employed. , The samples have to be conditioned by a cation exchange procedure prior to the reaction and excess reagent must be removed by passing the sample through a SiO2 column after the reaction. The resulting p-bromophenacyl esters may be measured by GC and/or HPLC.

Studies aiming to identify unknown compounds, including substituted organic acids, often employ a sequence of extraction and derivatization steps where different functional groups are marked with specific reagents and are then measured by GC/MS.41,45 For example, direct aqueous oximation of carbonyl functions with PFBHA is followed either by liquid_liquid extraction or by solid-phase extraction. Measurements of these extracts are used to identify aldehydes and ketones. Further methylation (e.g., BF3/methanol) of the extracts allows for identification of ketoacids whereas sylilation with «-(tert-butyldimethylsilyl)-A^-methylfluoracetamide (MTBSTFA) marks hydroxyl functions. Typical fragments when employing GC/MS with electron impact ionization (EI) are m/z 181 for carbonyl function, m/z 59 for methylesters, and m/z 75 for the sylilated hydroxyl functions. Measurements by GC/MS using chemical ionization (CI) result in dominant M+ ions, which can be used to determine the molecular weight of the unidentified compound. Samples such as ozonated drinking water, ozonated paper pulp, and oxidized isoprene have been investigated with this approach.41,45

b. High Performance Liquid Chromatography (HPLC)

HPLC methods are rarely applied to the analysis of short-chain organic acids due to their poor UV-absorbance and their nonfluorescent character. Only high concentrations can be measured directly by HPLC in combination with UV, diode array, or fluorescence detection. To enhance method sensitivity, organic acids may be derivatized in pre- or postcolumn reactions. Although an abundance of derivatization methods for various compounds is available, especially for

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