Diethylene Glycol (DEG) Residue Testing for Injectable Pharmaceutical Products

Diethylene glycol (DEG) residue testing is a critical safety and quality control for injectable pharmaceutical products, targeting trace-level contamination introduced through glycerin, propylene glycol, and polyethylene glycol excipients. Using validated gas chromatography (GC-FID) and GC-MS methods, accredited third-party laboratories and in-house QC laboratories can detect and quantify DEG at parts-per-million levels in accordance with USP-NF, Ph. Eur., and ICH requirements. This article explains the technical principles, sample types, analytical methods, acceptance criteria, and practical applications of DEG residue detection in injections and parenteral preparations.

Technical Overview

Diethylene glycol (DEG, CAS 111-46-6) is a colorless, hygroscopic, water-miscible glycol with no legitimate use as an active pharmaceutical ingredient. It has repeatedly entered the pharmaceutical supply chain as an economically motivated adulterant of glycerin and as a residual impurity or by-product in propylene glycol and polyethylene glycol (PEG) excipients. Because these excipients are widely formulated into parenteral products, DEG residue testing has become an essential element of pharmaceutical quality control.

Injectable preparations present a particularly high risk profile. Glycerin is commonly used as a tonicity-adjusting agent and vehicle, propylene glycol as a cosolvent for poorly water-soluble drugs, and PEG 300/400 as a solubilizer. Since injections bypass gastrointestinal barriers and first-pass metabolism, any DEG carried into the final product results in direct systemic exposure. DEG is a potent nephrotoxin and hepatotoxin; its metabolites, diglycolic acid and 2-hydroxyethoxyacetic acid, cause acute kidney injury, metabolic acidosis, liver damage, and peripheral neuropathy. Historical mass-poisoning events, beginning with the 1937 Elixir Sulfanilamide disaster, have established DEG control as a cornerstone of global pharmacopoeial and regulatory programs.

DEG residue testing for injectables is designed to achieve the following objectives:

• Verify that glycol-based excipients and finished injectable products are free from hazardous levels of DEG
• Demonstrate compliance with compendial monographs and regulatory guidance from agencies such as the FDA and WHO
• Support supplier qualification, batch release, stability evaluation, and out-of-specification investigations
• Protect patient safety by confirming that contamination remains within toxicologically justified limits at parts-per-million levels

Testing Samples

Finished Injectable Products

Small-volume parenterals (ampoules, vials, and pre-filled syringes) and large-volume infusions containing glycerin, propylene glycol, or PEG-based excipients are tested directly after appropriate dilution and matrix cleanup. The finished-product test confirms that no contamination was introduced during formulation, holding, or filling operations.

Pharmaceutical Excipients and Raw Materials

Glycerin, propylene glycol, PEG grades, sorbitol solution, and polysorbates received from suppliers represent the primary entry point for DEG. Incoming material testing supplements supplier certificates of analysis, in line with FDA guidance and pharmacopoeial requirements for identity and DEG limit confirmation on every lot.

In-Process Materials and Bulk Solutions

Excipient premixes, diluted bulk formulation solutions, and intermediates may be tested as in-process controls to detect contamination early and to segregate affected batches before filling.

Stability and Retention Samples

Commercial, accelerated, and long-term stability samples are analyzed to monitor whether glycol-related impurity levels change over the product shelf life, supporting shelf-life justification and post-market surveillance.

Testing Items

Testing programs for DEG residue in injectables typically encompass the following items, tailored to the product matrix and regulatory strategy:

• Identification of diethylene glycol by retention time and mass-spectral matching
• Quantitative determination of DEG content, reported in ppm (mg/kg) or percentage
• Simultaneous determination of ethylene glycol (EG), a structurally related co-contaminant frequently present alongside DEG
• Screening for triethylene glycol and other related glycol impurities where applicable
• Limit tests against compendial or toxicologically derived acceptance criteria
• Full impurity profiling of glycol-based excipients used in parenteral formulations

Where both DEG and EG are relevant, validated methods typically quantify both analytes in a single chromatographic run, providing an efficient impurity profile of the glycol matrix.

Testing Methods

Gas Chromatography with Flame Ionization Detection (GC-FID)

GC-FID is the primary compendial technique for glycol impurity analysis. Procedures described in the USP-NF monographs for Glycerin, Propylene Glycol, and Polyethylene Glycol, as well as the Ph. Eur. Glycerol monograph, employ capillary columns with polar polyethylene glycol phases, temperature programming, and external or internal standard calibration. For glycerin as a raw material, the compendial acceptance criterion for DEG and EG is typically not more than 0.1% (1,000 ppm) each. GC-FID provides robust quantification at the tens-of-ppm level and above.

