Propylene Glycol and Diethylene Glycol (DEG) Residue Testing: Analytical Methods, Standards, and Reg

Propylene glycol (PG) and diethylene glycol (DEG) residue testing is a critical quality and safety control for pharmaceuticals, cosmetics, food products, and their raw materials, where glycols serve as solvents, excipients, and humectants. Because DEG contamination of glycerin and propylene glycol has caused fatal poisoning outbreaks worldwide, regulators and pharmacopoeias such as the USP, Ph. Eur., FDA, and WHO enforce strict limits, typically not more than 0.1%, verified through validated GC-FID and GC-MS methods. This article outlines the scope, sample types, testing standards, analytical procedures, and applications of professional PG/DEG residue testing services.

Technical Overview

Propylene glycol (PG, propane-1,2-diol, CAS 57-55-6) is one of the most widely used low-molecular-weight glycols in the pharmaceutical, cosmetic, and food industries. It functions as a solvent, co-solvent, humectant, and preservative assistant in oral solutions, topical formulations, and food flavor carriers. PG is classified as a Class 3 residual solvent (low toxic potential) under ICH Q3C with a permitted daily exposure of 50 mg/day, and it is generally recognized as safe when used within established limits (E-number E1520, JECFA ADI 25 mg/kg body weight/day).

Diethylene glycol (DEG, CAS 111-46-6) is chemically related but toxic upon ingestion. Metabolized into acidic products including oxalate, DEG causes central nervous system depression, metabolic acidosis, acute kidney injury, and hepatotoxicity, and it can be fatal even at low doses, particularly in children. Historical tragedies linked to DEG-contaminated glycerin range from the 1937 Elixir Sulfanilamide disaster to fatal pediatric cough syrup outbreaks in Haiti (1996), Panama (2006), and the multiple WHO global medical product alerts issued in 2022–2023. The World Health Organization has stated that there is no safe level of DEG or ethylene glycol (EG) contamination in medicines.

DEG typically enters the supply chain as an adulterant or process impurity in substandard glycerin, propylene glycol, or polyethylene glycol raw materials, or through cross-contamination during manufacturing and transport. Consequently, compendial monographs and regulatory agencies require identity confirmation and limit testing of incoming glycerin and PG lots. The USP Glycerin monograph, the FDA Guidance for Industry on Testing of Glycerin for Diethylene Glycol, the European Pharmacopoeia, the Chinese Pharmacopoeia, and the Japanese Pharmacopoeia all specify DEG limits of not more than 0.1% (1000 ppm), while the EU Cosmetic Regulation restricts DEG to less than 0.1% in finished cosmetic products. Systematic PG/DEG residue testing is therefore essential for patient safety, regulatory compliance, and market access.

Testing Samples

Professional residue testing laboratories accept a broad range of sample matrices across the pharmaceutical, cosmetic, food, and chemical industries. Representative sample categories include:

Pharmaceutical Products and Excipients

• Oral liquid preparations, including cough syrups, elixirs, solutions, and suspensions
• Topical gels, creams, ointments, and transdermal formulations containing PG
• Suppositories and other excipient-rich dosage forms
• Raw material lots of glycerin, propylene glycol, and polyethylene glycols from every incoming container

Cosmetics and Personal Care Products

• Toothpaste, mouthwash, and other oral care products
• Lotions, creams, serums, facial masks, and deodorants containing glycerin or PG as humectants
• Glycerin-based cosmetic ingredients and semi-finished blends

Food, E-Liquid, and Veterinary Products

• Food-grade propylene glycol (E1520) and flavor or essence carriers
• Electronic cigarette liquids (PG/VG formulations) and nicotine solutions
• Veterinary oral preparations and feed additives containing glycols

Raw Materials and Industrial Intermediates

• Bulk glycerin, including biodiesel-derived crude and refined grades
• Propylene glycol production lots and co-product streams
• Unknown or suspect materials requiring contamination investigation

Testing Items

Core testing items for PG/DEG residue programs cover both toxic contaminant screening and excipient quality characterization:

• Diethylene glycol (DEG) content and compendial limit test
• Ethylene glycol (EG) content, which is typically co-tested with DEG
• Triethylene glycol (TEG) and other glycol-related impurities
• Propylene glycol identification (GC retention and spectroscopic confirmation) and quantitative assay
• Related substances and impurity profiling of glycerin and propylene glycol
• Unknown contamination screening in support of investigations and recalls

The most widely applied regulatory acceptance criterion is a DEG limit of not more than 0.1% (1000 ppm) in glycerin and PG raw materials, consistent with USP, Ph. Eur., and FDA guidance. For finished cosmetic products in the EU, DEG must be present at less than 0.1%. Many pharmaceutical manufacturers adopt tighter internal specifications, often an order of magnitude below the compendial limit, to provide additional safety margins for pediatric and high-dose products.

