3.2.4. Elution Solvent

The high recovery of analytes bonded with sorbent requires the selection of appropriate desorption solvent. Different factors should be taken into consideration, e.g., affinity of solvent to sorbent and analyte, its volatility and solubility of target compound.

Organic solvents with relatively low boiling point such as methanol, acetonitrile, ethanol and acetone are the most commonly used because they easily evaporate and therefore, the concentration of sample solution is possible without the risk of degradation the analyte. Sometimes, acids or buffers are added to change the ionization of investigated components or adsorbent and hereby, to decrease their interaction. As our study showed, the pure solvents had weak elution strength toward triterpenic acids bonded on Si-PANI. However, the additional low amount of ammonia, hydrochloric acid and ammonium acetate significantly increased the ability to elute of investigated triterpenes, probably as a result of ion exchange of bonded analyte on anion from mobile phase. The highest elution strength had 0.1 M methanolic solutions of investigated modifiers (Figure 7). The differences of elution from Si-PANI and Si-PANI (+) could be explain by stronger π-π interaction on deprotonated form. The higher % of recovery was obtained for protonated form of adsorbent (above 97%, 77%, and 80% for ammonia, HCl and ammonium acetate solution, respectively) comparing to deprotonated (ca. 89%, 55%, and 67% for ammonia, HCl and ammonium acetate solution, respectively).

Therefore, Si-PANI (+) was chosen for further experiments.

**Figure 7.** Effect of various solutions on the elution of retained analytes.
