*3.2.2 Solid phase extraction (SPE)*

To overcome the drawback of LLE method, solid phase extraction (SPE) has been developed and applied in sample preparation since the 1970s. In this method, extract is loaded onto a sorbent phase which will retain alkaloids. Then, interferences in extract are washed away and the analytes is eluted by suitable solvents [64]. In fact, cartridge is the most popular SPE type due to its convenience. The SPE procedure have five step including conditioning, loading, washing and elution step. Several factors can affect to the extraction efficiency such as concentration of analytes in solvent, loading solvent nature, sorbent types, particle size, volume used for loading – washing – eluting, flow rate and elution solvent. Each factor has specified role which depends on the affinity of analytes and solid phase. Because the chemical structures of alkaloids always have secondary or tertiary amine groups, the strong cation exchange (SCX) sorbents are an ideal choice for sample preparation. When using these sorbents, washing solvent will be water and organic solvent to eliminate both hydrophilic and hydrophobic from plant matrices. After that, alkaloids will be deprotonated for elution by alkalized solvent which has pH at 2 units above pKa of analytes and evaporated to enrich sample [64].

If the analytes are unstable in strong alkaline solutions, the weak cation exchange (WCX) will be use instead. The WCX sorbent has carboxylic acid as functional group which has pKa value about 4.8, so these sorbents should be conditioned by solutions having pH above 6.8 for sorbent ionization. In addition, the loading and washing solvent pH should be adjusted at the value above 6.8 and below 2 value of analytes's pKa to maintain the ionized state of both sorbent and analytes. Finally, the alkaloids will be eluted by the acidic solutions [63]. Besides, the C18 and C8 sorbents are also applied to extract aromatic alkaloids and eliminate hydrophilic interferences from matrices. In some case, those sorbents could be used for eliminating hydrophobic interferences by loading unretained alkaloids through cartridges [64].
