⚗️ Volatile organic compounds in water (BTEX, naphthalene, chlorinated hydrocarbons) — gas purging and trapping on an adsorbent (purge-and-trap), thermal desorption, GC-MS

Physicochemistry PN-EN ISO 15680

Overview

ISO 15680:2003 specifies a general method for determining volatile organic compounds (VOC) in water by isolation with the purge-and-trap technique and gas chromatography; annexes A, B and C give examples of analytes — from difluorodichloromethane (R-12) to trichlorobenzene, including all non-polar organic compounds of intermediate volatility (a VOC is defined as a generally non-polar compound with a boiling point of about −30 °C to 220 °C). Detection is best carried out with a mass spectrometer in the electron ionisation (EI) mode, but other detectors may be used (FID, ECD, PID, ELCD). The detection limit depends on the detector and the operating parameters — typically as low as 10 ng/l is achieved (3 standard deviations from 10 replicates), and the working range usually reaches 100 µg/l. The method applies to drinking water, groundwater, surface water, sea water and (diluted) wastewater. In Poland it is in force as PN-EN ISO 15680:2008 (Polish version; it replaced PN-EN ISO 15680:2004). The 2019 regulation (Dz.U. 2019 poz. 1311 [Journal of Laws 2019, item 1311]) names PN-EN ISO 15680 as the reference method of gas chromatography, among others for volatile hydrocarbons, trichlorobenzene and other chlorinated compounds in wastewater.

Interferences: in principle every purgeable compound with the same retention time and a similar mass spectrum; in practice the spectra of most analytes are characteristic, and the quantification ions are chosen so as to rule out co-elution. VOCs escape easily during sampling, transport, storage and preparation (results too low) or diffuse into the samples from the laboratory air and the refrigerator (results too high); the purge gas and the carrier gas, the laboratory air in the purge vessel and laboratory clothing (halogenated hydrocarbons) may all be sources of contamination. All tubing, seals and valves should be of stainless steel or glass (no plastics); vessels with a sintered glass frit carry the risk of transferring contamination; foaming of samples containing surfactants requires the purging to be stopped at once; "cold spots" in the transfer line adsorb VOCs, and insufficiently dried adsorbents block the cryogenic trap with ice; adsorbents age (their capacity and background change); samples in the autosampler are protected from light (amber vials).

Method principle

A fixed volume of the sample is purged with a fixed volume of inert gas, which blows out the volatile constituents; these are then retained either a) on a trap with a packed adsorbent (best combined with cryofocusing) or b) directly in a capillary cold trap. When the purging is finished, the trap is heated, desorbing the volatile constituents, which the GC carrier gas transfers onto a capillary chromatographic column (on-line or off-line system); for narrow injection bands cryofocusing at the adsorption trap is recommended, or transfer through an injector with a splitter of about 20:1 if the sensitivity allows. The constituents are separated in the GC with temperature programming and detected with a mass spectrometer; data are collected in full-scan mode or for a sufficient number of specific fragments to allow comparison with standards; a compound is regarded as present once the criteria of annex D are met (lowest identification concentration: the selected diagnostic ion of lowest intensity present at a signal-to-noise ratio > 3:1), and it is quantified from selected characteristic fragments. Reagents: water free of contamination (ISO 3696, from a single batch per series), methanol (or DMF, DMSO, acetone) as the solvent for the standards, sodium thiosulfate pentahydrate (about 100 mg/l) to remove chlorine or ozone, sodium hydrogen sulfate for acidification to pH 2 (preservation of aromatic compounds in surface waters), purge gas — helium or nitrogen of high purity (a purification cartridge may be used). Standards: stock solutions of 2 mg/ml (about 200 mg of each VOC in 100 ml of solvent; commercial certified solutions are best), internal standard solutions of 2 mg/ml (at least one for quantification, further ones as surrogates; deuterated compounds only for GC-MS — e.g. fluorobenzene, chlorobenzene-d5, 1,4-dichlorobenzene-d4, 1,4-difluorobenzene, toluene-d8), spiking solutions (dilution scheme in 100 ml of solvent; 5 µl per 100 ml of water gives 0–5 µg/l), calibration solutions (added to the water in the purge vessel; the organic solvent content of the final standard ≤ 2 % by volume) and a blank of unspiked water; intermediate standards are stored at about 4 °C and warmed to room temperature before use. Purge-and-trap apparatus: autosampler, purge vessel with a heating jacket, temperature control, gas supply, flow control and a timer, a condenser or a dry purging system, an adsorption trap (glass or steel, internal diameter 2–5 mm, packing 10–50 mm, at least 90 mg of polymeric, carbon or silica adsorbent, conditioned for at least 30 min above the desorption temperature in a stream of inert gas, with a blank run before use), a thermal desorption unit, a cold trap with coolant and heater, GC-MS with a data system. Samples are taken in accordance with ISO 5667-3 directly into the vessels (two samples are best), filling them without turbulence until they overflow and closing them without a headspace; where chlorine is present — thiosulfate; samples exceeding the range are not diluted (losses by evaporation) — instead the calibration is extended or headspace analysis is used (ISO 10301, ISO 11423-1); the analysis is best done on the day of sampling, no later than 5 days afterwards, with storage at about 4 °C in the dark in tightly closed vessels.

