Additives to mobile phase

The following are amongst the reagents that have been reported as being added to the mobile phase acids for quinine alkaloids [184], ninhydnn for amino acids [185 — 187], fluorescamine for biogenic amines [188] Fluorescein sodium [189], dichlorofluorescein [190], rhodamine 6G [191], ANS reagent [192] and bromine [193] have all been descnbed as additives to mobile phases... 

Chiral crown ethers can be generally utilized as chiral selectors. They have been used as additives to mobile phases or running buffer in MECK and capillary electrophoresis (CE) systems124,125 (see Section 3.1.6.4.). 

I.6.4. Enantioseparation via Chiral Additives to Mobile Phase or Running Buffer... 

In conclusion two facts should be noted. First,all the chromatograms quoted above have been followed with UV detectors. The usefulness of another detectors ( e.g. polarographic, refractive index) in CD solutions has not yet been proved. Second, CD additions to mobile phase solutions of RP systems are always followed by a loss of column efficiency (ca 30%).This problem however demands more detailed studies. 

M.A. Garcia, S. Vera and M.L. Marina, Determination of Micelle-Solute Association Constants of some Benzene and Naphthalene Derivatives by Micellar HPLC with Butanol and Sodium-Chloride Additives to Mobile Phase, Chromatographia, 32 148 (1991). 

Fig. 3.68. Analytical HPLC chromatograms with detection of diode array of 4.7 x 10"5mol/l of R3R dye curve (1) before and curve (2) after 180 min of photoelectrocatalysis on the Ti02 thin-film electrode biased at +1.0 V in NajSCT, 0.025 mol/l. Curve (4) before and curve (3) after photoelectrocatalysis in NaCl 0.022 mol/l and curve (5) after bleaching of 4.7 X 10-5 mol/l of R3R dye submitted to a chemical treatment by active chlorine addition. The mobile phase was methanol-water 80 20 per cent with a flow rate of 1 ml/min and controlled temperature at 30°C. The column was a Shimpack (Shimadzu) CLC-ODS, 5 /an (250 mm X 4.6 mm). Reprinted with permission from P. A. Cameiro el al. [138].

The addition of CO2 to mobile phases in normal phase chromatography using silica gel stationary phases was used as an adsorption-promoting solvent [56], Tetrahydrofiiran or chloroform with 3.5% ethanol was the organic components in... 

If the mobile phase is present in a significant concentration, as suggested by the results of solvent extraction studies (1,8), the practical meaning of the mobile phase to coal conversion processes may be profound. In coal liquefaction, two stage processes emphasizing the mobile phase and the macromolecular structure separately could well be most economical. In devolatilization kinetics, at least two sets of kinetic parameters are necessary to model the devolatilization phenomena associated with the mobile phase and the macromolecular structure respectively since the mobile phase components devolatilize at much lower temperatures than the macromolecular structure components 0. In addition, the mobile phase appears to have a significant influence on the thermoplastic properties of coal (0 and thereby on coke quality. 

The CLND is limited, of course, to mobile phases that do not contain nitrogen. Acetonitrile and amine modifiers, commonly used in HPLC, are therefore precluded. In addition, the CLND is not readily amenable to non-volatile buffers in the mobile phase. However, it is still possible to determine RRF values for samples run under these non-CLND-compatible HPLC conditions. In such cases, a two-step process is used. First, a CLND-compatible mobile phase (e.g., methanol/water/trifluoroacetic acid) is used to separate the compounds of interest and determine RRF values under those conditions (RRF ). Separately, the UV peak areas obtained using both the CLND-compatible and non-compatible HPLC conditions are compared by analyzing a common sample by both sets of HPLC conditions (apart from the CLND). The peaks of interest must, of course, be tracked to avoid misassignment (e.g., through UV spectra comparison). The relative response factor (RRF ) obtained for the CLND-compatible method can then be used to determine the relative response factor (RRF2)... 

Mobile phases for SEC fall into two broad categories aqueous buffers for GFC and organic solvents for GPC. In SEC, the mobile phase is selected not to control selectivity but for its ability to dissolve the sample. In addition, the mobile phase should have a low viscosity and be compatible with the detector and column packing. For example, polar solvents such as methanol... 

