🧪 Volatile Organic Compounds (VOC) in Water
Determination of volatile organic compounds (VOC) in drinking water and wastewater by purge-and-trap technique with GC-MS detection. The method allows identification and determination of dozens of chlorinated, aromatic, and aliphatic compounds.
Overview
Volatile Organic Compounds (VOCs) are a group of organic substances with boiling point 50–250°C and vapor pressure ≥0.01 kPa at 20°C. In waters they occur as anthropogenic contaminants (solvents, fuels, degreasers) and chlorine disinfection by-products (trihalomethanes — THM). Many VOCs exhibit carcinogenic, mutagenic, or hepatotoxic properties.
The purge-and-trap (P&T) technique involves purging water sample with inert gas (He), which extracts volatile analytes from aqueous phase and transfers them to sorbent trap (Tenax/activated carbon/silica gel). Then the trap is rapidly heated, desorbing concentrated analytes to gas chromatograph (GC) column coupled with mass detector (MS). The method allows simultaneous identification and quantitative determination of over 60 different VOCs.
The P&T/GC-MS technique is the reference method EPA 524.2/524.4 for drinking water and EPA 8260 for groundwater and wastewater. Standard PN-EN ISO 11423 covers analogous procedure compliant with European requirements. Typical LODs are 0.01–0.5 µg/L, allowing control of limits e.g., 1 µg/L for benzene or 100 µg/L for total THM.
The method requires specialized equipment (P&T concentrator, GC-MS), but is indispensable in trace VOC analysis in waters — offering simultaneous identification by mass spectrum and sensitivity unattainable by other techniques.
Method principle
Water sample (5 mL) is placed in purge vessel and purged with inert gas (helium, 40 mL/min, 11 min). Volatile organic compounds are purged from aqueous phase and transferred to sorbent trap (Tenax TA + activated carbon + silica gel). After purging, trap is rapidly heated (250°C, 4 min) in helium stream, desorbing analytes to gas chromatograph equipped with capillary column (e.g., DB-624, 30 m × 0.25 mm × 1.4 µm). GC temperature program separates analytes, which are identified based on mass spectra (MS in full scan mode or SIM). Quantification is performed from calibration curve with internal standard (e.g., fluorobenzene, 1,2-dichloroethane-d4).
Applications
- Drinking water quality monitoring — THM, chlorinated solvents (Journal of Laws 2017 item 2294)
- Groundwater testing in contaminated sites (fuel stations, chemical plants)
- Industrial wastewater control (dry cleaners, paint shops, printing houses)
- Environmental monitoring of landfills (leachates)
- Groundwater testing in wellhead protection zones
- VOC contamination source identification (fingerprinting)
- Treatment process control — chlorination by-products
- Swimming pool water analysis (THM, chloroform)
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.01–200 µg/L (depending on compound) |
| Limit of detection (LOD) | 0.01–0.5 µg/L |
| Number of analytes | 60+ VOCs in one analysis |
| Sample volume | 5 mL (typical) |
| Analysis time (P&T + GC) | 30–50 minutes |
| Identification | Mass spectrum + retention time (dual confirmation) |
Standard
- Standard number
- PN-EN ISO 11423:2002
- Title (PL)
- Jakość wody — Oznaczanie lotnych związków organicznych — Metoda chromatografii gazowej z techniką purge-and-trap i detekcją MS
- Title (EN)
- Water quality — Determination of volatile organic compounds — Purge-and-trap gas chromatography with mass spectrometric detection
Step-by-step procedure
1. Sample collection
Collect sample in 40 mL VOC vial with no headspace (no air bubbles). Add 25 mg ascorbic acid (chlorinated water). Store at 4°C, analyze within 14 days.
2. Standard preparation
From VOC concentrate in methanol prepare calibration series (5 points): 0.5; 1.0; 5.0; 10; 50 µg/L. Add internal standards (IS) to final concentration 10 µg/L.
3. P&T/GC-MS system calibration
Measure calibration series. Calculate response factors (RF) for each analyte. Check RSD RF < 20%. Average RF or linear regression.
4. Sample transfer to purge vessel
Transfer 5 mL sample to purge vessel. Add internal and surrogate standards (each 10 µg/L). Close vessel.
5. Purge phase
Purge sample with helium (40 mL/min) for 11 minutes. VOCs transfer from aqueous phase to sorbent trap (Tenax/carbon/silica).
6. Dry purge
Purge trap with dry helium for 1 min to remove water vapor before desorption.
7. Thermal desorption
Heat trap to 250°C for 4 minutes, desorbing analytes with helium stream onto GC column (DB-624). Transfer line 150°C.
8. GC separation
Temperature program: 35°C (2 min) → 8°C/min → 200°C (2 min). Carrier gas: He 1.2 mL/min. Run time ~25 min.
9. MS detection
MS in Full Scan mode (m/z 35–300) or SIM. Identification: retention time + mass spectrum (NIST library match >70%).
10. Calculations and quality control
Calculate concentrations based on IS. Check surrogate recovery (70–130%). Blank < LOD. BFB tune check every 12 h.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Gas chromatograph GC-MS | Agilent 8890 GC + 5977C MSD, Shimadzu GCMS-QP2020 NX, Thermo TRACE 1310 + ISQ 7000 | 400,000–800,000 PLN |
| Purge-and-trap concentrator | Teledyne Tekmar Atomx, EST Encon, OI Analytical Eclipse | 100,000–200,000 PLN |
| GC capillary column | Agilent DB-624 (30 m × 0.25 mm × 1.4 µm), Restek Rtx-VMS | 2,000–4,000 PLN |
| Autosampler for VOC vials | Teledyne Tekmar Versa, EST Centurion | 60,000–120,000 PLN |
| VOC vials 40 mL with PTFE septum | Glass vials with screw cap and PTFE/silicone septum, no headspace | 3–8 PLN/pc |
| Helium 5.0 cylinder | Helium 99.999%, carrier and purge gas, 50 L cylinder | 600–1,200 PLN/cylinder |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| VOC standard mix (EPA 524.2) | mixture | Certified standard 60+ VOCs in methanol, 200–2000 µg/mL, e.g., Supelco/Restek |
| Internal standards (IS) | — | Fluorobenzene (CAS 462-06-6), 1,2-dichloroethane-d4 (CAS 17060-07-0), chlorobenzene-d5 (CAS 3114-55-4) |
| Surrogate standards | — | 4-Bromofluorobenzene (CAS 460-00-4), dibromofluoromethane (CAS 1868-53-7), toluene-d8 (CAS 2037-26-5) |
| Anhydrous methanol (standard solvent) | 67-56-1 | Pesticide grade quality, free from VOCs |
| Ascorbic acid | 50-81-7 | Dechlorination of chlorinated drinking water samples, 25 mg per 40 mL vial |
| Helium 5.0 | 7440-59-7 | GC carrier gas and purge gas, purity 99.999% |
Health and safety (OHS)
- VOC standards in methanol — flammable, many compounds carcinogenic (benzene, chloroform, dichloromethane) — fume hood!
- Methanol — toxic by inhalation and skin contact, nitrile gloves
- VOC-contaminated water samples — potential toxic vapors, work in ventilated area
- Helium — asphyxiant, leak detector, room ventilation
- Collect VOC standard waste in halogenated/non-halogenated organic waste containers separately
- Secure gas cylinders against tipping, store vertically