🧬 THM / Trihalomethanes in Water
In short
Determination of trihalomethanes (chloroform, bromodichloromethane, dibromochloromethane, bromoform) in water by gas chromatography with ECD or MS detection. The test is performed according to PN-EN ISO 10301:2002; Linear range: 0.5–200 µg/L (typically). The procedure comprises 11 steps (about 4 h 55 min–5 h 55 min in total); it is used for: Drinking water quality control (THM sum ≤100 µg/L), Monitoring of chlorine disinfection by-products, Optimization of chlorination process (THM minimization).
At a glance
- Standard: PN-EN ISO 10301:2002
- Category: Physicochemistry
- Procedure steps: 11
- Total time of stages: 4 h 55 min–5 h 55 min
- Determined compounds: CHCl₃, CHBrCl₂, CHBr₂Cl, CHBr₃ (4 THM)
- Detection limit (headspace-GC-ECD): 0.1–0.5 µg/L
- Detection limit (purge & trap GC-MS): 0.01–0.05 µg/L
Overview
Trihalomethanes (THM) are a group of disinfection by-products (DBP) formed during chlorination of drinking water. Chlorine reacts with natural organic matter (humic, fulvic acids) present in raw water, forming volatile halogenated organic compounds. The four main THMs are: chloroform (CHCl₃), bromodichloromethane (CHBrCl₂), dibromochloromethane (CHBr₂Cl) and bromoform (CHBr₃). The sum of these four compounds (TTHM — Total Trihalomethanes) is legally regulated.
The permissible value of THM sum in drinking water is 100 µg/L (according to EU Directive 2020/2184 and MH Regulation). Chloroform is usually the dominant component (60–90% TTHM). Trihalomethanes show carcinogenic action (chloroform — group 2B according to IARC), mutagenic and hepatotoxic. Long-term exposure to THM in drinking water is associated with increased risk of bladder cancer.
The PN-EN ISO 10301 standard describes gas chromatography methods for determining volatile halogenated organic compounds in water, including THM. Two sample introduction techniques are used: liquid-liquid extraction (LLE) with pentane or hexane, followed by GC-ECD, and headspace analysis — static or dynamic (purge & trap), followed by GC-ECD or GC-MS. Electron capture detector (ECD) is particularly sensitive to halogenated organic compounds.
The headspace-GC technique is most commonly used due to simplicity, automation and no need to use organic solvents. Purge & trap offers the lowest detection limits (ng/L order), but is more time-consuming.
Method principle
Volatile THMs are released from the water sample into the gas phase — by headspace method (heating sample in closed vial, liquid-gas equilibrium) or purge & trap (purging with inert gas, sorption on trap, thermal desorption). The gas phase is introduced into a gas chromatograph (GC), where THMs are separated on a capillary column (e.g., DB-624, RTX-VRX) in a temperature program. Detection is performed using an electron capture detector (ECD) — highly selective and sensitive to halogen compounds — or mass detector (MS) providing identification based on mass spectrum. Concentrations are determined from calibration curve.
Applications
- Drinking water quality control (THM sum ≤100 µg/L)
- Monitoring of chlorine disinfection by-products
- Optimization of chlorination process (THM minimization)
- Testing of swimming pool water (THM in water and air)
- Water quality control after ozonation, UV and alternative disinfection
- Monitoring of surface waters as indicator of anthropogenic pollution
- Testing of bottled water
Key parameters
| Parameter | Value |
|---|---|
| Determined compounds | CHCl₃, CHBrCl₂, CHBr₂Cl, CHBr₃ (4 THM) |
| Detection limit (headspace-GC-ECD) | 0.1–0.5 µg/L |
| Detection limit (purge & trap GC-MS) | 0.01–0.05 µg/L |
| Linear range | 0.5–200 µg/L (typically) |
| Permissible THM sum (drinking water) | ≤100 µg/L |
| Detector | ECD (63Ni) or MS (quadrupole/ion trap) |
Standard
- Standard number
- PN-EN ISO 10301:2002
- Title (PL)
- Jakość wody — Oznaczanie wysoce lotnych związków chlorowcoorganicznych (LZWO) — Metody chromatografii gazowej
- Title (EN)
- Water quality — Determination of highly volatile halogenated hydrocarbons — Gas chromatographic methods
Step-by-step procedure
-
Sample collection
Fill 40 mL vial without air bubbles (THM volatile!). Add Na₂S₂O₃ (3 mg) before collection to remove Cl₂. Close tightly. Store at 4°C, analysis within 14 days.
-
Preparation of calibration standards
Prepare series 0, 5, 10, 25, 50, 100 µg/L TTHM in THM-free water. Add internal standard (IS). Add NaCl (3 g/10 mL).
-
Preparation of headspace vials
Into 20 mL vial introduce 10 mL sample, add NaCl (3 g) and 10 µL IS. Close with cap with PTFE/silicone septum. Leave no bubbles!
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Setting headspace autosampler
Thermostating temperature: 60–80°C, thermostating time: 20–30 min, transfer line temperature: 150°C.
-
GC setting
DB-624 column: temp. program 35°C (5 min) → 10°C/min → 200°C (2 min). Carrier gas: He 1.0 mL/min (GC-MS) or N₂ (GC-ECD). Injector: 200°C, split 1:10.
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Detector setting
ECD: temperature 300°C, make-up N₂ 30 mL/min. Or MS: SIM mode (m/z: 83, 85 for CHCl₃; 127, 129 for CHBrCl₂; 173 for CHBr₂Cl; 173, 252 for CHBr₃).
