SPME workshop focusing on the analysis of Coffee

Significance of the topic

The analysis of coffee aroma presents a significant challenge due to the complex mixture of volatile and semi-volatile compounds, now exceeding 1000 identified components. Solid phase micro-extraction (SPME) coupled with gas chromatography–mass spectrometry (GC-MS) offers a solvent-free, sensitive and reproducible approach to enrich and analyze these analytes, making it a crucial tool for quality control, flavor profiling and research in food and beverage industries.

Objectives and study overview

This application note reports on a series of workshops aimed at introducing both fundamental and advanced aspects of SPME, with a focus on ground coffee analysis. The study evaluated the impact of sample hydration, salt addition and pH adjustment on volatile extraction, compared dry and wet matrices, and assessed reproducibility under workshop conditions.

Methodology

One gram of ground coffee was placed in each 20 mL SPME vial and prepared under the following conditions:

• Dry coffee (no water)
• Hydrated with 5 mL water
• Hydrated with water plus 2 g NaCl
• Hydrated plus NaCl and 100 µL phosphoric acid (acidic)
• Hydrated plus NaCl and 100 µL NaOH (basic)

Each condition was run in duplicate alongside blank vials. Headspace SPME extractions were performed at 60 °C for 10 minutes with agitation using a DVB/CAR/PDMS fibre. Desorption into the GC injector occurred at 270 °C for 5 minutes.

GC-MS parameters:

• Injector temperature: 270 °C, split ratio 5:1
• Column: DB-Wax, 30 m × 0.25 mm × 0.5 µm
• Carrier gas flow: 1 mL/min
• Oven program: 35 °C (4 min), ramp at 10 °C/min to 220 °C, hold 7 min

Instrumentation

• GERSTEL Multipurpose Sampler MPS 2 XL with Maestro 1.4.8.14/3.5
• Gerstel Cooled Injection System (CIS 4)
• Agilent 7890A GC
• Agilent 5975C Inert XL MSD
• SPME fibres (DVB/CAR/PDMS) from Sigma-Aldrich

Main results and discussion

Comparison of dry and hydrated samples showed that water liberated certain volatiles (e.g., furan-3-methyl) while reducing headspace release of polar compounds such as acetol and nonanoic acid. Salt addition enhanced the extraction of less soluble analytes by salting-out, improving headspace yield and method reproducibility. pH adjustments influenced specific compounds: acidic conditions increased acetic acid response, whereas basic conditions favored 4,6-dimethyl pyrimidine. Duplicate runs demonstrated good chromatographic consistency across all sample types. Workshop time constraints limited extraction to 10 minutes; literature suggests that extending this to 30 minutes may achieve full equilibration and greater sensitivity.

Benefits and practical applications

SPME-GC/MS with DVB/CAR/PDMS allows rapid, solvent-free profiling of coffee volatiles with minimal sample preparation. The method is adaptable to varied sample matrices and provides reproducible results suitable for flavor research, quality control in coffee production and analytical method development workshops.

Future trends and potential applications

• Longer extraction times and optimized agitation for full equilibrium and enhanced sensitivity
• Automation and high-throughput analysis using robotic samplers
• Coupling with high-resolution mass spectrometry or two-dimensional GC for comprehensive aroma profiling
• Extension of methodology to other food and beverage matrices for comparative aroma studies

Conclusion

This study demonstrates that headspace SPME with a DVB/CAR/PDMS fibre is an effective and reproducible technique for coffee volatile analysis. Adjustments in hydration, salting-out and pH significantly affect analyte profiles, offering a versatile toolkit for both research and industrial quality control applications.

Reference

Carrier D., Roberts P., Buckendahl K. Chromatography Technical Note No AS120, Anatune Ltd., 2012