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

Abstract

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).