What are the first principles that FIDA is based on?

FIDA (Flow-Induced Dispersion Analysis) is based on two well-established physical principles:

1. Taylor Dispersion Analysis (TDA)
   In laminar flow through a capillary, solute molecules experience both convection (movement along the flow) and radial diffusion (movement across the flow). These combined effects lead to a predictable spreading of a solute pulse, known as Taylor dispersion. By monitoring how the concentration profile evolves during transit, it becomes possible to extract information about molecular diffusion. In other words:
   
   • In capillary flow under laminar conditions, the velocity profile is parabolic: molecules at the center move faster than those near the walls.
   • When a narrow plug of sample is injected into this flow, its molecules are carried forward by convection but at different speeds depending on radial position.
   • At the same time, radial diffusion drives molecules across streamlines, gradually redistributing them.
   • The interplay between convection and diffusion produces a broadened concentration profile — the Taylor dispersion profile.
   • By analyzing this profile, the effective diffusion coefficient of the molecules in solution can be determined.
2. The Stokes–Einstein Equation
   The Stokes–Einstein relation describes the connection between molecular diffusion and particle size in a liquid. By applying this equation to the dispersion profile obtained through TDA, FIDA calculates the hydrodynamic radius of molecules in absolute terms.
   
   • The Stokes–Einstein relation links the diffusion coefficient D of a particle in a fluid to its hydrodynamic radius
     where kB is the Boltzmann constant, T is the absolute temperature, and η is the viscosity of the medium.
   • This expression originates from kinetic theory and describes how random thermal motion (Brownian motion) is opposed by viscous drag.
   • Applying this relation to the diffusion coefficient obtained from Taylor dispersion yields the absolute hydrodynamic radius of the molecule.

Taken together, these two principles enable the determination of absolute molecular size in solution. Taylor dispersion provides the diffusion coefficient, and the Stokes–Einstein equation translates this into the hydrodynamic radius. These foundations ensure that FIDA measurements are firmly rooted in classical physical chemistry.

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