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Calibration
Measure standard series. Plot calibration curve (ratio of peak areas analyte/IS vs concentration). R² ≥0.998 for each THM.
-
Sample measurement
Place vials in autosampler. Instrument automatically thermostats, doses and analyzes. Analysis time approx. 50 min/sample (with thermostating).
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Identification and quantification
Identify THM peaks on chromatogram (retention times, MS spectra). Calculate concentrations from calibration curve. Calculate THM sum (TTHM).
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Quality control
Method blank, field blank, duplicate, fortified sample (spike 50 µg/L — recovery 80–120%), CRM control standard.
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Reporting
Report concentrations of individual THMs and TTHM sum in µg/L. For results <LOQ report "<LOQ". Include information on detection method (ECD/MS).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Gas chromatograph with ECD or MS detector | Shimadzu GC-2030 + ECD, Agilent 8890 GC + 5977B MS, Thermo TRACE 1310 | 100 000–350 000 PLN |
| Headspace autosampler | Shimadzu HS-20, Agilent 7697A, Teledyne Tekmar HT3 | 50 000–120 000 PLN |
| Purge & trap system (option) | Teledyne Tekmar Atomx, OI Analytical Eclipse | 80 000–150 000 PLN |
| Capillary column | Agilent DB-624 (30 m × 0.25 mm × 1.4 µm), Restek RTX-VRX | 1 500–3 500 PLN |
| Headspace vials 20 mL | Glass vials with crimp cap and PTFE/silicone septum | 2–5 PLN/pc. |
| Carrier gas — helium or nitrogen | Helium 5.0 (for GC-MS) or nitrogen 5.0 (for GC-ECD), 50 L cylinder | 300–1 500 PLN/cylinder |
| Gas for ECD — nitrogen or argon/methane | Nitrogen 5.0 (make-up gas) or Ar/CH₄ 95/5 | 300–600 PLN/cylinder |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| THM standards (mixture) | — | Certified mixture of 4 THM in methanol, e.g., Supelco EPA 501/601 THM Mix, 200 µg/mL each; store at –18°C |
| Internal standard (IS) | — | Fluorobenzene or 1,2-dibromoethane in methanol — for recovery and dosing volume correction |
| Sodium thiosulfate | 7772-98-7 | Na₂S₂O₃ — added to sample at sampling point (approx. 3 mg per 40 mL) to remove free chlorine and stop THM formation reaction |
| Sodium chloride | 7647-14-5 | NaCl — added to headspace vial (approx. 3–5 g per 10 mL sample) to increase THM release efficiency (salt effect) |
| Methanol analytical grade | 67-56-1 | For preparing standard dilutions; chromatography quality |
| THM-free water | — | Ultrapure degassed or boiled 15 min water — for preparing standards and blanks |
Health and safety (OHS)
- Chloroform — carcinogenic cat. 2, toxic (H351, H331, H302) — work under fume hood, gloves, goggles
- Methanol — toxic (H301, H311, H331), flammable — work under fume hood
- THM standards in methanol — store at –18°C, dose in fume hood
- Helium / nitrogen — gases under pressure, risk of asphyxiation in closed rooms
- THM-containing waste — collect as halogenated organic waste
- ECD detectors contain ⁶³Ni isotope — replacement and disposal according to radioactive source regulations
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 10301:2002 — “Water quality — Determination of highly volatile halogenated hydrocarbons — Gas chromatographic methods”.
How does this method work?
Volatile THMs are released from the water sample into the gas phase — by headspace method (heating sample in closed vial, liquid-gas equilibrium) or purge & trap (purging with inert gas, sorption on trap, thermal desorption). The gas phase is introduced into a gas chromatograph (GC), where THMs are separated on a capillary column (e.
What is the measuring range and accuracy?
Determined compounds: CHCl₃, CHBrCl₂, CHBr₂Cl, CHBr₃ (4 THM); Detection limit (headspace-GC-ECD): 0.1–0.5 µg/L; Detection limit (purge & trap GC-MS): 0.01–0.05 µg/L; Linear range: 0.5–200 µg/L (typically).
How long does the test take?
The times given for the individual stages add up to approximately 4 h 55 min–5 h 55 min. The procedure comprises 11 steps.
What equipment is required?
Gas chromatograph with ECD or MS detector, Headspace autosampler, Purge & trap system (option), Capillary column, Headspace vials 20 mL, Carrier gas — helium or nitrogen, Gas for ECD — nitrogen or argon/methane. Indicative cost of the core instrument (Gas chromatograph with ECD or MS detector): 100 000–350 000 PLN.
Where is this test used?
Drinking water quality control (THM sum ≤100 µg/L); Monitoring of chlorine disinfection by-products; Optimization of chlorination process (THM minimization); Testing of swimming pool water (THM in water and air); Water quality control after ozonation, UV and alternative disinfection; Monitoring of surface waters as indicator of anthropogenic pollution; Testing of bottled water.
What safety precautions apply?
Chloroform — carcinogenic cat. 2, toxic (H351, H331, H302) — work under fume hood, gloves, goggles; Methanol — toxic (H301, H311, H331), flammable — work under fume hood; THM standards in methanol — store at –18°C, dose in fume hood; Helium / nitrogen — gases under pressure, risk of asphyxiation in closed rooms; THM-containing waste — collect as halogenated organic waste; ECD detectors contain ⁶³Ni isotope — replacement and disposal according to radioactive source regulations.
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 10301.