Strategies to expand the applicability of LC/MS to the analysis of non-polar compounds are presented within this thesis: The most important techniques presented here are on-line electrochemical conversion of the analytes to more polar reaction products, atmospheric pressure electron capture negative ion-MS and coordination ionspray-MS. These techniques are presented in detail, compared and discussed critically with respect to their current status and future perspectives. Particular focus is directed from a chemical point of view on the substance groups which are accessible by each of the new approaches.
The coupling of electrochemistry to mass spectrometry was used for the on-line conversion of phenothiazines and polycyclic aromatic hydrocarbons (PAHs). LC/atmospheric pressure electron capture negative ion-MS was applied for the analysis of several nitroaromatic compunds, whereas LC/coordination ionspray-MS was used for the identification of reaction products in a model rubber vulcanization process. Soluble degradation products of a poly(ether ester) block copolymer were identified by means of LC/electrospray-MS in conjunction with the addition of ammonium and sodium ions to provide important complementary information on the number of monomer units.
The on-line electrochemical conversion of phenothiazine and its derivatives after liquid chromatographic separation has been studied by mass spectrometry and fluorescence spectroscopy. In an electrochemical cell consisting of a porous glassy carbon working electrode, the phenothiazines are readily converted to oxidized products, which can be detected by on-line fluorescence spectroscopy and mass spectrometry. The method allows rapid investigations on the electrochemical oxidation pathways, as demonstrated for phenothiazine itself. The phenothiazine derivatives are transferred into their strongly fluorescent sulfoxides. Based on this reaction, an LC/electrochemistry/ fluorescence method was developed.
An efficient way for the fast elucidation of electrochemical reactions of polycyclic aromatic hydrocarbons has been set-up by coupling electrochemistry on-line to mass spectrometry. With this set-up, an improvement of sensitivity in the LC/MS analysis of PAHs is observed. Due to their low redox potentials, the non-polar PAHs are converted into the respective radical cations, which may further react with constituents of the mobile phase and in additional electrochemical oxidation steps. Among other products, mono-, di- and trioxygenated species are observed in aqueous solutions, alkoxylated compounds in alcohols and solvent adducts in the presence of acetonitrile. Deuterated PAHs and deuterated solvents were used to gain additional information on the formation of the reaction products.
The determination of selected nitroaromatic compounds by means of LC/MS with electron capture ionization using a commercial atmospheric pressure chemical ionization (APCI) interface in the negative ion mode was carried out. The electron capture effect is observed for nitroaromatics which do not easily undergo deprotonation under these conditions. Depending on the structure of the analytes, either dissociative or non-dissociative electron capture is observed. Limits of detection and linear range for the determination of the analytes match those obtained for nitroaromatics which undergo deprotonation. The investigated substances comprise numerous substituted nitrobenzenes and nitrobenzoxadiazols including the amine derivatives of 4-chloro-7-nitro-2,1,3-benzoxadiazole (NBDCl) as well as the isocyanate derivatives of 4-nitro-7-piperazino-2,1,3-nitrobenzoxadiazole (NBDPZ). The parameters favoring electron capture mechanisms have been thoroughly investigated under consideration of the competing mechanism of deprotonation to allow a better understanding of the electron capture process and to improve sensitivity and selectivity of the analysis.
Liquid chromatography/coordination ionspray-mass spectrometry has been used for the identification of reaction products in a model rubber vulcanization process. After LC separation using reversed-phase conditions, AgBF4 was used to form a charged cation-analyte complex, which was detected by ESI-MS. Strong signals were observed for vulcanization accelerators and products of the reaction between these and alkenes. The method performs best for substances containing sulfur chains with chainlengths between two and eight sulfur atoms, but sulfur-free compounds containing triethoxysilyl groups were detected as well. For the latter, the post-column addition of NaBF4 proved to be a suitable alternative. Besides the vulcanization accelerators, various reaction products, including sulfur-chain bridged alkenes were identified.
A detailed study on the in vitro degradation of a biocompatible poly(ether ester) block copolymer, which is based on poly(ethylene glycol) and poly(butylene terephthalate), was carried out using LC/ESI-MS. All major degradation products and several side-products were identified using both the positive and the negative ion mode, respectively. The data indicate that degradation does not only occur in the “soft”, but also in the “hard” segment of the polymer. Liquid chromatographic separation is required to distinguish between isomers. The addition of ammonium and sodium ions provided important complementary information on the number of monomer units.