Low-Level Analysis of Sulfur Compounds in Beer by Purge and Trap

Significance of the Topic

Beer flavor and aroma are profoundly influenced by trace levels of volatile sulfur compounds. Even concentrations as low as 0.1 ng/mL can generate off-flavors and serve as indicators of inconsistencies in the brewing process. Reliable low-level monitoring of these species is therefore vital for quality control in modern breweries.

Objectives and Overview of the Study

This study aimed to develop and validate a fully automated purge-and-trap gas chromatography method with sulfur chemiluminescence detection (GC-SCD) for quantifying key sulfur compounds in beer at or below their sensory thresholds. A comparison between two capillary columns (DB-1 and GS-GasPro) evaluated chromatographic performance for routine brewery analysis.

Applied Methodology and Instrumentation

Sample preparation and instrumental workflow were automated to minimize manual handling:

• Beer samples (20 mL) were dispensed into amber vials, treated with defoamer and EDTA.
• An internal standard (isopropyl sulfide) was added automatically via the AQUATek 70 autosampler.
• Volatile sulfur species were purged for 7 minutes at ambient temperature, trapped on Tenax in the 3100 Sample Concentrator, then thermally desorbed at 225 °C.
• Analytes were separated on an Agilent 6890 GC connected to an Agilent Model 355 SCD.
• Two columns were tested: DB-1 (30 m × 0.53 mm, 5 µm) and GS-GasPro (60 m × 0.32 mm, 0.25 µm), each with optimized oven programs.

Results and Discussion

The method achieved quantification limits from 0.05 to 15 ng/mL, covering flavor and odor thresholds for compounds such as ethanethiol, dimethyl sulfide, and dimethyl trisulfide. Key findings included:

• Precision: %RSD at quantification levels ranged from 2% to 10% (n = 7).
• Repeatability on GS-GasPro: %RSD below 11% for primary analytes (n = 9).
• Accuracy: Spike recoveries of 4 ng/mL ranged from 75% to 89%.
• Column comparison: DB-1 provided superior peak resolution, while GS-GasPro offered sharper peaks and lower baseline bleed.

Benefits and Practical Applications

This automated P&T-GC-SCD approach delivers robust detection of sulfur compounds at or below sensory thresholds, ensuring consistent beer quality. The inert Silcosteel sample pathway minimizes analyte loss, reduces maintenance, and extends component lifetime, making the system well suited for high-throughput brewery QC laboratories.

Applied Instrumentation

• Tekmar AQUATek 70 autosampler
• Tekmar 3100 purge and trap Sample Concentrator
• Agilent 6890 Gas Chromatograph with Model 355 Sulfur Chemiluminescence Detector
• Columns: Agilent DB-1 (30 m × 0.53 mm, 5 µm) and GS-GasPro (60 m × 0.32 mm)

Future Trends and Opportunities

Emerging directions include integrating P&T-GC with high-resolution mass spectrometry for detailed sulfur speciation, deploying inline sensors for real-time process monitoring, and developing miniaturized detector modules for on-site brewery analysis. Advances in data analytics and chemometric tools may further enhance sensitivity and throughput.

Conclusion

The fully automated purge-and-trap GC-SCD method provides reliable, sensitive, and precise quantification of critical sulfur compounds in beer. Its high automation level supports routine QC workflows, enabling brewers to maintain flavor consistency and detect process deviations promptly.