, the apparent n-octanol/water partition coefficient (log D) presented a good linear correlation with log kw, S, and CHI, respectively, by the introduction of structure-related descriptors such as charge (ne), Abraham solvation parameters (A and B), and the polar surface area (PSA). Considering the differences in the log kw and S values obtained with different buffer salts and non-counter ions, the CHIs were relatively stable; therefore, the CHI is more suitable for establishing a quantitative structure-retention relationship (QSRR) model in IP-RPLC, compared to log kw and S. 在离子对反相液相色谱(IP-RPLC)分析中,溶质保留受对离子(counter ion)的影响比较受人关注,但鲜有研究流动相中缓冲盐类型和离子对试剂中非对离子(non-counter ion)对溶质保留行为的影响。鉴于此,该文以14种磺酸化合物为研究对象,甲醇为有机调节剂,分别考察了3种缓冲盐体系(磷酸二氢铵、氯化铵和乙酸铵)和5种离子对试剂体系(四丁基溴化铵、四丁基磷酸二氢铵、四丁基硫酸氢铵、四丁基硝酸铵和四丁基乙酸铵)下强离解酸性化合物的IP-RPLC保留行为,通过比较不同流动相条件下得到的溶质log kw(100%水相作流动相时的保留因子)、S(线型溶剂强度模型线性回归得到的常数),以及CHI(色谱疏水指数,log kw/S),寻找保留行为规律。研究表明,流动相中的缓冲盐类型和离子对试剂非对离子均会影响化合物的log kw和S值,所有化合物在氯化铵缓冲盐体系下具有最大的log kw值。相对于无机阴离子,离子对试剂中弱离解性有机阴离子(乙酸根)的存在有利于增加磺酸化合物的S值。通过对比不同条件下的保留行为,推测磺酸化合物的IP-RPLC保留机理中同时存在着离子对模型和动态离子交换模型。与log kw和S值不同,化合物的CHI值受缓冲盐类型以及离子对试剂非对离子的影响较弱。此外,研究发现化合物的表观正辛醇/水分配系数(log D)与log kw、S、CHI之间均具有良好的线性相关性。不同缓冲溶液和不同离子对试剂非对离子条件下获得的log kw和S值存在着一定的差异,而CHI值相对稳定,因此,CHI更适用于IP-RPLC中定量结构-保留行为关系模型的建立。
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