  Despite decades of research and development, the optimal efficiency of
slurry-packed HPLC columns is still hindered by inherent long-range flow
heterogeneity from the wall to the central bulk region of these columns. Here,
we show an example of how this issue can be addressed through the
straightforward addition of a semidilute amount (500~ppm) of a large, flexible,
synthetic polymer (18~MDa partially hydrolyzed polyacrylamide, HPAM) to the
mobile phase (1\% NaCl aqueous solution) during operation of a 4.6 mm $\times$
300 mm column packed with 10~$\mu$m BEH$^{\mathrm{TM}}$ 125~\AA \ Particles.
Addition of the polymer imparts elasticity to the mobile phase, causing the
flow in the interparticle pore space to become unstable above a threshold flow
rate. We verify the development of this elastic flow instability using pressure
drop measurements of the friction factor versus Reynolds number. In prior work,
we showed that this flow instability is characterized by large spatiotemporal
fluctuations in the pore-scale flow velocities that may promote analyte
dispersion across the column. Axial dispersion measurements of the quasi
non-retained tracer thiourea confirm this possibility: they unequivocally
reveal that operating above the onset of the instability improves column
efficiency by significantly reducing peak asymmetry. These experiments thereby
provide a proof-of-concept demonstration that elastic flow instabilities can be
harnessed to mitigate the negative impact of trans-column flow heterogeneities
on the efficiency of slurry-packed HPLC columns. While this approach has its
own inherent limitations and constraints, our work lays the groundwork for
future targeted development of polymers that can impart elasticity when
dissolved in commonly used liquid chromatography mobile phases, and can thereby
generate elastic flow instabilities to help improve the resolution of HPLC
columns.
