The Introduction of PDMS-Overcoated Adsorbent-Based Fiber Coatings

Importance of the Topic

A reliable solid-phase microextraction (SPME) fiber must resist fouling by non-volatile matrix components while maintaining efficient uptake of target analytes. The application of a thin polydimethylsiloxane (PDMS) overcoat on commercial PDMS-divinylbenzene (PDMS-DVB) fibers addresses these challenges by reducing surface adhesion of sample residues, extending fiber lifetime, and preserving analytical performance in complex matrices such as fruit juices.

Objectives and Study Overview

This study evaluated the impact of a proprietary 10 µm PDMS overcoat on standard PDMS-DVB fibers. Key goals included:

• Assessing analyte recovery for a panel of semi-volatile compounds extracted from grape juice and water.
• Determining optimal overcoat thickness to balance barrier properties with analyte uptake.
• Comparing fiber durability and response stability over repeated extractions.

Methodology and Used Instrumentation

Samples and Analytes

• 7 mL unadjusted grape juice or deionized water spiked with semi-volatile standards (nitroaromatics, pesticides, pharmaceuticals; see Table 1 summary).

Fiber Preparation and Extraction

• Standard PDMS-DVB vs. PDMS-DVB with 10 µm PDMS overcoat.
• Direct immersion at 35 °C, 300 rpm agitation for 40 min.
• Needle depth 11 mm, injection penetration 38 mm.
• Rinse: 30 s in water (additional 2 min in 20% methanol for Waterloo protocol).

Desorption and GC-MS Conditions

• Thermal desorption at 270 °C for 2 min into a split/splitless inlet (0.75 min splitless, then 30:1 split).
• Column: 30 m × 0.25 mm SLB-5ms, 0.25 µm film; helium at 1 mL/min.
• Oven program: 60 °C (1.5 min) → 180 °C @10 °C/min → 320 °C @20 °C/min (4.5 min).
• Detection: ion trap MS scanning m/z 70–340 at 0.2 s/scan.

Main Results and Discussion

Overcoat Thickness and Polarity

• 10 µm PDMS provided a sufficient barrier to matrix adhesion while minimizing recovery loss of polar analytes compared to 30 µm.

Rinse Step Impact

• A brief 30 s water rinse caused minimal signal reduction but slowed matrix buildup; prolonged rinsing led to greater analyte loss.

Salt Addition

• 25% salt in water enhanced polar analyte extraction, but in grape juice it reduced recovery of some compounds, likely due to matrix interactions.

Fiber Longevity

• Standard PDMS-DVB fibers lost ~50% response after 20 grape juice extractions.
• PDMS-overcoated fibers exhibited only ~20% response loss over the same cycle and retained 60% of initial response past 50 extractions.

Benefits and Practical Applications

• Fiber life extended by 75–100% in immersion mode, reducing replacement frequency.
• Sealed fiber ends prevent matrix wicking, improving quantification consistency.
• Slight polarity reduction is offset by increased capacity and durability.

Future Trends and Potential Applications

• Development of overcoats for alternative adsorbents (e.g., CAR/PDMS, DVB/CAR/PDMS).
• Integration of automated fiber cleaning or wiping mechanisms.
• Extension to highly complex matrices in environmental, food, and bioanalytical fields.

Conclusion

The incorporation of a thin PDMS overcoat on PDMS-DVB SPME fibers substantially enhances resistance to matrix fouling, prolongs fiber lifetime, and maintains analytical performance for semi-volatile extraction. This modification offers laboratories improved throughput and cost efficiency when analyzing challenging sample types.

Reference

Souza-Silva EA, Pawliszyn J. Analytical Chemistry, 2012, 84(15):6933–6938.