Differentiation of DOTP and DNOP in analysis of restricted phthalate esters using thermal desorption GC/MS

Importance of the Topic

Restricted phthalate esters are widely regulated in consumer products due to potential health and environmental concerns. Reliable analytical methods are essential for accurate identification and quantification of these compounds in polymers. Misidentification between structurally similar phthalates can lead to false positives, noncompliance with regulations, and compromised product quality.

Objectives and Study Overview

This study examines the ability of a thermal desorption gas chromatography/mass spectrometry (TD-GC/MS) method, following ASTM D7823-13 conditions, to distinguish di-(n-octyl) phthalate (DNOP) from bis(2-ethylhexyl) terephthalate (DOTP) in PVC samples. The focus is on overcoming challenges posed by co-elution and shared diagnostic ions in mass spectra.

Methodology and Instrumentation

Sample preparation involved placing approximately 70 µg of PVC containing high levels of DINCH and trace amounts of DNOP and DOTP into a pyrolysis cup. Thermal desorption was performed using a Multi-Shot Pyrolyzer (EGA/PY-3030D) directly coupled to the GC/MS inlet. The GC oven temperature was programmed from 80 °C to 200 °C at 50 °C/min, then to 320 °C at 15 °C/min. Separation employed a 30 m Ultra ALLOY+-5 column (0.25 mm i.d., 0.25 µm film). Split injection (1:100) ensured reproducible desorption and transfer to the MS detector.

Main Results and Discussion

Total ion chromatograms showed co-elution of DNOP, DOTP, and DINCH, preventing clear differentiation. Both DNOP and DOTP spectra contained m/z 149 and 279, invalidating extracted ion chromatogram (EIC) approaches for specificity. However, DOTP displayed a unique and abundant m/z 261 fragment absent in DNOP. By correlating retention time with the presence or absence of the m/z 261 ion, each compound was confidently identified and quantified under the standardized GC/MS conditions.

Benefits and Practical Applications

• Prevents false positives and inaccurate quantitation of restricted phthalates in polymer matrices.
• Aligns with existing ASTM D7823-13 protocols, facilitating rapid adoption in quality control and regulatory laboratories.
• Maintains simplicity of TD-GC/MS workflows without additional sample preparation or complex derivatization steps.

Future Trends and Potential Applications

Advancements may include high-resolution mass spectrometry to uncover further distinguishing ions, optimized column chemistries for enhanced separation, and automated data-analysis software for improved throughput. Emerging regulations on new plasticizers will drive demand for adaptable TD-GC/MS methods capable of resolving an expanding range of structurally related compounds.

Conclusion

This technical note demonstrates that selective monitoring of m/z 261 combined with retention time data under ASTM D7823-13 conditions effectively differentiates DNOP and DOTP in PVC samples. The approach enhances the reliability of restricted phthalate analysis in compliance testing and industrial quality assurance.

Reference

• ASTM International. ASTM D7823-13: Standard Test Method for Determination of Six Restricted Phthalate Ester Plasticizers in Poly(vinyl chloride) by Thermal Desorption Gas Chromatography/Mass Spectrometry.
• Frontier Laboratories Ltd. Technical Note PYA1-074E: Differentiation of DOTP and DNOP in Analysis of Restricted Phthalate Esters Using Thermal Desorption GC/MS.