Gas Chromatography-Mass Spectrometry (GC-MS)

GC-MS in selected-ion monitoring (SIM) mode is the method of choice for finished injectable products, where the excipient is present at low concentrations and expected DEG levels are in the low-ppm to sub-ppm range. Characteristic qualifier and quantifier ions provide unambiguous identification, and limits of quantification of approximately 1 ppm or lower are routinely achievable, enabling assessment against tightly derived product-specific limits.

HPLC with Refractive Index or Evaporative Light-Scattering Detection

HPLC-RI and HPLC-ELSD serve as alternative orthogonal techniques where gas chromatography is impractical, for example in thermally sensitive matrices. Their sensitivity is generally lower than GC-based methods, so they are mainly applied to screening or higher-level limit testing.

Sample Preparation

Sample handling is kept simple to minimize loss of the highly polar analyte: product or excipient samples are diluted in water, methanol, or acetonitrile, filtered through low-extractable membranes, and injected directly. For complex injectable matrices, liquid-liquid extraction or solid-phase extraction cleanup may be applied before analysis to remove interfering excipients and drug substance.

Method Validation

All methods are validated in accordance with ICH Q2(R2) and USP General Chapters 621 and 1225, covering specificity, linearity, range, accuracy, repeatability, intermediate precision, limit of detection, limit of quantification, robustness, and solution stability. System suitability criteria, including resolution between DEG, EG, and matrix peaks, are verified in every sequence, and analyses are performed under ISO/IEC 17025-accredited quality systems where third-party testing is required.

Applications

DEG residue testing supports the entire lifecycle of injectable drug products and their excipients:

• Batch release testing of injectable formulations containing glycerin, propylene glycol, or PEG excipients
• Incoming material control and supplier qualification for glycol-based excipients on a lot-by-lot basis
• GMP compliance support during regulatory inspections and product dossier submissions
• Out-of-specification and contamination investigations, including root-cause tracing to raw material sources
• Long-term and accelerated stability programs to confirm impurity control over the product shelf life
• Import and export compliance for products placed on markets with pharmacopoeial DEG limits
• Generic drug development and quality-by-design studies evaluating excipient risk in parenteral formulations

Finished-product limits are typically derived from the excipient specification scaled to the formulation. For example, if glycerin complying with the compendial DEG limit of 0.1% is used at a defined concentration in an injection, the corresponding maximum DEG level in the product is calculated from the excipient content, and tighter toxicology-based limits may be applied where daily doses are high.

FAQ

Why is DEG testing required when the excipient supplier provides a certificate of analysis?

Regulatory guidance, including FDA recommendations issued after international poisoning incidents, requires manufacturers to confirm identity and DEG limits on every lot of glycerin by a validated method rather than relying solely on supplier documentation. Economically motivated adulteration can occur anywhere in the supply chain, and sole reliance on certificates of analysis has been a contributing factor in past contamination events.

What are the applicable standards and limits?

The USP-NF Glycerin monograph limits DEG and EG to not more than 0.1% each, with equivalent requirements in the Ph. Eur., JP, and Chinese Pharmacopoeia monographs. ICH Q3C classifies ethylene glycol as a Class 2 residual solvent with a concentration limit of 620 ppm, and comparable risk-based limits are commonly derived for DEG in finished products based on the excipient content and maximum daily dose.

Which analytical method should be chosen, GC-FID or GC-MS?

GC-FID is well suited to excipient and bulk testing where expected levels are near the 0.1% compendial limit. GC-MS in SIM mode is preferred for finished injectables because its lower limits of quantification and superior specificity allow reliable measurement at low-ppm levels in complex formulation matrices.

How are limits established for the finished injectable product?

Limits are calculated by multiplying the DEG limit of the excipient by its concentration in the formulation, and are then compared with toxicologically acceptable exposure estimates for the intended daily dose. The stricter of the two values is normally adopted as the product specification.

Can DEG appear even if raw materials initially passed testing?

DEG is introduced primarily through contaminated or adulterated excipients, and it may also be present in PEG grades as a residual by-product of polymerization. Lot-by-lot testing, combined with supplier audits and appropriate stability monitoring, provides layered protection against contamination reaching the finished injection.