Is eddy diffusion independent of flow rate?

Eddy diffusion, the “A” term, a constant, does not change as the mobile linear flow rate changes.

Is eddy diffusion dependent on flow rate?

Another common conclusion today is that the contribution of eddy diffusion broadening does exhibit a slight dependence on flow rate. The usual form of this is that the dependence is v^1/3, and you might often see books or articles that include the following term in the overall band broadening equation.

How does flow rate affect longitudinal diffusion?

If the flow rate is too low, the longitudinal diffusion factor (6/u) will increase significantly, which increases plate height. At low flow rates, the analyte spends more time at rest in the column and therefore longitudinal diffusion becomes a more significant problem.

What do you understand by eddy diffusion in chromatography?

In liquid chromatography, eddy diffusion is responsible for a major part of the band broadening in the column. Since eddy diffusion is a combination of diffusion and convection, the term eddy dispersion might be more correct than eddy diffusion.

What is the difference between molecular and eddy diffusion?

If the convective motion is laminar (non-turbulent), you use the molecular diffusion coefficient. If the motion is turbulent, you use the eddy diffusivity, which is supposed to simulate the enhancing effect of turbulence.

Fundamentals of HPLC 29 - Eddy Diffusion and Flow

What is eddy and longitudinal diffusion?

The longer the path length of the molecule, the more be elution time and hence band broadening occurs. This effect is called Eddy's diffusion. Eddy's diffusion occurs in HPLC as the packed column is used in this technique. The longitudinal diffusion is zero/negligible.

How does flow rate affect chromatography?

A higher than usual flow rate may adversely affect the quality of the chromatography not giving the analyte sufficient time to interact with the stationary phase. Faster isn't always better. A lower than usual flow rate may leave the analyst waiting for the peak to appear at the detector.

What affects longitudinal diffusion?

The concentration of analyte is less at the edges of the band than at the center. Analyte diffuses out from the center to the edges. This causes band broadening. If the velocity of the mobile phase is high then the analyte spends less time on the column, which decreases the effects of longitudinal diffusion.

How does flow rate affect band broadening?

At high flow rates, B/v gets smaller, h is smaller, and the contribution of longitudinal diffusion to peak broadening is smaller.

What causes eddy diffusion?

Eddy diffusion, eddy dispersion, or turbulent diffusion is a process by which substances are mixed in the atmosphere, the ocean or in any fluid system due to eddy motion. In other words, it is mixing that is caused by eddies that can vary in size from subtropical ocean gyres down to the small Kolmogorov microscales.

What is the value of eddy diffusion?

Turbulence, or eddy diffusion, differs fundamentally from molecular diffusion in that all properties (e.g., heat, salt, nutrients, and dissolved gases) have the same eddy diffusion coefficient; a typical value for horizontal eddy diffusivity in the ocean is ∼500 m² s⁻¹, a value that is 10⁹ times greater than molecular ...

Does diffusion depend on pressure?

Diffusion coefficients are inversely proportional to total pressure or total molar density and are therefore reported by convention at a standard pressure of one atmosphere. Doubling the pressure of a diffusing mixture halves the diffusion coefficient, but the actual rate of diffusion remains unchanged.

What factors affect the column efficiency in 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.

Which term in van Deemter equation is most important at low flow rates Why?

Stationary phase particle size is one of the most important factors in the van Deemter equation. For a given column length, the plate number (N_th) is inversely related to the particle size of the column packing.

Why is longitudinal diffusion a more serious problem in GC than in LC?

Why is longitudinal diffusion a more serious problem in gas chromatography than in liquid chromatography? Diffusion coefficients of gases are 10^4 times greater than those of liquids. Therefore, longitudinal diffusion occurs much faster in gas chromatography than in liquid chromatography.

What does the retention factor k describe?

However, in column chromatography, the retention factor or capacity factor (k) is defined as the ratio of time an analyte is retained in the stationary phase to the time it is retained in the mobile phase, which is inversely proportional to the retardation factor.

How does flow rate affect separation?

Isocratic Separation

Changes in flow-rate will change the retention and dead times proportionally. For example, a 10% reduction in flow-rate will increase both values by 10%, so k remains unaffected by flow-rate.

How does flow rate affect column chromatography?

If the solvent flow rate is used very slowly in column chromatography, the diffusion processes will lead to band widening. For columns of smaller diameter, the optimal rate is lower than columns with a larger diameter. Therefore, compared with smaller columns the larger columns can be run with the higher flow rate.

Does flow rate affect peak area?

The effect of flow rates and the slope of the linear concentration gradient on peak areas in high pressure liquid chromatography was investigated. It was found that within limits the slope of the gradient had no effect on the peak areas. However, the peak areas were inversely proportional to the flow rates.

What is longitudinal diffusion in gas chromatography?

Longitudinal diffusion is a chief cause of band broadening in Gas Chromatography, as the diffusion rates of gaseous species are much higher than those of liquids. It is for this reason that longitudinal diffusion is less of an issue in liquid chromatography.

Why is the van deemter B term in GC much greater than in HPLC?

Why is the van Deemter B term in GC much greater than in HPLC? Diffusion of molecules in the gas phase is much greater than in the liquid phase.

Why does band width increase as retention time increases?

A longer column generally improves the separation. The trade-off is that the retention time increases proportionally to the column length and a significant peak broadening will be observed as well because of increased longitudinal diffusion inside the column.