Ethylene glycol residue testing by high-performance liquid chromatography (HPLC) is a critical quality control procedure for detecting and quantifying residual ethylene glycol in pharmaceuticals, cosmetics, polymers, and chemical products. This article provides a comprehensive technical overview of HPLC-based residual ethylene glycol determination, covering sample types, chromatographic conditions, method validation, regulatory limits, and practical applications. Whether you are establishing an in-house method or outsourcing analysis, this guide explains the key technical considerations for accurate and compliant residual solvent testing.
Technical Overview
Ethylene glycol (EG, CAS No. 107-21-1) is a colorless, odorless, and highly hygroscopic dihydroxy alcohol widely used as a reaction solvent, chemical intermediate, cryoprotectant, and raw material in the production of polyethylene terephthalate (PET), polyester resins, and polyethylene glycols. Because ethylene glycol is metabolized in the human body to glycolic acid and oxalic acid, which can cause central nervous system depression, metabolic acidosis, and acute renal failure, residual ethylene glycol in consumer and pharmaceutical products must be strictly controlled.
The International Council for Harmonisation (ICH) guideline Q3C (R8) classifies ethylene glycol as a Class 2 residual solvent, assigning a permitted daily exposure (PDE) of 6.2 mg/day and a concentration limit of approximately 620 ppm for drug products administered at a maximum daily dose of 10 g. Similar controls are imposed by pharmacopoeial monographs and cosmetics regulations, particularly for glycerin- and propylene glycol-based formulations where ethylene glycol and diethylene glycol (DEG) contamination incidents have occurred historically.
Since ethylene glycol is a small, highly polar molecule with no significant UV chromophore and poor volatility characteristics in gas chromatography, high-performance liquid chromatography (HPLC) coupled with refractive index detection (RID), evaporative light scattering detection (ELSD), charged aerosol detection (CAD), or mass spectrometry (MS) is the preferred analytical platform for sensitive and selective residue determination.
Testing Samples
Pharmaceutical Samples
Active pharmaceutical ingredients (APIs), synthetic intermediates, process residues, oral liquid preparations such as elixirs and syrups, and pharmaceutical excipients including glycerin, propylene glycol, and low-molecular-weight polyethylene glycols are routinely screened for ethylene glycol residues in accordance with ICH Q3C and pharmacopoeial requirements.
Cosmetic and Personal Care Products
Toothpaste, mouthwash, skin creams, lotions, and humectant-containing formulations are tested to verify the absence or controlled level of ethylene glycol and related diols, supporting compliance with safety assessments required by cosmetic regulatory frameworks.
Polymer and Chemical Products
PET chips, polyester resins, textile fibers, antifreeze formulations, brake fluids, heat transfer fluids, and unsaturated polyester products may retain unreacted ethylene glycol monomer, which must be quantified to confirm product specification and process completeness.
Food Contact and Environmental Samples
Food packaging materials, food additives, and migrating species from PET containers, as well as water and wastewater samples from industrial discharge, can be analyzed using the same HPLC methodology after appropriate sample preparation.
Testing Items
The scope of a typical ethylene glycol residue testing program includes the following analytical items:
- Quantitative determination of residual ethylene glycol content (reported in ppm, mg/kg, or percentage).
- Simultaneous detection of diethylene glycol (DEG) and triethylene glycol (TEG) as related impurities.
- Multi-component residual solvent profiling for Class 2 and Class 3 solvents where required.
- Purity assessment and impurity identification in glycerin, propylene glycol, and PEG excipients.
- Method development, method validation, and system suitability verification in accordance with ICH Q2 (R2).
Testing Methods
Chromatographic Principle
The method separates ethylene glycol from the sample matrix and co-existing glycols using a ligand-exchange and ion-exclusion mechanism on a sulfonated polystyrene-divinylbenzene resin column operated in the hydrogen form. Because ethylene glycol lacks a UV-absorbing chromophore, a universal detector such as refractive index (RID) is most commonly employed, with ELSD, CAD, and MS offered as higher-sensitivity alternatives.
Typical HPLC Conditions
The following chromatographic parameters represent a widely adopted configuration and may be optimized for specific matrices:
- Column: Sulfonated polystyrene-divinylbenzene cation-exchange column, hydrogen form, 300 mm × 7.8 mm, 8 to 9 μm particle size or equivalent carbohydrate analysis column.
- Mobile phase: 0.005 mol/L dilute sulfuric acid in water, isocratic elution, freshly prepared and degassed.
