Theoretical Modeling and Experimental Evaluation of a Microscale Molecular Mass Sensor

Abstract

A theoretical model for a recently developed microscale molecular mass sensor (micro-MMS) is presented. The micro-MMS employs a widely applicable technique of measuring the refractive index gradient (RIG) in a microchannel created after two adjacent streams merge: a "sample stream" containing analyte(s) of interest in a host solvent and a "mobile-phase" stream containing only the host solvent. Because the flow in the microchannel is laminar, the analytes in the sample stream mix with the mobile-phase stream primarily by diffusion. The diffusion-induced RIG in the microchannel is measured by monitoring the deflection angle of a diode laser probe beam, which is orthogonal to both the direction of flow and the direction of analyte diffusion. The micro-MMS samples the RIG with probe beams at two positions along the direction of flow, and the ratio of the downstream to the upstream signal monitors the diffusion coefficient. Following calibration for a given class of compounds, the molecular mass of an analyte of interest can be determined. Along with the analyte diffusion coefficient, the theoretical model indicated three other specific parameters are important to interpret the micro-MMS output: the radius of the interrogating light probe beams, the time intervals between each of the detection positions, and the merge point relative to the detection positions. A series of experiments were conducted at different beam radii and flow rates to investigate these parameters, and the results are consistent with the model. The model shows that by using smaller beam radii and altering flow rates the molecular mass range of the micro-MMS can be, in principle, tuned from less than 10(2) g/mol to greater than 10(8) g/mol. The ratio data from the micro-MMS is also demonstrated to readily provide a "universal calibration", from which the determination of unknown diffusion coefficients can be readily obtained.

Mass-Transfer Characteristics of a Double-Y-Type Microchannel Device

“…However, flow splitting in microchannel devices is rarely found in published papers. In reported works for the analytical application of microchannels, the solute's concentration profile was directly measured within the channel using a laser-based refractive index detector (Costin and Synovec, 2002) or a microscope for measuring fluorescence emission (Costin et al, 2004). A strategy for flow splitting has not yet been developed for the application of microchannel devices to analytical chemistry.…”

Mass Tansfer Characteristics of a Microchannel Device of Split‐Flow Type

“…However, flow splitting in microchannel devices is rarely found in published papers. In works reported for analytical application of microchannels, the profile of the solute concentration was directly measured within the channel using a laser-based refractive index detector [9] or a microscope for measuring fluorescence emission [10]. A strategy for flow splitting has not yet been developed for the application of microchannel devices in analytical chemistry.…”

Noninvasive Diagnosis of a Single Cell with a Probe Beam

“…Either a concentration or temperature gradient will induce a refractive index gradient, which in turn causes deflection of a probe beam (8)(9)(10)(11)(12)(13)(14)(15). Recently, we have applied the optical beam deflection (OBD) method for exploring reaction-heat-induced temperature gradients (16)(17)(18)(19)(20)(21)(22)(23) and reaction-induced concentration gradients (24,25).…”

“…On the other hand, any chemical reaction will generate a concentration change and reaction‐heat‐induced temperature gradient in the reaction medium or its vicinity. Either a concentration or temperature gradient will induce a refractive index gradient, which in turn causes deflection of a probe beam (8–15). Recently, we have applied the optical beam deflection (OBD) method for exploring reaction‐heat‐induced temperature gradients (16–23) and reaction‐induced concentration gradients (24, 25).…”