Ethylene glycol (EG) residue testing by high-performance liquid chromatography (HPLC) is a critical quality control procedure for detecting and quantifying toxic glycol impurities in pharmaceuticals, polymer materials, cosmetics, and medical devices. This technical article outlines the analytical principles, applicable sample types, chromatographic methods, method validation requirements, and regulatory considerations involved in accurate EG residual determination.
Technical Overview
Ethylene glycol (EG, CAS No. 107-21-1) is a low-molecular-weight dihydric alcohol widely used as a raw material in polyester (PET) production, antifreeze formulations, humectants, and as a byproduct or residual monomer in polyethylene glycol (PEG) excipients. Due to its documented nephrotoxicity and central nervous system toxicity, even trace levels of residual EG in pharmaceuticals, food-contact materials, and consumer products pose a significant safety risk. Global contamination incidents involving oral liquid medications have further heightened regulatory scrutiny of glycol impurities.
Because ethylene glycol lacks a strong UV chromophore and is highly polar, liquid chromatography coupled with universal detection—most commonly refractive index detection (RID), but also evaporative light scattering detection (ELSD), charged aerosol detection (CAD), or mass spectrometry (MS)—represents the most reliable analytical platform for its quantification. A well-developed HPLC method can selectively separate EG from structurally related impurities such as diethylene glycol (DEG), triethylene glycol, and glycerol, achieving quantification limits at the low ppm level in complex matrices.
Accredited testing laboratories perform EG residue analysis under GLP/GMP-aligned quality systems, applying method validation principles consistent with ICH Q2(R2) to ensure specificity, linearity, accuracy, precision, and robustness. Results are reported against established acceptance criteria drawn from pharmacopeial monographs (USP–NF, Ph. Eur., ChP) and WHO guidance, which typically impose limits at or below 0.1% (1000 ppm), and as low as 0.25% combined for EG and DEG in excipient-grade polyethylene glycols.
Testing Samples
Pharmaceutical Raw Materials and Excipients
Polyethylene glycols (PEG 200, 300, 400, 600, and higher grades) are routinely screened for residual EG introduced during polymerization. Testing verifies compliance with pharmacopeial monographs prior to release for formulation use.
Pharmaceutical Finished Products
Oral solutions, cough syrups, elixirs, topical gels, and suppositories are analyzed to detect EG contamination arising from excipients, manufacturing equipment, or intentional adulteration with glycol-based diluents.
Polymer Materials and Packaging
Cosmetics and Personal Care Products
Humectants, creams, lotions, toothpaste, and glycol-based cosmetic ingredients are evaluated for EG content in accordance with regional cosmetic safety regulations.
Industrial Chemicals and Medical Devices
Chemical intermediates, antifreeze concentrates, dialysis-related consumables, and polyurethane- or polyester-based medical device materials may be tested to support material qualification and biocompatibility assessments.
Testing Items
Standard testing programs for ethylene glycol residue determination typically include the following deliverables:
- Quantitative determination of residual ethylene glycol content (reported in ppm, mg/kg, or percentage).
- Simultaneous determination of diethylene glycol (DEG) and, where required, triethylene glycol (TEG).
- Separation and identification of EG from matrix interferences and structurally similar glycols.
- Assessment of results against pharmacopeial limits (USP, Ph. Eur., ChP) and WHO-recommended acceptable limits.
- Full method validation packages covering specificity, linearity, range, accuracy, precision, LOD/LOQ, and robustness.
Testing Methods
HPLC with Refractive Index Detection (HPLC–RID)
The reference method for glycol quantification employs an ion-exclusion or carbohydrate analysis column (sulfonated polystyrene–divinylbenzene resin in H+ form, or a calcium-loaded carbohydrate column, typically 300 mm × 7.8 mm). A dilute sulfuric acid mobile phase (approximately 0.005 mol/L) or pure water is delivered isocratically at 0.5–0.6 mL/min, with the column maintained at 50–85 °C depending on the stationary phase. The refractive index detector, operated under strict temperature control, provides stable and reproducible quantification using an external standard calibration curve.
