A comparison of Direct Immersion and Headspace SPME Sampling of Whiskey Samples

Importance of the Topic

Solid Phase Microextraction (SPME) is a solvent-free, non-exhaustive sampling technique widely used for profiling volatile and semi-volatile compounds in complex food and beverage matrices such as whiskey. Accurate flavor characterization is crucial for quality control, product development, and authentication in distilleries. Comparing headspace and direct immersion SPME techniques helps laboratories select the optimal approach for target analytes, balancing sensitivity, fiber life, and throughput.

Objectives and Study Overview

This application note explores and compares two SPME sampling modes—headspace and direct immersion—using whiskey as a model matrix. The primary goals are:

• Evaluate extraction efficiency for volatile and heavier flavor components.
• Optimize SPME conditions on an automated autosampler platform.
• Assess the trade-offs between analyte coverage and fiber longevity.

Instrumentation Used

An automated EST Analytical FLEX Series autosampler was configured with a 50/30 µm DVB/CAR/PDMS SPME fiber installed on an Agilent 7890A gas chromatograph coupled to a 5975 inert XL mass spectrometer. A Restek Stabilwax DA column (30 m × 0.25 mm × 0.25 µm) facilitated chromatographic separation. Key GC/MS parameters included:

• Injection mode: pulsed splitless, inlet at 220 °C.
• Oven program: 45 °C hold 2 min, 20 °C/min to 100 °C, then 5 °C/min to 240 °C, 10 min final hold.
• Carrier gas: helium at 1.0 mL/min, total flow 14 mL/min.
• MS scan range: m/z 50–300 at ~5.5 scans/sec.

Methodology

Sample preparation differed by mode:

• Headspace SPME: 5 mL whiskey plus 1 g NaCl in a 20 mL vial. Incubation at 60 °C for 20 min with medium stirring, fiber exposed at 50% depth for 20 min.
• Direct Immersion SPME: 10 mL whiskey in a vial. Incubation at 60 °C for 20 min without stirring, fiber immersed to full depth for 20 min.

After extraction, fibers were desorbed at 250 °C in the GC inlet for 2 min. The FLEX software managed method sequences for both techniques.

Main Results and Discussion

Chromatographic profiling revealed distinct extraction patterns:

• Headspace SPME selectively captured lighter volatiles (e.g., small esters, aldehydes) present in the gas phase above the liquid.
• Direct Immersion SPME recovered heavier, less volatile compounds (e.g., long-chain esters, alcohols, lactones) directly from the liquid matrix.

Overlay of total ion chromatograms demonstrated that immersion mode delivers broader analyte coverage, whereas headspace mode offers reproducible extraction of low-boiling components. However, repeated immersion cycles accelerated fiber aging, while headspace sampling extended fiber lifetime.

Contributions and Practical Applications

This comparison provides clear guidance for analytical laboratories and distilleries:

• Choose headspace SPME for routine monitoring of key aroma volatiles with high sample throughput and minimal fiber wear.
• Use direct immersion SPME when profiling medium- to high-boiling flavor markers or trace non-volatiles critical to product differentiation.
• Leverage automation to improve reproducibility and reduce hands-on time in QA/QC and R&D workflows.

Future Trends and Potential Applications

Advancements likely to enhance SPME analysis include:

• Development of novel fiber coatings with tailored selectivity for specific compound classes.
• Integration with high-resolution accurate mass spectrometry for improved compound identification.
• On-line coupling of SPME with other separation techniques (e.g., two-dimensional GC) for deeper flavor profiling.
• Application expansion into other beverages, food matrices, and environmental monitoring.

Conclusion

The EST Analytical FLEX Series autosampler effectively automates both headspace and direct immersion SPME, delivering reliable profiling of whiskey flavor compounds. Direct immersion maximizes extraction of heavier analytes, while headspace extends fiber life and maintains high throughput for volatile targets. The choice of sampling mode should align with the desired analyte range and operational demands.