Quantitation of 3,3’-Dichloro-4,4’- Diaminodiphenylmethane (MOCA) in the Work Environment by GC/MS

Importance of the Topic

3,3’-Dichloro-4,4’-diaminodiphenylmethane (MOCA) is widely used as a hardener in urethane resin manufacturing. MOCA is regulated due to its toxicity, with a time-weighted average exposure limit of 0.005 mg/m3. Reliable workplace monitoring methods are essential to ensure regulatory compliance and safeguard worker health.

Objectives and Study Overview

This study presents a new analytical method developed by the Japan Industrial Safety and Health Association for quantifying MOCA in workplace air. The method uses solid adsorption sampling followed by gas chromatography–mass spectrometry (GC–MS) analysis with trifluoroacetyl (TFA) derivatization.

Instrumentation Used

• Gas chromatograph–mass spectrometer: GCMS-QP2020 NX
• Analytical column: SH-Rxi-1HT (15 m × 0.25 mm, 0.10 μm film thickness)
• Injector: splitless inlet at 280 °C with a single taper liner
• Carrier gas: helium, constant linear velocity (60.4 cm/s)
• Oven program: 100 °C (1 min) to 300 °C at 20 °C/min (3 min)
• MS modes: scan (m/z 40–700) and selected ion monitoring (SIM)

Methodology

• Standard Preparation:
  • MOCA stock solution (5000 μg/mL in methanol).
  • Internal standard (3,3’-dichlorobenzidine) diluted to 10 μg/L in toluene.
  • Calibration solutions (0.5–50 μg/L) prepared and derivatized with MBTFA.
• Sample Collection and Extraction:
  • Air samples collected on sulfuric acid-treated glass fiber filters.
  • Elution with water and NaOH, extraction into toluene containing IS.
  • Centrifugation and dehydration prior to derivatization.
• Derivatization:
  • Addition of MBTFA to toluene extract, reaction for 30 min.
• GC–MS Analysis:
  • Injection of 1 μL splitless.
  • Detection in SIM mode monitoring m/z 409, 444, 446 for IS and 423, 458, 460 for MOCA-TFA.

Main Results and Discussion

• Calibration linearity: correlation coefficient greater than 0.9997 over the 0.5–50 μg/L range.
• Limit of quantitation: 0.5 μg/L (1/50 of the regulatory limit).
• Repeatability: RSD of approximately 1.5% for five replicate analyses at 0.5 μg/L.
• Recovery: 95–110% for spike levels corresponding to 0.5 and 20 μg/L; RSD 0.12–2.5%.
• No target compound detected in method blanks, confirming method specificity.
• Batch analysis efficiency: sample extraction overlapped with calibration run to maximize throughput.

Benefits and Practical Applications

This validated method offers high sensitivity, precision, and accuracy for MOCA monitoring at or below regulatory limits. Its streamlined workflow supports routine environmental and occupational safety laboratories, enabling rapid and reliable compliance testing.

Future Trends and Opportunities

• Development of portable GC–MS systems for on-site real-time monitoring.
• Automation of sample preparation to further increase throughput and reduce manual handling.
• Expansion to multi-analyte panels to assess co-exposure to related aromatic amines.
• Investigation of alternative derivatization reagents to improve reaction efficiency and reduce solvent use.
• Integration of advanced data processing and AI-driven peak identification.

Conclusion

The GC–MS method with TFA derivatization provides a reliable, efficient, and regulatory-compliant approach for monitoring MOCA in workplace air. Its excellent linearity, precision, and recovery rates make it suitable for routine occupational health assessments.

Reference

• Keita I., Osamu N., Akito T., Yuichiro K., Shinobu Y., Hiroyuki M., Ginji E. Development of a new method for working environment measurement of 3,3’-dichloro-4,4’-diaminodiphenylmethane (MOCA), Shimadzu News No. M302, First Edition, Sep. 2020.