Internal control testing of diethylene glycol (DEG) residue is a critical quality assurance practice used by manufacturers of PET polymers, pharmaceutical excipients, cosmetics, and food-contact materials to verify that DEG levels stay within self-imposed specification limits that are typically stricter than regulatory maximums. This article provides a comprehensive technical overview of DEG residue verification, covering sample types, analytical methods such as GC-FID and GC-MS, applicable standards including USP, Ph. Eur., EN 13130, and GB series methods, and practical guidance for establishing robust in-house release criteria.
Technical Overview
Diethylene glycol (DEG, CAS No. 111-46-6) is a colorless, hygroscopic, water-miscible organic compound widely used as a coolant, humectant, and chemical intermediate. Although industrially valuable, DEG is toxic to humans: ingestion can cause central nervous system depression, metabolic acidosis, acute kidney injury, and hepatic damage. Historical contamination tragedies involving DEG-adulterated pharmaceutical syrups, glycerin excipients, and toothpaste have made DEG one of the most closely monitored processing-related impurities and residual solvents in global supply chains.
DEG enters products through two principal routes. First, in the manufacture of polyethylene terephthalate (PET) and other polyesters, DEG forms naturally as a byproduct of polycondensation and side reactions, typically at levels of 1.0–2.0 wt%. Second, DEG appears as an adulterant or contaminant in glycerin, propylene glycol, and sorbitol-based raw materials when economically motivated substitution occurs upstream in the supply chain.
Internal control testing of DEG residue refers to the manufacturer's own in-house verification program, in which every production batch or incoming raw material lot is tested against internally defined action limits. These internal limits are deliberately set tighter than compendial or regulatory maxima to provide a safety margin, enable trend analysis, support out-of-specification (OOS) investigation, and satisfy due-diligence obligations under quality management systems such as ISO 9001 and GMP. Testing may be performed in the manufacturer's own laboratory or outsourced to accredited third-party laboratories, provided the method is validated and the results are traceable.
The regulatory landscape surrounding DEG is extensive and includes the following key references:
- USP–NF monograph for Glycerin, which requires identification and limitation of diethylene glycol, supported by FDA industry guidance on testing glycerin for DEG contamination.
- European Pharmacopoeia (Ph. Eur.) glycerol monographs, which impose related-substances limits on DEG.
- Commission Regulation (EU) No 10/2011 on plastic food-contact materials, which sets a combined specific migration limit (SML) of 30 mg/kg for ethylene glycol and diethylene glycol into food simulants.
- EN 13130-21, which specifies the determination of ethylene glycol and diethylene glycol in food simulants for compliance testing.
- GB/T 14190 and GB/T 17931, which cover test methods and product specifications for PET resins, including DEG content requirements relevant to bottle-grade materials.
- WHO medical product alerts and guidance on the prevention of DEG- and EG-contaminated oral liquid medicines.
Testing Samples
PET Resins and Chips
Bottle-grade, film-grade, and fiber-grade PET chips are the most common polymer matrices tested for DEG. Both free (extractable) DEG and total DEG incorporated into the polymer chain may be characterized. Bottle-grade PET specifications under GB/T 17931 and equivalent internal standards typically control DEG content in the range of approximately 1.0–1.5 wt%, with exact limits confirmed against the applicable current product standard and customer agreement.
Polyester Fibers, Films, and Textiles
Polyester staple fiber, filament yarn, and BOPET film samples are tested to verify residual glycol content arising from polymerization and finishing processes. Sample preparation generally involves cutting or milling the material to increase surface area before solvent extraction.
Pharmaceutical Raw Materials and Excipients
Glycerin (glycerol), propylene glycol, polyethylene glycols, sorbitol solutions, and maltitol syrups are high-risk materials for DEG adulteration. Each incoming lot should be tested for DEG identity and content as part of supplier qualification and routine incoming-material control under GMP.
Oral Liquid Preparations
Cough syrups, elixirs, and other oral liquid dosage forms containing glycerin or propylene glycol are tested for DEG contamination. These products are of particular concern for pediatric populations, where low body weight amplifies toxicity risk.
Cosmetics and Personal Care Products
Toothpaste, mouthwash, creams, and lotions containing glycerin or glycol-based humectants are tested to ensure DEG residues remain below internal safety limits derived from safety assessments and regional cosmetic regulations.
Food-Contact Materials and Packaging
PET bottles, trays, films, and coatings intended for direct food contact undergo migration testing to demonstrate compliance with the combined EG/DEG specific migration limit of 30 mg/kg under Regulation (EU) No 10/2011 and equivalent requirements in other jurisdictions.
Testing Items
The scope of an internal control DEG residue testing program typically includes the following determination items:
- DEG residue content in polymer materials, expressed as wt% or mg/kg (ppm), covering both extractable and total DEG.
- DEG specific migration into aqueous, acidic, alcoholic, and fatty food simulants, expressed in mg/kg or mg/dm², per EN 13130-21 and Regulation (EU) No 10/2011.
- DEG content in pharmaceutical excipients and oral liquids, expressed as % w/w or mg/kg against compendial limits.
- Related glycol impurities, including ethylene glycol (EG) and triethylene glycol (TEG), which frequently co-occur with DEG and are often limited simultaneously.
- Identity confirmation of DEG in suspect raw materials using mass-spectrometric detection to support adulteration investigations.
