Effect of parallel segmented flow chromatography on the height equivalent to a theoretical plate. I—Performance of 4.6 mm × 30 mm columns packed with 3.0 μm Hypurity-C18 fully porous particles

• Parallel segmented flow chromatography consists in splitting the exit flow of mobile phase.
• About half the stream flowing through the central region of the column is sent to the detector.
• The stream flowing along the less well packed wall region of the column is sent to waste.
• Elimination of the stream flowing along the wall cancels the effect of bed heterogeneity.
• The apparent column HETP and the signal to noise ratio are reduced by about 30–50%.

The mass transfer kinetics in short and wide 4.6 mm × 30 mm columns packed with 3.0 μm Hypurity-C18 fully porous particles were measured for three different configurations of the inlet sample distribution and outlet sample collection: (1) both the inlet and outlet column endfittings are standard, (2) the inlet endfitting is standard while the outlet endfitting allows parallel segmentation of the exiting flow between a central and a peripheral region across the column diameter, and (3) both the inlet and outlet endfittings allow a parallel segmentation of the flow entering and exiting the column, respectively. The total reduced heights equivalent to a theoretical plate (HETP) were carefully measured, using the first and second central moments of the elution band profiles, obtained by its accurate numerical integration. The longitudinal diffusion term was measured at the lowest experimental reduced velocity applied. The solid–liquid mass transfer resistance was estimated from measurements of the intra-particle diffusivity using the Torquato's model of effective diffusion in packed beds. The trans-channel and short-range interchannel eddy diffusion HETP terms were taken from data obtained by solving numerically the Navier–Stokes equations and simulating advective-diffusive transport in computer-generated random sphere packings. The results clearly show that the trans-column eddy diffusion HETP term can account for up to 85% of the total intrinsic HETP (corrected for extra-column contributions) of the standard columns. Parallel segmented flow chromatography can reduce this contribution by half at high velocities, by eliminating most of the baseline peak tailing. This holds true irrespective of the retention factor of the analyte. It was found also that segmenting the inlet/outlet flow increases detection sensitivity by 25–50% for peaks with large to small retention factors, respectively. In practice, the advantage of parallel segmented flow chromatography in gradient elution (thin peak widths) is essentially limited by post-column bandspreading and diffusion in the dwell volume of the instrument used. Analyst should minimize post-column bandspreading (caused by connectors and detection cell volume) and synchronize the eluent composition in the peripheral and central inlet ports of the column, by using two separate pumps with appropriate dwell volumes.