Table of Contents:
Within this dissertation, a method for the ultra trace determination of fluorobenzoic acids in complex aqueous sample matrices using isotope dilution mass spectrometry (IDMS) with gas chromatography or ion chromatography with mass selective detection is presented.
To minimize possible interferences of the sample matrix and to enrich the analytes simultaneously, a previously developed solid-phase extraction method was optimized and characterized.
The necessary separation of the analyte mixture obtained after solid-phase extraction, has been achieved by the development of two chromatographic methods.
Since only the volatile esters are suitable for use in gas chromatography, a synthesis was developed and optimized, which permits the quantitative formation of the methyl esters by using BF3 ·MeOH on a micro scale, directly in a GC vial at 64°C within 24 hours. For their separation a GC column functionalized with polydimethylsiloxane containing 50 % phenyl was chosen and a temperature gradient to give the best separation at a minimal runtime was developed. A characterization of the fragmentation pattern after electron impact ionization was necessary in the subsequent creation of a selected ion method (single ion monitoring, SIM) to enhance the instrumental detection limit and further minimization of the matrix influence.
In case of ion chromatography, after a screening of dierent types of separation columns with dierent functionalization and a comparison of dierent eluent ions and concentrations, varying the modifier percentage and the separation temperature, the method that worked out best was developed. The use of a 250 mm column of the latex type in combination with a 0,63 mM sodium carbonate eluent and 18 % acetonitrile as the organic modifier at 45 °C resulted in the best possible separation at minimum runtime under isocratic conditions. For the required mass-selective detection electrospray ionization was used. To characterize the ionization behavior of the acids in the electrospray, with the aim to improve the instrumental detection limit by a maximum degree of ionization, the influence of capillary voltage, fragmentor voltage and spray composition was studied. The final development of a selected ion method analogous to gas chromatography, completed the method development and yielded in a maximum of sensitivity with a simultaneous minimization of interferences.
For stable isotopic labeling of fluorobenzoic acids, an inexpensive and quick H/D exchange reaction without the use of catalyst was realized. Solely by an oil bath, drying oven or in a synthesis microwave a thermally induced electrophilic aromatic substitution in concentrated D2SO4 managed an exchange of the aromatic protons with deuterium. By the example of six FBAs, the reaction time and reaction temperature has been optimized to the highest possible purity in a single reaction to acheive a minimum mass shift of
In conclusion, the suitability of the isotopically labeled fluorobenzoic acids in the single steps of the analysis method SPE, esterfication using BF3 · MeOH, gaschromatographic separation, electron impact ionization, ionchromatographic separation and electrospray ionization were controlled and an isotope dilution analysis in real samples was carried out.