Turbulent flow chromatography is often used for on-line sample cleanup of biological matrices in liquid chromatography–mass spectrometry applications. However, the general mechanisms are not well represented in the literature and there is a lot of misunderstanding of turbulent flow chromatography's basic principles. This column installment will explore the technique's theoretical concepts, explain how they can be applied, and discuss common practice through examples in the literature.
Turbulent flow was first defined over 100 years ago by British physicist Osborne Reynolds (1). Reynolds discovered that the flow of a fluid through a conduit becomes turbulent when the momentum of the fluid exceeds its resistance to flow by a factor of 2000–3000. The ratio of these opposing forces, known as the Reynolds number (R e), is expressed in equation 1:
According to equation 2, turbulence occurs more readily in columns packed with large particles — 35–75 µm — than those packed with small particles — 3–10 µm. Experimental observations of solute band broadening relative to column retention have indicated that the transition from laminar to turbulent flow occurs as R e' increases beyond a value greater than 1 and that virtually all of the flow paths within a column become turbulent as R e' exceeds a value of 10 (2).
Solute band broadening relative to column retention is measured as the height equivalent to a theoretical plate (H). Typically, H increases from a minimum as the mobile phase velocity increases as long as the flow remains laminar. As the flow becomes turbulent through more and more channels in the packed column, H begins to decrease and reaches a minimum when all of the flow paths in the column become turbulent. Compared to flow through a straight hollow tube, transition from laminar to turbulent flow is much more gradual in a packed column as the flow rate increases.