Comparing a 250 mm x 4.6 mm column versus a 250 mm x 2.0 mm column with identical packing material and particle size, the optimum linear velocity of the mobile phase is the same. The internal volume of a 2.0 mm ID column is however 5 times smaller compared to a 4.6 mm ID column.
This means that when both columns are used at their optimum flow rate, the flow rate for the 2.0 mm column will be 5 times lower compared to the 4.6 mm ID column, while retention times will remain the same
This results in a 5 times lower solvent consumption for 2.0 mm internal diameter columns vs. 4.6 mm columns. For 1.0 mm ID columns the solvent reduction is 20 fold.
So, by using smaller ID columns, a substantial reduction in solvent consumption can be achieved, reducing costs and cutting down on environmental pollution