Experimental Study on Diethylene Glycol (DEG) Residue Content: Analytical Methods, Standards, and Qu

Diethylene glycol (DEG) residue determination is a critical safety and quality parameter for polyester polymers, pharmaceutical excipients, cosmetics, and food contact materials. This experimental study evaluates validated gas chromatography-based procedures for quantifying DEG from trace contaminant levels to minor comonomer levels, aligned with GB, ASTM, ISO, and pharmacopoeial standards. The results offer practical guidance for manufacturers, regulatory teams, and accredited third-party laboratories seeking reliable DEG residue control.

Technical Overview

Diethylene glycol (DEG, CAS 111-46-6) is a colorless, hygroscopic, water-miscible diol formed as a co-product in ethylene glycol manufacture and widely used as a solvent, humectant, and polyester modifier. Although structurally similar to ethylene glycol, DEG is substantially more toxic: its metabolic conversion produces renal tubular injury and hepatic damage, and historical contamination of medicines, toothpaste, and wine has caused fatal mass-poisoning incidents worldwide. As a result, DEG residues are strictly regulated in pharmaceuticals, cosmetics, food contact materials, and polymer products.

In polyethylene terephthalate (PET) production, DEG is deliberately incorporated as a comonomer, typically at 1.0–3.0 wt%, to improve dye uptake, flexibility, and melt processability, while additional free DEG may remain as a processing residue. In glycerin, propylene glycol, and sorbitol solution excipients, DEG is a hazardous contaminant restricted by pharmacopoeial limits of typically 0.10% (w/w). In European food-contact plastics legislation, DEG carries a specific migration limit of 30 mg/kg food. Accurate residue measurement therefore underpins both routine quality control and regulatory compliance, and this study summarizes the experimental approaches used to achieve it.

Testing Samples

Polyester (PET) Chips, Fibers, and Films

Bottle-grade and fiber-grade PET chips, staple fibers, filament yarns, and BOPET films were collected from production lots. Fiber-grade chips usually contain higher DEG (approximately 1.5–3.0 wt%) than bottle-grade chips (approximately 1.0–2.0 wt%), because DEG content is intentionally adjusted to modify crystallization behavior and dyeability.

Pharmaceutical Excipients and Formulations

Refined glycerin, propylene glycol, polyethylene glycols, and sorbitol solutions were sampled across the supply chain, together with oral solutions, syrups, and gelatin capsules in which glycol-based excipients or plasticizers are used. These matrices require the lowest reporting limits because pharmacopoeial restrictions apply at the 0.1% level and below.

Food Contact Materials and Food Simulants

Packaging films, coatings, adhesives, and printing ink layers were tested, along with the aqueous, acidic, alcoholic, and fatty food simulants used in standardized migration experiments performed according to GB 31604 series and GB 5009.156 migration testing protocols.

Cosmetics, Personal Care, and Consumer Products

Toothpaste, mouthwash, creams, and humectant-containing formulations were included, since DEG adulteration of glycerin has repeatedly entered cosmetic supply chains and several jurisdictions prohibit DEG as an ingredient outright.

Testing Items

• Free and total DEG content in PET and other polyesters, expressed as weight percent (wt%).
• DEG impurity level in glycerin, propylene glycol, and other glycol-based excipients (% w/w).
• Specific migration of DEG into food simulants (mg/kg food), evaluated against a 30 mg/kg migration limit.
• DEG residue in processed textiles, nonwovens, and finished packaging articles (mg/kg).
• Full glycol distribution profile, including ethylene glycol (EG), DEG, and triethylene glycol (TEG), to distinguish comonomer design from contamination.

Testing Methods

Gas Chromatography with Flame Ionization Detection (GC-FID)

GC-FID is the primary quantitative technique for DEG in all sample categories. A polar polyethylene glycol-modified capillary column (30 m × 0.32 mm i.d., 0.25 µm film) provided baseline separation of EG, DEG, and TEG. Typical conditions include a split injector at 250 °C, an FID at 280 °C, and an oven program from 60 °C (2 min hold) ramped at 10 °C/min to 240 °C. Quantification uses an internal standard, commonly 1,4-butanediol or 1,3-propanediol, which corrects for injection variability and the moderate volatility of DEG.

