Determination of Ethylene Glycol Residue by Gas Chromatography: Experimental Method and Technical Gu

Determination of ethylene glycol residue by gas chromatography (GC) is an essential analytical procedure for verifying product safety and regulatory compliance in pharmaceuticals, medical devices, polymers, and packaging materials. This technical article describes the experimental principle, chromatographic conditions, sample preparation workflow, calibration strategy, and validation requirements for accurate quantification of residual ethylene glycol. Laboratory analysts and quality assurance professionals can use this guide to establish a reliable, reproducible, and compliance-ready GC testing protocol.

Technical Overview

Ethylene glycol (EG; CAS No. 107-21-1; molecular formula C2H6O2; molecular weight 62.07 g/mol) is a colorless, odorless, and strongly hygroscopic diol with a boiling point of 197.3 °C and complete miscibility with water and lower alcohols. Its combination of high polarity and relatively low volatility directly influences the design of the gas chromatographic method used for its quantification, favoring polar stationary phases and direct liquid injection techniques.

Residual ethylene glycol originates from several common industrial and pharmaceutical processes. It may remain as a process solvent in the synthesis of active pharmaceutical ingredients and intermediates, form as a hydrolysis byproduct of ethylene oxide sterilization in medical devices, persist as a residual species in polyester and PET polymer production, or enter products through contaminated raw materials and coolant systems.

From a toxicological standpoint, ethylene glycol is metabolized to glycolic and oxalic acids, which can cause central nervous system depression, metabolic acidosis, and severe renal injury. Consequently, it is classified as a Class 2 residual solvent under the ICH Q3C guideline, with a permitted daily exposure of 6.2 mg/day and a concentration limit of 620 ppm in drug products. Gas chromatography, typically coupled with flame ionization detection (FID), is the method of choice for detecting and quantifying ethylene glycol at these stringent regulatory levels.

Testing Samples

Pharmaceutical Products and Active Ingredients

Oral solutions, suspensions, tablets, capsules, and active pharmaceutical ingredients in which ethylene glycol was used as a synthesis or recrystallization solvent are routinely tested. Screening for related glycols such as diethylene glycol is frequently performed in parallel due to documented historical contamination incidents.

Medical Devices and Sterilization Products

Ethylene oxide sterilized single-use devices, including catheters, syringes, infusion sets, dialyzers, breathing circuits, and surgical kits, are tested because ethylene glycol forms when residual ethylene oxide reacts with moisture during sterilization or storage. Residue evaluation supports compliance with ISO 10993-7 requirements.

Polymer, Resin, and Packaging Materials

PET granules, polyester films and fibers, engineering resins, adhesives, coatings, and food-contact articles are analyzed to quantify residual glycol species that may migrate into food, beverages, or drug products.

Cosmetics and Personal Care Products

Creams, lotions, gels, and other formulations containing glycol-based humectants or solvents are tested to confirm that ethylene glycol contamination from raw materials remains below specification limits.

Industrial Chemicals and Fluids

Antifreeze concentrates, heat transfer fluids, brake fluids, and polyester intermediates are tested for ethylene glycol purity and for cross-contamination during production, storage, and transport.

Testing Items

The core testing item is the quantitative determination of residual ethylene glycol content, typically reported in mg/kg (ppm), mg/g, or as a percentage of the sample mass. Depending on the study objectives, the following items are commonly included in a complete test package:

• Quantification of ethylene glycol residue against an established specification limit.
• Optional simultaneous screening of diethylene glycol and propylene glycol.
• System suitability verification, including resolution, theoretical plates, and peak tailing.
• Detection and quantification limits demonstrating method sensitivity relative to the regulatory threshold.
• Confirmation of peak identity by retention time matching or GC-MS when matrix interference is suspected.

Testing Methods

Method Principle

The sample is dissolved or extracted in a suitable diluent such as purified water, methanol, or dimethylformamide. An aliquot of the resulting solution is injected into a gas chromatograph equipped with a polar polyethylene glycol (PEG) capillary column and a flame ionization detector. Ethylene glycol is separated from matrix components and related glycols, and quantification is achieved using an external standard calibration curve or an internal standard ratio method.

Instruments and Reagents

• Gas chromatograph equipped with FID (GC-MS optional for confirmation).
• Polar capillary column, for example a PEG (WAX-type) phase, 30 m × 0.32 mm I.D. × 0.25 µm film.
• Ethylene glycol reference standard with certified purity of 99.5 percent or higher.
• Internal standard such as 1,4-butanediol or 1,3-propanediol, if the internal standard method is applied.
• Chromatography-grade water, methanol, or DMF as diluent; membrane filters (0.22 or 0.45 µm); sonicator; analytical balance; volumetric glassware.

