. 2022 Oct 5;38(12):1457–1487. doi: 10.1007/s44211-022-00190-8

Table 2.

Various retention modes and typical steps that carried out during the SPE process

Sorbent phase type		Bed sorbent	Structure of bonded phases	Bonded function	Target analyte	Solvents
Sorbent phase type		Bed sorbent	Structure of bonded phases	Bonded function	Target analyte	Conditioning	Rinse	Elution
Normal-phase		Amino (-NH₂)	-(CH₂)NH₂	Polar interactions Hydrogen bonding Dipolar attraction	Exhibits polar functionalities (hydroxyl, carbonyls, amines, double bonds, compounds with heteroatoms)	Nonpolar solvent (hexane, chloroform)	Nonpolar solvent	Strong polar eluent (such as methanol)
		Diol (C–OH C–OH)	Spaced-bonded propanediol	Polar interactions Hydrogen bonding Dipolar attraction
		Cyano (-CN)	-(CH₂)₃CN	Polar interactions Adsorption
		Silica gel	-SiOH
		Florisil	Mg₂SiO₃
		Alumina	Al₂O₃
Reversed-phase		Triacontyl	-C₃₀H₆₁	Adsorption/hydrophobic interaction van der Waals forces	Slightly non-polar Moderately non-polar Nonpolar	MeOH/H₂O ACN/H₂O with appropriate pH	Polar solvent With appropriate pH	Strong non-polar (hexane, chloroform) Polar to semi-polar (methanol)
		Octadecyl (C₁₈)	-(CH₂)₁₇CH₃
		Octyl (C₈)	-(CH₂)₇CH₃
		Ethyl (C₂)	-CH₂CH₃
		Cyclohexyl	CH₂-Cyclohexyl
		Phenyl	CH₂-Phenyl
Ion exchange	SCX	Sulfonic	Propanesulfonic acid	Electrostatic attraction (cation exchange-ionic base)	Positive charge analytes	Water or buffer pH > 7 (pH = pKa − 2)	pH = pKa-2	Eluent with ionic strength (pH = pKa + 2)
	SCX	Sulfonic	Benzenesulfonic acid
	WCX	Carboxylic acid	Carboxylpropyl
	SAX	Quaternary ammonium	Trimethylaminopropyl	Electrostatic attraction (anion exchange-ionic acid)	Negative charge analytes	Water or buffer pH < 7 pH = pKa + 2	pH = pKa + 2	Eluent with high ionic strength (pH = pKa-2)
	WAX	Primary, secondary, or tertiary amines		Electrostatic attraction (anion exchange-ionic acid)	Negative charge analytes	Water or buffer pH < 7 pH = pKa + 2	pH = pKa + 2	Eluent with high ionic strength (pH = pKa-2)