⛽ Hydrocarbon Oil Index in Water — Petroleum Hydrocarbons (GC-FID)
In short
Determination of mineral oil index (C₁₀–C₄₀) in water by solvent extraction, Florisil cleanup, and gas chromatography GC-FID. The test is performed according to PN-EN ISO 9377-2:2003; Hydrocarbon range: C₁₀ (n-decane) to C₄₀ (n-tetracontane). The procedure comprises 8 steps (about 2 h 5 min–2 h 20 min in total); it is used for: Monitoring of wastewater from refineries and petrochemical plants, Control of groundwater (fuel contamination from leaking tanks), Study of surface waters (oil spill, ecological disasters).
At a glance
- Standard: PN-EN ISO 9377-2:2003
- Category: Physicochemistry
- Procedure steps: 8
- Total time of stages: 2 h 5 min–2 h 20 min
- Hydrocarbon range: C₁₀ (n-decane) to C₄₀ (n-tetracontane)
- Limit of quantification: 0.1 mg/L
- Working range: 0.1–1 000 mg/L (with dilution)
Overview
The hydrocarbon oil index (HOI) is a key parameter for assessing water contamination with petroleum hydrocarbons. The ISO 9377-2 method measures the sum of non-polar hydrocarbons with chains from n-decane (C₁₀) to n-tetracontane (C₄₀), covering a wide range of petroleum substances: gasoline, diesel, lubricants, machine oils, fuel oil.
This standard is one of the most commonly used standards in environmental analysis in Europe. The HOI index is a mandatory parameter in monitoring wastewater from petroleum industry facilities, fuel stations, automotive workshops, ports, and in assessing groundwater contamination with petroleum substances.
The method includes extraction of hydrocarbons from water with organic solvent (n-pentane, n-hexane or their mixtures), cleanup of extract on Florisil (magnesium silicate adsorbent) to remove polar substances (fats, fatty acids, resins), and subsequent GC-FID chromatographic analysis.
In the literature the same result is sometimes called TPH (Total Petroleum Hydrocarbons). The standard defines the hydrocarbon oil index through the procedure itself: it is the sum of non-polar compounds extractable with a solvent, not adsorbed on Florisil and eluting on a non-polar GC column between n-decane and n-tetracontane — so the result depends on strict adherence to the procedure, not on the type of oil in the sample.
Cleanup on Florisil is the step that decides the reliability of the result: it is what distinguishes petroleum hydrocarbons from natural organic substances — animal and vegetable fats. Omitting this step inflates the result by biogenic substances, which in wastewater from food-processing plants or car washes can change the assessment of compliance with the permit.
Method principle
The method consists of three stages: (1) extraction, (2) Florisil cleanup, (3) GC-FID analysis.
1. Extraction: Water sample is extracted with hydrocarbon solvent (boiling range 36–69°C, e.g., n-pentane or n-hexane). Petroleum hydrocarbons pass into organic phase.
2. Florisil cleanup: Extract is passed through Florisil column (MgSiO₃, 150–250 µm, activated at 140°C/16h). Florisil retains polar compounds (fats, esters, acids), passing non-polar hydrocarbons.
3. GC-FID analysis: Cleaned extract is injected on capillary GC column with non-polar phase (e.g., DB-1 or HP-5). Flame ionization detector (FID) records signal proportional to mass of hydrocarbons burned in H₂/air flame. Total peak area is integrated between retention times of n-decane (C₁₀) and n-tetracontane (C₄₀).
HOI [mg/L] = (sample area − blank area) × calibration factor / sample volume
Applications
- Monitoring of wastewater from refineries and petrochemical plants
- Control of groundwater (fuel contamination from leaking tanks)
- Study of surface waters (oil spill, ecological disasters)
- Monitoring of stormwater from fuel stations and parking lots
- Control of wastewater from automotive workshops and car washes
- Assessment of remediation of sites contaminated with petroleum substances
- Assessment of the effectiveness of petroleum substance separators
- Study of leachate from waste landfills
- Monitoring of wastewater from metalworking plants
Key parameters
| Parameter | Value |
|---|---|
| Hydrocarbon range | C₁₀ (n-decane) to C₄₀ (n-tetracontane) |
| Limit of quantification | 0.1 mg/L |
| Working range | 0.1–1 000 mg/L (with dilution) |
| Boiling temperature of analytes | 175–525°C |
| Sample volume | 1 L (typically) |
| Precision (CV) | 5–15 % at concentrations above 1 mg/L |
| Detector | FID (Flame Ionization Detector) |
| GC column | Capillary, non-polar phase (DB-1, HP-5), 10–30 m |
| GC analysis time | 5–20 min (with oven cooling) |
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
-
Sample preparation
Collect sample in 1–2 L glass bottle (not PE/PP — adsorb hydrocarbons). Acidify with HCl to pH 2. Do not filter. Analysis within 7 days, store at 4°C.
-
Extraction
Transfer 1 L sample to 2 L separatory funnel. Add 30 mL n-pentane. Shake 5 min. Separate phases. Repeat 2× with 20 mL. Combine pentane extracts.
-
Drying of the extract
Pass the combined extracts through a funnel with anhydrous Na₂SO₄ (about 20 g, calcined). Water remaining in the organic phase spoils the separation on the column and increases detector noise.
-
Florisil cleanup
Pass extract through column with 2 g Florisil (saturated with solvent). Elute with 20 mL solvent. Florisil retains polar contaminants.