Figure 18 CEC separation of explosives with addition of SDS to mobile phase. Column 34 cm x 75 mm i.d., 21 cm packed with 1.5-pm nonporous ODS II particles. Mobile phase 20% methanol, 80% 10 mM MES, 5 mM SDS. Running potential 12 kV (480 V/cm in packed portion). Injection 2 s at 2 kV of 12.5 mg/L (each component) sample. (Reprinted from Ref. 89, with permission.)...

Tauc, R, Cochet, S., Algiman, E., CaUebaut, 1., Cartron, J.-R, Brochon, J.C., and Bertrand, O. 1998. Ion-exchange chromatography of proteins Modulation of selectivity by addition of organic solvents to mobile phase Application to single-step purification of a protei-... 

The prominent niobium and lead spikes of continental materials are not matched by any of the OIBs and MORBs reviewed here. They are, however, common features of subduction-related volcanic rocks found on island arcs and continental margins. It is therefore likely that the distinctive geochemical features of the continental crust are produced during subduction, where volatiles can play a major role in the element transfer from mantle to crust. The net effect of these processes is to transfer large amounts of lead (in addition to mobile elements like potassium and rubidium) into the crust. At the same time, niobium and tantalum are retained in the mantle, either because of their low solubility in hydrothermal solutions, or because they are partitioned into residual mineral phases such as Ti-minerals or certain amphiboles. These processes are the subject of much ongoing research, but are beyond the scope of this chapter. 

The separation of enantiomers is especially important in the pharmaceutical field, because drag enantiomers may produce different effects in the body. Enantiomer separations by chromatography require one of the components of the phase system to be chiral. This can be achieved by (a) the addition of a chiral compound to the mobile phase, which is then used in combination with a nonchiral stationary phase, or (b) the use of a chiral stationary phase in combination with a nonchiral mobile phase. The chiral phase can either be a solid support physically coated with a chiral stationary phase liquid or a chemically bonded chiral phase. For mobile-phase compatibility reasons, a chiral stationary phase is preferred in LC-MS. However, most chiral stationary phases have stringent demands with respect to mobile-phase compositiorr, which in turn may lead to compatibility problems. Three types of phase systems are applied in LC-MS ... 

LC operation No restrictions to mobile phase composition Gradient elution Volatile and nonvolatile additives (buffers, ion-pair reagents) Free choice of LC column dimensions Flow-rate between 1 pl/min and 2 ml/ntin... 

As was the case for the forced-decomposition samples, the solid-state stability samples should be monitored using both PDA and MS detection to ensure that specificity of the candidate method is maintained. Analysis of the samples using an orthogonal method to further verify specificity is also recommended. If the method is, at this point, shown to provide separation among the API, all drug synthesis process impurities, and all degradation products, no additional method development is required at this juncture. When coelution is observed between components, development using additional column/mobile-phase combinations may be considered (see Section III.E). Alternatively, the use... 

The addition of surfactants to mobile phases has also been used to prevent on-column precipitation of proteins. The surfactants serve to keep the proteins in solution, allowing the direct injection of plasma samples (52). 

Selection of the mobile phase is critical in the characterization of silica sols by SEC. As with the other separation methods, pH should be slightly basic, and low ionic strength must be used to prevent particle aggregation. In addition, the mobile phase must interact with the surface of the packing-particle pores to neutralize undesirable charge effects. Negatively charged surfaces within the pore can result in ion-exclusion effects whereby... 

Over the last decade, the most significant developments in liquid chromatography, specifically in HPLC, have been in the areas of instrumentation and equipment [49-52], advances in stationary phase chemistry and their design [51, 52], improvements in column design and dimensions [53, 54], use of different additives in mobile phases [55] and sample preparation techniques [38,39]. All of these factors have helped to significantly improve the way separation-scientists develop their methods. 

The mobile phase itself can generate additional peaks. Since the composition of the sample and the mobile phase are by definition different, the injection of the sample disturbs the equilibrium of the column with the mobile phase. Especially in cases where mobile-phase additives are used at low concentration, extra peaks corresponding to mobile-phase constituents can occur. This phenomenon is commonly observed in ion-pair diromatography. Both negative and positive peaks are possible. 

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