Mobile-phase controlling

Development in TLC is the process by idiich the mobile phase moves through the sorbent layer, thereby inducing differential migration of the suple components. The principal development modes used in TLC are linear, circular and anticircular with the velocity of the mobile phase controlled by capillary forces or forced-flow conditions. In any of these modes the development process can be extended by using continuous development or multiple development. 

The mobile phase controls HPLC separation While the HPLC stationary phase provides retention and influences the separation mechanism, it is the mobile phase which controls the overall separation. HPLC method development efforts focus on finding a set of mobile phase conditions that provide adequate separation of the analyte peak(s) from other components in the sample. 

The composition of the mobile phase controls the product isolation procedure. Isolation from re versed-phase PHPLC fractions requires evaporation,... 

Capillary electrochromatography (CEC) combines some aspects of mobile phase control and separation by capillary electrophoresis with the wider range of separation mech-... 

In MLC, the retention is influenced by two competing equilibria of solute interactions with micelles in the mobile phase (controlled by K m) and their partitioning into the stationary plmse (controlled by P s) (see eq. 9.11). Both partitioning processes depend on the hydrophobicity of die solute. The reciprocal of the intercept in eq. 9.11 is the retention factor at zero micelle concentration, ko, which is a parameter similar to, and as shown below, is useful in hydrophobicity measurements. 

Docetaxel, taxoid anticancer agent was extracted from plasma and analyzed on a Cig column (A = 227nm) with paclitaxel as IS [1394]. Good peak shapes and resolution were obtained in 12-min analysis using a 43/57 acetonitrile/water (20 mM aimnonium acetate pH 5) mobile phase. Control standards were run from 5 to lOOOng/mL with a reported quantitation limit of 5ng/mL. 

The mobile phase of HPLC is usually a mixture of two miscible solvents. In gas chromatc raphy the carrier gas plays no role in determining retention times, but in HPLC the mobile phase controls retention times and selectivity. 

For LC, temperature is not as important as in GC because volatility is not important. The columns are usually metal, and they are operated at or near ambient temperatures, so the temperature-controlled oven used for GC is unnecessary. An LC mobile phase is a solvent such as water, methanol, or acetonitrile, and, if only a single solvent is used for analysis, the chromatography is said to be isocratic. Alternatively, mixtures of solvents can be employed. In fact, chromatography may start with one single solvent or mixture of solvents and gradually change to a different mix of solvents as analysis proceeds (gradient elution). 

Retention and stereoselectivity on the BSA columns can be changed by the use of additives to the aqueous mobile phase (30). Hydrophobic compounds generally are highly retained on the BSA, and a mobile-phase modifier such as 1-propanol can be added to obtain reasonable retention times. The retention and optical resolution of charged solutes such as carboxyUc acids or amines can be controlled by pH and ionic strength of the mobile phase. 

THIN-LAYER CHROMATOGRAPHY OF BENZOIC ACIDS WITH EXTERNAL CONTROL PROPERTIES OF THE MOBILE PHASE... 

In the course of mixture separation, the composition and properties of both mobile phase (MP) and stationary phase (SP) are purposefully altered by means of introduction of some active components into the MP, which are absorbed by it and then sorbed by the SP (e.g. on a silica gel layer). This procedure enables a new principle of control over chromatographic process to be implemented, which enhances the selectivity of separation. As a possible way of controlling the chromatographic system s properties in TLC, the pH of the mobile phase and sorbent surface may be changed by means of partial air replacement by ammonia (a basic gaseous component) or carbon dioxide (an acidic one). 

Distribution of benzodiazepines in I-octanol - water system was investigated by a direct shake flask method at the presence of the compounds used in HPLC mobile phases the phosphate buffer with pH 6,87 (substances (I) - (II)), acetic and phosphate buffer, perchloric acid at pH 3 (substances (III) - (VI)). Concentrations of substances in an aqueous phase after distribution controlled by HPLC (chromatograph Hewlett Packard, column Nucleosil 100-5 C, mobile phase acetonitrile - phosphate buffer solution with pH 2,5, 30 70 (v/v)). 

When the silica surface is in contact with a solvent, the surface is covered with a layer of the solvent molecules. If the mobile phase consists of a mixture of solvents, the solvents compete for the surface and it is partly covered by one solvent and partly by the other. Thus, any solute interacting with the stationary phase may well be presented with two, quite different types of surface with which to interact. The probability that a solute molecule will interact with one particular type of surface will be statistically controlled by the proportion of the total surface area that is covered by that particular solvent. 

Retention is controlled by solute interactions with both the mobile phase and the stationary phase and each will be discussed in this chapter. Interactions in the mobile... 

Dispersion in the mobile phase is again diffusion controlled and, so, again reiterating equation (7),... 

Thus, for significant values of (k") (unity or greater) the optimum mobile phase velocity is controlled primarily by the ratio of the solute diffusivity to the column radius and, secondly, by the thermodynamic properties of the distribution system. However, the minimum value of (H) (and, thus, the maximum column efficiency) is determined primarily by the column radius, secondly by the thermodynamic properties of the distribution system and is independent of solute diffusivity. It follows that for all types of columns, increasing the temperature increases the diffusivity of the solute in both phases and, thus, increases the optimum flow rate and reduces the analysis time. Temperature, however, will only affect (Hmin) insomuch as it affects the magnitude of (k"). 

The conditions required to minimize tube dispersion are clearly indicated by equation (10). Firstly, as the column should be operated at its optimum mobile phase velocity and the flow rate, (0) is defined by column specifications it is not a variable that can be employed to control tube dispersion. Similarly, the diffusivity of the solute (Dm)... 

It follows from equation (2) that the sample load will increase as the square of the column radius and thus the column radius is the major factor that controls productivity. Unfortunately, increasing the column radius will also increase the volume flow rate and thus the consumption of solvent. However, both the sample load and the mobile phase flow rate increases as the square of the radius, and so the solvent consumption per unit mass of product will remain the same. 

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