Development of selected sulphur compounds and oxygenated volatile organic compounds reference gas mixtures for air quality monitoring

Highly accurate analysis for the quantification of sulphur compounds and oxygenated volatile organic compounds are crucial for the adherence of the legislation in different environmental sectors. The sulphur compounds and oxygenated volatile organic compounds measurements are challenging, due to various factors such as molecules being adsorbed on the inner surfaces of cylinders. It is therefore important to produce accurate and reliable reference gas mixtures with mole fraction at ambient levels for the air quality monitoring and field of gas sensing in South Africa. The challenges in producing sulphur compounds and oxygenated volatile organic compounds reference gas mixtures are that the overall process from gravimetric preparation steps until the comparison analysis process and the stability of mixture in the gas cylinder, results in the large measurement uncertainties. In order to produce reference gas mixtures of the highest level, three important steps are followed: purity assessment of starting material, gravimetric preparation, and verification/validation of prepared gas mixtures. The purity analysis of high purity starting materials was determined using gas chromatography coupled with various detectors and Karl Fischer for determination of moisture content in high purity chemicals. The sulphur compounds and oxygenated volatile organic compounds to be developed in this study were hydrogen sulphide, sulphur dioxide, acetone, methanol, ethanol, isopropanol, and n-butanol. These components were produced following the International Organisation for Standardisation documents at mole fraction of 10 µmol/mol for sulphur compounds and 5 µmol/mol for oxygenated volatile organic compounds. The preparation of sulphur compounds reference gas mixtures was done with a static gravimetric method using a direct method where a target component was transferred directly into the cylinder. The preparation of oxygenated volatile organic compounds used an indirect method whereby a target liquid component from high purity chemicals was transferred into a cylinder using a gas-tight syringe.The comparison between the reference gas mixtures was validated using Non-Dispersive Ultra-Violet analysers (NDUV), gas chromatograph coupled with pulsed discharge helium ionisation detector (GC-PDHID, UV fluorescence analysers for sulphur compounds and gas chromatograph coupled with flame ionisation detector (GC-FID) for the oxygenated volatile organic compounds. A multi-point calibration method was used to analyse sulphur dioxide and hydrogen sulphide on the NDUV analyser, and the single-point calibration method was used for analysis on the gas chromatography and UV fluorescence where a sample mixture is analysed against a reference mixture with a similar mole fraction. The statistical data considered during analysis included calculation of the instrument drift and percentage relative standard deviation to check measurements repeatability, reliability, and measurement uncertainty. The gravimetric results of prepared sulphur compounds at 10 µmol/mol gave a percentage relative expanded uncertainty of 0.041 % REU for hydrogen sulphide, 0.12 % REU for sulphur dioxide. The gravimetric results of prepared oxygenated volatile organic compounds at 5 µmol/mol showed a percentage relative expanded uncertainty 0.068 to 0.35 % REU for isopropanol and ethanol respectively and less than 2.4 % REU for multi component of oxygenated volatile organic compounds. Finally, the primary standard gas mixtures of sulphur compounds and oxygenated volatile organic compounds were developed with the highest metrological measurement uncertainty level of (k=2).Environmental SciencesM. Sc. (Environmental Sciences

Development of benzene, toluene, ethyl benzene, m-xylene, p-xylene and o-xylene (BTEX) gas reference standards for air pollution monitoring in South Africa

Direct monitoring of volatile organic compounds emitted from industrial sources as well as the monitoring of ambient levels thereof in the atmosphere play an important role in providing data for various legislative requirements. There are many volatile organic compounds emitted to the atmosphere, from biomass burning, power stations, and many other sources. Primary reference gas mixtures of volatile organic compounds containing six components, namely benzene, toluene, ethylbenzene, o-xylene, m-xylene, and p-xylene (BTEX) were prepared by gravimetric methods at nominal amount of substance fractions of 10 µmol/mol. The preparation was performed using the stainless steel (loop) method and direct liquid injection method using gas tight syringe connected to a dedicated syringe heater for comparison purposes. The results of the gravimetric values for both methods were within 1 % of the nominal amount of substance fraction. After the preparation, comparison was achieved by analysis of the BTEX gas mixtures using gas chromatography with flame ionisation detection (GC-FID) and showed comparability to within 2 % of the gravimetric values. The contributions to the uncertainty of measurement were from weighing stainless steel tubes, syringes, and gas cylinders, as well as purity data. The contribution to the uncertainty of measurement from the analysis by GC-FID were repeatability and reproducibility. The combined expanded uncertainty of measurement was between 1.09 % and 3.51 %. The larger deviation from the gravimetric values and associated uncertainty was observed for benzene due to its higher volatility compared to the other components. The BTEX reference gas mixtures prepared in this study are currently being used as a source of traceability in South Africa for air pollution monitoring purposes.Open access funding provided by Mintek.http://link.springer.com/journal/769Chemistr

