Lesson 8 - Sample introductino technique in GC II - Solid Phase Micro Extraction

ADVANCED INSTRUMENTAL ANALYSIS

Lesson 8: Solid Phase Micro-Extraction

Discovery of SPME

  • Inventor: Dr. Janusz Pawliszyn (1990) at University of Waterloo, Canada.

  • Purpose: Develop a fast, solvent-free, and field-compatible sample preparation method to improve efficiency in industrial applications.

Lesson Structure

  1. Introduction to SPME

  2. Operating Principles of SPME

  3. Factors Affecting Performance of SPME

  4. Applications & Advantages of SPME

  5. Summary

Objectives of the Lesson

  • Explain the operating principles of SPME.

  • List the advantages and disadvantages of SPME.

  • State the applications of SPME.

Introduction to SPME

  • Main Feature: Fiber coated with liquid polymer, solid sorbent, or both.

  • Components: 1cm, fused silica fiber bonded to stainless steel plunger, resembling a modified microlitre syringe.

Components of a Manual SPME Holder

  • Parts:

    • Plain Hub

    • O-ring

    • Adjustable needle guide/depth gauge

    • Plunger

    • Plunger retaining screw

    • Septum piercing needle

    • Exposed fiber in headspace/liquid sample

Selecting SPME Fiber

  • Factors:

    • Polarity of analyte: Must match fiber's coating.

    • Volatility and molecular size:

      • Thicker film (100μm) for volatile compounds.

      • Thinner film (7-30μm) for larger molecules.

      • Porous fibers can retain smaller analytes (C2 to C6).

Operating Principles of SPME

Steps in SPME Procedure

  1. Extraction/Sampling:

    • Fiber introduced to sample via direct or headspace SPME; allows time to reach equilibrium.

  2. Transfer:

    • Moves absorbed analytes to conditions suitable for desorption in chromatography.

  3. Desorption:

    • Fiber exposed to conditions to release solutes for analysis by chromatographic instruments.

Sampling Methods for SPME

  • Direct SPME: Fiber exposed to liquid sample, adsorbing only the component of interest.

  • Headspace SPME: Fiber positioned in the air gap above liquid; must ensure vaporization of analyte is achievable and sample is suitable.

How SPME Works

  • Process:

    1. Analytes adsorbed onto fiber coating.

    2. After equilibrium (2-30 min), the fiber is drawn back and injected into GC/LC for desorption.

    3. For GC, select a temperature high enough to release all adsorbed samples without damaging fiber.

    4. For HPLC, ensure the mobile phase desorbs compounds without damaging fiber coating.

Factors Affecting SPME

  1. Fiber Polarity and Surface Area:

    • Match fiber coating to analyte’s polarity. Increase area for smaller molecule extraction.

  2. Fiber Coating Thickness:

    • Thicker fibers hold more analytes; thinner fibers allow faster diffusion.

  3. Sample Agitation:

    • Enhances extraction and speeds up the process, especially for higher molecular weight analytes.

  4. Immersion vs. Headspace Sampling Effects:

    • High salt concentration and pH adjustments can enhance extraction efficiency.

Salt Concentration Effects

  • Salting out Effect: Increased ionic strength reduces solubility of organic compounds, enhancing their absorption onto the fiber.

pH Influence on Absorption

  • Absorption efficiency changes significantly with pH: higher at pH 2 compared to pH 7; hydrophobicity plays a role.

SPME Fibers Available

  • Coating Types:

    • PDMS, PDMS/DVB, Polyacrylate, CAR/PDMS, CW/DVB, CW/TPR, StableFlex DVB/CAR/PDMS.

  • Phase Stability Considerations: Various levels of stability with different solvent types.

Maintenance on SPME

  • Precautions: Avoid chlorinated solvents. Carefully handle fibers to not strip the coating.

  • Cleaning Protocols:

    • Bonded fibers can be thermally cleaned. Non-bonded fibers must adhere to specific thermal treatment protocols.

Applications & Advantages of SPME

  • Applications: Environmental pollutants, pharmaceutical impurities, pesticides, fragrances, chemical warfare agents.

  • Advantages:

    • Fast, labor-efficient, minimal sample volume needed, solvent-free, minimizes background interference, portable, excellent for trace analysis.

Disadvantages of SPME

  • Certain skill level required, limited lifespan (300 adsorption/desorption cycles), need for fiber conditioning pre/post analysis.

Summary

  • SPME Overview: A method for concentrating volatile or non-volatile compounds suitable for GC or HPLC; crucial to optimize desorption conditions based on the fiber and analyte specificities for consistent results.