# Optimum flowrate

I'm interested in knowing what disadvantage there is to reducing flowrates. Common wisdom says that linear velocity in HPLC is "better" (minimum in vanDeemter curve) for large analytes (peptides/proteins) if it's low; and for small molecules, if it's high. Since UHPLC has such broad minimums in the vanDeemter curve, what disadvantage(s) is/are there to using say, 0.2 mL min for a small-molecule substance? Small molecules are less sensitive to tiny changes in gradient composition (a greater risk with low flowrates), and after all, one of the biggest advantages of UHPLC is its low solvent consumption. Just curious.

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## Answers

Basically, there is an optimal linear velocity for a given separation. This optimal flow rate depends upon the analyte, conditions and column. Change one of these parameters (e.g., temperature, organic modifier, particle size/chemistry, column dimensions, etc) and you need to re-confirm that you are at the optimal linear velocity for this 'new' separation.

You mention van Deemter curves. Assuming you are familiar with them, you know that there is an A-term, B-term and C-term associated with the van Deemter equation. At very low linear velocities, the B-term (axial or longitudinal diffusion) can begin to influence the observed plate height (H). The B-term is affected by analyte k', Diffusion Coefficients (D

_{S}, D_{M}) and Obstruction Factors (g_{M}, g_{S}). It isof particle size: B = 2gindependent_{M}D_{M}+ 2g_{S}D_{S}k'. See (J. H. Knox, J. Chrom. A, 831 (1999) 3-15) for more information. So, if you increase the flow rate, the B-term's influence on H decreases; decrease the flow rate, the B-term's influence on observed H increases; and if you REALLY decrease the flow rate, the B-term can have a large influence on the observed plate height.So, to answer your question, if you go too slow, you are eventually running at sub-optimal linear velocities and can lose efficiency (increase H) due to the B-term. Conversely, if you go too fast, you can also lose efficiency but for different reasons (C-term). Other disadvantages to running at sub-optimal flow rates is unnecessarily long analysis times, sensitivity loss (peak height) and possible solvent waste. The only true way to see what the lowest optimal linear velocity is for your separation is to generate your own van Deemter curve for your analyte(s) of interest. You then balance that with what I just mentioned.

Good luck!!

--Doug