| Summary: | A Dissertation submitted in fulfillment of the requirements for the degree
Master of Science
In the Faculty of Sciences
at the University of the Witwatersrand, Johannesburg
Johannesburg, January 2015 === Three methods, for the analyses of low levels of moisture in gas samples,
were developed and optimized. The analytical techniques included Fourier
Transform Infrared Spectroscopy (FTIR) and Pulsed Discharge Helium
Ionization/Gas Chromatography (PDHID/GC).
The methods included the direct analyses of moisture in gas samples
using FTIR as well as the analysis of acetylene (C2H2) by FTIR and
GC/PDHID. For the latter methods, the purpose was to convert the
moisture in a gas sample to C2H2 by hydrolization of the calcium carbide
(CaC2) with moisture to C2H2 and then analyze the resulting C2H2 content
by FTIR or GC/PDHID. The C2H2 result was then converted back to
moisture to obtain the moisture content of the sample.
The FTIR moisture method developed provided eleven different
wavenumbers for quantitation providing a wide analytical scope,
specifically in complex gas matrices, where there is often peak overlap
between matrix and moisture. A heated eight meter glass long path gas
cell and a mercury cadmium telluride (MCT) detector were utilized. The
FTIR method required much greater volumes of sample than the GC
method but allowed for direct analysis of moisture without prior conversion
to acetylene. Moisture permeation standards were used for calibration and
the LOD’s ranged from 0.5 to 1 ppm with quantification possible from 0.5
to 10ppm.
For the FTIR C2H2 method various concentration ranges were established
from 50 up to 2000 ppm. Three wavenumbers were evaluated for C2H2
and methane was introduced as an internal standard. The use of methane
as an internal standard provided better r2 values on the calibration data
than for the tests run without internal standard.
A gas chromatographic (GC), pulsed discharge helium ionization detector
(PDHID) method for the determination of moisture content in small
quantities of gases, based on the conversion of the moisture to acetylene
(C2H2) prior to analysis, was developed. The method developed on the
GC/PDHID for C2H2, provided a quantitation range from 0.6 to 7.7 ppm.
Conversion of the moisture to acetylene was achieved by hydrolysing an
excess of calcium carbide (CaC2) in a closed reaction vessel with a
measured volume of a sample containing a known quantity of moisture.
The gaseous reaction mixture was transferred, using helium (He) carrier
gas, to a GC/PDHID, set up with “sample injection and heart cut to
detector” to prevent matrix disturbances on the PDHID, for analysis. The
acetylene concentration values thus obtained were converted back to
moisture values and percentage recoveries calculated. A similar
conversion process was applied on FTIR.
The conversion of moisture to C2H2 using CaC2 was tested and proven to
be viable. Quantification was not possible as the available sample holder
could not be adequately sealed to prevent air ingress. This led to higher
C2H2 values than expected. This process can be optimized by the design
and production of a sealed sample holder.
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