(209e) Microreactors with Integrated Porous Silicon Layer for Reaction Studies by Matrix-Free Mass Spectrometry

Porous silicon can be used as matrix material for laser desorption/ionisation on silicon-mass spectrometry (DIOS-MS) [1] and porous silicon supports compatible with MALDI plates are commercially available. The DIOS-MS technique diminishes the utilization of a matrix ? chemical substance that absorbs the laser beam energy and facilitates vaporization and ionization. Furthermore utilization of porous silicon overcomes the problem of interference of the matrix with sample ions and therefore enables to identify low molecular weight compounds. Here we report on the integration of porous silicon spots in a silicon-glass to perform a chemical reaction in a continuous flow regime and in-line investigation of product formation by MS. The reaction products leave the microchannel network through the open outlet and the eluent is placed on the porous silicon spot on a front side of the chip (Figure 1). Two types of microreactors were designed and fabricated. The reactor of type I consists of an inlet mixer (A) where two reactants are mixed together and the chemical reaction is initiated. Subsequently, the active mixture passes into the reaction chamber. Both reactants are pumped into chip with the same speed (constant volume pressure driven flow induced by syringe pump). To avoid the mixture of undefined reagent ratio contaminating the reaction coil (B), an additional under-pressure line is introduced. By applying a small under-pressure all solutions will be removed from the chip to the waste location. The residence time is defined as the period between shutting-off the waste line and the time the active mixture spends in the reaction coil. The reaction is terminated by solvent evaporation on the porous silicon spot. When the droplet of eluent is formed on the outlet of the channel, the flows are stopped and the droplet evaporates. Then the residual liquids from the channels are removed from the chip by waste line. Subsequently, the chip is removed from the holder and placed inside the chip-tailored steel MALDI plate and delivered into the standard MALDI TOF MS device where the analysis of product is conducted. After the experiment the chips are flushed with fresh solvents and cleaned in ultrasonic bath (to remove the residuals deposited on porous silicon spot). The detailed operation regime is presented on Figure 2. In the microreactor type II an additional quenching line and mixer are integrated (Figure 1-II). The reaction time is defined by the geometry ? the time difference between the initiation of the reaction in the inlet mixer and termination in quenching mixer (where the active mixture is combined with chemical inhibitor of the reaction). The waste line operates identically as in design I. When all flows are stabilized, the waste line is closed and the inactive mixture runs through the outlet to be deposited on the porous silicon spot. Two types of outlet configurations were proposed (C). The conventional via-hole with the outlet opening in the center of the porous silicon spot and the 4-outlet located close to the perimeter of the porous silicon area (see Figure 1). In case of problems with homogeneity of the product concentration in evaporated eluent, four droplets growing on the outlet will reach the size when they will merge and cover the central part of the porous silicon spot. The silicon-glass microreactors were fabricated by standard microfabrication processes [2]. The channels 50µm width and 50µm depth were formed on the back side of 4 inch highly p-doped (0.001 Ù∙cm) (100) silicon wafer by deep reactive ion etching (BOSCH process). The porous silicon spots (thickness of ca. 0.9 µm, diameter of 2 mm) were created locally via hydrofluoric acid anodization process [3] using PECVD silicon nitride mask. The inlets and via-holes were powder-blasted in glass and silicon respectively. The silicon wafer was anodically bonded to Pyrex glass and diced into separate chips. The feasibility of the porous silicon spots was tested by spotting a 20mM solution of Angiotensin I human (M=1296.48) on microreactor-DIOS plate and conventional MALDI plate without matrix. The correct mass spectrum was observed from porous silicon spots, while no signal was observed in case of a MALDI plate without the matrix utilization. The opportunity of integration of porous silicon spots with multiline microreactors reduces the experimental time and effort. The concept can be easily extended to a higher number of reaction channels in parallel, as was reported recently [4]. In this case a wide range of residence time points on a reaction kinetic curve can be covered just by a single flow rate. [1] J. Wei, J.M. Buriak and G. Siuzdak, Desorption?ionization mass spectrometry on porous silicon, Nature 399, 1999, pp. 243?246. [2] J. G.E. Gardeniers, R.E. Oosterbroek, A. van den Berg, Silicon and glass micromachining for µTAS, in: Lab-on-a-chip: Miniaturized systems for (bio)chemical analysis and synthesis, eds. R.E. Oosterbroek and A. van den Berg, Elsevier, Amsterdam, 2003, pp. 37-67. [3] R.W. Tjerkstra, M.J. de Boer, J.W. Berenschot. J.G.E. Gardeniers, A. van den Berg and M.C. Elwenspoek, Electrochim. Acta, 1997, 42, pp. 3399-3406. [4] W. P. Bula, W. Verboom, D. N. Reinhoudt and J.G.E. Gardeniers, Lab Chip, 2007, 7, pp. 1717-1722