Simple chip-based interfaces for on-line monitoring of supramolecular interactions by nano-ESI MS

Two simple interfaces were designed and realized, enabling on-line coupling of microfluidics reactor chips to a nanoflow electrospray ionization (NESI) time-of-flight (TOF) mass spectrometer (MS). The interfaces are based on two different approaches: a monolithically integrated design, in which ionization is assisted by on-chip gas nebulization, and a modular approach implying the use of commercially available Picospray(TM) tips. Using reserpine as a reference compound in a 1: 1 mixture of acetonitrile and water revealed that both interfaces provide a remarkably stable mass spectrometric signal ( standard deviations lower than 8% and 1% for the monolithic and modular approaches, respectively). Glass microreactors, containing mixing zones, were fabricated and coupled to the modular interface with perfluoroelastomer Nanoport(TM) fluidics connectors, providing a tool to study chemical reactions on-line. Investigation of the mixing dynamics showed that complete on-chip reagents mixing is achieved within a few tens of milliseconds. Metal - ligand interactions of Zn-porphyrin 1 with pyridine ( 2), 4-ethylpyridine ( 3), 4-phenylpyridine ( 4), N-methylimidazole ( 5), and N-butylimidazole ( 6) in acetonitrile as well as host - guest complexations of beta-cyclodextrin ( 7) with N-(1-adamantyl) acetamide ( 8) or 4-tert-butylacetanilide ( 9) in water were studied by mass spectrometry using themodular NESI-chip interface. From on-chip dilution-based mass spectrometric titrations of Zn-porphyrin 1 with pyridine ( 2) or 4-phenylpyridine ( 4) in acetonitrile K-a-values of 4.6 +/- 0.4 x 10(3) M-1 and 6.5 +/- 1.2 x 10(3) M-1, respectively, were calculated. The K-a-values are about four times larger than those obtained with UV/vis spectroscopy in solution, probably due to a higher ionization efficiency of complexed compared to uncomplexed Zn-porphyrin. For the complexation of N-( 1-adamantyl) acetamide ( 8) with beta-cyclodextrin (7), a K-a-value of 3.6 +/- 0.3 x 10(4) M-1 was obtained, which is in good agreement with that determined by microcalorimetry.