Testing Methods

A robust testing strategy combines compendial quantitative analysis with confirmatory mass spectrometry and matrix-validated sample preparation, ensuring reliability across simple raw materials and complex finished products.

GC-FID Quantitative Method (Primary Technique)

Gas chromatography with flame ionization detection (GC-FID) is the pharmacopoeial reference approach for glycol analysis. Samples are diluted in water or methanol, optionally spiked with an internal standard such as 1,3-butanediol, and injected onto a polar polyethylene glycol (wax) capillary column, typically 30 m in length. A temperature program, for example from approximately 60°C ramped to 230°C, separates ethylene glycol, propylene glycol, diethylene glycol, and triethylene glycol with baseline resolution. Quantification against certified reference standards typically achieves a limit of quantitation of approximately 50–100 ppm (0.005–0.01%), well below the 0.1% regulatory limit, and system suitability requires adequate peak resolution and repeatable response factors in every sequence.

GC-MS Confirmatory Analysis

Gas chromatography–mass spectrometry (GC-MS) in selected ion monitoring mode provides unambiguous structural confirmation of DEG, EG, and related glycols through mass spectra and retention time matching. GC-MS achieves parts-per-million to sub-parts-per-million detection capability, supports identification of unknown glycol contaminants, and is the preferred technique for out-of-specification investigations and regulatory dispute resolution.

Alternative and Complementary Techniques

For matrices that are difficult to chromatograph directly, such as highly sugared syrups or toothpaste, validated sample preparation including dilution, liquid-liquid extraction, solid-phase extraction cleanup, or standard addition recovery correction is applied. High-performance liquid chromatography with refractive index detection (HPLC-RI) and LC-MS may be used as orthogonal techniques, while two-dimensional GC can resolve co-eluting matrix interferences in complex formulations.

Method Validation and Quality Assurance

Residue testing methods are validated in accordance with ICH Q2(R2), demonstrating specificity, linearity (typically r ≥ 0.999 across 50–150% of the specification level), accuracy with recovery within acceptable compendial ranges, precision, limit of detection and quantitation, range, and robustness. Testing is performed under ISO/IEC 17025-accredited or cGMP-compliant quality systems with NIST-traceable standards, calibrated instrumentation, control charts, and electronic data integrity compliant with 21 CFR Part 11, ensuring results are defensible for regulatory submissions and audits.

Applications

PG and DEG residue testing supports multiple quality, regulatory, and investigational objectives across the product lifecycle:

• Incoming excipient control, including identity and DEG limit testing of every container of glycerin as recommended by the FDA
• Finished product batch release testing and Certificate of Analysis generation
• Supplier qualification, raw material changeover, and vendor audit support
• Regulatory submissions, including CMC documentation, DMFs, and stability programs
• Import and export compliance testing for international market access
• Out-of-specification and contamination investigations, root cause analysis, and corrective action verification
• Response to WHO medical product alerts, market surveillance programs, and product recalls

FAQ

Why is diethylene glycol so dangerous at low levels?

DEG is metabolized in the liver to acidic products that cause metabolic acidosis and deposit as oxalate crystals in the kidneys, producing acute renal failure, liver injury, and neurological damage. Children are especially vulnerable, and the WHO considers DEG and EG unsafe at any level in medicinal products, which is why sensitive analytical verification is mandatory.

What are the key regulatory limits for DEG and PG?

The USP Glycerin monograph, Ph. Eur., the Chinese and Japanese pharmacopoeias, and FDA guidance all limit DEG to not more than 0.1% (1000 ppm) in glycerin and related raw materials. The EU Cosmetic Regulation restricts DEG to less than 0.1% in finished cosmetics. Propylene glycol itself is permitted as an excipient and food additive, with ICH Q3C assigning it Class 3 status and a permitted daily exposure of 50 mg/day.

Which analytical technique is used, and what sensitivity is achievable?

GC-FID is the primary quantitative method and reliably quantifies DEG at 50–100 ppm or lower, far beneath the 0.1% specification. GC-MS confirmation delivers parts-per-million or lower detection limits and definitive identification, which is critical for investigations and regulatory reporting.

What are the sample submission requirements?

Typical requirements are 10–20 g or 10–20 mL of sample in sealed, inert containers such as amber glass or HDPE bottles, accompanied by the certificate of analysis, formulation or composition information where available, and the applicable specification or regulatory limit for evaluation.

Can complex finished products such as syrups or toothpaste be tested?

Yes. Validated sample preparation protocols, including matrix dilution, extraction, cleanup, and recovery correction by standard addition, allow accurate DEG and PG determination in high-sugar syrups, viscous gels, and surfactant-rich oral care products.

What is the typical turnaround time?

Standard testing is generally completed within five to seven working days from sample receipt, with expedited 48–72 hour services available for urgent release, investigation, or recall support scenarios.