Analytical Solutions for Sustainable PET Production

Plastic pollution has become one of the most pressing environmental issues of our time, with millions of tons entering landfills and oceans annually. Widely used in bottles and packaging, polyethylene terephthalate (PET) is at the forefront of global recycling initiatives due to its favorable properties and established collection infrastructure. However, the environmental benefits of recycled PET (rPET) hinge on rigorous quality assurance, such as distinguishing recycled from virgin PET and ensuring reliable performance in new products.¹

A key indicator of rPET quality is the concentration of cyclic oligomers, byproducts of PET polymerization that are typically present at lower levels in recycled material. Accurate quantification of these oligomers is essential, not only for regulatory compliance but also for maintaining the integrity of recycled goods across diverse industries, from textiles to food packaging.²

In response to these demands, the Japanese Ministry of the Environment has established comprehensive guidelines for evaluating rPET, emphasizing the need for rapid, reliable analytical methods. This article presents a fast, selective, and quantitative high-performance liquid chromatography (HPLC) method for cyclic oligomer analysis. This approach not only meets regulatory standards but also advances the practical adoption of recycled plastics in a sustainable future.

Experimental

Sample Preparation:

Following the official guidelines, rPET samples undergo a straightforward pretreatment:³ dissolution in hexafluoroisopropanol (HFIP), dilution in acetonitrile (ACN), and filtration, as visualized in Figure 1.

To address the challenge that strongly hydrophobic oligomers tend to stick to the hydrophobic separation column (C18) in a reversed-phase HPLC method, resulting in excessive analysis time in an isocratic assay (Figure 2[a]), gradient elution was employed instead. Ramping the organic solvent concentration up to 100% at the end of the method ensured complete elution of all target analytes and reduced total analysis time (Figure 2[b]).

Analytical Method:

Analytical Conditions of the Analysis of Cyclic Oligomers in Recycled PET: System: i-Series (LC 2070C) compact HPLC (Shimadzu); Column: 250 × 4.6 mm, 5-µm Shim-pack Scepter C18-120 (Shimadzu); Flow rate: 1.5 mL/min; Mobile phase (isocratic elution): 30:70 (v/v) H₂O–ACN; Mobile phase (gradient elution): A) H₂O B) ACN, 70% B (0 min) → 100% B (17.5–20 min) → 70% B, 5 min reequilibration at 70% B; Injection volume: 20 µL; Detection: UV detection at λ = 242 nm.

Quantitative Analysis:

Obtaining pure standards for each cyclic oligomer is impractical. Instead, according to the guideline², dimethyl terephthalate serves as a universal calibration standard. By configuring the chromatographic data system to use dimethyl terephthalate’s calibration curve for all oligomers (with a correction factor of 1), accurate, convenient quantification across the entire oligomer spectrum can be achieved.

The content of each cyclic oligomer in rPET is calculated using the following formula:

Where:

1. C = Concentration determined by HPLC (mg/L)
2. V = Specified volume (mL)
3. m = Sample weight (mg)

This approach yielded robust quantitative data, supporting both regulatory compliance and industrial quality assurance. Calculated concentrations in rPET are stated in Table 1.

Method Transfer to UHPLC:

In the attempt to further reduce analysis time and solvent consumption, the method was transferred from the original column size of 250 x 4.6 mm, 5-µm to a 100 x 3.0 mm, 1.9-µm column with the same column chemistry.³ All method parameters were adjusted accordingly, resulting in a total analysis time of 12 min instead of 25 min, requiring only a quarter of the solvent, as well as a quarter of the sample used with the original method. Even strongly retained long-chain oligomers, such as cyclic octamers, could be efficiently eluted in less than 7 min (Figure 3). Details of the ultrahigh-pressure liquid chromatography (UHPLC) method can be found below.

Analytical Conditions of the Analysis of Cyclic Oligomers in Recycled PET: System: i-Series (LC 2070C) compact HPLC (Shimadzu); Column: 100 × 3.0 mm, 1.9-µm Shim-pack Scepter C18-120 (Shimadzu); Flow rate: 0.8 ml/min; Gradient: A) H₂O B) ACN, 70% B (0 min) → 90% B (6 min) → 100% B (6.01–8.5 min) → 70% B, 3.5 min re-equilibration at 70% B; Injection volume: 5 µL; Detection: UV detection at λ = 242 nm.

The revised method also demonstrated excellent repeatability, while offering higher throughput and reduced solvent consumption, making it ideal for routine industrial and research applications.

Conclusion

This advanced HPLC method provides an efficient, practical solution for the routine quality control of recycled PET. The approach integrates straightforward sample preparation, guideline-based quantification with dimethyl terephthalate, and a robust gradient separation to enable consistent measurement of cyclic oligomers, including higher molecular weight species.

Transitioning from a conventional 250 × 4.6 mm, 5-µm column to a 100 × 3.0 mm, 1.9-µm UHPLC column reduces analysis time from 25 to 12 min and lowers solvent use by approximately 75%, while maintaining repeatability. These improvements support increased laboratory throughput and reduced environmental impact.

Cyclic oligomers are recognized as relevant markers for rPET quality, with implications for mechanical properties and, in food-contact applications, for migration and safety assessments. A rapid, quantitative, and reproducible method helps meet regulatory requirements and aligns with international standards.

For recyclers, converters, and brand owners, standardized HPLC/UHPLC analysis of cyclic oligomers supports quality assurance and transparent recycled-content claims. This method contributes to reliable rPET assessment and supports broader efforts toward a circular, sustainable plastics economy.

References

1. PlasticsEurope. Plastics — the Facts 2023: An Analysis of European Plastics Production, Demand and Waste Data; PlasticsEurope: Brussels, 2023. https://plasticseurope.org/
2. Ministry of the Environment (Japan). Guidelines for Ensuring the Reliability of Indications of Specified Procured Goods, etc. Appendix 3: “4.8. Explanation of Survey Methods for Products Containing Recycled Plastic (by Resin Type)”; Ministry of the Environment: Tokyo, 2025.
3. Shimadzu Corporation. Analysis of Cyclic Oligomers in Recycled PET Using Integrated LC System; Application Note; Shimadzu Corporation: Kyoto, 2025.