Mixing the mobile phase

Mitragyna alkaloids 314 Mixing the mobile phase 132 Molybdatophosphoric acid see Phosphomo-lybdic acid... 

T.575 gbf-8ittmonium fDrmate was made up to 500 cm-te a graduated flask. To this solution, 50 cm3 of ethanol was added, and after mixing the mobile phase was placed in the solvent reservoir and pumping was commenced at 2 cm3 min"1 . 

If you do not have a gradient system, I have developed a fast isocratic scouting technique. You select the same column and detector wavelength, but equilibrate the column in 80% acetonitrile in water for our first injection. A strong solvent composition is selected to blow everything off quickly. Look at the peaks if they are resolved, quit. If they are still unresolved, mix the mobile phase with an equal volume of water making 40% acetonitrile, reequilibrate, and shoot again. This time, the peaks should be much farther apart. If not, do another equal dilution with water to 20%, reequilibrate, and reinject the sample. 

Choosing a Mobile Phase Several indices have been developed to assist in selecting a mobile phase, the most useful of which is the polarity index. Table 12.3 provides values for the polarity index, P, of several commonly used mobile phases, in which larger values of P correspond to more polar solvents. Mobile phases of intermediate polarity can be fashioned by mixing together two or more of the mobile phases in Table 12.3. For example, a binary mobile phase made by combining solvents A and B has a polarity index, of... 

The most commonly used gradients are linear gradients where the starting solvent is gradually mixed with a second gradient-forming solvent at the column entrance to yield a volume fraction

mobile phase modulator that increases huearly with time ... 

This type of chromatographic development will only be briefly described as it is rarely used and probably is of academic interest only. This method of development can only be effectively employed in a column distribution system. The sample is fed continuously onto the column, usually as a dilute solution in the mobile phase. This is in contrast to displacement development and elution development, where discrete samples are placed on the system and the separation is subsequently processed. Frontal analysis only separates part of the first compound in a relatively pure state, each subsequent component being mixed with those previously eluted. Consider a three component mixture, containing solutes (A), (B) and (C) as a dilute solution in the mobile phase that is fed continuously onto a column. The first component to elute, (A), will be that solute held least strongly in the stationary phase. Then the... 

Concentrations of moderator at or above that which causes the surface of a stationary phase to be completely covered can only govern the interactions that take place in the mobile phase. It follows that retention can be modified by using different mixtures of solvents as the mobile phase, or in GC by using mixed stationary phases. The theory behind solute retention by mixed stationary phases was first examined by Purnell and, at the time, his discoveries were met with considerable criticism and disbelief. Purnell et al. [5], Laub and Purnell [6] and Laub [7], examined the effect of mixed phases on solute retention and concluded that, for a wide range of binary mixtures, the corrected retention volume of a solute was linearly related to the volume fraction of either one of the two phases. This was quite an unexpected relationship, as at that time it was tentatively (although not rationally) assumed that the retention volume would be some form of the exponent of the stationary phase composition. It was also found that certain mixtures did not obey this rule and these will be discussed later. In terms of an expression for solute retention, the results of Purnell and his co-workers can be given as follows,... 

When u E, this interstitial mixing effect was considered complete, and the resistance to mass transfer in the mobile phase between the particles becomes very small and the equation again reduces to the Van Deemter equation. However, under these circumstances, the C term in the Van Deemter equation now only describes the resistance to mass transfer in the mobile phase contained in the pores of the particles and, thus, would constitute an additional resistance to mass transfer in the stationary (static mobile) phase. It will be shown later that there is experimental evidence to support this. It is possible, and likely, that this was the rationale that explains why Van Deemter et al. did not include a resistance to mass transfer term for the mobile phase in their original form of the equation. 

This is an oversimplified treatment of the concentration effect that can occur on a thin layer plate when using mixed solvents. Nevertheless, despite the complex nature of the surface that is considered, the treatment is sufficiently representative to disclose that a concentration effect does, indeed, take place. The concentration effect arises from the frontal analysis of the mobile phase which not only provides unique and complex modes of solute interaction and, thus, enhanced selectivity, but also causes the solutes to be concentrated as they pass along the TLC plate. This concentration process will oppose the dilution that results from band dispersion and thus, provides greater sensitivity to the spots close to the solvent front. This concealed concentration process, often not recognized, is another property of TLC development that helps make it so practical and generally useful and often provides unexpected sensitivities. 

For this technique, the upper plate has a mobile phase inlet channel on one edge and a slit at the opposite edge for directing the mobile phase toward the next plate (33). The slit (width approx. 0.1 mm) can be produced by cutting the layers with a sharp blade this enables easy passage of mobile phase and separated compounds without any mixing. The cushion of the OPLC instrument is applied to the uppermost layer only, and each plate presses on to the sorbent layer below. 

Virtually all interactive mechanisms that control retention in chromatography are, in fact, mixed interactions as shown by the previous application examples. It has already been suggested that reverse phases can exhibit almost exclusively dispersive interactions with solutes. However, as they are almost always employed with aqueous solvent mixtures then, polar and dispersive interactions will still be operative in the mobile phase. Consequently, the examples given here will be taken where the mixed interactions are either unique or represent a separation of special interest. 

The selection of proper mobile phase in TLC exerts a decisive influence on the separation of inorganic ions. With a particular stationary phase, the possibility of separation of a complex mixture is greatly improved by the selection of an appropriate mobile phase system. In general, the mixed aqueous-organic solvent systems containing an acid, a base, or a buffer have been the most favored mobile phases for the separation of ionic species. The mobile phases used as developers in inorganic PLC include ... 

Mobile phase The HPLC mobile phase is made up as follows. Prepare 2 L of acetate buffer by dissolving 13.6 g of sodium acetate and 6 mL of glacial acetic acid in 2 L of deionized water. Adjust the solution to pH 4.8 with concentrated sodium hydroxide solution (or glacial acetic acid) if necessary. Mix 2 L of buffer with 1.6-2 L (the amount depends on the particular commodity) of methanol. Eilter the solution through a 0.22-pm Nylon 66 filter membrane before using the mobile phase Absolute ethanol Aaper Alcohol and Chemical Co. (200 proof)... 

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