磷酸根离子在阴离子交换树脂上的保留行为及其机理探讨

首次发现磷酸根离子在阴离子交换柱上以两个色谱峰流出。在研究磷酸根离子的保留行为的基础上，提出了Ｈ２ＰＯ－４在固定相中进一步离解的保留机理，即Ｈ２ＰＯ－４在与阴离子交换树脂交换基进行离子交换的过程中，由于树脂交换基和淋洗离子的电荷相互作用促使一部分Ｈ２ＰＯ－４进行第２级离解。由于Ｈ２ＰＯ－４和ＨＰＯ２－４在阴离子交换树脂上的保留值不同，导致磷酸根离子出现“双峰”。

Retention Mechanism of Phosphate Anion on an Anion Exchange Column

The retention mechanism of phosphate anion on an anion exchange column was studied. As a common consideratoin, an analyte ion was retained on ion exchanger as the form existed in the mobile phase. However, two peaks for phosphate anion were observed. The measured charges corresponding to the two peaks were 1.06 and 1.87 respectively. Obviously, the first peak is the monovalent H 2PO - 4 and the second peak is the divalent HPO 2- 4. By using EDTA eluent with pH 6.0, the phosphate anion can be retained on the same column as its divalent anion HPO 2- 4 but not H 2PO - 4, because the measured charge for the peak is 1.86. At pH 6.0, the dissociation of H 2PO - 4 is very weak, existing in mobile phase as monovalent anion H 2PO - 4 mainly. This means that for the weak inorganic acids with multi stepwise dissociation constant, the retention valence on ion exchanger depends not only on the existing forms in the mobile phase, but also on the change of exsiting of analyte on the stationary phase. We consider that in the ion exchange process of H 2PO - 4 with ion exchange group on the stationary phase, the secondary dissociation of H 2PO - 4 was made up due to the electrostatic interaction with ion exchange group on stationary phase and eluent ion. At the system using PyA as eluent, the further dissociation of H 2PO - 4 produced on the statonary phase resulting from the electrostatic interaction between H 2PO - 4 and anion exchange sites, and that between H 2PO - 4 and eluent ion. Because the capacity factors of H 2PO - 4 and HPO 2- 4 are different, two separated peaks were observed though only H 2PO - 4 was injected into the mobile phase.