Diethylene glycol (DEG) residue testing is a critical safety and quality control step for cosmetic raw materials such as glycerin, propylene glycol, polyethylene glycols, and sorbitol solutions, where DEG may occur as a toxic manufacturing by-product or adulterant. Accredited third-party laboratories apply gas chromatography-based analytical methods to detect and quantify DEG down to low-ppm levels, supporting compliance with pharmacopeial monographs and cosmetic regulations worldwide. This technical article explains the scope, analytical procedures, acceptance limits, and practical considerations for DEG residue analysis in cosmetic ingredients.
Technical Overview
Diethylene glycol (DEG, C4H10O3, molecular weight 106.12) is a colorless, odorless, hygroscopic liquid that is fully miscible with water and many organic solvents. Although chemically similar to glycerin in its humectant properties, DEG is significantly toxic: it is metabolized in the body to oxalic acid and other reactive intermediates that cause acute kidney injury, central nervous system depression, and liver damage. Numerous historical poisoning incidents involving DEG-contaminated glycerin in pharmaceutical syrups, toothpaste, and other consumer products have made it one of the most closely monitored process-related impurities in personal care supply chains.
DEG can enter cosmetic ingredients through several routes. It is formed as a co-product during ethylene oxide hydrolysis and glycol production, it can remain as a residual impurity in glycerin and propylene glycol refining, and it has been used deliberately as an inexpensive adulterant for glycerin, a practice known as economically motivated adulteration. Because DEG is rarely declared and cannot be identified visually, instrumental residue testing of incoming raw materials is the only reliable control measure.
The principal regulatory benchmarks include USP General Chapter <462> (Diethylene Glycol and Ethylene Glycol Impurity Tests) and the European Pharmacopoeia glycerol monograph, both of which set a limit of not more than 0.1% (1,000 mg/kg) DEG and ethylene glycol in glycerin. In the European Union, the Scientific Committee on Consumer Safety (SCCS) concluded that DEG up to 0.1% in oral care products does not pose a safety concern, a threshold widely applied in safety assessments under Regulation (EC) No 1223/2009. In China, the national toothpaste material standard GB 22115 restricts DEG impurities to no more than 0.1%, and safety dossiers reviewed under the Safety and Technical Standards for Cosmetics routinely require DEG data for glycerin-containing formulations. Many global brands and retailers impose substantially stricter internal specifications at the low-ppm level.
Testing Samples
Glycerin and Glycerin-Based Humectants
High-purity glycerin (99.5%), aqueous glycerin solutions (typically 80% to 95%), and botanical extracts using glycerin as a carrier solvent are the highest-risk sample categories. Because DEG adulteration of glycerin is the most frequently documented failure mode, these materials should be tested on a lot-by-lot basis with certificate-of-analysis verification.
Propylene Glycol and Other Glycol Ingredients
Propylene glycol, dipropylene glycol, butylene glycol, pentylene glycol, and hexanediol are common solvents and humectants in serums, creams, and deodorants. DEG may be present as an unintended co-product of synthesis and should be screened during supplier qualification and periodic incoming inspection.
Polyethylene Glycols, Polysorbates, and Sorbitol Solutions
Liquid polyethylene glycol grades (such as PEG-8 through PEG-400), PEG esters, polysorbate surfactants, and 70% sorbitol solution can carry low-level glycol impurities, including ethylene glycol, DEG, and triethylene glycol, originating from their ethylene oxide-based or hydrogenation-based production routes.
Finished Cosmetic Products
Toothpaste, mouthwash, oral care gels, moisturizing creams, lotions, sheet masks, and wet wipes may be tested directly to verify that combined raw material contributions keep the finished product within applicable limits, or to support root-cause investigation of a suspect batch.
Testing Items
The core analytical target is the diethylene glycol residue content, reported in mg/kg (ppm) or as a percentage by weight. Testing is typically extended to the following related items in a single chromatographic run:
- Diethylene glycol (DEG) residue content in the raw material or finished product
- Ethylene glycol (EG) residue, routinely quantified together with DEG under pharmacopeial limit tests
- Triethylene glycol (TEG) and other homologous glycol impurities where relevant
- Glycerin identity and assay, recommended alongside DEG testing to detect adulteration or substitution
- Interpretation against acceptance criteria, for example not more than 1,000 mg/kg (0.1%) per USP <462> and the European Pharmacopoeia, 0.1% in finished oral care products per the SCCS assessment, or brand-specific limits such as 50 to 100 mg/kg
Testing Methods
Sample Preparation
Raw materials are accurately weighed and diluted in a suitable solvent such as water, methanol, or acetonitrile. Finished products typically undergo ultrasonic extraction of DEG from the formulation matrix, followed by centrifugation and filtration through a 0.22 to 0.45 micrometer membrane. For trace-level analysis in complex matrices, optional silylation derivatization (for example with BSTFA and a catalyst) improves chromatographic behavior and sensitivity. Because glycerin and glycols are strongly hygroscopic, samples must be handled in airtight containers to prevent moisture uptake that affects weighing accuracy.
