Anal. Chem. 2000, 72, 3266-3271
Liquid Chromatography/Electrospray Mass Spectrometry of Organoselenium Compounds with Postcolumn Crown Ether Complexation Wilson Z. Shou, Michelle M. Woznichak, Sheldon W. May, and Richard F. Browner*
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
Postcolumn addition is an effective means of alleviating or solving ionization-related problems in liquid chromatography/electrospray ionization mass spectrometry (LC/ ESIMS). In the current study, initial attempts to develop a direct LC/ESIMS method for an organoselenium compound, 4-hydroxyphenyl 2-methyl-2-aminoethyl selenide (HOMePAESe), were unsuccessful because of extensive fragmentation which occurred even under the mildest insource collision-induced dissociation (CID) conditions. To reduce the extent of compound fragmentation, a crown ether, 18-crown-6, was added postcolumn to the system, forming a complex with HOMePAESe, which survived the electrospray ionization process with reduced fragmentation and hence improved sensitivity for the major ions. The general applicability of this crown ether complexation approach to clinical samples was demonstrated by the analysis of HOMePAESe in human urine, using a structural analogue, 4-fluorophenyl 2-aminoethyl selenide (FPAESe) as an internal standard. The limit of detection for HOMePAESe, based on a signal-to-noise ratio of 3:1, was estimated to be 5 pg/µL in urine. The potential application of this approach to the general analysis of other amine-containing compounds was also evaluated. Electrospray ionization mass spectrometry (ESIMS) is widely used for liquid chromatography/mass spectrometry (LC/MS) interfacing. However, its use often places restrictions on the options generally available to LC systems with UV detection. High concentrations of nonvolatile buffers and additives and operating with high water percentages in the mobile phase are all conditions generally incompatible with the electrospray process and can cause significant signal suppression. To allow independent optimization, it is sometimes desirable to decouple the separation and the electrospray ionization steps by the means of postcolumn addition of “electrospray-compatible” solvents and additives, which are intended to facilitate the ionization step and improve detection.1-3 There are also instances where specific analytes do not give adequate electrospray signals because of either low polarity or extensive fragmentation. Various modification schemes such as (1) Voyksner, R. D. In Electrospray Ionization Mass Spectrometry; Cole, R. B., Ed.; Wiley: New York, 1997; p 332. (2) Chassaigne, H.; Lobinski, R. J. Chromatogr., A 1998, 829, 127. (3) Kuhlmann, F. E.; Apffel, A.; Fischer, S. M.; Goldberg, G.; Goodley, P. C. J. Am. Soc. Mass Spectrom. 1995, 6, 1221.
3266 Analytical Chemistry, Vol. 72, No. 14, July 15, 2000
cationization by metals,4,5 adduct formation by halogens6 (under negative ionization mode), and formation of ternary complexes7-9 have been used to address these problems. Many of these modifications were also carried out postcolumn in order to decouple the ionization process from the conditions used in the chromatographic separation procedures. Karlsson10 and Kohler et al.11 used postcolumn cationization to analyze mixtures of relatively nonpolar carbohydrates, whereas Shen al.12 added a Co+ and pyridine solution postcolumn, forming ternary complexes with tetracycline. For the interface designs, Kohler et al. and Karlsson used a concentric sheath flow type sprayer to add metal solutions to column effluents close to the nebulizer tip, whereas Shen et al. mixed the column effluents and modifiers with a Y-shaped junction located just before the ion spray probe. In the present study, we evaluated the utility of postcolumn addition of a polyether, 18-crown-6, to overcome the extensive fragmentation that we had observed in direct LC/ESIMS analysis of some small, fragile, low-mass (
