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Van Deemter Equation Pdf Free [VERIFIED]

To describe the contribution of the above factors, several so-called plate height equations have been developed. A plate height equation expresses the correlation between plate height and mobile phase velocity. Best known is the van Deemter equation, which describes the various contributions to plate height (H). In this equation the parameters that influence the overall peak width are expressed in three terms:

Van Deemter Equation Pdf Free

Peak height and peak broadening are governed by kinetic processes in the column such as molecular dispersion, diffusion and slow mass transfer. Identical molecules travel differently in the column due to probability processes. The three processes that contribute to peak broadening described in the van Deemter equation are:

The aim of this module is to illustrate the principle of band broadening in HPLC. The van Deemter equation and the terms of the equation will be defined and explained. The effects of eddy diffusion, longitudinal diffusion, and mass transfer on the efficiency of chromatographic peaks will be explored.

Stationary Phase: Fully porous silica based stationary phases are most commonly used which can be of various pore sizes suitable for different MW range of proteins. Although the modern, highly pure silica-based phases are made extremely inert but silica is inherently acidic in nature and the free silanol groups can lead to some degree of chemical interaction between the proteins and stationary phase. For this reason, sometimes polymeric stationary phases are used, or the better way is to optimize the mobile phase conditions to reduce these secondary interactions.

Chromatographers are always interested in higher efficiency. This is motivated by the resolution equation, which shows that increases in column efficiency always result in improved resolution. Chromatographic efficiency is affected by a large number of experimental variables and its optimization can be achieved in many different ways, depending upon how many variables one is willing to adjust. These include pressure, temperature, particle size, column length, and eluent velocity. In the early days of high performance liquid chromatography (HPLC), the selection of column formats (particle size, type, and column diameters) was rather limited and thus, optimization often was done by adjusting operational variables such as eluent velocity, column temperature, and operating pressure. Nowadays the selection of column formats is substantially wider and one can find a number of particle sizes between 1.7 μm and 5 μm, and numerous column lengths are achievable by coupling columns in series. This makes optimization of these nominally "discrete" variables possible (that is, particle size and column length).

One-Parameter Optimization: In this case, one has already chosen a particle size and column length. The only variable left to optimize is velocity. Here, the van Deemter equation (1) is used to calculate the eluent velocity that gives the minimal plate height, provided that this velocity (and flow rate) can be reached within the pressure limit and flow limit of the instrument. Equations 1 and 2 give the van Deemter equation and the optimal linear velocity, respectively, where Dm is the solute diffusion coefficient, dp is particle size, and A, B, and C are van Deemter equation coefficients. It should be noted that throughout this column, it is assumed that the plate height is related to velocity through the van Deemter equation. This is always a rather good approximation as long as one does not work far from the minimum in the plate height curve.

However, the problem of operating at the van Deemter optimum velocity specified by equation 2 is that one frequently cannot achieve the separation in the desired time (using t0 as a proxy for the analysis time) because column length is already chosen. To achieve the target t0, one needs to operate at the velocity given by equation 3, where L is the column length, εt and εe are the total and interstitial porosities of the column, respectively (typically about 0.55 and 0.38), and λ is the ratio of the porosities.

The resulting plate count obtained at this velocity can be calculated easily by inserting equation 3 into equation 1. This plate count is always lower than the plate count calculated based upon the van Deemter optimum velocity; however, it is also the more realistic estimate.

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