GC, GC×GC, and TOFMS for Characterization and Roast Level Differentiation of Coffee

Importance of the Topic

Coffee represents one of the most widely consumed beverages worldwide, and its complex aroma profile reflects bean variety, origin, processing, roasting, and brewing conditions. Understanding these volatile and semi-volatile components is essential for quality control, process optimization, flavor profiling, and guiding consumer preferences. Non-targeted analytical approaches such as gas chromatography coupled with time-of-flight mass spectrometry (GC-TOFMS) and comprehensive two-dimensional GC (GC×GC-TOFMS) combined with headspace solid phase microextraction (HS-SPME) address the challenge of characterizing hundreds of aroma compounds without prior selection.

Objectives and Study Overview

This study aimed to differentiate coffee brews prepared from six bean types—medium-roasted beans from Peru, Costa Rica, Colombia, and Kona, plus dark roasts from Costa Rica and Kona—by profiling their volatile constituents. Using a non-targeted workflow, analytes were extracted by HS-SPME, separated by GC or GC×GC, and detected by Pegasus BT 4D TOFMS. The goal was to compare roast level effects independently of geographical origin and highlight the advantages of GC×GC in resolving complex mixtures.

Methodology and Instrumentation

Sample Preparation:

• Grind whole beans and brew by French press (15 g beans per 118 mL boiling water, 4 min steep).
• Transfer 4 mL of brewed coffee to 20 mL vial; incubate at 60 °C for 5 min.
• Extract headspace using DVB/CAR/PDMS fiber for 5 min at 60 °C.

GC-TOFMS and GC×GC-TOFMS Conditions:

• Instrument: LECO Pegasus BT 4D with LECO GC×GC quad jet modulator and L-PAL 3 autosampler.
• Columns: Rxi-5Sil MS (30 m × 0.25 mm × 0.25 µm) primary; Rxi-17Sil MS (0.3 m × 0.25 mm × 0.25 µm) secondary.
• Carrier gas: He at 1.4 mL/min; injection split 5:1 at 250 °C.
• Temperature program: 40 °C (3 min) to 250 °C at 10 °C/min, hold 5 min; secondary oven +10 °C, modulator +15 °C, 1.2 s modulation.
• Mass spectrometer: 33–510 m/z, acquisition 10 spectra/s (GC), 100 spectra/s (GC×GC).

Main Results and Discussion

GC×GC-TOFMS markedly increased peak capacity, signal-to-noise (S/N) and produced structured chromatograms:

• Coelution resolved: A single GC peak split into 2 compounds by GC×GC (e.g., 2,3-dimethylpyridine and 5-methyl-2(5H)-furanone), improving library match scores from poor to >800.
• S/N enhancement: Cryogenic focusing at the modulator revealed 5-hydroxymethylfurfural in addition to 3-phenylfuran, boosting detectability.
• Structured bands: Compound classes (alkanes, furans, pyridines, etc.) formed visual bands in the 2D chromatogram, aiding rapid classification.

Roast-related differences emerged across origins:

• Medium-roasted samples exhibited higher levels of caramel- and nut-associated ketones (e.g., 2,3-pentanedione) and certain alkanes in the Peru sample.
• Dark roasts showed elevated pyridines with smoky, roasted notes (e.g., 3-ethylpyridine) especially in Kona and Costa Rica dark beans.

Benefits and Practical Applications

The non-targeted GC×GC-TOFMS workflow enables comprehensive profiling of coffee volatiles, supporting:

• Quality control and authentication by distinguishing roast levels independently of origin.
• Process optimization through monitoring flavor-active analytes.
• Product development and consumer guidance via detailed aroma characterization.

Future Trends and Opportunities

Advances may include:

• Integration with chemometric models for robust classification of origin and roast.
• Expansion of spectral libraries and retention index databases for more confident identifications.
• Automation and high-throughput screening in industrial settings.
• Coupling with complementary detectors (e.g., olfactometry) to correlate chemical and sensory profiles.

Conclusion

The Pegasus BT 4D GC×GC-TOFMS platform combined with HS-SPME offers a powerful non-targeted approach to unravel complex coffee aroma profiles. Enhanced separation, sensitivity, and structured chromatograms reveal hundreds of analytes and enable clear differentiation of roast levels across bean origins.

References

• LECO Corporation. Application Note: GC, GC×GC, and TOFMS for Characterization and Roast Level Differentiation of Coffee. Form No. 203-821-556, 2019.