Zwitterionic ion chromatography of common inorganic anions using stationary phases modified with carboxybetaine-type surfactants
O'Riordain, Colman (2006) Zwitterionic ion chromatography of common inorganic anions using stationary phases modified with carboxybetaine-type surfactants. PhD thesis, Dublin City University.
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Carboxybetaine-type zwitterionic surfactants were employed to dynamically modify reversed-phase columns (both particle-packed and monolithic in structure), before investigating their use as stationary phases for the determination of common inorganic anions by zwitterionic ion chromatography (ZIC). Both surfactants utilised (namely dodecyldimethylaminoacetic acid and N-dodecyl-N,N- (dimethylammonio)undecanoate (DDMAU)) had a similar structure, with an internal, positively charged ammonium group, and an external, negatively charged carboxylate group, but DDMAU was found to exhibit superior column coating stability, due to its increased hydrophobic character. Both surfactants were found to have a pH dependent effective ion-exchange capacity, with increased eluent pH values resulting in reduced analyte anion retention times, due to increased protonation of the weak acid terminal group of the surfactant molecules. The suitability of these carboxybetaine-modified stationary phases to the determination of nutrient anions in high-ionic strength environmental sample matrices (e.g. seawater samples) has been demonstrated. The ability to apply dual gradient programs (i.e. with a combined eluent pH and flow gradient) provided the opportunity to expedite the elution, and improve the efficiency, of later eluting analyte peaks (i.e. polarisable anions such as iodide and thiocyanate), without sacrificing the resolution and efficiency of earlier eluting anions (e.g. iodate, nitrite etc.). The applicability of DDMAU-modified stationary phases to capillary IC using direct contactless conductivity detection was established, with modified capillary monoliths facilitating the separation of a mixture of 8 anions using flow rates of 1 jiL/min and less. Effective column lengths of the DDMAU-modified capillary monolith, and, therefore, analyte retention, could be adjusted through relocation of the contactless conductivity detector utilised.
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