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Concentration (optional)
If concentration low: concentrate extract with N₂ stream to ~1 mL (T < 40°C). Do not evaporate to dryness (loss of volatile hydrocarbons!).
-
GC-FID analysis
Inject 1–2 µL on capillary column. Temperature program: 40°C (1 min) → 10°C/min → 320°C (5 min). FID detector 350°C. Gases: H₂ 40 mL/min, air 400 mL/min.
-
Integration and calculations
Determine C₁₀–C₄₀ retention window based on n-alkane standards. Integrate total peak area in this window. Calculate HOI from calibration curve.
-
Quality control
Blank sample (dist. water through entire procedure). Control standard every 10 samples. CRM. Duplicate every 20 samples. Recovery from fortified sample 70–120%.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Gas chromatograph with FID | Agilent 8890, Shimadzu GC-2030, Thermo TRACE 1310 | 120 000–300 000 PLN |
| Capillary GC column | Agilent DB-1 / DB-5, Restek Rtx-1, 10–30 m × 0.25 mm × 0.25 µm | 1 500–3 000 PLN |
| Autosampler | Agilent 7693A, Shimadzu AOC-20i | 40 000–80 000 PLN |
| Extraction system | 2 L separatory funnels or automated liquid-liquid extraction system | 200–500 PLN (funnels) / 30 000–80 000 PLN (automated) |
| Florisil column | Glass column 10 mm × 200 mm, Florisil 150–250 µm (60–100 mesh) | 100–500 PLN (column) + 300–800 PLN/kg (Florisil) |
| Evaporator (optional) | N₂ blowdown system or Kuderna-Danish for extract concentration | 3 000–15 000 PLN |
| Ready-made Florisil cartridges (alternative to glass column) | Florisil SPE cartridges 6 mL / 1 g | 15–40 PLN/pc |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| n-Pentane or n-hexane | 109-66-0 / 110-54-3 | Purity for hydrocarbon analysis (pesticides/GC grade). Extraction solvent, boiling range 36–69°C |
| Florisil | 1343-88-0 | 150–250 µm (60–100 mesh). Activate at 140°C for 16h. For cleanup of extract from polar substances |
| n-Alkane standards (C₁₀, C₄₀) | — | For determining retention window. n-Decane (C₁₀) and n-tetracontane (C₄₀) in solvent |
| HOI calibration standard | — | Mineral oil mixture (e.g., Type B per ISO 9377-2) in solvent, certified concentration |
| Anhydrous sodium sulfate (Na₂SO₄) | 7757-82-6 | Calcined (400°C, 4h). For drying organic phase |
| Hydrogen and synthetic air | — | Gases for FID detector. H₂ 5.0, synthetic air (hydrocarbon-free) |
| Dilute hydrochloric acid (HCl) | 7647-01-0 | For acidifying the sample to pH < 2 before extraction |
| Nitrogen 5.0 | 7727-37-9 | For concentrating the extract in a nitrogen stream, purity 99.999 % |
Health and safety (OHS)
- n-Pentane/n-hexane — extremely flammable! Flash point < -20°C. Work away from ignition sources, fume hood
- n-Hexane — neurotoxic with chronic exposure. Work in well-ventilated room
- Hydrogen (H₂) — extremely flammable and explosive. Check GC installation tightness
- Florisil — irritating dust to respiratory tract. Dust mask when handling
- Pentane/hexane waste — flammable organic waste, collect separately
- CO₂ or powder extinguisher available near GC station
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 9377-2:2003 — “Water quality — Determination of hydrocarbon oil index — Part 2: Method using solvent extraction and gas chromatography”.
How does this method work?
The method consists of three stages: (1) extraction, (2) Florisil cleanup, (3) GC-FID analysis. 1.
What is the measuring range and accuracy?
Hydrocarbon range: C₁₀ (n-decane) to C₄₀ (n-tetracontane); Limit of quantification: 0.1 mg/L; Working range: 0.1–1 000 mg/L (with dilution); Boiling temperature of analytes: 175–525°C.
How long does the test take?
The times given for the individual stages add up to approximately 2 h 5 min–2 h 20 min. The procedure comprises 8 steps.
What equipment is required?
Gas chromatograph with FID, Capillary GC column, Autosampler, Extraction system, Florisil column, Evaporator (optional), Ready-made Florisil cartridges (alternative to glass column). Indicative cost of the core instrument (Gas chromatograph with FID): 120 000–300 000 PLN.
Where is this test used?
Monitoring of wastewater from refineries and petrochemical plants; Control of groundwater (fuel contamination from leaking tanks); Study of surface waters (oil spill, ecological disasters); Monitoring of stormwater from fuel stations and parking lots; Control of wastewater from automotive workshops and car washes; Assessment of remediation of sites contaminated with petroleum substances; Assessment of the effectiveness of petroleum substance separators; Study of leachate from waste landfills; Monitoring of wastewater from metalworking plants.
What safety precautions apply?
n-Pentane/n-hexane — extremely flammable! Flash point < -20°C. Work away from ignition sources, fume hood; n-Hexane — neurotoxic with chronic exposure. Work in well-ventilated room; Hydrogen (H₂) — extremely flammable and explosive. Check GC installation tightness; Florisil — irritating dust to respiratory tract. Dust mask when handling; Pentane/hexane waste — flammable organic waste, collect separately; CO₂ or powder extinguisher available near GC station.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 9377-2.