Sample Preparation

• Polyesters: total DEG is liberated by sealed-vessel methanolysis, in which ground PET reacts with methanol under transesterification conditions so that ester-bound DEG is released and quantified; alternatively, free DEG can be extracted after dissolution of the polymer in hexafluoroisopropanol followed by precipitation, consistent with the approach in GB/T 14190 for fiber-grade PET chips.
• Glycol excipients: direct dilution in water or methanol with internal standard addition is sufficient given the simple matrix.
• Food simulants: direct injection for aqueous simulants; liquid-liquid extraction or concentration steps are applied to fatty simulants.
• Trace-level matrices: silylation derivatization (for example with BSTFA and a catalytic admixture) improves peak shape and sensitivity for DEG at sub-mg/kg levels.

GC-MS Confirmation

Gas chromatography coupled with mass spectrometry in electron-impact mode confirms identity using characteristic ions such as m/z 45, 31, and 74, with selected-ion monitoring employed for trace quantification. GC-MS is recommended for regulatory disputes, adulteration investigations, and any result approaching a statutory limit, because it adds mass-spectral specificity that FID alone cannot provide.

Reference Methods and Method Validation

The experimental procedures were benchmarked against GB/T 14190 for PET chips, the GB/T 14571 series and ASTM E202 for glycol purity analysis, applicable ISO test methods for polyester and glycol characterization, GB 31604 series migration protocols for food contact materials, and the GC-based limit tests described in the USP, Ph. Eur., and Chinese Pharmacopoeia monographs for glycerin. Validation results across matrices were as follows:

• Linearity: correlation coefficient r² ≥ 0.9995 over the calibrated range of 0.001–1.0 mg/mL.
• Limits of detection and quantification: LOD approximately 1.5–5 mg/kg depending on matrix; LOQ approximately 5–15 mg/kg, equivalent to about 0.01 wt% in polymer digests.
• Recovery: 92.6–105.8% at three spiking levels in PET, glycerin, and simulant matrices.
• Precision: repeatability RSD of 1.8–4.5% (n = 6) and intermediate precision RSD below 6% over different analysts and days.

Applications

• Polymer quality control: continuous monitoring of DEG comonomer content to stabilize melt viscosity, crystallization behavior, and dye uniformity in PET chips and fibers.
• Pharmaceutical supply chain compliance: incoming identity and purity testing of glycerin and propylene glycol under GMP to screen for DEG adulteration before production.
• Food contact compliance: migration testing of packaging laminates, coatings, and adhesives against regulatory migration limits for market access dossiers.
• Cosmetic and consumer product safety: verification that humectant raw materials and finished formulations are free of prohibited DEG.
• Research and development: studying the influence of DEG level on polyester hydrolytic stability, thermal properties, and copolymer architecture.
• Regulatory and forensic investigation: definitive GC-MS confirmation of DEG in adulterated or non-conforming goods.

FAQ

What level of DEG is normally present in PET?

Fiber-grade PET typically contains 1.5–3.0 wt% DEG as an intentional comonomer, while bottle-grade PET generally falls between 1.0 and 2.0 wt%. These levels are design parameters rather than safety failures, provided migration from food-contact articles remains within regulatory limits.

What are the main regulatory limits for DEG residues?

Pharmacopoeias restrict DEG in glycerin to no more than 0.10% (w/w), European food-contact plastics legislation sets a specific migration limit of 30 mg/kg food, and many cosmetic regulations prohibit DEG as an intentional ingredient. Specific limits should always be confirmed against the applicable standard for the target market.

Why is GC-FID preferred over HPLC for DEG determination?

DEG lacks a strong chromophore, which limits UV-based HPLC sensitivity, whereas GC-FID provides excellent response for glycols, simple calibration with internal standards, and simultaneous separation of EG, DEG, and TEG. HPLC with refractive index detection remains a viable alternative when derivatization is not feasible.

How should insoluble PET samples be prepared?

Two validated routes exist: sealed-vessel methanolysis, which measures total DEG by chemically liberating bound DEG, and direct dissolution in a fluorinated solvent such as hexafluoroisopropanol followed by polymer precipitation and GC analysis of the extract for free DEG. Cryogenic grinding before preparation improves reproducibility.

How often should the DEG method be revalidated?

Full validation is expected when the method is first introduced, when the matrix or instrument configuration changes, and periodically as part of laboratory quality systems. Accredited third-party laboratories typically verify calibration, recovery, and precision with each analytical batch through blanks, spiked controls, and duplicate analyses.