What Is the Formula for Retention Factor

Although the term retention factor is sometimes used as a synonym for delay factor compared to planar chromatography, the term is not defined in this context. However, in column chromatography, the retention factor or capacitance factor (k) is defined as the ratio between the time an analyte is maintained in the stationary phase and the time it is maintained in the mobile phase[3], which is inversely proportional to the delay factor. (^2)Sometimes the (R_f) is called a delay factor because it is a measure of how the movement of spots is slowed or delayed. In chromatography, the delay factor (R) is the proportion of an analyte in the mobile phase of a chromatographic system. [1] In particular in planar chromatography, the RF delay factor is defined as the ratio between the distance traveled between the center of a point and the distance traveled by the solvent front. [2] Ideally, the values for HF correspond to the R values used in column chromatography. [2] For the retention factor (k), the delay factor (R) is defined as follows: An RF value is always between 0 and 1. When the substance moves, it can only move in the direction of the solvent flow and cannot move faster than the solvent. For example, if a particular substance in an unknown mixture exceeds 2.5 cm and the solvent front exceeds 5.0 cm, the delay factor would be 0.50. One can choose a mobile phase with different properties (in particular polarity) to control the distance traveled by the substance studied. A convenient way for chemists to report the results of a TLC plate in lab notebooks is a “retention factor” value, (^2) or (R_f) that quantifies the motion of a compound (equationref {2}). RF delay factor is often used in paper chromatography and thin layer chromatography for the analysis and comparison of different substances.

It can be described mathematically by the following ratio:[2] An RF value is characteristic of each given connection (provided that the same stationary and mobile phases are used). It can provide corroborating evidence of the identity of a compound. If the identity of a compound is suspected but not yet proven, an authentic sample of the compound or standard is discovered and guided on a TLC plate side by side (or on top of each other) with the compound in question. Note that this identity check must be performed on a single disk because it is difficult to accurately duplicate all the factors affecting RF from one experiment to another. To measure the distance traveled by a connection, the distance between the original position of the connection (the baseline marked with pencil) and the position of the connection after elution is measured (the approximate center of the point, Figure 2.14a). Due to the approximate nature of this measurement, ruler values should only be recorded with millimeter accuracy. To measure the distance travelled by the solvent, the distance between the baseline and the solvent front is measured. Although a TLC can theoretically be guided to any height, it is common to run the solvent approximately (0.5 : text{cm}) from the top of the plate to minimize errors in calculations (R_f) and achieve the best separation of mixtures. A TLC plate should not be allowed to flow completely towards the top of the plate, as this can affect the results. However, if a saturated and sealed TLC chamber is used, the (R_f) can still be calculated. The value (R_f) is a ratio and represents the relative distance travelled by the point from the distance it could have travelled if it had moved with the solvent front.

A (R_f) of 0.55 means that the point (55%) has moved to the front of the solvent or just over half. [R_f = dfrac{text{distance travelled by the compound}}{text{distance travelled from the solvent front}} label{2}] Since a (R_f) is essentially a percentage, it is not particularly important to run a TLC at a certain height on the TLC plate. In Figure 2.15, a sample of acetophenone was eluted at different altitudes, and the (R_f) was calculated as similar, but not identical. Slight variations of (R_f) are caused by errors in the measurements of the ruler, but also by different amounts of water adsorbed on the TLC plates, which modify the properties of the adsorbent. (R_f) values should always be considered approximate. The solvent front (Figure 2.14b) is essential for this calculation (R_f). When removing a TLC plate from its chamber, the forehead of the solvent should be immediately marked with pencil, as the solvent often evaporates quickly. .

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