Van deemter equation

What are the three terms that make up the van deemter equation?

The Van Deemter equation is governed by three cumulative terms: (A) eddy diffusion, (B) longitudinal diffusion, and (C) mass transfer. A loss in peak efficiency can be observed as a wider analyte band, and therefore, these three terms can also be viewed as factors that contribute to band broadening.

What is longitudinal diffusion?

A molecule flowing through a column has two means of movement. One is the physical flow that is taking place. But the other is still its ability to diffuse in a random manner from one point to another. All compounds moving through a chromatographic column must exhibit some degree of longitudinal diffusion broadening.

What is the rate theory?

Rate Theory, GC and HETP (Packed Columns) where A, B and C are constant for a column, where experimental parameters remain unaltered during the run. Chromatographic columns with small H values bring about compact peaks. Rate theory considers the on column broadening of the chromatographic to arise from, a.

What is band broadening?

Band-broadening is a general term used to describe the overall dispersion or widening of a sample peak as it passes through a separation system.

How is Hetp calculated?

In elution chromatography, in which the peak develops on a time scale, an equivalent form of the above equation is HETP = L σt2/tr2, in which L is now the column length, tr the time of retention of the peak by the column, and σt the standard deviation of the peak measured in units of time.

How does pore size affect chromatography?

Although the surface area decreases with increasing pore size, large-pore silica gel products are popular for various separation purposes. A wrong pore size, however, gives poor chromatographic performance. Large molecules cannot enter the pores and merely interact with the ligands on the stationary phase surface.

What useful information can be found from a van deemter plot?

The van Deemter equation in chromatography, named for Jan van Deemter, relates the variance per unit length of a separation column to the linear mobile phase velocity by considering physical, kinetic, and thermodynamic properties of a separation.

What is Hetp?

Simplified peak evaluation by measurement of. peak width and asymmetry factor. Peak broadening is typically described as plate number N or as height equivalent to a theoretical plate (HETP). This concept is equivalent to a tanks-in-series model reflecting the number of equilibrium stages represented by the column.

What is column efficiency?

Column efficiency, indicated as the number of theoretical plates per column, is calculated as N = 5.54 (tR / w0.5)2 where tR is the retention time of the analyte of interest and w0.5 the width of the peak at half height.

What are Eluents?

noun, plural: eluents. A substance that separates and moves constituents of a mixture through the column of a chromatograph. Supplement. The eluent in liquid chromatography is a liquid solvent whereas in gas chromatography is a carrier gas.

How do you calculate flow rate in HPLC?

Flow rate is mainly effected by column diameter. For example, a 4.6 mm diameter column will have a linear flow rate just above 1 mL/min (so we use 1 mL/min). A 2.1 mm ID column will have a linear velocity/flow rate of around 208 ul/min. A 10 mm ID column ~ 5 ml/min.

What does number of theoretical plates mean?

Theoretical plate number (N) is an index that indicates column efficiency. It describes the number of plates as defined according to plate theory, and can be used to determine column efficiency based on calculation in which the larger the theoretical plate number the sharper the peaks.

What is band spreading?

Before the sample/analyte band reaches the detector, it will pass through multiple components of the chromatographic system that will contribute to the distortion and broadening of the chromatographic band [Figure 7]. This phenomenon is referred to as band spreading.

What causes peak broadening?

The sample injection volume is related to broadening of the sample zone in the first stage of column separation. Therefore, increasing the injection volume can result in peak broadening. In particular, the higher the ratio of strong solvent in the sample solvent, the greater the effects.

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