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 4.6 mm internal diameter (ID) is used. By scaling down to a 3.2 mm 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 4.6 mm ID column is being used with a 1.0 mL/min. flow rate, the same linear velocity is achieved with a 3.2 mm column using a flow rate to 0.48 mL/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.
Note that while solvent use could actually be reduced 80% by using 2.1 mm ID columns, the entire analytical system must be able to accommodate these narrow bore columns. This means that any extra column volume must be minimized (in tubing, connectors, etc.) and a microflow cell must be used in the HPLC detector. This is especially critical when using gradient mobile phase programs because the system dwell volume, or the volume contained between the pumps and the analytical column, becomes a significant factor. Large dwell volumes cannot be swept through quickly enough with low flow rates, making gradients impractical with narrow bore columns.
To adjust the effective system dwell volume, simply scale it down by the ratio of the column volumes (Vc2/Vc1). Assuming the approximate volumes of a 4.6 mm ID column is 0.1 mL/cm, a 3.2 mm ID column is 0.048 mL/cm and a 2.1 mm ID column is 0.021 mL/cm, we can simply apply ratio of the column volumes as 0.48 and 0.21, respectively. Hypothetically, if the system dwell volume is 1 mL, which is common for binary systems at maximum pressure, then the dwell volume needs to be reduced to 0.48 mL for 3.2 mm ID columns and to 0.21 mL for 2.1 mm ID columns. Reducing system dwell volume is generally accomplished by using smaller internal diameter tubing or smaller volume mixing chambers. For more information on lowering system dwell volumes, consult your instrument manufacturer.