Separative power

Beyond certain limits increase of the reflux ratio does not appreciably increase the separating power or efficiency of the column. As a rough guide, if the column has an efficiency of n plates at total reflux, the reflux ratio should be between 2>t/3 and 3n/2. 

The separating power of a chromatographic process arises from the development of many theoretical plates to achieve adsorption equiUbrium within a column of moderate length. Even though the separation factor between two components may be small, any desired resolution may be achieved with sufficient theoretical plates. 

The second term in brackets in equation 36 is the separative work produced per unit time, called the separative capacity of the cascade. It is a function only of the rates and concentrations of the separation task being performed, and its value can be calculated quite easily from a value balance about the cascade. The separative capacity, sometimes called the separative power, is a defined mathematical quantity. Its usefulness arises from the fact that it is directly proportional to the total flow in the cascade and, therefore, directly proportional to the amount of equipment required for the cascade, the power requirement of the cascade, and the cost of the cascade. The separative capacity can be calculated using either molar flows and mol fractions or mass flows and weight fractions. The common unit for measuring separative work is the separative work unit (SWU) which is obtained when the flows are measured in kilograms of uranium and the concentrations in weight fractions. 

Selectivity. The relative separation, or selectivity, Ot of a solvent is the ratio of two components in the extraction-solvent phase divided by the ratio of the same components in the feed-solvent phase. The separation power of a hquid-liquid system is governed by the deviation of Ot from unity, analogous to relative volatility in distillation. A relative separation Ot of 1.0 gives no separation of the components between the two liquid phases. Dilute solute concentrations generally give the highest relative separation factors. 

State of the Art A desirable gas membrane has high separating power (ot) and high permeability to the fast gas, in addition to critical requirements discussed below. The search for an ideal membrane produced copious data on many polymers, neatly summarized by Robeson [J. Membrane ScL, 62, 165 (1991)]. Plotting log permeability versus log selectivity (ot), an upper bound is found (see Fig. 22-73) which all the many hundreds of data points fit. The data were taken between 20-50°C, generally at 25 or 35°C. 

SEC columns have become much more efficient since they were introduced in the late 1950s. The major factor for this has been the ability of synthetic polymer chemists to produce smaller particle sizes of column packing materials. The first sorbents were several 100 /mm wide in diameter (20), whereas modem columns are filled with particles in the range between 3 and 20 /mm, which caused an immense improvement in separation power. The major drawback... 

To select a column for a particular analytical problem, the first step is to make a choice about the pore size(s) to be used for the separation. In general, one cannot expect that a single pore size will fulfill the needs of a separation. In size exclusion chromatography, it is more common that columns of different types are combined with each other to deliver the separation range needed for a particular analysis. Therefore, column banks with different pore sizes are frequently combined with each other to maximize the separation power for... 

A theoretical model whereby maximum peak capacity could be achieved by the use of 3-D planar chromatographic separation was proposed by Guiochon and coworkers (23-27). Unfortunately, until now, because of technical problems, this idea could not be realized in practice. Very recently, however, a special stationary phase, namely Empore silica TLC sheets, has now become available for realization of 3-D PC. This stationary phase, developed as a new separation medium for planar chromatography, contains silica entrapped in an inert matrix of polytetrafluoroethy-lene (PTFE) microfibrils. It has been established that the separating power is only ca. 60% of that of conventional TLC (28) this has been attributed to the very slow solvent migration velocity resulting from capillary action. 

Current interest is, however, mainly in the coupling of HPLC and TLC, to which considerable attention has been devoted for the solution of difficult separation problems. Since Boshoff et al. (39) first described the direct coupling of HPLC and TLC, several papers (40-43) have been published describing the on-line coupling of liquid chromatographic methods and PC, usually with different interfaces, depending on the first technique applied. If PC is used as the second method, all the MD methods discussed above can be applied to increase the separating power. 

Multidimensional HPLC offers very high separation power when compared to monodimensional LC analysis. Thus, it can be applied to the analysis of very complex mixtures. Applications of on-line MD-HPLC have been developed, using various techniques such as heart-cut, on-column concentration or trace enrichment applications in which liquid phases on both columns are miscible and compatible are frequently reported, but the on-line coupling of columns with incompatible mobile phases have also been studied. 

The applieation range of eoupled-eolumn teehnology is determined by the separation power of the first eolumn. In general, it ean be said that low resolution favours multiresidue methods (MRMs), while high resolution leads to methods for a single analyte or for a group of analytes with similar properties. 

A commonly used system in environmental analysis is the heart-cutting technique which uses the separation power of the first column to obtain a higher selectivity than with the previously described precolumn enrichment. The two columns are coupled via a switching valve, as shown in Figure 13.5. 

Figure 4.29 Electrochemical reactor with alternating conducting and insolating porous sections each connected to separate power supplies [65].

Solvent selectivity refers to the ability of a chromatographic system to separate two substances of a mixture. It depends on the chemistry of the adsorbent surface, such as the layer activity and type of chemical modihcation. The separation power or resolution is given by Equation 4.8 [27] ... 

The separating power of a column can Ise expressed as its peak capacity defined as the number of peaks that can be resolved, at any specified resolution level, in a given separation time. For the general case it can be calculated using equation (1.49)... 

Principles and Characteristics High-performance thin-layer chromatography (HPTLC), also known as planar chromatography, is an analytical technique with separation power and reproducibility superior to conventional TLC, which was first used in 1938 [7] and modified in 1958 [8]. HPTLC is based on the use of precoated TLC plates with small particle sizes (3-5 xm) and precise instruments for each step of the chromatographic process. 

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