This plot shows the relationship between HETP (height equivalent to a theoretical plate) and linear velocity of the mobile phase. A smaller HETP value indicates higher efficiency. The optimum linear velocity, which offers the maximum column efficiency, is shown in the graph. Reducing the particle size offers lower HETP values resulting in higher column efficiency. For columns packed with 1.8 µm particles, when the linear velocity of the mobile phase is increased to two or three times its original value, the HETP value is maintained at a much lower level compared to a similar increase when using columns with larger particles. This demonstrates that the flow rate can be increased and the analysis time shortened without sacrificing the column efficiency. The use of smaller particles, however, requires increased pumping pressure. The UHPLC™ is capable of delivering solvents at pressures up to 15,000 psi.