Improving the Sensitivity of Detection of 3-chloropropane-1,2-diol (3-MCPD) using Gas Chromatography with Negative Chemical Ionisation Mass Spectrometry detection

Importance of the topic

3-Chloropropane-1,2-diol (3-MCPD) is a carcinogenic contaminant formed during acid hydrolysis of vegetable proteins used in products such as soy sauce and hydrolysed vegetable protein (HVP). Regulatory agencies have set limits (e.g., 0.02 mg/kg in the EU) to protect consumer health. Sensitive and specific analytical methods are essential to ensure compliance and minimize dietary exposure.

Objectives and Study Overview

This study evaluated the improvement in detection sensitivity of 3-MCPD by comparing gas chromatography–mass spectrometry with negative chemical ionisation (NCI-MS) against conventional electron impact ionisation (EI-MS). It aimed to develop a robust workflow including sample extraction, derivatisation, and quantitation using internal and external calibration.

Used Instrumentation

• Shimadzu GCMS-QP5050A gas chromatograph–mass spectrometer
• Shimadzu AOC-20i/s autosampler
• DB-5 capillary column (30 m × 0.25 mm i.d. × 0.25 μm)
• Helium carrier gas; isobutane reagent gas for NCI
• GCMSsolution data acquisition and processing software

Methodology

Sample Preparation:

• Soy sauce samples (8 g) spiked with isotopically labeled 3-MCPD-d5 internal standard, salted to 20 g, sonicated, and loaded onto Extrelut™ 20 columns.
• Non-polar matrix eluted with hexane–diethyl ether (90:1), 3-MCPD extracted with diethyl ether, dried and concentrated to 10 mL.

Derivatisation:

• Aliquots of extract or calibration standards reacted with N-heptafluorobutyrylimidazole (HFBI) at 70 °C for 20 min, quenched with water, vortexed, separated, dried over Na₂SO₄, and injected (1 μL).

Chromatography and Mass Spectrometry:

• Oven program: 50 °C (1 min) to 90 °C at 2 °C/min, then to 270 °C at 40 °C/min.
• Injector 270 °C; interface 270 °C (EI) or 200 °C (NCI).
• EI mode: full scan and selected ion monitoring (SIM) at m/z 253, 275, 289, 291, 453.
• NCI mode: isobutane reagent gas, full scan (m/z 154–514) and SIM at m/z 446, 482, 502 (d0) and 449, 486, 507 (d5).

Main Results and Discussion

EI-SIM linear calibration was achieved over 5–15 000 ng/mL (r² ≥ 0.9998); detection limit ~2 ng/mL. NCI-scan provided qualitative spectra with characteristic high-mass ions (m/z 502, 482, 446, 213) and detection limits of 0.25–1 ng/mL depending on the ion. NCI-SIM achieved superior sensitivity with a linear range of 2–10 ng/mL (r² ≥ 0.99999) and a detection limit of ~0.1 ng/mL using m/z 446 (d0) and 507 (d5). Spiked soy sauce samples (10 μg/kg) yielded recoveries between 135 % and 150 %, confirming method applicability.

Benefits and Practical Application

• NCI-MS significantly lowers the detection limit for 3-MCPD compared to EI-MS, enhancing regulatory compliance testing.
• Derivatisation with HFBI and use of an isotopic internal standard ensure accurate quantitation in complex food matrices.
• Method is compatible with routine quality control laboratories for monitoring 3-MCPD in food products.

Future Trends and Potential Applications

• Integration of tandem mass spectrometry (GC-MS/MS) to further improve specificity and reduce matrix interferences.
• Development of rapid on-line derivatisation systems to increase sample throughput.
• Extension of the NCI-MS approach to other alkyl chlorides and halogenated food contaminants.

Conclusion

NCI-MS with HFBI derivatisation provides a rapid, robust, and highly sensitive method for quantitative detection of 3-MCPD in soy sauce and similar products. The approach outperforms traditional EI-MS in detection limit while maintaining linearity and reproducibility, meeting stringent regulatory requirements.

References

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