Sammendrag
Diethanolamine (DEA) and N-methyldiethanolamine (MDEA) are used in post combustion CO2 capture technology and to remove impurities (e.g. H2S, SO2 and CO2) in natural gas. Their small molecular size and high polarity means DEA (mw 105) and MDEA (mw 119) are highly soluble in water. LC-ToF-MS provides high sensitivity and reproducibility for the analysis of DEA and MDEA in deionised water standards. However, high concentrations of salt in a sample (e.g. seawater) cause a significant problem in the LC-ToF-MS. Dilution (x100) of a 5 ppm MDEA in seawater sample with deionised water proved a suitable for achieving good LC-ToF-MS analysis although, this method is limited when the MDEA concentration is low. In the case of DEA in seawater significantly reduced response and poor peak shape were observed in LC-ToF-MS chromatograms. A range of techniques were investigated to extract DEA from seawater and provide sample concentration. Liquid-liquid extraction was unsuitable given the solubility of DEA in water and the lack of a non-miscible polar solvent into which DEA would partition. Initial studies using solid phase extraction (SPE) found that stationary phases such as C18-bonded silica, and Hypercarb did not adsorb DEA. As DEA is a very small and polar molecule, it is likely to be present in ionic form in water and therefore Strata-X-C (500 mg; Phenomenex) cationic exchange SPE cartridges were investigated. This type of solid phase yielded partial extraction of the DEA, confirming it was present in ionic form. However, recovery values were still significantly below those for deionised water standards. Experiments indicated that DEA recovery is highly dependent upon the amount of absorbent and that DEA molecules are competing with dissolved salt ions in the seawater for adsorption sites. This process also resulted in a high amount of salt in the sample extract which caused significant ion suppression in the LC-ToF-MS. Attempts to improve/optimise the extraction and analysis me