- Method validation parameters, including specificity, linearity, accuracy, precision, limit of detection (LOD), and limit of quantitation (LOQ), documented in the internal control protocol.
Testing Methods
Gas chromatography (GC) is the platform of choice for DEG residue determination because of its sensitivity, selectivity, and suitability for volatile and semi-volatile glycols. Typical analytical configurations and workflows are described below.
GC-FID with Internal Standard Quantitation
The most widely applied method uses gas chromatography with flame ionization detection (GC-FID) and an internal standard such as 1,4-butanediol. For polymer samples, DEG is liberated by reflux extraction with methanol or water, or by complete dissolution of the PET in a suitable solvent such as hexafluoroisopropanol followed by reprecipitation of the polymer. For excipients and oral liquids, samples are diluted directly in an appropriate solvent, derivatized if required, and injected. Quantitation is performed against a multi-point calibration curve, and DEG content is calculated on a sample mass basis. Typical LOQ values of 5–10 mg/kg are achievable for polymer extracts and significantly lower for neat liquid matrices.
GC-MS Confirmation
Gas chromatography coupled with mass spectrometry (GC-MS) is employed for identity confirmation, trace-level analysis, and investigation of suspect or out-of-specification results. Characteristic ions of DEG are monitored in selected-ion monitoring (SIM) mode, providing enhanced specificity in complex cosmetic or pharmaceutical matrices.
HPLC with Refractive Index Detection
High-performance liquid chromatography with refractive index (RI) detection offers a non-GC alternative for the determination of free DEG in PET and glycols in liquid matrices, and is referenced in some national test-method standards. Sensitivity is generally lower than GC-FID, so this technique is best suited to higher-level determinations.
Migration Testing for Food-Contact Materials
In accordance with EN 13130-21 and Regulation (EU) No 10/2011, migration testing exposes the food-contact article to food simulants, typically 10% ethanol (aqueous foods), 3% acetic acid (acidic foods), 20% and 50% ethanol (alcoholic foods), and vegetable oil or its substitutes (fatty foods). Time–temperature conditions (for example, 10 days at 40°C for long-term room-temperature storage) are selected according to intended use. The EG and DEG transferred into the simulants are quantified by GC-FID or HPLC, and the combined result is compared against the 30 mg/kg SML.
Method Validation and Quality Control
Regardless of the technique selected, an internal control method should be validated for specificity, linearity across the expected concentration range, recovery (typically 90–110%), repeatability and intermediate precision, and documented LOD/LOQ. System suitability checks, calibration verification standards, procedural blanks, and fortified sample controls should accompany each analytical sequence, and all data should be retained for batch-release decisions and regulatory audit.
Applications
Internal control DEG residue testing supports quality and compliance objectives across multiple industries:
- Polymer manufacturing: batch release of bottle-grade PET chips, monitoring of polycondensation stability, and verification of customer and national product specifications.
- Pharmaceutical manufacturing: incoming-lot testing of glycerin and propylene glycol excipients, in-process control, and finished-product release of oral liquid medicines in accordance with GMP and pharmacopoeial requirements.
- Cosmetics production: verification of glycerin-based raw materials and finished personal care products against internal safety limits.
- Food packaging compliance: demonstration of conformity with specific migration limits for EG and DEG in PET bottles, films, and coated articles destined for regulated markets.
- Supply-chain risk management: supplier qualification, periodic requalification, and rapid screening in response to WHO alerts concerning contaminated raw materials.
- Investigative support: root-cause analysis of OOS results, batch comparability studies, and due-diligence evidence for customs, regulatory submissions, or litigation defense.
FAQ
What internal control limit should be set for DEG in PET?
Most bottle-grade PET producers control DEG within approximately 1.0–1.5 wt%; fiber-grade specifications may be somewhat broader. The internal limit should be set tighter than the applicable product-standard or customer limit, for example by reserving a defined margin, so that normal process variation does not result in nonconforming batches.
How low can DEG be detected?
With GC-FID and a validated extraction procedure, an LOQ of approximately 5–10 mg/kg is realistic for polymer extracts. GC-MS in SIM mode can extend quantitation to the low mg/kg range or below in liquid matrices. The laboratory should always report the method-specific LOD and LOQ alongside the result.
Why is internal control testing stricter than regulatory testing?
Regulatory and compendial limits represent maximum acceptable levels, whereas internal limits incorporate a deliberate safety margin, account for measurement uncertainty, enable early detection of adverse trends, and protect against upstream supply-chain adulteration. This layered approach is a fundamental expectation of modern quality management systems.
Can DEG testing be performed in-house?
Yes, provided the laboratory holds suitable GC instrumentation, validated methods, trained analysts, and a documented quality system. Many organizations complement in-house screening with periodic confirmatory testing at accredited third-party laboratories to ensure independent verification and method comparability.
How frequently should DEG residue be tested?
For high-risk materials such as pharmaceutical-grade glycerin, every incoming lot should be tested before release. For PET production, DEG is typically monitored per production campaign or per batch as defined in the internal control plan, with increased frequency during process changes, start-ups, or OOS investigations.
What should a DEG residue test report include?
A compliant report should identify the sample and batch, describe the sampling and analytical method, state the standard or internal limit applied, report the quantitative result with units, list the LOD and LOQ, include QC and recovery data, and provide a clear conformity judgment signed by the authorized analyst.