Overview
EN ISO 9289:1995 (ISO 9289:1991) defines a validated laboratory method for the determination of free residual hexane in oilseed residues after extraction with hydrocarbon-based solvents. The standard covers the analytical principle, required reagents and materials, apparatus, sampling and sample storage, a head-space desorption and gas‑chromatographic test procedure, expression of results and reporting requirements. The method targets the fraction of hexane desorbed rapidly by heating (the “free residual hexane”), measured as mg n‑hexane/kg of sample.
Key topics and technical requirements
- Principle: desorption of hexane by heating a sealed sample at 80 °C and analysis of the head‑space vapour by gas chromatography with flame ionization detection (GC‑FID).
- Sampling & storage: follow ISO 5500 sampling; laboratory samples must be hermetically sealed (preferably crimped metal boxes) and stored at 4 °C; avoid plastic containers to prevent loss of hexane.
- Sample prep & test routine:
- Typical test portion: 5 g of oilseed residue in a 20 ml septum flask.
- Heat sealed flask at 80 °C for 60 min; withdraw 1 ml head‑space with a heated gas syringe and inject to GC.
- Add internal standard (cyclohexane or n‑heptane) to calibration and sample flasks; calibration uses oilseed residues of the same origin/moisture with defined solvent additions.
- Apparatus & GC conditions:
- GC with FID and integrator/recorder; either a packed column (~2 m × 3.2 mm) coated with SE‑30 or a capillary column (~30 m × 0.3 mm) with SE‑30 film.
- Injector/detector ≈ 120 °C; oven ≈ 40 °C; carrier gas pressure ≈ 0.3 bar; 1/100 split injection for capillary columns.
- Calibration & quality control: calibration factor F determined from a set of calibration flasks with known solvent additions; recommended use of technical hexane for calibration.
- Results & reporting: express free residual hexane in mg/kg; report method details, operating conditions and any deviations or incidents.
Practical applications and users
- Laboratories performing quality control on oilseed meal, extracted cakes and processing residues.
- Oilseed processors and solvent‑extraction plant quality assurance teams monitoring solvent carry‑over.
- Food safety and regulatory agencies assessing residual solvent levels in feed/food production streams.
- Analytical service providers offering residue testing using head‑space GC methods.
Benefits: the method isolates the rapidly desorbable (free) fraction of hexane, providing actionable data for process control, worker safety assessments and regulatory compliance.
Related standards
- ISO 5500 - sampling of oilseed residues (referenced for sampling protocol).
- ISO 8892 (referred to in the standard) - method for total residual hexane (used to compare free vs total residual hexane).
- ISO 5725 - precision statistics for repeatability and reproducibility (inter‑laboratory performance assessment).
Keywords: EN ISO 9289:1995, free residual hexane, oilseed residues, hexane determination, head‑space GC, gas chromatography, sampling, calibration, ISO 9289:1991.
