4. Types of Sorbents

4-1. Sorbent Materials

Early SPE used only a few sorbents (e.g., bonded silica gel, alumina). As the technique become more widely used, sorbents
with enhanced separation characteristics were developed, resulting in a wide range of products for diverse applications:

• Polymer: styrene–divinylbenzene (SDVB), divinylbenzene (DVB), methacrylate types
• Silica gel: chemically bonded silica gel; unmodified silica gel
• Magnesium silicate (Florisil)
• Alumina: acidic, neutral, basic
• Graphitized carbon
• Activated carbon
• Cellulose
• Polyamide

4-2. Characteristics of Silica-Based Sorbents

Typical properties:

4-3. Silica-Based Sorbents by Separation Mode

Sorbents can be classified by separation mode into reversed-phase sorbents (non-polar interactions), ion-exchange sorbents, and normal-phase sorbents (polar interactions). In addition to silica-based sorbents, normal-phase sorbents such as magnesium silicate (Florisil) and alumina (acidic/neutral/basic) are also commercially available.

4-4. Characteristics of Polymeric Sorbents

In the 1990s, C18 was often used to preconcentrate pesticides such as asulam and oxine-copper in water. However, variability between manufacturers and lots—and unstable recoveries (e.g., for oxytetracycline)—were noted with C18. To address these issues, SPE cartridges packed with styrene–divinylbenzene (SDVB) copolymers came into wide use and are now broadly adopted in official methods. High-purity polymer sorbents are largely free from the trace-metal effects seen with silica-based phases.

4-5. Polymer Sorbents by Separation Mode

Initially, commercial polymer sorbents were mostly SDVB. With advances in synthesis and the spread of LC/MS(/MS), new reversed-phase polymers incorporating functional groups with affinity for highly polar compounds were introduced—e.g., N-containing methacrylates and N-vinylpyrrolidone. Functional polymeric sorbents were also developed, including
methacrylate-based ion-exchange phases with very weak hydrophobicity and mixed-mode ion-exchange phases with strong hydrophobic interactions.

4-6. Commercial SPE Product Forms

Common commercial formats include standard cartridges, disks, and 96-well SPE plates compatible with high-throughput analysis (standard 96-well footprint).