GC-FID Quantitative Method
Gas chromatography with flame ionization detection (GC-FID) is the primary quantitative technique, consistent with USP <462> and pharmacopeial glycerol monographs. Analysis is performed on a polyethylene glycol (wax-type) capillary column, for example 30 m length with 0.32 mm internal diameter, using split injection, helium or hydrogen carrier gas, and a temperature program ramping from approximately 60 degrees Celsius to 230 degrees Celsius. Quantification employs external calibration with certified reference standards and, where specified, an internal standard such as 1,4-butanediol or tetraethylene glycol. Typical quantification limits are in the range of 10 to 20 mg/kg with injection-to-injection precision typically below 3% RSD.
GC-MS Confirmatory Method
Gas chromatography coupled with mass spectrometry (GC-MS) in electron ionization mode provides unambiguous identity confirmation through retention time and mass spectral matching, monitoring characteristic ions such as m/z 45, 74, and 103 for DEG and corresponding fragments for ethylene glycol. GC-MS achieves detection limits down to approximately 1 to 2 mg/kg and is preferred for regulatory dispute resolution, dispute-sensitive batches, and complex finished-product matrices.
Alternative HPLC Methods
Where volatility or matrix interferences limit GC performance, high performance liquid chromatography with refractive index detection (HPLC-RI) or evaporative light scattering detection (HPLC-ELSD) on an aminopropyl or carbohydrate column offers a complementary screening approach, particularly for high-boiling polyether ingredients.
Method Validation and Quality Control
Testing is performed by ISO/IEC 17025-accredited third-party laboratories with methods validated in line with ICH Q2 principles covering specificity, linearity (typically r-squared not less than 0.995), accuracy (spike recovery of 85% to 110%), precision (RSD below 5%), limits of detection and quantitation, range, and robustness. Each analytical sequence includes certified reference materials, procedural blanks, duplicate preparations, and matrix spike recovery checks. Reports state the sample identity, lot number, analytical method, result in mg/kg, detection limit, and a clear pass or fail conclusion against the applicable specification.
Applications
DEG residue testing supports multiple points across the cosmetics quality and regulatory lifecycle:
- Incoming raw material lot release and certificate-of-analysis verification for glycerin, propylene glycol, PEGs, polysorbates, and sorbitol solutions
- Supplier qualification, periodic requalification, and detection of economically motivated adulteration
- Safety assessment and dossier support for product notification and registration in China, the European Union, and other markets, including toxicological justification of impurity levels
- Finished product verification for oral care and leave-on categories subject to the 0.1% DEG threshold
- Root-cause investigation, retain-sample testing, and market surveillance following customer complaints or regulatory inquiries
- Export clearance, customs compliance, and support for retailer and brand audit programs
FAQ
What is the regulatory limit for DEG in cosmetic ingredients?
Major pharmacopeias limit DEG and ethylene glycol in glycerin to not more than 0.1% (1,000 mg/kg). The SCCS assessment supports up to 0.1% DEG in finished oral care products in the EU, and GB 22115 applies the same 0.1% ceiling for toothpaste in China. DEG is not permitted as an intentional cosmetic ingredient; it is tolerated only as an unavoidable trace impurity within these limits, and many brands enforce stricter internal specifications at the 50 to 100 mg/kg level.
Which cosmetic ingredients should be tested for DEG?
Priority materials are glycerin and glycerin-based extracts, followed by propylene glycol and related glycols, polyethylene glycols, polysorbates, PEG derivatives, and sorbitol solutions. Any finished product in which these ingredients are used at significant levels, especially oral care products, should also be considered for verification testing.
What detection limits can be achieved?
GC-FID methods typically achieve quantification limits of 10 to 20 mg/kg, while GC-MS confirmation can reach 1 to 2 mg/kg, both well below the 1,000 mg/kg pharmacopeial limit and suitable for verifying tighter brand specifications.
How should samples be submitted?
A minimum of 20 to 50 g or 20 to 50 mL per lot is recommended. Samples must be sealed in airtight, preferably amber, containers to prevent moisture absorption, clearly labeled with product name, lot number, and date, and accompanied by the supplier certificate of analysis. Samples are stable at ambient temperature under proper sealing.
What is the typical turnaround time?
Standard DEG residue testing by GC-FID is generally completed within 5 to 10 working days after sample receipt, with GC-MS confirmation or finished product extraction adding a few additional days where required.
Why are DEG and ethylene glycol tested together?
Both compounds arise from the same ethylene oxide-based chemistry, present overlapping toxicological concerns, and share chromatographic behavior on wax columns, allowing a single validated method to quantify both impurities efficiently in one run.