CCQM-K120 (Carbon dioxide at background and urban level)

CCQM-K120.a comparison involves preparing standards of carbon dioxide in air which are fit for purpose for the atmospheric monitoring community, with stringent requirements on matrix composition and measurement uncertainty of the CO2 mole fraction. This represents an analytical challenge and is therefore considered as a Track C comparison. The comparison will underpin CMC claims for CO2 in air for standards and calibrations services for the atmospheric monitoring community, matrix matched to real air, over the mole fraction range of 250 μmol/mol to 520 μmol/mol. CCQM-K120.b comparison tests core skills and competencies required in gravimetric preparation, analytical certification and purity analysis. It is considered as a Track A comparison. It will underpin CO2 in air and nitrogen claims in a mole fraction range starting at the smallest participant's reported expanded uncertainty and ending at 500 mmol/mol. Participants successful in this comparison may use their result in the flexible scheme and underpin claims for all core mixtures This study has involved a comparison at the BIPM of a suite of 44 gas standards prepared by each of the participating laboratories. Fourteen laboratories took part in both comparisons (CCQM-K120.a, CCQM-K120.b) and just one solely in the CCQM-K120.b comparison. The standards were sent to the BIPM where the comparison measurements were performed. Two measurement methods were used to compare the standards, to ensure no measurement method dependant bias: GC-FID and FTIR spectroscopic analysis corrected for isotopic variation in the CO2 gases, measured at the BIPM using absorption laser spectroscopy. Following the advice of the CCQM Gas Analysis Working Group, results from the FTIR method were used to calculate the key comparison reference values

International Comparison CCQM-K51 - Carbon Monoxide (CO) in Nitrogen (5 µmol mol-1)

The first key comparison on carbon monoxide (CO) in nitrogen dates back to 1992 (CCQM-K1a). It was one of the first types of gas mixtures that were used in an international key comparison. Since then, numerous national metrology institutes (NMIs) have been setting up facilities for gas analysis, and have developed claims for their Calibration and Measurement Capabilities (CMCs) for these mixtures. Furthermore, in the April 2005 meeting of the CCQM (Consultative Committee for Amount of Substance) Gas Analysis Working Group, a policy was proposed to repeat key comparisons for stable mixtures every 10 years. This comparison was performed in line with the policy proposal and provided an opportunity for NMIs that could not participate in the previous comparison. NMISA from South Africa acted as the pilot laboratory. Of the 25 participating laboratories, 19 (76%) showed satisfactory degrees of equivalence to the gravimetric reference value. The results show that the CO concentration is not influenced by the measurement method used, and from this it may be concluded that the pure CO, used to prepare the gas mixtures, was not 13C-isotope depleted. This was confirmed by the isotope ratio analysis carried out by KRISS on a 1% mixture of CO in nitrogen, obtained from the NMISA. There is no indication of positive or negative bias in the gravimetric reference value, as the results from the different laboratories are evenly distributed on both sides of the key comparison reference value.JRC.H.2-Air and Climat

International comparison CCQM K52: carbon dioxide in synthetic air

61 p. : il.The first key comparison on carbon dioxide in nitrogen dates from 1993-1994 (CCQM-K1b) [1]. It is in fact one of the first types of gas mixtures that was used in an international (key) comparison. Since then, numerous National Metrology Institutes (NMIs) have been setting up facilities for gas analysis, and have developed claims for their Calibration and Measurement Capabilities (CMCs) for these mixtures. Also, in 2003 a comparison (CCQM-P41 [2, 3]) was carried out between NMIs and WMO laboratories for the determination of greenhouse gases showing a good overall agreement between the participants. In the April 2005 meeting of the CCQM Gas Analysis Working Group, a policy was proposed to repeat key comparisons for stable mixtures every 10 years. Consequently, this comparison is consistent with the proposed policy and enables NMIs that could not participate in the previous comparison to take part. This report describes the results of a key comparison for carbon dioxide in synthetic air (oxygen + nitrogen). The amount–of–substance fraction level of carbon dioxide chosen for this key comparison (360 μmol/mol) represents the ambient level of this component in air. This key comparison aims to support CMC-claims for carbon dioxide in both nitrogen or air (synthetic and purified) from 100 μmol/mol to 20 cmol/mol