Cascade optical chromatography for sample fractionation

Abstract

Optical chromatography involves the elegant combination of opposing optical and fluid drag forces on colloidal samples within microfluidic environments to both measure analytical differences and fractionate injected samples. Particles that encounter the focused laser beam are trapped axially along the beam and are pushed upstream from the laser focal point to rest at a point where the optical and fluid forces on the particle balance. In our recent devices particles are pushed into a region of lower microfluidic flow, where they can be retained and fractionated. Because optical and fluid forces on a particle are sensitive to differences in the physical and chemical properties of a sample, separations are possible. An optical chromatography beam focused to completely fill a fluid channel is operated as an optically tunable filter for the separation of inorganic, polymeric, and biological particle samples. We demonstrate this technique coupled with an advanced microfluidic platform and show how it can be used as an effective method to fractionate particles from an injected multicomponent sample. Our advanced three-stage microfluidic design accommodates three lasers simultaneously to effectively create a sequential cascade optical chromatographic separation system.

Optical chromatographic sample separation of hydrodynamically focused mixtures

“…In a previous separation, 18 we used the same device but relied on particles in flow that were separated regardless of their position in the flow. This method was used successfully for several different samples including the cleanup of biological samples for PCR.…”

“…When a similar separation was attempted using the previous method that lacked hydrodynamic focusing, the sample was poorly separated, yielding two samples that were 60%-70% pure in each separated component. 18 In the current experiment, the mixed sample was hydrodynamically focused prior to separation, resulting in two 100% separated component streams. In replicate runs, the separation remained consistent and was not affected by the occasional doublet or triplet that occurred if the rate of particle release from the wall was increased.…”

“…The microfluidic flowcell, similar in construction to previous work, 18 shown in Figure 1, consisted of three plates of fused silica. The 250 lm thick center plate was wet etched in hydrofluoric acid (HF) to yield a structure of etched trenches, 85 lm wide and 40-70 lm deep, on both sides of the plate with several 50 lm diameter capillary thru-holes connecting the trenches on either side of the plate (Translume Inc, Ann Arbor, MI, USA).…”

“…17 It has even been shown that with multiple lasers and a complex microfluidic design, multi-component biological samples can be fractionated. 18 In this paper, we expand on our previous work by improving the separation capability of the system through the development of a novel "trap and release" technique. By first hydrodynamically focusing an injected sample before it encounters the separation laser, we are able to reproducibly and completely separate a sample that was previously poorly separated.…”

Microfluidic sorting with a moving array of optical traps

“…In the absence of fluid flow, one may make use of a Bessel beam where one set of objects that experiences a lower level of optical forces can jump between bright rings of the Bessel beam to eventually get collected at the central bright core [2,3], or one may use a periodically moving or vibrating light interference pattern where one type of particles sees a higher level of optical force and follows the moving pattern to be sorted towards the output direction [4][5][6][7]. In the presence of fluid flow, passive sorting can be realized with a focused light beam propagating against the fluid flow to trap particles at different axial locations where the optical force on them is balanced by the fluid drag force (optical chromatography) [8]. The other approach is to exploit the difference in degree of interactions of flowing colloidal particles (having varying size, shape, or composition) with the array of traps to selectively deflect a particular set of particles away from the fluid flow direction.…”