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

Physicochemistry PN-ISO 11423-1

Overview

STATUS (as of 03.11.2026). PN-ISO 11423-1:2002 — Polish version, published on 16.12.2002, implements ISO 11423-1:1997 [IDT]; the PKN catalogue card has no note of withdrawal. ISO 11423-1:1997 (first edition) has the stage “confirmed after review” (90.93) in the ISO catalogue. Part 2 (extraction and gas chromatography) has no Polish edition. The description agrees with the description of the same standard in the method “Benzene and benzene derivatives in water”.

SCOPE (clause 1). Benzene, methylbenzene (toluene), dimethylbenzenes (xylenes) and ethylbenzene — the standard calls this group BTX — in homogeneous samples of water and waste water at concentrations above 2 µg/l. In samples polluted with organic compounds the limit of determination may be higher, and high concentrations are determined after dilution. Volatile organic compounds other than BTX are covered by other standards: PN-EN ISO 15680 (purge-and-trap) and PN-EN ISO 20595 (static head-space analysis with mass spectrometric detection) — separate methods in the Knowledge Base.

PRINCIPLE (clause 2). A defined volume of unfiltered sample is heated in a gas-tight septum-covered vial; after equilibrium between the gaseous and liquid phases has been established, an aliquot of the gaseous phase is transferred to a gas chromatograph. The compounds are separated on two capillary columns of different polarity and determined with a flame ionization detector (FID); a mass spectrometer or a photo-ionization detector may be needed for more reliable identification (7.2).

SAMPLES (clauses 3, 6). BTX are lost during sampling, transport and storage, and volatile compounds in ambient air contaminate samples, so samples are taken into glass vials or bottles of non-actinic glass, without contact with plastics and without composite samples; an air blank and a reagent blank are taken in parallel. If possible, the analysis is started within 2 days; samples are stored at 4 °C in the dark. Adding potassium carbonate (about 7–8 g per 5 ml of sample) shifts the equilibrium towards the gas phase and lowers the limit of detection; in gaseous waters free carbon dioxide is neutralised with potassium carbonate.

DETERMINATION (7.1–7.4). The static equilibrium is checked by the laboratory — at least 60 °C for at least 1 h has been found sufficient, the same for samples and blanks. Identification is by retention time (differences within a series of no more than ± 0.02 min at comparable concentrations) on at least two columns of different polarity; if the results from the two columns differ, the lower value is usually the more accurate one. With GC-MS confirmation the ratio of the main ion to a secondary ion is within ± 10 % of the value for the standard. Blanks are run before and during the series, because benzene is ubiquitous.

CALIBRATION (clause 8). The total method is calibrated with an external standard — aqueous solutions from a stock solution (100 µl of each compound in 5 ml of dimethylformamide; alternatively acetone or methanol), prepared immediately before use and never by diluting other calibration solutions. E.g. 50 µl of stock solution in 1000 ml of water gives 878 µg/l of benzene; the concentrations are calculated from the densities of the compounds at 20 °C given in Table 3 of the standard (benzene 0.878 g/ml, toluene 0.867, o-xylene 0.881, m-xylene 0.865, p-xylene 0.861, ethylbenzene 0.867 g/ml).

REGULATIONS. The parametric value for benzene in water intended for human consumption is 1.0 µg/l (Dz.U. 2026 item 748, Annex 1 part B item 7); the method must have a limit of quantification of no more than 30 % of the parametric value (0.3 µg/l) and a measurement uncertainty of no more than 40 % (Annex 4 part B). The range of applicability of this standard begins above 2 µg/l, i.e. above the required limit of quantification for benzene. The regulation does not list toluene, ethylbenzene or xylenes.

WHAT WE DO NOT GIVE HERE. The publicly available text of the standard ends in clause 8 with the table of densities. The calculation of the result, precision data, examples of columns (Annex B) and of chromatograms (Annex C) are outside it — which is why we do not give column dimensions and phases, temperature programmes, retention times, limits of detection of individual compounds or precision data.

Method principle

Benzene, toluene, ethylbenzene and xylenes pass from the heated water sample into the gas phase above it in a closed vial, and after equilibrium has been reached an aliquot of this phase is introduced into a gas chromatograph. The compounds are separated on two capillary columns of different polarity and determined with a flame ionization detector (FID), and when the identification is uncertain — with a more selective detector (MS or photo-ionization). The concentration is calculated from the calibration of the total method with aqueous standard solutions.

Applications

• Water and waste water — homogeneous samples, BTX concentrations above 2 µg/l (clause 1)
• Water intended for human consumption — benzene: parametric value 1.0 µg/l, limit of quantification of the method ≤ 30 % of this value (Dz.U. 2026 item 748, Annex 1 part B item 7, Annex 4 part B)
• Gaseous waters — after neutralising free carbon dioxide with potassium carbonate (clause 6)
• Samples polluted with organic compounds — the limit of determination may be higher; high concentrations after dilution (clause 1)

Key parameters

Standard

Standard number

PN-ISO 11423-1:2002

Title (PL)

Jakość wody — Oznaczanie benzenu i niektórych pochodnych — Część 1: Metoda analizy fazy nadpowierzchniowej z zastosowaniem chromatografii gazowej

Title (EN)

Water quality — Determination of benzene and some derivatives — Part 1: Head-space gas chromatographic method

Step-by-step procedure

1. Preparation of containers

   Keep bottles and vials upside down for 1 h at 150 °C, then protect them from contamination and close them when cool (clause 4).

2. Sampling

   Sample into head-space vials or into bottles of non-actinic glass, without turbulence and without contact with plastics; in parallel an air blank and a reagent blank (clause 6).

3. Storage

   If possible, start the analysis within 2 days; store the samples at 4 °C in the dark (clause 6).

4. Vial with the sample

   Transfer an aliquot of the sample into a vial, if necessary with potassium carbonate (about 7–8 g per 5 ml), crimp the cap and check its tightness (clause 6).

5. Equilibrium

   Heat the vials under conditions ensuring static equilibrium (at least 60 °C for at least 1 h has been found sufficient), the same for samples and blanks (7.1).

6. Chromatography

   Introduce an aliquot of the gas phase into the chromatograph; separate on two columns of different polarity and measure with the FID; calibration solutions and blanks at the beginning and at the end of the series (7.1, 7.2).

7. Identification

   Compare the retention times with the calibration solutions (± 0.02 min); a peak on both columns means that the presence is very probable; in case of doubt confirm by GC-MS (ion ratio ± 10 %) (7.4).

8. Calibration

   Immediately before use prepare aqueous calibration solutions from the stock solution (e.g. 50 µl per 1000 ml of water — 878 µg/l of benzene) and calibrate the total method; do not dilute calibration solutions (clause 8).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Benzene — harmonised CLP classification: H225, H304, H315, H319, H340 (may cause genetic defects), H350 (may cause cancer), H372
• N,N-dimethylformamide — harmonised CLP classification: H312, H319, H332, H360D (may damage the unborn child)
• Methanol (alternative to dimethylformamide) — harmonised CLP classification: H225, H301, H311, H331 (toxic if swallowed, in contact with skin and if inhaled), H370 (causes damage to organs)

Frequently asked questions