Stationary phase requirements

Chromatographic separations are accomplished by continuously passing one sample-free phase, called a mobile phase, over a second sample-free phase that remains fixed, or stationary. The sample is injected, or placed, into the mobile phase. As it moves with the mobile phase, the sample s components partition themselves between the mobile and stationary phases. Those components whose distribution ratio favors the stationary phase require a longer time to pass through the system. Given sufficient time, and sufficient stationary and mobile phase, solutes with similar distribution ratios can be separated. 

The selection of solvents for LLPC is similar to the selection of solvents in liquid-liquid extraction systems. The solid support has little effect upon the selection of the solvent pair, except for the obvious fact that a hydrophilic support for a polar stationary phase requires a hydrophobic... 

Each type of gas chromatograph has its own set of operating instructions, but general experimental conditions are appropriate for all instruments. Three important factors must always be considered when a GC analysis is to be completed (1) selection of the proper column (2) choice of temperatures for injector, oven, and detector and (3) adjustment of gas flow. Because hundreds of stationary phases are available, it is impossible to outline the characteristics of each. Selecting the stationary phase requires some knowledge of the nature of the sample to be analyzed. 

The solubility of polymers is, for thermodynamic reasons, more restricted than the solubility of low-molecular compounds and, consequently, the choice of solvents is limited. Potential solvents for most synthetic polymers are of moderate polarity. Alcohols and liquids of similar polarity are precipitants for many synthetic polymers. The search for a mobile phase that enables RPC through solvophobic interactions between the polymer and the nonpolar stationary phase requires attempts to make the mobile phase an unfavorable environment for the solute. This easily conflicts with the narrow limits of solubility of the polymer under investigation. Solubility effects are known to occur even in low-molecular RPC 92 94), but in polymer RPC they even may govern retention. 

Every change in the mobile or stationary phase requires a long equilibration time. 

The electroneutrality of the adsorbed monolayer on the stationary phase requires that... 

Even with overloaded conditions the ratio between sample mass and the amount of stationary phase required is low and uneconomical, as in most cases it will not be... 

A method to select the appropriate stationary phase for analysis of a sample mixture is to consider the polar characteristics of the analytes and select a stationary phase of similar polarity. An analyte with similar polar character to the stationary phase will be well retained, the principle of like attracts like applies, and useful retention is then likely to occur leading to adequate selectivity and separation of the analytes primarily on the basis of volatility. Conversely, if the solute is immiscible with the stationary phase then little or no retention difference will be obtained. Further useful indication of retention characteristics may be obtained by analysing a sample on a non-polar and polar column, for example, a dual column GC fitted with Apiezon/OVlOl and Carbowax 20M columns temperature programmed 50-220°C at 10°Cmin with a final hold of 10 min. The chromatogram will indicate the polarity of stationary phase required for the components and the analysis can be repeated with columns of differing polarity, e.g. OV17, OV1701. Tables of RIs for various classes of compounds have been published, mainly for squalane, the reference non-polar stationary phase and Apiezon L, with RI values for Carbowax 20M as a reference polar stationary phase [10]. 

Even with overloaded conditions the ratio between sample mass and the amount of stationary phase required is low and uneconomical, as in most cases it will not be higher than 10 mg g. A totally different approach for preparative liquid chromatography is displacement chromatography (Fig. 20.5). In this method, comparatively enormous sample loads are brought on to the chromatographic bed. As the different compounds of the sample mixture show different affinities towards the stationary phase, they displace each other from the adsoptive (in a... 

For HPLC, the compatibility between the chemical parameters necessary for efficient separation and sensitive detection is of paramount importance. A compromise between two optima is often required. For example, peroxyoxalate reactions need alkaline conditions but reverse-phase separations (using silica-based stationary phases) require an eluent pH... 

Owing to the different densities and specific surface areas of supports, the retention diagrams of the same solute substance injected on the same stationary phase covering various supports are not identical. The higher the specific smface areas of the support, the higher the percent stationary phase required to ensure the invariance of retentoin volumes contributed by dissolution. 

Packing the Column. The quantity of stationary phase required is determined by the sample size. A common rule of thumb is to use a weight of packing material 30-100 times the weight of the sample to be separated. Columns are usually built with roughly a 10 1 ratio of height to diameter. In the microscale laboratory, two standard chromatographic columns are used ... 

The stationary phases requirements of selectivity and higher thermal stability then became more clearly defined the process of stationary-phase selection and classification became logical after the studies of McReynolds (28) and Rohrschneider (29,30) were pubUshed, both of which were based on the retention index (31). The Kovats retention index procedure and McReynolds constants are discussed in detail in the following section. Kovats retention indices today remain a widely used technique for reporting retention data, while every stationary phase developed for packed and capillary GC has been characterized by generation of its McReynolds constants. 

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