Determining Differences in Control vs Off-Odor Water ® Samples Using GCxGC-TOFMS and ChromaTOF Tile

Summary: GCxGC-TOFMS and ChromaTOF Tile for Differentiating Control vs Off-Odor Water Samples

Importance of the topic

Identifying chemical causes of off-odors in water is a common and practical quality-control problem across beverage, municipal water, and process-water applications. Non-targeted comparative analysis that reliably highlights which volatiles and semi-volatiles are elevated in a problematic sample accelerates root-cause investigations, supports corrective actions, and improves product quality. Two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GCxGC-TOFMS) combined with targeted comparison software (ChromaTOF Tile) provides enhanced separation, sensitivity, and rapid discovery of differentiating chemical features compared to single-dimension GC or purely visual inspection of chromatograms.

Objectives and overview of the study

The study aimed to demonstrate a practical workflow to detect analytes elevated in an off-odor water sample relative to a control water sample. The approach combined headspace solid-phase microextraction (HS-SPME) sampling, GCxGC-TOFMS analysis (LECO Pegasus BTX 4D), and ChromaTOF Tile fold-change comparisons to rapidly pinpoint retention windows and m/z values that differed between samples and to tentatively identify likely odor-causing compounds.

Methodology and instrumentation

Sample preparation and extraction

• Sample volume: 2.0 mL water per 20 mL SPME vial.
• Incubation: 2 min at 40 °C.
• Extraction: 5 min HS-SPME using a tri-phase fiber (PDMS / DVB / C-WR) at 40 °C.
• Desorption: 2 min in the GC inlet, split 10:1.

GCxGC and MS conditions (overview)

• Autosampler: LECO L-PAL 3.
• GCxGC system: LECO QuadJet thermal modulator, modulation period 2.5 s; secondary oven offset +10 °C.
• Columns: 1D Rxi-5ms (30 m × 0.25 mm i.d., 0.25 µm); 2D Rxi-17Sil ms (0.6 m × 0.25 mm i.d., 0.25 µm).
• Carrier gas: He at 1.4 mL/min; inlet temperature 250 °C.
• GC temperature program: 40 °C (2 min), 6 °C/min to 200 °C, then 24 °C/min to 250 °C.
• Transfer line: 250 °C.
• Mass spectrometer: LECO Pegasus BTX 4D, ion source 250 °C, mass range 35–500 m/z, acquisition rate 100 spectra/s.
• Retention indexing: alkane standard run in the same conditions for RI determination.

Data processing strategy

Rather than performing full peak finding on each sample, ChromaTOF Tile was used in fold-change mode to compare raw m/z-resolved chromatographic tiles across the two datasets. This highlights chromatographic windows (retention time regions) and specific m/z values with large relative intensity differences, focusing subsequent spectral matching and RI checks on the most relevant features.

Main results and discussion

General observations

• Contour plots of the two samples showed many similar features; obvious differences were limited on visual inspection.
• Siloxane peaks from sampling/analytical background were prominent but did not differ between samples and therefore were not responsible for the odor differences.

Features elevated in the off-odor sample

• ChromaTOF Tile quickly highlighted multiple chromatographic windows with significantly higher signal in the off-odor sample. Tentative identifications (library match + RI comparison) pointed primarily to aldehydes — examples include hexanal, heptanal, octanal, and nonanal — compounds commonly associated with grassy, fatty, or rancid notes.
• Table content summary: the detected differentiating features included tentative names, spectral match scores, RI agreement information, relative intensity heat-map trends (control vs off-odor), and aroma descriptors to contextualize potential odor contributions.

Representative detection examples

• Nonanal: Large increase in the off-odor sample; visible in both TIC and XIC (m/z 98). Easily noted visually in contour plots when abundant.
• Octanal: Lower signal-to-noise and partially obscured by siloxane tail in the TIC; detected through XIC (m/z 84) and resolved in the 2nd-dimension separation, illustrating the GCxGC advantage for separating analytes from matrix artefacts.
• Heptanal: Buried beneath a coeluting background peak in TIC; revealed by ChromaTOF Tile and confirmed by reviewing an XIC unique to heptanal.

Processing speed and sensitivity gains

Using ChromaTOF Tile fold-change comparison was faster and more directed than independent full peak finding for each dataset. The method uncovered lower S/N and partially coeluting features that would be difficult or time-consuming to locate by visual inspection alone.

Benefits and practical applications of the method

• Rapid differential screening: Efficiently prioritizes candidate compounds responsible for sensory defects in water and beverage QC.
• Improved chemical coverage: GCxGC increases peak capacity and helps separate analytes from interferences.
• Enhanced sensitivity for discovery: TOFMS with full-spectrum acquisition and tile-based m/z comparisons reveals low-abundance differentiators.
• Reduced processing time: Tile-based comparative workflows focus analyst effort on chemically meaningful regions rather than exhaustive peak lists.

Instrumentation used

• Autosampler: LECO L-PAL 3.
• GCxGC system and modulator: LECO QuadJet thermal modulator, Pegasus BTX 4D platform.
• Columns: Rxi-5ms (1D) and Rxi-17Sil ms (2D).
• Mass spectrometer: LECO Pegasus BTX 4D TOFMS (35–500 m/z, 100 spectra/s).
• Data processing software: ChromaTOF and ChromaTOF Tile (fold-change comparison mode).

Future trends and potential applications

• Automated suspect/suspect-similarity screening: Integration of fold-change outputs with expanded curated libraries to accelerate confident identifications of odorants.
• AI-assisted prioritization: Machine-learning models could rank candidates by odor potency, detection thresholds, and chemical class to guide confirmatory testing.
• Quantitation and targeted follow-up: After discovery, developing targeted GC/MS or GCxGC-MRM methods for quantitation of odorants will aid regulatory and corrective actions.
• Higher throughput & sensitivity: Advances in modulators, faster acquisition electronics, and improved SPME chemistries will further enhance detection of trace odorants.
• Broader applications: The same workflow can be applied to any pairwise non-targeted comparison (environmental, food authenticity, process control, contamination tracing).

Conclusion

The combined workflow of HS-SPME sampling, GCxGC-TOFMS analysis on the Pegasus BTX 4D, and ChromaTOF Tile fold-change comparison efficiently identified volatile and semi-volatile features elevated in an off-odor water sample. The approach outperformed purely visual comparisons by revealing low S/N and coeluting analytes (notably several aldehydes) likely contributing to the odor. Tile-based comparisons reduce data-processing time and focus confirmatory efforts on the most relevant chemical features, making this a robust approach for routine QC, troubleshooting, and broader non-targeted pairwise comparisons.

References

• LECO Corporation. Determining Differences in Control vs Off-Odor Water Samples Using GCxGC-TOFMS and ChromaTOF Tile. Application Note, Form No. 203-821-706, April 2025.