Chromatography: Theoretical Plates Calculation

Theoretical Plates and Resolution

• Number of theoretical plates (N) is calculated by: N = (tr2 – tr1) (Wb2 + Wb1)/2
• N is a measure of the column's efficiency
• Resolution (Rs) is a measure of how well two peaks are separated, and is preferred over N since it considers both retention time and column efficiency
• Rs ≥ 1.5 represents baseline resolution, or complete separation of two neighboring solutes
• Rs ≥ 1.0 is considered adequate for most separations

Plate Height and Van Deemter Equation

• Plate height or height equivalent of a theoretical plate (H or HETP) is a measure of the column's efficiency
• H = L/N, where L is the column length and N is the number of theoretical plates
• The Van Deemter equation relates flow rate to H: H = A + B/m + Cm
• H is used to relate kinetic processes to band broadening

Sources of Band Broadening

• Eddy diffusion is a source of band broadening
• Mobile phase mass transfer is a source of band broadening
• Stagnant mobile phase mass transfer is a source of band broadening due to differences in diffusion rates between the mobile phase outside and inside the pores of the support
• Stationary phase mass transfer is a source of band broadening
• Longitudinal diffusion is a source of band broadening

Chromatography Introduction

• Chromatography is a separation technique based on the interactions of compounds with two phases, a mobile phase and a stationary phase
• A chromatogram is a graph of concentration versus elution time
• Types of chromatography include gas chromatography (GC) and liquid chromatography (LC)
• The capacity factor (k') is a useful parameter in understanding retention mechanisms, defined as: k' = Moles Astationary phase / Moles Amobile phase