Weight stationary liquid phase

Weight of stationary liquid phase. WL. Weight of liquid phase in the column. 

Liquid-Liquid Chromatography. Liquid-liquid chromatographic (LLC) separations result from partitioning of solute (analyte) molecules between two immiscible liquid phases (10). The liquid mobile and liquid stationary phases ideally have little or no mutual solubility. The stationary liquid phase is dispersed on a column of finely divided support. The use of a nonpolar mobile phase and a polar stationary phase is referred to as normal phase LLC. Under these conditions, less polar solutes are preferentially eluted from the column. Reverse phase chromatography employs a nonpolar stationary phase and a polar mobile phase. Generally, polar compounds elute more rapidly with this technique. Reverse phase chromatography, useful for the separation of less polar solutes, has found increased application in occupational health chemistry. It is optimally suited to the separation of low-to-medium molecular weight compounds of intermediate polarity. 

The HcReynolds abroach, which was based on earlier theoretical considerations proposed by Rohrschneider, is formulated on the assumption that intermolecular forces are additive and their Individual contributions to retention can be evaluated from differences between the retention index values for a series of test solutes measured on the liquid phase to be characterized and squalane at a fixed temperature of 120 C. The test solutes. Table 2.12, were selected to express dominant Intermolecular interactions. HcReynolds suggested that ten solutes were needed for this purpose. This included the original five test solutes proposed by Rohrschneider or higher molecular weight homologs of those test solutes to improve the accuracy of the retention index measurements. The number of test solutes required to adequately characterize the solvent properties of a stationary phase has remained controversial but in conventional practice the first five solutes in Table 2.12, identified by symbols x through s have been the most widely used [6). It was further assumed that for each type of intermolecular interaction, the interaction energy is proportional to a value a, b, c, d, or e, etc., characteristic of each test solute and proportional to its susceptibility for a particular interaction, and to a value x, X, Z, U, s, etc., characteristic of the capacity of the liquid phase... 

Mass transfer (the C term), which involves collisions and interactions between molecules, applies differently to both packed and capillary columns. Packed columns are mostly filled with stationary phase so liquid phase diffusion dominates. The mass transfer is minimized by using a small mass of low-viscosity liquid phase. Capillary columns are mostly filled with mobile phase, so mass transfer is important in both the gas and liquid phases. A small mass of low-viscosity liquid phase combined with a low-molecular weight carrier gas will minimize this term. 

The area available for the stationary phase will directly affect the phase ratio. If a solid material is used as the stationary phase in a packed column, if a liquid phase is deposited on a solid adsorbent with a constant film thickness, or if chemically bonded phases are employed, the phase ratio (through VJ will be directly proportional to the available surface area. The surface area of an adsorbent is usually given per unit weight (i.e. the specific surface area in m2/g). However, it should be noted that the relevant quantity is the surface area per unit volume (m2/ml) in the packed column. 

Figure 2.4 shows schematically a packed column in a longitudinal cross section. The column itself is usually made of stainless steel and is packed tightly with stationary phase on an inert solid support of diatomaceous earth coated with a thin film of liquid. The liquid phase typically constitutes 3, 5, or 10% by weight of the total stationary phase. 

The liquid phase used in a GC column is the principal factor determining the nature of the separations that can be achieved. Non-polar silicone liquid phases, such as SE-30 , OV-V JXR or QF-1 , permit the separation of fatty acid esters mainly on the basis of their molecular weights, when in packed columns. However, there can be separation of unsaturated fatty acids of the same chain-length with WCOT columns (c.f. Figure 5.7 below) or when the amount of the stationary phase on the support (in packed columns) is low (1-3 %). Non-polar phases are of value in the analysis of fatty acid derivatives of higher than normal molecular weight, especially with WCOT columns. 

The solid support is subsequently impregnated by a liquid stationary phase. While many solid supports can carry up to 25% by weight of a liquid phase before becoming visibly wet, much lower phase loadings (a few percent) are used in practice. Both the amount and the chemical type of a stationary phase are crucial to the separation characteristics (efficiency and sample capacity) of a chromatographic column. Packed columns are considered to be low-efficiency, high-capacity GC columns. While their best efficiencies amount to no more than a few thousand theoretical plates, packed columns can tolerate microgram amounts of samples. Only carefully and totally packed columns yield the expected efficiencies. 

The liquid stationary phase can be applied to the solid support in a number of ways, but usually by a batch coating procedure. Previously washed and deactivated support is suspended in a solution of the liquid phase in a volatile solvent and the solvent removed by evaporation. The amount of stationary phase added to the solvent is expressed in weight per cent of a solid support, and this phase loading is selected to give the required characteristics of the column. 

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