Typical Chromatographic Conditions

• Column: polyethylene glycol (WAX) capillary column, 30 m × 0.32 mm × 0.25 µm or equivalent.
• Carrier gas: high-purity nitrogen or helium at a constant flow of approximately 1.0 to 2.0 mL/min.
• Inlet temperature: 230 to 250 °C; split injection with a split ratio of 10:1; injection volume 1.0 µL.
• Oven program: initial 100 °C held for 1 min, ramped at 15 °C/min to 220 °C, held for 5 min.
• FID temperature: 260 to 280 °C, with standard hydrogen, air, and makeup gas flows.

Sample Preparation

For pharmaceutical samples, an accurately weighed portion of 0.5 to 1.0 g is transferred to a volumetric flask, diluted to volume with the selected diluent, sonicated for approximately 30 minutes, and filtered or centrifuged before injection. For medical devices and polymeric materials, the sample is cut into small pieces, a defined mass is extracted with water at 37 °C for one hour (or sonicated for 30 minutes), and the extract is analyzed. Standard solutions are prepared gravimetrically at a minimum of five concentration levels bracketing the specification limit.

Calibration and Quantification

A calibration curve is constructed by plotting peak area (or peak area ratio against the internal standard) versus concentration. The correlation coefficient should typically be 0.999 or better. The ethylene glycol concentration in the sample solution is interpolated from the regression equation, and the residual content is calculated by accounting for the dilution volume and sample mass, with results expressed in ppm or mg/g. Duplicate preparations and injections are recommended to confirm measurement stability.

Method Validation Parameters

• Specificity: resolution greater than 1.5 between ethylene glycol and adjacent peaks such as propylene glycol and diethylene glycol.
• Linearity: at least five concentration levels covering 50 to 150 percent of the limit, with r of 0.999 or higher.
• Accuracy: mean recovery within 95 to 105 percent at three spike levels.
• Precision: repeatability RSD of 5.0 percent or lower for six independent preparations.
• Sensitivity: LOD and LOQ established at signal-to-noise ratios of approximately 3 and 10, respectively.
• Robustness and solution stability: verified under small deliberate variations of flow rate, oven program, and preparation hold time.

Applications

The GC-based ethylene glycol residue test supports a wide range of quality and regulatory activities across multiple industries:

• Batch release testing and routine quality control of pharmaceutical products and active ingredients.
• Ethylene oxide sterilization validation and routine residue monitoring for medical devices under ISO 10993-7 and ISO 11135 frameworks.
• Compliance demonstration for food-contact materials and packaging migration studies.
• Stability studies evaluating glycol residue changes over shelf life and storage conditions.
• Extractables and leachables studies in packaging compatibility assessments.
• Root-cause investigation of out-of-specification results and supplier raw material qualification.

FAQ

Question 1: Why is direct injection usually preferred over static headspace GC for ethylene glycol?

Answer: Ethylene glycol has a high boiling point and low vapor pressure, which results in poor partitioning into the headspace phase and insufficient sensitivity under standard headspace conditions. Direct liquid injection of an aqueous or solvent extract onto a polar column provides substantially better detection limits, while derivatization followed by GC-MS can be used when additional sensitivity or confirmatory information is required.

Question 2: What regulatory limit applies to ethylene glycol in pharmaceuticals?

Answer: Under ICH Q3C, ethylene glycol is a Class 2 residual solvent with a permitted daily exposure of 6.2 mg/day, corresponding to a concentration limit of 620 ppm for drug products administered at a maximum daily dose of 10 g. For medical devices, acceptable residue levels are defined in ISO 10993-7 according to device contact category and duration.

Question 3: Can the method distinguish ethylene glycol from diethylene glycol and propylene glycol?

Answer: Yes. A polar polyethylene glycol column provides adequate retention and separation of these glycols, and specificity should be confirmed during validation with a resolution of at least 1.5. When coelution or complex matrices are a concern, identity can be confirmed using a second column of different polarity or by GC-MS.

Question 4: What precautions are important during sample handling?

Answer: Because ethylene glycol is highly hygroscopic, standards should be prepared gravimetrically in a controlled environment, and samples should be stored in tightly sealed containers to prevent moisture uptake and cross-contamination. Extracts should be analyzed promptly or their solution stability should be demonstrated during method validation.