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 Sciences / M. Sc. (Environmental Sciences)
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:unisa/oai:uir.unisa.ac.za:10500/27315 |
| Date | 05 1900 |
| Creators | Leshabane, Nompumelelo |
| Contributors | Tshilongo, J., Moja, S. J. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | 1 online resource (xvi, 249 leaves) : color illustrations; color graphs, application/pdf |
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