Tech Tip — Reducing Column Internal Diameter

Larger diameter columns require higher flow rates, and thus larger volumes of mobile phase, to reach the desired linear velocity. Typically, a conventional analytical column of 4.0 or 4.6mm internal diameter (ID) is used. By scaling down to a 3.2mm ID column, we can significantly reduce the flow rate and solvent volume needed to reach the same optimal linear velocity, without increasing run time. The new flow rate can be easily determined using Equation 1.

For example, if a 4.6mm ID column is being used with a 1.0mL/min. flow rate, the same linear velocity is achieved with a 3.2mm column using a flow rate to 0.48mL/min. This results in a mobile phase reduction of 52%--a considerable solvent cost savings. It also results in less solvent waste generation, another cost savings, and better system performance. Typically, 2-3 fold increases in sensitivity can be expected when injecting the same sample mass on a smaller ID column. The performance of most LC/MS interfaces is enhanced by lower flow rates as well.