Diethylene Glycol (DEG) Residue Analysis in Active Pharmaceutical Ingredients: Methods, Standards, a

Diethylene glycol (DEG) residue analysis is a critical quality control test for active pharmaceutical ingredients (APIs), excipients, and liquid dosage forms, protecting patients from a contaminant responsible for multiple fatal poisoning incidents worldwide. This technical article explains the origins and toxicology of DEG contamination, the sample types and test items involved, and the validated GC-FID, GC-MS, and HPLC methods used by pharmaceutical QC departments and accredited third-party laboratories to quantify DEG down to low-ppm levels in accordance with USP, Ph. Eur., ICH, and WHO requirements.

Technical Overview

Diethylene glycol (DEG, 2,2'-oxydiethanol, CAS 111-46-6, molecular formula C4H10O3, molecular weight 106.12) is a colorless, hygroscopic, highly water-miscible liquid with a sweet taste. Within the pharmaceutical supply chain, DEG typically enters products as an adulterant of glycerin or propylene glycol, as a residual process solvent used in API synthesis or crystallization, as a degradation product of polyethylene glycol (PEG), or through cross-contamination on shared manufacturing equipment.

DEG is severely toxic to humans. After absorption, it is metabolized to acidic intermediates that cause acute tubular necrosis, acute kidney injury, central nervous system depression, metabolic acidosis, and hepatotoxicity. Historic poisoning tragedies, including the 1937 sulfanilamide elixir disaster in the United States, the 1990s pediatric syrup incidents in Haiti, the 2006 cough syrup contamination in Panama, and the WHO medical product alerts issued between 2022 and 2023 concerning contaminated pediatric syrups, have driven strict pharmacopoeial controls. The most widely applied acceptance criterion is not more than 0.1 percent (1000 ppm) DEG in glycerin-type excipients, and regulators worldwide recommend 100 percent screening of every incoming lot of glycerin and propylene glycol.

Because DEG is not listed as a residual solvent under ICH Q3C, any DEG residue in an API must instead be evaluated as a process-related impurity under ICH Q3A and ICH Q3B, qualified on the basis of toxicological data, and controlled through a justified specification. Testing is performed under GMP conditions, frequently in ISO/IEC 17025-accredited laboratories, to support release decisions, regulatory submissions, and contamination investigations.

Testing Samples

APIs and Synthetic Intermediates

• APIs manufactured, purified, or recrystallized using DEG or glycol-based solvent systems
• Synthetic intermediates, mother liquors, and process streams evaluated for mass balance and impurity fate
• PEGylated APIs and PEG-containing conjugates in which oxidative or thermal degradation may generate DEG
• Cleaning validation samples, including equipment rinse solutions and surface swabs

Excipients and Starting Materials

• Glycerin, propylene glycol, liquid polyethylene glycols, and sorbitol or maltitol solutions, which represent the highest risk of economically motivated adulteration
• Bulk solvent shipments received by tanker or drum at goods-in inspection
• Materials tested against USP, Ph. Eur., and ChP monograph limit requirements

Finished Dosage Forms

• Oral solutions, syrups, elixirs, tinctures, and suspensions, with particular emphasis on pediatric products
• Topical and transdermal liquid preparations
• Retained samples, complaint samples, and market surveillance specimens collected during contamination investigations

Testing Items

Residual DEG testing programs are structured around clearly defined qualitative and quantitative items, each traceable to pharmacopoeial monographs or scientifically justified internal specifications:

• DEG identification: qualitative screening based on retention time matching and mass spectral confirmation
• DEG quantification: determination of DEG content in percent weight by weight or mg/kg (ppm)
• Pharmacopoeial limit test: compliance evaluation against USP, Ph. Eur., or ChP limits, most commonly not more than 0.1 percent
• Related glycol determination: simultaneous quantification of ethylene glycol, triethylene glycol, and propylene glycol
• Bundled residual solvents panel: screening of ICH Q3C Class 1, 2, and 3 solvents alongside DEG where required
• Method development and validation: establishment of a matrix-specific procedure with documented specificity, sensitivity, and accuracy

Testing Methods

Gas Chromatography with Flame Ionization Detection (GC-FID)

GC-FID is the primary pharmacopoeial technique for DEG limit tests and quantitative assays. Samples are diluted in a suitable solvent such as methanol or water and introduced by direct liquid injection onto a polar polyethylene glycol (wax-type) capillary column. Quantitation is typically performed with an internal standard such as 1,3-butanediol. Practical quantification limits generally fall in the range of 10 to 100 ppm, which is well below the 0.1 percent monograph limit and suitable for routine batch release testing.