- Flow rate: 0.5 to 0.6 mL/min.
- Column temperature: 50 to 65 °C.
- Detector: Refractive index detector, maintained at 33 to 40 °C for baseline stability.
- Injection volume: 10 to 20 μL.
- Run time: 20 to 30 minutes, depending on column dimensions and matrix complexity.
Sample Preparation
Solid samples are accurately weighed and extracted or dissolved in water or mobile phase with the aid of sonication. Liquid samples and viscous formulations are diluted gravimetrically with the mobile phase. The resulting solutions are centrifuged if necessary and filtered through a 0.45 μm (or 0.22 μm) hydrophilic membrane filter before injection. For RID-based methods, careful control of temperature and consistent reference cell equilibration are essential to minimize baseline drift.
Calibration and Quantification
Quantification is performed using the external standard method with a multi-point calibration curve prepared from an ethylene glycol reference standard across the expected concentration range, typically covering 50 percent to 150 percent of the specification level. Linearity is confirmed with a correlation coefficient of not less than 0.999. Where required, diethylene glycol is calibrated and quantified in the same injection sequence.
Method Validation
A validated method suitable for regulatory submission should demonstrate the following performance attributes:
- Specificity: Complete resolution of ethylene glycol from the sample matrix, DEG, and adjacent glycol peaks.
- Linearity: Established over the analytical range with acceptable residuals and y-intercept.
- Accuracy: Mean recovery between 95 and 105 percent at spike levels of 50, 100, and 150 percent of the limit.
- Precision: Repeatability (six independent preparations) with RSD typically not more than 5 percent, plus intermediate precision across analysts, days, and instruments.
- Sensitivity: Limit of detection (LOD) and limit of quantification (LOQ) established from signal-to-noise ratios of 3:1 and 10:1 respectively, with the LOQ comfortably below the regulatory limit.
- Robustness: Evaluation of small, deliberate variations in flow rate, column temperature, and mobile phase concentration.
- Solution stability: Verification that standard and sample solutions remain stable for the duration of the analytical sequence.
Applications
HPLC-based ethylene glycol residue testing supports a broad range of industrial and regulatory applications:
- Pharmaceutical quality control and batch release testing of APIs, intermediates, and oral liquid dosage forms.
- Excipient qualification for glycerin, propylene glycol, and polyethylene glycol supply chains.
- Cosmetic product safety evaluations and supplier verification programs.
- Polymer and PET manufacturing process monitoring to confirm monomer conversion and product specification.
- Regulatory dossier support, including impurity profiling and stability-indicating method data for new drug applications.
- Research and development activities, including formulation screening, process optimization, and extractables and leachables studies.
- Environmental monitoring of industrial effluents and assessment of migration from food contact materials.
FAQ
Why is HPLC preferred over GC for ethylene glycol residue testing?
Ethylene glycol is highly polar, non-volatile, and thermally labile in a way that produces broad, tailing peaks in gas chromatography, often requiring derivatization. HPLC with RID or ELSD allows direct aqueous injection without derivatization, delivering better reproducibility and simpler sample preparation.
What is the regulatory limit for residual ethylene glycol?
Under ICH Q3C (R8), ethylene glycol is a Class 2 solvent with a PDE of 6.2 mg/day, corresponding to a concentration limit of approximately 620 ppm for a drug product with a maximum daily intake of 10 g. Product-specific monographs may impose tighter limits.
What is the typical detection limit of the HPLC method?
With RID, a quantification limit of approximately 10 to 50 ppm is routinely achievable for clean matrices. With ELSD, CAD, or LC-MS detection, quantification limits at the low ppm level or below can be obtained, depending on the matrix.
How much sample is required for testing?
Generally, 1 to 5 g of solid or 5 to 10 mL of liquid sample is sufficient for a full assay including duplicate preparation and spiked recovery verification. Additional quantity is recommended if method development or validation is required.
What is the typical turnaround time?
Standard residue testing using an established method is typically completed within 5 to 10 working days after sample receipt. Method development and full validation projects may require 3 to 6 weeks depending on complexity and regulatory expectations.
Can ethylene glycol and diethylene glycol be determined in a single run?
Yes. On ion-exclusion and ligand-exchange columns with RID detection, ethylene glycol, diethylene glycol, and triethylene glycol are well resolved and can be quantified simultaneously in a single chromatographic injection, which improves efficiency for glycerin and glycol excipient screening.