Chromatography - Plate height | posavski-obzor.info
If the length of the column is L, then the HETP (height equivalent to the theoretical plate) is; Resolution can be defined as the ratio of the difference in retention time Smaller size of particle (large surface area) reduces size of theoretical. The height equivalent to a theoretical plate, as discussed above, is defined as the is HETP = L σt2/tr2, in which L is now the column length, tr the time of retention of through the tortuous pathways of the granular bed of the packing particles. To introduce and explain the concept of Chromatographic Resolution (RS). • To define the The retention factor is equal to the ratio of retention time of the analyte on the column to the retention time of a .. Plate (HETP)'. These two column itself. The quality of the column packing, the particle size, the column dimensions.
It is often found that the analyte of interest must be separated from the myriad of individual compounds that may be present in a sample. As well as providing the analytical scientist with methods of separation, chromatographic techniques can also provide methods of analysis.
Chromatography involves a sample or sample extract being dissolved in a mobile phase which may be a gas, a liquid or a supercritical fluid. The mobile phase is then forced through an immobile, immiscible stationary phase. The phases are chosen such that components of the sample have differing solubilities in each phase.
A component which is quite soluble in the stationary phase will take longer to travel through it than a component which is not very soluble in the stationary phase but very soluble in the mobile phase. As a result of these differences in mobilities, sample components will become separated from each other as they travel through the stationary phase. Techniques such as H.
High Performance Liquid Chromatography and G. Gas Chromatography use columns - narrow tubes packed with stationary phase, through which the mobile phase is forced.
Chromatography - Introductory theory
The sample is transported through the column by continuous addition of mobile phase. This process is called elution. The average rate at which an analyte moves through the column is determined by the time it spends in the mobile phase. Distribution of analytes between phases The distribution of analytes between phases can often be described quite simply. An analyte is in equilibrium between the two phases; Amobile Astationary The equilibrium constant, K, is termed the partition coefficient; defined as the molar concentration of analyte in the stationary phase divided by the molar concentration of the analyte in the mobile phase.
The time between sample injection and an analyte peak reaching a detector at the end of the column is termed the retention time tR. Each analyte in a sample will have a different retention time. The time taken for the mobile phase to pass through the column is called tM. A term called the retention factor, k', is often used to describe the migration rate of an analyte on a column.
You may also find it called the capacity factor. Longitudinal diffusion contributes to peak broadening only at very low flow rates below the minimum optimum plate heigth.
The Van Deemter equation
Longitudinal diffusion is the result of concentration differences in the mobile phase. In the center of the peak zone the concentration is at its maximum. The concentration before and after the peak zone is lower. This results in diffusion, both in the direction of the mobile phase flow as well as in the opposite direction.
This effect will be relatively great at long residence times in the column, which is the case at low flow rates.
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As the flow rate increases, this effect will contribute less to the total peak broadening. In practice, it is best to select flow rates that minimise the effect of longitudinal diffusion on column efficiency.
Increased temperature and diffusion coefficient increase the B term, while increased viscosity decreases it. Diffusion coefficients for components in liquids are generally very small.
A function of chromatographic theory has been twofold: This is inadequate at high velocities, however, and is replaced by the equation Knowledge of the component terms in such equations allows one to optimize chromatographic operating conditions. Applications Chromatographic methods will separate ionic species, inorganic or organic, and molecular species ranging in size from the lightest and smallest, helium and hydrogento particulate matter such as single cells.
No single configuration will accomplish this, however. Little preknowledge of the constituents of a mixture is required. At its best, chromatography will separate several hundreds of components of unknown identity and unknown concentrations, leaving the components unchanged. Amounts in the parts per billion range can be detected with some detectors.
The solutes can range from polar to nonpolar—i.