⛽ Hydrocarbon Oil Index in Water (GC-FID)

Physicochemistry PN-EN ISO 9377-2

Overview

Hydrocarbon Oil Index (HOI or TPH — Total Petroleum Hydrocarbons) is an indicator of water contamination with petroleum substances — oils, greases, fuels, and their degradation products. Standard PN-EN ISO 9377-2 defines HOI as sum of non-polar organic compounds extractable with hexane, not adsorbed on Florisil, eluting on non-polar GC column in range between n-decane (C10) and n-tetracontane (C40).

The method involves extracting water sample with hexane (or pentane), cleaning extract on Florisil column (magnesium silicate) to remove polar compounds (fats, esters, fatty acids, resins), and GC-FID analysis. Chromatogram is integrated as sum of all peaks between n-C10 and n-C40. Limit of quantification is 0.1 mg/L.

HOI is key parameter in industrial wastewater control (refineries, mechanical plants, fuel stations), surface water monitoring (fuel leaks, ecological disasters), and oil separator effectiveness assessment. Permissible concentration in wastewater discharged to sewage system is regulated by regulation on wastewater discharge conditions.

Florisil cleanup is crucial step — it distinguishes petroleum hydrocarbons from natural organic substances (animal and vegetable fats). Without this step, result would be overestimated by biogenic substances.

Method principle

Water sample (1 L) is acidified with HCl to pH < 2 and extracted twice with hexane (2 × 50 mL) in separatory funnel. Organic phases are combined and dried over anhydrous Na₂SO₄. Extract is cleaned by passing through Florisil column (150–250 µm, activated at 140°C/16 h) — polar compounds (fats, esters, alcohols, acids) are adsorbed, while hydrocarbons pass through. Cleaned extract is concentrated under N₂ stream to 1 mL and analyzed by gas chromatograph with flame ionization detector (FID). Non-polar capillary column (e.g., DB-1, 10–30 m). Temperature program: 40°C → 300–350°C. Peak area between n-C10 (Tr ~5 min) and n-C40 (Tr ~30 min) is integrated and calculated to mg/L from calibration curve (mineral oil mixture).

Applications

• Industrial wastewater control — refineries, workshops, fuel stations
• Surface water monitoring — fuel leaks, ecological disasters
• Oil separator effectiveness assessment
• Groundwater testing in petroleum-contaminated sites
• Stormwater control from industrial areas and roads
• Wastewater from car washes and metalworking plants monitoring
• Landfill leachate testing

Key parameters

Standard

Standard number

PN-EN ISO 9377-2:2003

Title (PL)

Jakość wody — Oznaczanie indeksu oleju mineralnego — Część 2: Metoda z zastosowaniem ekstrakcji rozpuszczalnikiem i chromatografii gazowej

Title (EN)

Water quality — Determination of hydrocarbon oil index — Part 2: Method using solvent extraction and gas chromatography

Step-by-step procedure

1. Sample preparation

   Collect 1 L sample in glass bottle. Acidify with HCl to pH < 2. Do not filter — analyze whole sample with suspension. Store at 4°C, analyze within 7 days.

   🌡 Temperature: 4°C

2. Hexane extraction I

   Transfer sample to 2 L separatory funnel. Add 50 mL n-hexane. Shake vigorously 5 min (carefully — gas release). Let phases separate (10 min).

   ⏱ Time: 15 min

3. Hexane extraction II

   Drain aqueous phase. Repeat extraction with second 50 mL hexane portion. Combine organic phases.

   ⏱ Time: 15 min

4. Extract drying

   Pass combined hexane extracts through funnel with anhydrous Na₂SO₄ (20 g). Collect dry extract in flask.

   ⏱ Time: 10 min

5. Florisil cleanup

   Pass extract through Florisil column (2 g, activated 140°C/16 h). Elution with 20 mL hexane. Florisil retains polar compounds (fats, esters).

   ⏱ Time: 15 min

6. Extract concentration

   Concentrate cleaned extract under gentle N₂ stream to 1 mL volume. Do not evaporate to dryness!

   ⏱ Time: 20–40 min

7. GC-FID analysis

   Inject 1 µL extract (split 1:10). Temp. program: 40°C (1 min) → 10°C/min → 320°C (5 min). Carrier gas H₂ 2 mL/min. FID detector 350°C.

   ⏱ Time: 35 min

8. Chromatogram integration

   Integrate sum of peaks between n-C10 and n-C40 (baseline-to-baseline method). Subtract blank (hexane). Read result from calibration curve.

   ⏱ Time: 5 min

9. Calibration

   Calibration curve: 0, 0.5, 1, 5, 10, 50 mg/L mineral oil mixture in hexane. Min. 5 points, R² > 0.999.

   ⏱ Time: 2 h

10. Quality control

    Hexane blank < LOQ. CRM (if available) or spike (recovery 70–120%). Duplicate: RPD < 25%.

    ⏱ Time: 30 min

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• n-Hexane — highly flammable, neurotoxic (polyneuropathy) — work exclusively in fume hood!
• Hydrochloric acid — corrosive, irritant vapors, use eye and hand protection
• Florisil — dust irritant to respiratory tract, use dust mask
• Hydrogen (FID) — highly flammable and explosive, check installation tightness
• Collect hexane waste in flammable organic waste container
• Safety glasses, lab coat, and nitrile gloves mandatory

Frequently asked questions