Determining Phenolic Compounds in Whisky using Direct Large Volume Injection and Stir Bar Sorptive Extraction

Significance of the Topic

Phenolic compounds such as phenol, cresols, guaiacol, and ethyl derivatives are key contributors to the smoky and peaty flavor profile of Scotch whisky. Accurate measurement of these analytes supports quality control, flavor profiling and process optimization in distilleries. This study contrasts two gas chromatography–mass spectrometry (GC-MS) based approaches—direct large volume injection (LVI) and stir bar sorptive extraction coupled with thermal desorption (SBSE-TD)—to determine seven major phenolic compounds in whisky, highlighting sensitivity, reproducibility and practical suitability.

Aims and Study Overview

The primary objective was to develop and validate quantitative methods for seven phenolic markers in Scotch whisky using:

• LVI-GC-MS as a reference technique, enabling direct injection of up to 20 µL whisky samples with solvent venting.
• SBSE-TD-GC-MS employing novel ethylene glycol–modified silicone (EG-Silicone) Twisters for enhanced extraction efficiency.

The methods were benchmarked in terms of linearity, detection/quantification limits, precision, recovery, and comparability of results across two commercial single malt brands (40–46 % v/v ethanol).

Methodology and Instrumentation

Sample Preparation:

• Matrix-matched calibration standards prepared in ethanol/water (20–40 % v/v, pH 3 for SBSE) spiked with phenol, o-/m-/p-cresol, guaiacol, 4-ethylphenol and 4-ethylguaiacol.
• LVI: 20 µL injections into a PTV inlet with solvent venting at 20 °C, followed by temperature ramp to 320 °C.
• SBSE: Extraction of 5 mL diluted whisky on EG-Silicone Twisters (1 h stirring at 800 rpm), thermal desorption in TDU solvent-vent mode, cryofocusing in CIS, and GC transfer.

Instrumental Setup:

• Thermal Desorption Unit (TDU) and Cooled Injection System (CIS 4) with solvent vent accessory (Gerstel).
• Agilent 6890N GC coupled to 5795B inert XL MSD (EI Scan 30–350 amu).
• CP-FFAP narrow-bore column (25 m×0.15 mm×0.25 µm film).
• MAESTRO and Agilent ChemStation integrated control; IFD™ deconvolution software for reconstructed ion chromatograms.

Main Results and Discussion

Calibration and Linearity:

• LVI: Range 0.1–2.0 ng/µL, R² 0.991–0.999.
• SBSE: Range 8–100 ng/mL pre-dilution, R² 0.999–1.000.

Detection and Quantification Limits (pure whisky basis):

• LVI LODs 90–210 ng/mL; LOQs 240–530 ng/mL.
• SBSE LODs 1.2–6.9 ng/mL; LOQs 3.3–18.7 ng/mL.

Precision:

• LVI %RSD 1.6–6.2 (n = 3 injections per whisky brand).
• SBSE %RSD 0.8–5.4 (n = 3 extractions per brand).

Recovery (SBSE, EG-Silicone Twister): 12.2 % (guaiacol) to 56.8 % (4-ethylphenol), RSD 4.2–8.9 %.
Whisky Analysis:

• Concentration results by LVI and SBSE agreed within ±20 % for most compounds.
• Levels in both brands corresponded to medium-peated whisky (15–30 ppm total phenols).

Benefits and Practical Applications

• LVI offers minimal sample prep and fast throughput but lower sensitivity and requires frequent inlet maintenance or automated liner exchange to avoid contamination.
• SBSE achieves up to 100× better sensitivity, broadens the range of extractable compounds (non-polar and polar), and minimizes matrix contamination in the GC inlet. Twister reuse (50–100 runs) and parallel extraction on multi-position stirrers enhance cost-effectiveness.
• Deconvolution-based quantification improves selectivity for coeluting analytes masked by matrix.

Future Trends and Possibilities

• Further development of polar-modified Twisters and alternative sorbent phases for extended coverage of flavor-active compounds.
• Integration with high-resolution MS or GC×GC to resolve complex matrices and trace analytes.
• Automation of SBSE workflows and incorporation of inline cleanup to increase throughput in industrial QA/QC.
• Application to other spirit types and non-alcoholic beverages for comprehensive aroma profiling.

Conclusion

This comparative study demonstrates that SBSE-TD-GC-MS with EG-Silicone Twisters provides superior sensitivity and selectivity for phenolic compounds in whisky, while direct LVI-GC-MS offers simplicity and speed. Both approaches deliver robust quantification, supporting flavor quality assessment and routine analysis in research and production environments. The choice depends on sensitivity requirements, sample throughput, and maintenance considerations.

References

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