⛽ Hydrocarbon Oil Index in Water (GC-FID)
Determination of hydrocarbon oil index (C10–C40 hydrocarbons) in surface waters and wastewater by hexane extraction, Florisil cleanup, and gas chromatography with FID detector.
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
| Parameter | Value |
|---|---|
| Measurement range | 0.1–1,000 mg/L (with dilution) |
| Limit of quantification (LOQ) | 0.1 mg/L |
| Hydrocarbon range | n-C10 (decane) to n-C40 (tetracontane) |
| Analyte boiling point | 175–525°C |
| Precision (CV) | 5–15% at concentration >1 mg/L |
| Sample volume | 1 L (typical) |
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.
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).
3. Hexane extraction II
Drain aqueous phase. Repeat extraction with second 50 mL hexane portion. Combine organic phases.
4. Extract drying
Pass combined hexane extracts through funnel with anhydrous Na₂SO₄ (20 g). Collect dry extract in flask.
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).
6. Extract concentration
Concentrate cleaned extract under gentle N₂ stream to 1 mL volume. Do not evaporate to dryness!
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.
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.
9. Calibration
Calibration curve: 0, 0.5, 1, 5, 10, 50 mg/L mineral oil mixture in hexane. Min. 5 points, R² > 0.999.
10. Quality control
Hexane blank < LOQ. CRM (if available) or spike (recovery 70–120%). Duplicate: RPD < 25%.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Gas chromatograph with FID | Agilent 8890 GC, Shimadzu GC-2030, Thermo TRACE 1310 | 150,000–400,000 PLN |
| Non-polar capillary column | Agilent DB-1 (10 m × 0.32 mm × 0.1 µm), Restek Rtx-1 | 1,500–3,000 PLN |
| Liquid autosampler | Agilent 7693A, Shimadzu AOC-20i, Thermo AI/AS 1310 | 40,000–80,000 PLN |
| Glass separatory funnels 1–2 L | DURAN, Simax, borosilicate with PTFE stopcock | 100–300 PLN/pc |
| Florisil columns | Ready SPE Florisil cartridges 6 mL/1 g or glass columns with Florisil | 15–40 PLN/pc (cartridges) |
| Nitrogen evaporator | Organomation N-EVAP, TurboVap LV, N₂ stream | 5,000–30,000 PLN |
| Hydrogen/air for FID | Hydrogen generator (Linde HiQ H₂) + synthetic compressed air | H₂ generator: 15,000–40,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| n-Hexane (C₆H₁₄) | 110-54-3 | Pesticide grade / picograde quality, extraction solvent, 100 mL per extraction |
| Florisil (magnesium silicate) | 1343-88-0 | Granulation 150–250 µm (60–100 mesh), activated 140°C/16 h, cleanup adsorbent |
| Anhydrous sodium sulfate (Na₂SO₄) | 7757-82-6 | Analytical grade, drying agent, calcined 400°C/4 h |
| Diluted hydrochloric acid (HCl) | 7647-01-0 | For sample acidification to pH < 2 before extraction |
| Mineral oil standard mixture | — | Equilibrated mixture of two mineral oils (type A + type B), certified, e.g., Carl Roth, Merck |
| n-Decane (C10) and n-Tetracontane (C40) | 124-18-5 / 4181-95-7 | Integration range markers, purity >99% |
| Nitrogen 5.0 | 7727-37-9 | For extract concentration under N₂ stream, purity 99.999% |
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