Packing-material particle size linear velocity, column

FIGURE 14.8 Relationship of HETP and mobile phase linear velocity for column packing materials of 2, 3, 5, and 8 /an ROSIL C18 particle size. Mobile phase was 75 25 acetonitrile water. Sample test probe was pyrene at k = 6. 

Other potential improvements in the method could include using a smaller (e.g., 3 mm) i.d. column while using the same length column and particle size of the packing material. A 3.0-mm-i.d. column can be used to reduce solvent waste, since columns with smaller diameters have reduced column volume and require use of lower flow rates, and therefore they can decrease solvent waste by at least 60%. A simple calculation to achieve equivalent retention on a smaUer-i.d. column at the same linear velocity is shown in equation (8-2). 

HPLC is a well-established analytical technique that has been used in laboratories globally over the past 35 years. One of the major drivers for the development of this method has been the evolution of packing materials used to effect the separation. The fundamental principles of this evolution are governed by the van Deemter equation, which is an empirical formula that defines the correlation between linear velocity (flow rate) and plate height (HETP or column efficiency). Since particle size is one of the variables, a van Deemter curve can be used to investigate chromatographic performance. 

UHPLC is characterized by the use of chromatographic conditions that employ smaller particle size packing materials and instmmentation capable of operating at higher pressures and higher linear velocities than those typically encountered in HPLC. Most UHPLC columns also have smaller column diameters than those typical of HPLC methods. 

Indeed, the 30-plus year progress of liquid chromatography is a history of the development of packing materials. The particle size directly influences the column efficiency and thus further affects the separation results. The underlying theory is the van Deemter equation, which is the empirical formula that shows the relationship between linear velocity (flow rate) and plate height (column efficiency). From the van Deemter equation we can know that, as the particle diameter decreases, there is a significant gain in efficiency even when the flow rates are increased. When the particle size was reduced to sub-2 (jim, the analytical process was speeded up by a factor of nine without compromise of efficiency, or in other words, the efficiency was increased by a theoretical ninefold for a similar run time (22). 

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