Lesson 13 - Solid Phase Extraction I

ADVANCED INSTRUMENTAL ANALYSIS (LS5013FP)

Lesson 13: Solid Phase Extraction I

1. Introduction to SPE

  • Solid Phase Extraction (SPE): Chromatographic technique to prepare samples by removing interfering substances.

  • Involves two phases: solid phase and liquid phase.

  • Allows isolation of specific analytes from sample solutions.

  • Extraction can be achieved by:

    • Retaining the analyte of interest and washing off impurities.

    • Retaining impurities and eluting the target analyte.

2. Types of SPE

  • SPE is categorized based on the type of sorbent surface:

    • Reversed Phase (RP)

    • Normal Phase (NP)

    • Ion Exchange (IE)

      • Cation Exchange SPE

      • Anion Exchange SPE

3. Key Operating Principles

Objectives

  • After completing this lesson, students should be able to:

    1. Explain the operating principles of SPE.

    2. List the available types of SPE.

    3. Describe the role of pH in SPE.

4. Mechanisms of Retention

Retention Mechanisms in SPE

  • Most common mechanisms:

    • Van der Waals Forces (non-polar interactions)

    • Hydrogen Bonding

    • Dipole-Dipole Forces (polar interactions)

    • Cation-Anion Interactions (ion-ion interactions)

5. Reversed Phase SPE (RP)

  • Characteristics:

    • Utilizes non-polar modified solid phase and a polar liquid phase.

    • Target analytes are typically mid- to non-polar.

  • Retention Mechanism:

    • Hydrophobic Interactions involving non-polar-non-polar interactions and van der Waals forces.

    • Use non-polar solvent for elution to disrupt binding forces.

6. Normal Phase SPE (NP)

  • Characteristics:

    • Involves polar stationary phase and mid-polar to non-polar liquid phase.

    • Target analytes are typically mid- to polar.

  • Retention Mechanism:

    • Involves polar functional group interactions (dipole-dipole, hydrogen bonding).

7. Ion Exchange SPE (IE)

  • Suitable for charged compounds in solution.

  • Retention is based on electrostatic attraction between charged functional groups of the compound and bonded groups on the sorbent surface.

  • Elution Process:

    • Requires pH adjustments to ensure both the analyte and sorbent groups are charged.

    • Neutralizing one of the charges disrupts binding, allowing elution.

    • Alternatively, high ionic strength solutions can be used for elution.

8. Role of pH in SPE

  • SPE solutions have a broad pH range:

    • Silica-Based Packing: Operates optimally in the pH range of 2 – 7.5.

    • Out of this range:

      • Risks hydrolysis and cleavage of the bonded phase.

      • Potential dissolution of silica material.

9. Separation Techniques

  • Three methods for separating target compounds from impurities:

    1. Selective Extraction: Choose SPE sorbent to bind target compounds or impurities.

    2. Selective Washing: Retain both target and impurities; apply washing solutions strong enough to remove impurities without affecting target compounds.

    3. Selective Elution: Elute adsorbed target compounds in solvent while retaining impurities.

10. Visual Schemes for Separation Techniques

Scheme 1: Selective Extraction

  • Retention of target components as the sample passes through SPE tube.

  • Option to elute or discard impurities after retention.

Scheme 2: Selective Washing

  • Both target compounds and impurities are retained on SPE packing.

  • Use a strong wash solution to remove impurities, leaving target compounds behind.

Scheme 3: Selective Elution

  • Compounds of interest are eluted in a suitable solvent, leaving impurities behind.