Applications

• Drinking water — BTEX, naphthalene, chlorinated aliphatic and aromatic hydrocarbons (THM, vinyl chloride, trichloroethene, tetrachloroethene)
• Groundwater at filling stations, landfills and chemical plants — monitoring of VOC contamination
• Surface water, sea water and diluted wastewater — the GC reference method for volatile hydrocarbons under Dz.U. 2019 poz. 1311
• Trace analysis (down to 10 ng/l) with GC-MS and deuterated standards; screening of many analytes from a single injection
• Laboratories with automatic on-line purge-and-trap systems and an autosampler

Key parameters

Standard

Standard number

PN-EN ISO 15680:2008 (ISO 15680:2003)

Title (PL)

Jakość wody — Oznaczanie wybranych jednopierścieniowych węglowodorów aromatycznych, naftalenu i niektórych związków chlorowanych metodą chromatografii gazowej z zastosowaniem techniki wypłukiwania i wychwytu (purge-and-trap) oraz desorpcji termicznej

Title (EN)

Water quality — Gas-chromatographic determination of a number of monocyclic aromatic hydrocarbons, naphthalene and several chlorinated compounds using purge-and-trap and thermal desorption

Step-by-step procedure

1. Sampling and preservation

   Take the sample in accordance with ISO 5667-3 directly into the vessels (two samples are best), filling them without turbulence until they overflow and closing them without a headspace; where chlorine or ozone is present add about 100 mg/l of sodium thiosulfate; for aromatic compounds in surface waters lower the pH to 2 with sodium hydrogen sulfate. Store at about 4 °C, in the dark, and analyse preferably on the day of sampling (within 5 days). Do not dilute samples of high concentration.

2. Preparation of the trap and the system

   Condition the adsorption trap for at least 30 min above the desorption temperature in a stream of inert gas and carry out a blank run; make sure that all parts are of steel or glass, with no cold spots in the transfer line; check the purity of the purge gas and the carrier gas.

3. Standards and calibration

   From the stock solutions of 2 mg/ml (of the analytes and the internal standards) prepare the spiking solutions according to the scheme (e.g. 5 µl into 100 ml of water → 0–5 µg/l, solvent ≤ 2 % by volume); add them to the water in the purge vessel or in the autosampler vessel; keep a portion of the water as a control blank; check fresh standards against the previous ones.

4. Purging and trapping

   Add the internal standard solution to the fixed volume of the sample; purge with a fixed volume of helium/nitrogen at a controlled temperature, retaining the VOCs on the adsorption trap (with cryofocusing) or in the cryogenic trap; if foaming occurs (surfactants), stop the purging at once; dry the trap before desorption (risk of blocking with ice).

5. Desorption and GC-MS

   Heat the trap and transfer the desorbed constituents with the carrier gas onto the capillary column (on-line or off-line; a splitter of about 20:1 if needed); separate with temperature programming and record the MS spectra (full scan or selected fragments).

6. Identification and quantification

   Regard a compound as present when it meets the criteria of annex D (relative retention times, diagnostic ions, S/N > 3:1); calculate the concentrations from the selected characteristic fragments against the internal standard and the calibration function (ISO 8466-1); monitor the blanks; for samples above the range — an extended calibration or headspace (ISO 10301, ISO 11423-1).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• VOC standards (benzene, chlorinated hydrocarbons) are toxic and carcinogenic — work in a fume hood, gloves, minimum quantities
• Methanol and other solvents — flammable and toxic; a solvent-free laboratory is also needed to avoid contaminating the samples
• Liquid nitrogen / coolant of the cryogenic trap — risk of frostbite and of oxygen deficiency in small rooms
• Hot parts of the desorber and of the GC injector — take care during servicing

Frequently asked questions