Development of primary reference gas mixtures for volatile organic compound monitoring in South Africa

Abstract

Direct monitoring of volatile organic compounds (VOCs) emitted from industrial sources and the monitoring of ambient levels thereof in the atmosphere, play an essential role in providing data for various legislative requirements. Many volatile organic compounds are emitted into the atmosphere from biomass burning, power stations, and many other sources. Once in the atmosphere, the volatile organic compounds are known to play a role in the generation of ozone by reaction with other constituents of air. There are many observed adverse effects on the environment due to air pollutants and their reaction products, such as reduced visibility due to heavy fog, which includes emitted particles, photochemical smog, acid rain due to the emissions of nitrogen and sulphur oxides, and ozone depletion. Gas reference standards are used for the calibration of equipment used to measure the VOC pollutants. The gas reference standards are also a source of traceability, linked directly to the international system of units (SI) due to the realisation of measurement capabilities through primary methods such as gravimetry and coulometry. In this study, reference gas mixtures containing benzene, toluene, ethylbenzene, o-xylene, m-xylene, and p-xylene (BTEX), were prepared by gravimetric methods at mole fractions of between 5 nmol/mol to 30 µmol/mol in this study. The preparation was performed using three different techniques: the micro-gravimetric method using small glass capillary tubes, developed at the National Institute of Standards and Technology (NIST), the stainless steel (loop) method with the dedicated minimum dead volume connector developed at National Physical Laboratory (NPL) and the direct liquid injection method using a dedicated heated syringe heater to volatilise components and flushed with small amounts of nitrogen developed at Korean Research Institute of Standards and Science (KRISS). After the preparation, a comparison was achieved by analysis of the BTEX gas mixtures using gas chromatography with flame ionisation detection (GC-FID). The NIST method was used to directly prepare 500 nmol/mol, which showed large uncertainty values due to poor weighing repeatability. With the NPL method, the 30 µmol/mol [BTX (benzene, toluene and p-xylene)], and the 10 µmol/mol BTEX were achieved. The KRISS method was used to prepare 10 µmol/mol of BTEX. The NPL and the KRISS methods were compared by GC-FID analysis and showed comparability to within 2% of the gravimetric values. Stability studies on 30 µmol/mol (BTX) gas mixtures showed minimal loss of benzene, toluene and p-xylene over 24 months. However, the long-term stability analysis was affected by the demand for cylinders for other vital projects. Unfortunately, many of the gas mixtures were discarded to allow the cylinders to be used in other projects. The reference gas standards were internationally benchmarked through the National Metrology Institute of South Africa (NMISA) participation in key comparisons Asia Pacific Metrology Programme (APMP.QM-S12, APMP.QM-S14) and Consultative Committee for Amount of Substance (CCQM-K10). In the two APMP comparisons, the NMISA results had large deviations from the comparison reference values, mainly from the poor analysis methods due to the inefficiency of trapping of components using pre-concentrators. However, in the CCQM-K10, NMISA showed improved results, with most values within 0,7 nmol/mol at 5 nmol/mol nominal mole fraction, compared to 30 nmol/mol obtained in APMP.QM-S12 and 10 nmol/mol were obtained in APMP.QM-S14, both at 100 nmol/mol. This improvement in CCQM-K10 was attributed to the improved analysis of BTEX at nmol/mol by optimizing the pre-concentrator conditions, which improved both the repeatability and the accuracy of BTEX standards used to analyse the comparison gas mixture. The contributions to the uncertainty of measurement were from the weighing of capillary tubes, weighing of stainless steel loops, weighing of syringes, purity data, and GC-FID verification data. The BTEX reference gas mixtures prepared in this study will be used as a source of traceability in South Africa for air pollution monitoring.