A powerful new hyphenated technique based on the combination of LC, on-line electrochemical ("coulometric") oxidation and ES- or APCI-MS has been developed. Simple and commercially available instrumentation has been used to set up the analytical system.
Ferrocenecarboxylic acid chloride was prepared from ferrocenecarboxylic acid and used as derivatizing reagent for the synthesis of several alcohol and phenol derivatives that were characterized by IR, EI-MS and 1H-NMR. These ferrocenecarboxylic acid esters have been separated by reversed-phase LC and oxidized (ionized) "coulometrically" prior to single quadrupole MS analysis using electrospray ionization and atmospheric pressure chemical ionization interfaces. The dependency of the ionization on the electrochemical pretreatment was demonstrated. Limits of detection for selected derivatives ranged from 3 nM to 0.4 µM depending on the individual compound and the selected interface. To test the applicability of the new hyphenation technique for fast LC separations, conventional guard columns were employed for the separation of mixtures containing up to nine alcohol or phenol derivatives that could be accomplished in a time of 1 - 1.5 min depending on the number and structure of the individual compounds. Nanospray and electrospray ion sources were used in combination with tandem mass spectrometry to study the fragmentation pathway of the ferrocenecarboxylic ester derivatives. Common fragmentation products for each substance class enabled the use of precursor ion scans. LC/electrochemistry/ES(+)-MS chromatograms were recorded in multiple reaction monitoring mode. The limits of detection for selected phenol derivatives were below 1 nM. Seven gasoline and four diesel samples were then analyzed by LC/electrochemistry/ APCI(+)-MS. In the resulting complex chromatograms, selected mass traces were used to identify several groups of alcohols and alkylphenols. Only a sum parameter for alcohols and phenols of the same molecular mass could be obtained because of the large number of structural isomers that were not chomatographically resolved in the mass traces. Apart from the alcohols and phenols, several other compounds were found in the samples. Many of these unknown compounds could be arranged in four series with a mass difference of 14 mass units. Tandem-MS experiments were performed to identify the unknown substances but only little information was gained. Finally, to validate the method two oil samples that are used as standard reference materials were analyzed by LC/electrochemistry/MS after a derivatization step with ferrocenecarboxylic acid chloride and a short sample preparation. Phenol and the C1 to C9 alkylphenols were detected in the resulting chromatograms using selected ion monitoring. Quantitation was performed by external calibration and the results were compared to data obtained by the NIST and another research group employing independent methods.