Stir Bar Sorptive Extraction: Recovery of Organic Acids and Amines

Importance of the Topic

The extraction of volatile and semivolatile compounds from aqueous matrices is fundamental in environmental, food, and pharmaceutical analysis. Stir Bar Sorptive Extraction (SBSE) offers a solvent-minimized, sensitive, and quantitative approach. However, ionizable compounds such as organic acids and amines present unique challenges due to their pH-dependent partitioning.

Study Objectives and Overview

This study investigates SBSE recovery of representative organic acids and amines under varying pH conditions. It aims to:

• Assess extraction efficiency of fatty acids and long-chain amines using PDMS-coated stir bars.
• Correlate recovery with analyte dissociation constants and octanol–water partition coefficients (Kow).
• Develop pH-based strategies to enhance extraction of ionizable species.

Methodology and Instrumentation

Samples containing acids or amines (100–500 µg/L) were prepared in unbuffered water or in 25 mM buffer solutions at pH 2, 3, 5, 7, and adjusted to pH 10 with KOH. Twister stir bars performed extractions at room temperature for four hours. After extraction, bars were rinsed and thermally desorbed.

Used Instrumentation

• GC 6890 (Agilent Technologies) with programmed temperature vaporization inlet (CIS4, Gerstel).
• Thermal Desorption System TDS 2/TDSA (Gerstel) with autosampler.
• Detectors: FID for acids on DB-FFAP column; MSD (5973N) for amines on HP-5 column.
• Helium carrier gas, splitless or split (20:1) mode.

Main Results and Discussion

• Direct Thermal Desorption: Both fatty acids and their methyl esters showed comparable transfer to the GC, confirming suitability for SBSE analysis.
• Extraction Kinetics: Equilibrium for C14 and C18 acids required extended extraction (4 h) compared to typical 60–90 min for smaller analytes.
• Acid pH Dependence: Maximum recovery at low pH when acids are protonated. Octanoic acid showed negligible recovery at pH 5; C14 acid was fully recovered; C18 acid exhibited reduced recovery, likely due to aggregation or adsorption.
• Amine pH Dependence: Highest recoveries of long-chain amines occurred at high pH (neutral form). Butylamine was not recovered due to high polarity. Trioctylamine recovery decreased at extremes, attributed to surface adsorption.

Benefits and Practical Applications

SBSE with PDMS stir bars enables trace-level analysis of neutral and ionizable compounds with minimal solvent. Precise pH control simplifies sample preparation in environmental monitoring, beverage quality control, and process analytics. The approach avoids laborious liquid–liquid extraction, enhancing throughput and sensitivity.

Future Trends and Use Opportunities

• Extending SBSE protocols to a wider range of ionizable analytes and complex matrices.
• Surface passivation of sampling vessels to minimize adsorption losses.
• Integration with high-resolution mass spectrometry for structural elucidation.
• Development of mixed-phase coatings to broaden polarity coverage.

Conclusion

Effective SBSE of organic acids and amines requires controlling their ionization state, typically by buffer selection, to maintain analytes in their neutral form. Very hydrophobic compounds may require matrix modifications or surface treatments to prevent loss. SBSE thus provides a robust, sensitive technique for diverse analytical applications.

References

1. Baltussen E. et al. Stir Bar Sorptive Extraction (SBSE), a Novel Extraction Technique for Aqueous Samples: Theory and Principles. J. Microcolumn Separations. 11(10):737–747 (1999).
2. KowWin software, EPI Suite™, USEPA.
3. Ochiai N. et al. Determination of Trace Amounts of Off-Flavor Compounds in Drinking Water by SBSE and Thermal Desorption GCMS. The Analyst. 126:1652 (2001).