Shi, Qiang (2008) Reproducibility and accuracy of halocarbon analyses of seawater with GC/MS. (Diploma thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 68 pp.

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Abstract

Brominated halogenated methanes (halocarbons: CHBr3, CH2Br2 and CHBr2CI) are atmospheric trace gases that act as major carriers of bromine to the atmosphere. The major sources of these brominated halocarbons are natural, including production by marine organisms, however there is also production associated with industrial cooling waters. At IFM-GEOMAR, these compounds are measured with the purge and trap method, gas chromatograph (GC), electron capture detector (ECD) and mass spectrometry (MS). With the purge and trap method the halocarbons are extracted from the samples, and transferred to the gas chromatograph. They are separated via gas chromatography and detected using an electron capture detector or mass spectrometer. Calibration of these systems is difficult, and seawater analyses are frequently calibrated with liquid standards. Measurement uncertainties arise due to uncertainty concerning the stability and accuracy of these standards. In this Diploma thesis I used the measurement system to examine the stability of halocarbons (CH3I, CHBr3, CH2Br2 and CHBr2CI) in liquid standard solutions. I used repeated measurements of different standard solutions (pentane, methanol and water) at three different temperatures and in glass bottles with different surface-area to volume ratios to test whether the concentrations of halocarbons are stable. The concentrations of the halocarbons were found to decrease significantly with time in all cases. For example, only 25% of the original bromoform could be observed in the methanol solution after 90 days and the largest decrease took place in the first 9 days. The likely reasons for the decreases of halocarbons are discussed and I conclude that absorption/adsorption of the glass wall is likely to be the main process responsible.

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