🧪 Benzene and Benzene Derivatives in Water — Head-space Analysis (HS-GC)

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 (ISO 11423-2 — extraction and gas chromatography) has no Polish edition: the PKN search for “ISO 11423” returns only part 1.

WHAT THE STANDARD COVERS (clause 1, introduction). Benzene, methylbenzene (toluene), dimethylbenzenes (xylenes) and ethylbenzene — abbreviated 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 of the sample. The method can also be used for further derivatives and nonpolar compounds with similar physical properties — its applicability is verified for the particular sample. The standard is not a method for volatile organic compounds in general: purge-and-trap with thermal desorption is described in PN-EN ISO 15680, and static head-space analysis with mass spectrometric detection for a wider set of volatile compounds in PN-EN ISO 20595 (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. Benzene and its derivatives are separated on two capillary columns with stationary phases of different polarity (e.g. by simultaneous splitting) and determined with a suitable detector — in the standard a flame ionization detector (FID); a more selective detector (mass spectrometer, photo-ionization detector) may be needed for more reliable identification.

EQUILIBRIUM AND SALTING OUT (clause 6, 7.1). At the start the laboratory checks whether the chosen conditions ensure a static equilibrium — at least 60 °C for at least 1 h has been found sufficient; time and temperature must be the same for samples and the blank. Adding potassium carbonate (about 7–8 g per 5 ml of sample, so that some remains undissolved) shifts the equilibrium towards the gas phase and lowers the limit of detection; the quantities of salt and the volumes of samples and blanks must be identical. In gaseous waters free carbon dioxide is neutralised with potassium carbonate (about 1 % by mass of carbonate ion in the vial), and the calibration includes this step.

IDENTIFICATION (7.2, 7.4). A compound is identified by its retention time compared with the calibration solutions — differences within a series of no more than ± 0.02 min at comparable concentrations. A peak on one column means that the compound may be present; a peak also on a column of different polarity means that its presence is very probable. If the results from the two columns differ, peak overlap may be the reason and the lower value is usually the more accurate one; with GC-MS confirmation the ratio of the main ion to a secondary ion has to be within ± 10 % of the value for the standard (with a photo-ionization detector — within 20 %).

CALIBRATION (clause 8). The total method is calibrated with an external standard — aqueous solutions of the compounds to be determined, prepared from a stock solution in dimethylformamide (alternatively in acetone or methanol; no more than 1 ml of solvent per litre of water). Stock solution: 5 ml of solvent and 100 µl of each compound in a 10 ml vial, preferably stored at −20 °C in the dark (stable for at least a week). Calibration solution: an appropriate amount of stock solution (usually 50 µl — giving 878 µg/l of benzene) is dosed into 1000 ml of water stirred with a magnetic stirrer and stirred for another hour, and 5 ml is transferred to a vial (with potassium carbonate if used). Calibration solutions are not prepared by diluting other calibration solutions; they are prepared immediately before use. In routine work the calibration function is checked with two solutions (1–20 % and about 80 % of the working range) analysed twice.

INTERFERENCES AND BLANKS (clause 3, 7.3). BTX are lost during sampling, transport, storage and sample preparation, and volatile organic compounds in ambient air contaminate samples and the water used for blanks; samples must not come into contact with plastics. A second liquid phase prohibits head-space analysis, and superposition of other hydrocarbons (e.g. mineral oil constituents) hinders the determination. Benzene is ubiquitous, so blanks are run before and during a series, covering all steps; the blank value is subtracted only if its standard deviation does not significantly exceed the standard deviation of the calibration function.

WHAT WE DO NOT GIVE HERE. The publicly available text of the standard ends in clause 8 with the table of densities of the compounds. 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, limits of detection of individual compounds or precision data.

Method principle

Benzene and its derivatives 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. Separation is carried out on two capillary columns of different polarity, and detection with a flame ionization detector (FID). The content is calculated from the calibration of the total method with aqueous standard solutions; adding potassium carbonate shifts the equilibrium towards the gas phase and lowers the limit of detection.

Applications

• Water and waste water — benzene, toluene, xylenes and ethylbenzene at concentrations above 2 µg/l (clause 1)
• Further benzene derivatives and nonpolar compounds with similar physical properties — after verifying applicability for the particular sample (clause 1)
• Gaseous waters — after neutralising free carbon dioxide with potassium carbonate (clause 6)

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 cleaned bottles and vials upside down for 1 h at 150 °C in a ventilated drying oven, after removal protect them from contamination (e.g. with aluminium foil) and close them when cool (clause 4).

2. Sampling

   Sample directly into head-space vials or into bottles of non-actinic glass filled without turbulence; avoid plastics and composite samples; in parallel take an air blank and a reagent blank (clause 6).

3. Storage

   If possible, start the analysis within 2 days of sampling; store the samples at 4 °C in the dark, and for longer than 2 days — in the bottles (clause 6).

4. Vial with the sample

   Immediately after arrival transfer an aliquot of the sample into a vial — if necessary with potassium carbonate (about 7–8 g per 5 ml) — crimp the cap with the septum, shake and check the tightness of the cap (clause 6).

5. Equilibrium and dosing

   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, then introduce an aliquot of the gas phase into the chromatograph; place calibration solutions and blanks at the beginning and at the end of the series (7.1).

6. Chromatography and identification

   Separate on at least two capillary columns of different polarity and measure with the FID; if the identification is doubtful use a more selective detector (MS, photo-ionization detector) (7.2, 7.4).

7. Blanks

   Run blanks with water before and during the series; if they exceed 10 % of the lowest results, check every step; subtract the blank value only if its standard deviation does not significantly exceed that of the calibration function (7.3).

8. Calibration

   Immediately before use prepare aqueous calibration solutions from the stock solution in dimethylformamide (e.g. 50 µl per 1000 ml of water — 878 µg/l of benzene) and calibrate the total method; check the calibration function with two solutions (1–20 % and about 80 % of the range) analysed twice (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