Gas Chromatography with Mass Spectrometry (GC-MS)

GC-MS provides unambiguous confirmatory identification using electron impact ionization and selected ion monitoring, with characteristic DEG fragment ions including m/z 45, 74, and 75. This technique achieves low-ppm or sub-ppm quantification limits and is the method of choice for complex syrup matrices, out-of-specification investigations, and disputed results requiring forensic-level confidence.

Alternative and Supportive Techniques

• HPLC with refractive index, evaporative light scattering, or charged aerosol detection for laboratories requiring a non-GC workflow
• Derivatization strategies, such as silylation or boronate ester formation, to improve peak shape and detectability in GC-MS confirmation work
• Headspace GC screening of volatile co-solvents to complete the overall residual solvent picture

Method Validation and Data Quality

Validated methods must satisfy ICH Q2(R2) and USP General Chapter 1225 requirements, covering specificity, linearity across the reporting range (typically correlation coefficient not less than 0.999), accuracy demonstrated by spike recovery, precision expressed as relative standard deviation, and LOD and LOQ below the applicable reporting threshold. Robustness, system suitability criteria, and solution stability are documented to ensure reproducible transfer between laboratories. All analytical work should be performed under a GMP-aligned quality system, ideally within ISO/IEC 17025-accredited facilities, with results issued in a clear certificate of analysis and out-of-specification outcomes investigated in accordance with regulatory expectations.

Applications

• GMP batch release and certificate of analysis generation for APIs and glycol-based excipients
• Incoming material control, including 100 percent identity and DEG limit testing of every glycerin and propylene glycol lot, as recommended by major regulators
• Supplier qualification, vendor audits, and import-export regulatory compliance support
• Contamination investigations, root-cause analysis, and batch genealogy studies following customer complaints or WHO medical product alerts
• Stability studies and shelf-life confirmation for liquid formulations containing glycols or PEGs
• Regulatory dossier support, including DMF, ASMF, and CEP documentation containing justified impurity control strategies
• Pediatric formulation safety programs and research into PEG degradation pathways under forced degradation conditions

FAQ

Why is DEG testing required for APIs when DEG is not listed in ICH Q3C?

Although DEG is not classified as a residual solvent under ICH Q3C, it may still be present as a process-related impurity or contaminant. ICH Q3A and Q3B require that any such impurity be identified, qualified using toxicological principles, and controlled in the specification, which makes dedicated DEG residue analysis essential whenever glycol-based materials or solvents are involved in manufacture.

What is the typical regulatory limit for DEG?

The most widely applied pharmacopoeial limit is not more than 0.1 percent (1000 ppm) in glycerin-type excipients, tested according to the relevant USP, Ph. Eur., or ChP monograph. For APIs and finished products, acceptance criteria must be established case by case based on the maximum daily dose and the qualified acceptable daily intake of DEG.

Which analytical technique should be selected?

GC-FID is recommended for routine high-throughput QC due to its robustness and pharmacopoeial status, while GC-MS is preferred when confirmatory identification, very low detection limits, or analysis of complex syrup matrices is required. HPLC with refractive index detection serves as an acceptable alternative where GC is impractical.

How much sample is needed and how long does testing take?

Typical sample requirements are in the range of 1 to 5 grams for APIs and excipients and 10 to 20 mL for liquid formulations. Standard turnaround for a validated routine method is usually 5 to 10 working days, while investigative work involving method development and confirmation may require additional time.

Can finished syrup products be tested directly?

Yes. Oral liquids are diluted in an appropriate solvent and analyzed directly by GC-FID, with GC-MS confirmation employed when matrix interference is suspected. Sample preparation is designed to account for viscosity and high sugar content so that recovery remains within validated acceptance limits.