HILIC with Evaporative and Aerosol Detection
Hydrophilic interaction liquid chromatography (HILIC) on amide or bare silica columns, using acetonitrile-rich mobile phases, offers improved retention of EG and superior compatibility with ELSD and charged aerosol detection. CAD in particular delivers significantly enhanced sensitivity and a wide dynamic range, making it suitable for matrices requiring quantification below 10 ppm.
LC–MS Confirmation
For complex matrices or investigative cases, HILIC–LC–MS/MS monitoring the protonated molecular ion of EG (m/z 63) provides unambiguous confirmation of identity and trace-level sensitivity. LC–MS is frequently employed as an orthogonal confirmatory technique alongside RID screening results.
Sample Preparation
Liquid samples are typically diluted with water or mobile phase, vortexed, and filtered through a 0.22 μm or 0.45 μm membrane prior to injection. Viscous PEG excipients are accurately weighed, dissolved in a defined solvent system, and centrifuged or filtered to remove the polymer matrix. Solid polymers and packaging materials undergo aqueous extraction, reflux extraction, or accelerated solvent extraction to liberate residual glycols. Solid-phase extraction (SPE) cleanup is applied where endogenous matrix components interfere with chromatographic separation.
Method Validation
Validated methods must demonstrate the following performance characteristics in accordance with ICH Q2(R2):
- Specificity: complete resolution of EG from DEG, TEG, glycerol, and matrix peaks.
- Linearity: correlation coefficient typically r ≥ 0.999 across the validated working range.
- Accuracy: mean recovery within 95–105% at spiked concentration levels.
- Precision: repeatability RSD ≤ 3% and intermediate precision RSD ≤ 5%.
- Sensitivity: LOD and LOQ established at signal-to-noise ratios of 3:1 and 10:1, respectively.
- Robustness and solution stability evaluated across reasonable variations in method parameters.
Applications
Ethylene glycol residue testing by liquid chromatography supports a broad range of quality, safety, and regulatory objectives across multiple industries:
- Release testing and batch certification of pharmaceutical excipients and finished dosage forms.
- Root-cause investigation and contamination screening following glycol-related safety alerts.
- Migration studies for PET packaging in contact with food, beverages, and drug products.
- Stability programs monitoring glycol impurity behavior over shelf life.
- Cosmetic ingredient safety evaluation and supplier qualification.
- Medical device material characterization and extractables/leachables studies.
- Research and development support for polymerization process optimization and impurity profiling.
FAQ
Q: Why is ultraviolet (UV) detection not suitable for ethylene glycol analysis?
A: Ethylene glycol contains no conjugated chromophore and therefore exhibits negligible UV absorption at routine detection wavelengths. Universal detectors such as RID, ELSD, CAD, or mass spectrometric detection are required for direct quantification, although pre-column derivatization can be used to enable UV or fluorescence detection when higher sensitivity is needed.
Q: Can ethylene glycol and diethylene glycol be determined in a single analysis?
A: Yes. Validated HPLC methods employing ion-exclusion or carbohydrate columns provide baseline separation of EG, DEG, and related glycols within a single chromatographic run, allowing simultaneous quantification of all relevant impurities.
Q: What detection limits can be achieved?
A: With RID, practical quantification limits typically fall in the range of 10–50 ppm depending on the matrix. HILIC–CAD and LC–MS/MS methods can extend sensitivity to the low ppm or sub-ppm level, which is sufficient to demonstrate compliance with the strictest pharmacopeial and WHO limits.
Q: How much sample is required for testing?
A: In most cases, 1–5 g (or an equivalent liquid volume) of homogenized sample is sufficient, including material needed for replicate preparation and confirmatory testing.
Q: What is the typical turnaround time?
A: Standard projects are generally completed within 5–10 working days after sample receipt, while urgent testing and investigative analyses can often be accommodated on an expedited schedule by prior arrangement.
Q: Which regulatory standards govern acceptable ethylene glycol levels?
A: Acceptance criteria are matrix-dependent and are defined by pharmacopeial monographs such as USP–NF and Ph. Eur. for PEG excipients, as well as WHO guidance for contaminated medicines, which recommends controlling EG and DEG to stringent percentage-level limits. Testing laboratories tailor acceptance criteria to the specific product, intended route of administration, and applicable regulatory framework.