Mobility control background

If the mixture to be separated contains fairly polar materials, the silica may need to be deactivated by a more polar solvent such as ethyl acetate, propanol or even methanol. As already discussed, polar solutes are avidly adsorbed by silica gel and thus the optimum concentration is likely to be low, e.g. l-4%v/v and consequently, a little difficult to control in a reproducible manner. Ethyl acetate is the most useful moderator as it is significantly less polar than propanol or methanol and thus, more controllable, but unfortunately adsorbs in the UV range and can only be used in the mobile phase at concentrations up to about 5%v/v. Above this concentration the mobile phase may be opaque to the detector and thus, the solutes will not be discernible against the background adsorption of the mobile phase. If a detector such as the refractive index detector is employed then there is no restriction on the concentration of the moderator. Propanol and methanol are transparent in the UV so their presence does not effect the performance of a UV detector. However, their polarity is much greater than that of ethyl acetate and thus, the adjustment of the optimum moderator concentration is more difficult and not easy to reproduce accurately. For more polar mixtures it is better to explore the possibility of a reverse phase (which will be discussed shortly) than attempt to utilize silica gel out of the range of solutes for which it is appropriate. 

When electrochemical detection systems are used, it is essential that the mobile phases used are electrically conductive but only result in a low background current at the voltage selected. Cell volumes are of the order of 1 /d and the control of flow rate, pH and temperature is critical for reliable detection. 

In MACE, the alteration of the ionic mobility as a factor of the tenside concentration in the background electrolyte solution is a measure of the strength of interaction, which may be evaluated graphically. In Fig. 1, a schematic representation of MEKC is given for the separation of micelle and EOF markers as well as drug solutes of different lipophilicity. If the substances are neutral, their position between the EOF marker and the micelle marker is given only by their lipophilicity, which controls their affinity to the micellar phase. This means that S3 in Fig. 1 has the lowest hydrophilicity. 

The support can then be mechanically coated with a variety of liquid stationary phases. The mobile phase most commonly used in packed column GC is nitrogen with a flow rate of ca 20 ml/min. Packed column GC affords a relatively low degree of resolution compared to capillary GC typically 4000-6000 plates for a 2 m column compared to > 100 000 plates for a 25 m capillary column. The high temperature limit of packed columns is ca 280°C beyond this temperature the liquid stationary phase evaporates at a rate which creates a large background signal. However, for many routine quality control operations, they are quite adequate. 

The solution contained within the capillary in which the separation occurs is known as the background electrolyte (BGE), carrier electrolyte, or, simply, the buffer. The BGE always contains a buffer because pH control is the most important parameter in electrophoresis. The pH may affect the charge and thus the mobility of an ionizable solute. The electro-osmotic flow (EOF) is also affected by the buffer pH. Table 1 contains a list of buffers that may prove useful in high-performance capillary electrophoresis (HPCE). As will be seen later, only a few of these buffers are necessary for most separations. 

The thermal stability of a conductivity detector is of great importance. Effective thermostating is highly required, as the temperature greatly affects the mobility of ions and, therefore, conductivity. A 0.5-3% increase of conductivity is usually expected per degree Celsius. Close temperature control is necessary to minimize background noise and maximize sensitivity this is an especially important issue if nonsuppressed eluents are used. 

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