Time-Resolved FT-IR Spectroscopy of Chemical Reactions in Solution by Fast Diffusion-Based Mixing in a Micromachined Flow Cell

Abstract

A new concept for the study of chemical reactions in solution by time-resolved Fourier transform infrared spectroscopy (TR/FT-IR) is presented. The key element of this concept is a micromachined mixing unit for fast and highly reproducible diffusion-based mixing that is incorporated in a flow cell for transmission measurements and operated in the stopped-flow mode. The mixing unit achieves multilamination of two liquid streamlines inside the flow cell. When the flow in both feeding channels is maintained, there is almost no mixing of the liquids, because of the short residence time inside the mixer, hence allowing for the recording of a reference spectrum of the reactants prior to reaction. When the flow is stopped by rapid switching of a dedicated injection valve, highly reproducible diffusion-controlled mixing takes place inside the flow cell so that spectral changes induced by the reaction under investigation can be directly followed. The total volume required for one experiment is ∼ 5 μL, and mixing times achieved so far are in the millisecond range. Factors governing time resolution in this new concept are the time required to stop the flow, the spacing of the individual streamlines, the diffusion coefficients of the reactants involved, and the signal strength of the spectral changes induced by the reaction under study. In this paper, the possibilities and limitations of the new concept are studied with the use of three model reactions, which are an acid-base neutralization reaction, the addition of sulfite to formaldehyde, and the basic hydrolysis of methyl monochloroacetate. In addition, the complete mixing process in the system was studied by computational fluid dynamics (CFD) simulations, which provided valuable insights into details of the mixing process itself as well as confirming the experimental results obtained.

Microfluidic Devices for Studying Kinetics

“…63,[65][66][67] Another effective approach is to integrate free jet turbulent mixing with a CCD array detector, which allows all points along the reaction channel to be monitored simultaneously; however, this approach comes at a cost to sensitivity. 30 Other analytical approaches incorporated into microfluidic devices for studying kinetics include small angle X-ray scattering (SAXS), 68 FT-IR, 82 and conductivity measurements. 64 It should be noted that most of these approaches push the limit of the "on-chip" definition as implemented, since only optical carrier components (i.e., fiber optics) and occasionally lenses are actually directly integrated into the device.…”

Fabrication of miniaturized fluidic devices using SU-8 based lithography and low temperature wafer bonding

“…This mixing device is used for the investigation of chemical reactions by time resolved FTIR-spectroscopy combined with in situ mixing of two reactants [4,6,7].…”

“…Then the flow is stopped, the two reactants are mixed by diffusion and the chemical reaction takes place. This reaction is investigated by time resolved FTIR-spectroskopy [7]. The stop of the flow is the "trigger" for the start of the reaction.…”

Micellar electrokinetic chromatography with on‐line Fourier transform infrared detection

“…The IR beam from an external optical port of the spectrometer was focused on the cell, by means of an off-axis parabolic mirror, and a high-sensitivity mercury cadmium telluride (MCT) detector. An external optical focusing unit, constructed in-house [12], enabled correct focusing of the IR beam on the IR window of the capillary. This unit was placed in a Perspex box and purged with dry air.…”