Eluent anion, choice

The choice of eluent system depends on the polymer type. For most non-ionic hydrophilic polymers, water can be used. However much more complex eluent systems are needed, for anionic and cationic polymers where interactions with the column based on ion exclusion, inclusion and exchange, adsorption by hydrogen bonding or hydrophobic interactions and intramolecular electrostatic effects, are possible. This can often make method development in aqueous SEC extremely difficult and time-consuming. 

Table 3-7 presents an overview of the various eluents being compatible with suppressor systems and their elution power. With the eluents listed in Table 3-7 and with the aid of organic additives described above, nearly all anions that are detected via conductivity may be analyzed using one of the many available anion exchangers. This multitude of stationary phases, with their different selectivities, enables the vast number of potential eluents to be reduced to a few versatile systems. Therefore, the statement that suppressor systems limit the choice of eluents is not valid. 

The other consideration in the choice of separator column is the operating pH range of the eluent used for chromatography. Generally, the pH of cation eluent will be between 2 and 5 and the pH of the anion eluent will be between 8 and 12. The proper separator column material must then be able to handle such extremes... 

The preferred eluents for anions are dilute carbonate-bicarbonate mixture, sodium hydroxide and, for common alkali metals and simple amines, dilute mineral acids (HCl, HNO3, BaCl2, AgN03, amino acids, alkyl and aryl sulfonic acids). The most common choice is HCl, but in the case of divalent ions, an eluent of much higher affinity for the ion-exchange resin, such as AgN03, must be used. 

Since sample ions are both taken up and eluted in pairs with water as the eluent, the chromatographic peak for a sample cation (C ) wiU actually be a cation-anion pair (C A. The retention time for such a peak will be a function of how tightly A, as well as C, is retained by the phosphocholine. The retention time for C can be either increased or decreased by the choice of the accompanying anion,... 

On the other hand, the amine component of sulfonamides makes cation exchange possible. Because all sulfonamides possess an aromatic ring system, reversed-phase interactions also contribute to the retention mechanism. Due to the huge selectivity differences between the various separator columns, the choice of stationary phase very much depends on the sample matrix. As an example. Figure 6.72 illustrates the separation of sulfonamides on OmruPac PCX-500 with a combined sodium acetate/acetonitrile eluent imder gradient conditions. Perchlorate anions added to the mobile phase form stable ion pairs, so that those interactions also contribute to the retention mechanism in this case. 

---