Elimination of Polar Matrix Components Prior to GC Analysis using Stir Bar Sorptive Extraction (SBSE)

Importance of the Topic

Gas chromatographic analysis of complex samples containing polar matrix components often requires extensive cleanup to prevent poor peak shapes, column degradation and detector interference. Polar compounds such as water, acetic acid, ethanol, glycols, surfactants and emulsifiers can co‐elute with target analytes and compromise sensitivity. Stir Bar Sorptive Extraction (SBSE) using a nonpolar PDMS phase offers a simplified approach for selective enrichment of nonpolar analytes while excluding polar interferences.

Objectives and Study Overview

The main goal of this application study was to demonstrate elimination of polar matrix components prior to GC–MS analysis in various sample types. Key aims included comparison of SBSE to conventional liquid–liquid extraction, evaluation of matrix removal in acidic, alcoholic, surfactant‐rich and glycol‐based samples, and illustration of improved chromatographic performance and analyte detection.

Methodology and Sample Preparation

Two preparation methods were compared:

• Conventional liquid–liquid extraction (LLE) using ethyl acetate or methylene chloride, often requiring dilution and extended emulsion‐breaking steps.
• SBSE: samples (diluted 1:10 in water unless noted) were stirred with a PDMS‐coated Gerstel Twister™ bar for 1 hour at room temperature, rinsed, dried and directly thermally desorbed into the GC inlet.

Used Instrumentation

• Gas chromatograph: Agilent 6890 with He carrier gas, 30 m × 0.25 mm × 0.25 µm HP-5 column.
• Detector: Agilent 5973 mass selective detector.
• Thermal desorption: Gerstel TDS-2 with TDSA autosampler and CIS4 PTV inlet.
• Injection conditions: PTV inlet desorption at 250 °C, split ratio 30:1, SBSE desorption splitless from 30 °C to 250 °C.

Main Results and Discussion

Extraction performance was evaluated in four sample categories:

• Acidic matrices (balsamic vinegar): SBSE eliminated acetic acid (pKow = 0.09) interference, yielding clear identification of esters (isobutyl acetate, isoamyl acetate, phenethyl acetate) that were obscured in ethyl acetate extracts.
• Ethanolic matrices (whiskey): Direct injection of neat spirit produced a large ethanol peak and baseline elevation. SBSE removed most ethanol (pKow = –0.14) and water, revealing C6–C12 esters and higher alcohols with improved sensitivity.
• Surfactant/emulsifier‐rich samples (dish detergent, hand soap): Solvent extractions suffered long emulsion‐breaking times. SBSE discriminated against nonionic surfactants and charged detergents, enabling direct extraction of fragrance compounds (limonene, linalool, ethyl vanillin) and trace antibacterial Triclosan without emulsion formation.
• Glycol‐based matrices (automotive antifreeze): LLE recovered primarily ethylene and diethylene glycols (pKow = –1.2, –1.47) with no trace additives. SBSE revealed low‐level additives and degradation products (benzothiazole, menthol, siloxanes) in new and used antifreeze.

Benefits and Practical Applications

SBSE with PDMS bars offers multiple advantages:

• Elimination of water and volatile acids/alcohols prior to GC, improving peak shape and protecting columns and detectors.
• Minimal solvent consumption and simplified workflow without lengthy emulsion‐breaking steps.
• Predictable recoveries based on octanol–water partition coefficients (Kow), enabling method optimization for target analytes.

Future Trends and Potential Applications

Further developments may include integration of SBSE into automated high-throughput platforms, expanded application to environmental, food and biomedical matrices, and use of software tools to predict extraction efficiencies. pH adjustment strategies can extend SBSE to weakly charged species, broadening its analytical scope.

Conclusion

Stir Bar Sorptive Extraction using PDMS‐coated bars provides a robust, selective and solvent-saving approach for removal of polar matrix components prior to GC–MS analysis. It outperforms traditional liquid–liquid extraction in diverse sample types by improving detection of nonpolar analytes and simplifying sample preparation.

References

1. Baltussen E., Sandra P., David F. and Cramers C.: Journal of Microcolumn Separations 11 (1999) 737.
2. Dugay J., Miege C. and Hennion M.-C.: Journal of Chromatography A 795 (1998) 27.
3. De Bruin L.S., Josephy P.D. and Pawliszyn J.B.: Analytical Chemistry 70 (1998) 1986.
4. Beltran J., Lopez F.J., Cepria O. and Hernandez F.: Journal of Chromatography A 808 (1998) 257.