⛽ BTEX in Water (Benzene, Toluene, Ethylbenzene, Xylenes)

Physicochemistry PN-EN ISO 11423-1

Overview

BTEX is an acronym for four volatile aromatic hydrocarbons: benzene (B), toluene (T), ethylbenzene (E), and xylenes (X — ortho, meta, para). These compounds are main components of gasoline, organic solvents, and petrochemical feedstocks. They are common groundwater contaminants around fuel stations, refineries, fuel depots, and petrochemical plants.

Benzene is classified as group 1 carcinogen (IARC), and its permissible concentration in drinking water is 1 µg/L according to Ministry of Health Regulation and EU Directive. Toluene, ethylbenzene, and xylenes are less toxic but exhibit neurotoxic and hepatotoxic effects. Permissible concentrations in groundwater are regulated by Ministry of Environment Regulation on groundwater quality standards.

Headspace (HS) technique involves thermal equilibration of water sample in closed vial, then sampling portion of gas phase (headspace) and introducing it to gas chromatograph. For increased sensitivity, headspace-SPME (solid phase microextraction) with CAR/PDMS or DVB/CAR/PDMS fiber is used, lowering LOD to ng/L level.

GC-MS detection ensures unambiguous identification of individual BTEX isomers, while GC-FID is simpler and cheaper alternative for routine determinations. The method is fast (15–30 min per sample), requires no solvent extraction, and generates minimal waste.

Method principle

Water sample (10 mL) is placed in tightly sealed headspace vial (20 mL) with NaCl addition (3 g, salting-out effect). Vial is thermostated (60–80°C, 15–20 min) in headspace module until vapor equilibrium of BTEX between liquid and gas phase is established. Gas phase portion (1 mL) is automatically sampled through heated needle (110°C) and injected onto GC capillary column (e.g., DB-624, 20 m × 0.18 mm × 1.0 µm). Temperature program separates BTEX isomers: benzene (Tr ~3 min), toluene (~5 min), ethylbenzene (~7 min), m/p-xylene (~7.5 min), o-xylene (~8 min). Detection: MS (m/z 78, 91, 106) or FID. In HS-SPME technique, fiber (CAR/PDMS) exposed in headspace (25°C, 15 min) adsorbs analytes, then thermally desorbed in GC injector (290°C, 2 min).

Applications

• Groundwater monitoring in fuel-contaminated sites (fuel stations, fuel depots)
• Drinking water quality control — benzene max 1 µg/L (Journal of Laws 2017 item 2294)
• Industrial wastewater testing (refineries, petrochemicals, paint shops)
• Fuel-contaminated site remediation assessment
• Surface water monitoring around petrochemical plants
• Groundwater control in wellhead protection zones
• Underground storage tank (UST) leak identification

Key parameters

Standard

Standard number

PN-EN ISO 11423-1:2002

Title (PL)

Jakość wody — Oznaczanie benzenu i wybranych pochodnych — Część 1: Metoda analizy fazy nadpowierzchniowej (headspace) z detekcją GC-MS

Title (EN)

Water quality — Determination of benzene and some derivatives — Part 1: Headspace method with GC-MS detection

Step-by-step procedure

1. Sample collection

   Collect sample in 40 mL VOC vial with no headspace. Avoid contamination — do not use plastic containers. Store at 4°C, analyze within 14 days.

   🌡 Temperature: 4°C

2. HS vial preparation

   Transfer 10 mL sample to 20 mL headspace vial. Add 3 g NaCl (salting-out effect). Add 10 µL IS solution (fluorobenzene 10 mg/L). Close immediately with crimp cap with PTFE/butyl septum.

   ⏱ Time: 3 min

3. Standard preparation

   BTEX calibration series: 0.1; 0.5; 1.0; 5.0; 10; 50 µg/L in water with NaCl. Add IS to each standard. Prepare in HS vials.

   ⏱ Time: 15 min

4. Thermostatting in HS module

   Place vial in headspace autosampler. Thermostat: 80°C, equilibration time 15 min. Heated needle 110°C, transfer line 120°C.

   ⏱ Time: 15 min • 🌡 Temperature: 80°C

5. Injection onto GC column

   Autosampler withdraws 1 mL gas phase and injects onto DB-624 column. Split 1:10 or splitless (low concentrations).

   ⏱ Time: 1 min

6. Chromatographic separation

   Temperature program: 40°C (1 min) → 15°C/min → 120°C → 35°C/min → 200°C (1 min). Carrier gas He 1.0 mL/min. Run time ~10 min.

   ⏱ Time: 10 min

7. MS detection

   SIM mode: benzene m/z 78, toluene m/z 91+92, ethylbenzene m/z 91+106, xylenes m/z 91+106. Or Full Scan m/z 35–200.

   ⏱ Time: continuous

8. HS-SPME (alternative for ultra-low concentrations)

   Expose CAR/PDMS fiber in vial headspace (25°C, 15 min, no agitation). Desorb in GC injector (290°C, 2 min).

   ⏱ Time: 17 min

9. Calculations

   Calculate BTEX concentrations by internal standard method: c = (A_analyte/A_IS) × (c_IS/RF). Result in µg/L.

   ⏱ Time: 5 min

10. Quality control

    Blank < LOD, spike (recovery 80–120%), duplicate (RPD < 20%). Control standard every 10 samples — deviation max ±20%.

    ⏱ Time: 10 min

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Benzene — group 1 carcinogen (IARC), work exclusively in fume hood with ventilation!
• Toluene, ethylbenzene, xylenes — neurotoxic, irritant, avoid vapor inhalation
• Methanol — toxic, flammable, nitrile gloves and safety glasses
• Fuel station samples — may contain high BTEX concentrations, open in fume hood
• Collect BTEX standard waste in non-halogenated organic waste containers
• Secure gas cylinders (He) against tipping

Frequently asked questions