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Solid-phase microextraction and on-line methylation gas chromatography for aliphatic carboxylic acids
Yaqian Liu1, Seong R Cho, Neil D Danielson
1Department of Chemistry and Biochemistry, Miami University, Oxford, OH 45056, USA.
Analytical and Bioanalytical Chemistry
|May 16, 2002
Summary
Solid-phase microextraction (SPME) coupled with phenyltrimethylammonium hydroxide (PTMAH) offers an efficient on-fiber methylation technique for carboxylic acids. This method simplifies analysis and improves detection limits for fatty acids.
Area of Science:
- Analytical Chemistry
- Chromatography
- Organic Chemistry
Background:
- Direct methylation of carboxylic acids using phenyltrimethylammonium hydroxide (PTMAH) is a useful derivatization technique.
- Sample matrix effects can complicate direct injection methods, necessitating sample cleanup or alternative approaches.
Purpose of the Study:
- To develop and optimize an on-line methylation method for carboxylic acids using solid-phase microextraction (SPME).
- To evaluate the effectiveness of different SPME fibers for simultaneous extraction and derivatization.
- To compare SPME-based methylation with traditional syringe injection methods.
Main Methods:
- Solid-phase microextraction (SPME) with carboxene/PDMS fibers was used for simultaneous extraction of carboxylic acids (RCOOH) and PTMAH.
- Optimization of SPME parameters including fiber type, absorption time, desorption temperature, and reagent ratio.
- Gas chromatography-mass spectrometry (GC-MS) was employed for analysis of methylated fatty acids.
Main Results:
- The carboxene/PDMS fiber demonstrated superior extraction efficiency for the RCOOH-PTMAH mixture compared to PDMS and PA fibers.
- Optimal conditions included a 20-min absorption time, 5-10 min desorption at 280°C, and a PTMAH/RCOOH ratio of 125:1.
- Linearity was established for C18:0 (stearic acid) from 3.3 x 10⁻⁶ to 3.3 x 10⁻⁴ M with a detection limit of 1 µM.
- The SPME method proved effective for various fatty acids (C18:1, C18:2, C18:3) and yielded comparable results to direct injection for olive oil transesterification.
Conclusions:
- SPME-based on-line methylation provides a convenient and effective alternative to direct injection for carboxylic acid analysis.
- This technique minimizes instrumental issues related to sample matrices and enhances analytical efficiency.
- The optimized SPME method is suitable for the determination of fatty acids in complex samples like oils.