Mobile phases attributes

The effect of the mobile phase modifier was investigated for a series of phen-oxypropionic acid (PEA) herbicides on a teicoplanin CSP [85, 86] an increasing enantioselectivity was found with increasing MeOH content in the mobile phase, attributed to restriction of the solute association with the TE CSP, which led to favorable stereoselective interactions. 

Nonetheless, these results are partial and can be seen only as a test study, and clearly many improvements will be considered. For example, the decision at each node should not be restricted to the only use of molecular key attributes, but should also take into account the mobile phase constituents. Future works will also extend this approach to the full database and will probably lead to the introduction of knowledge rules in CHIRBASE. Knowledge rules will help the users not only in the choice of a wide range of columns but also in the selection of appropriate experimental conditions. 

Fig. 6-6. Overload elution profiles of D,L-PA injected on a column (125 4 mm) packed with the L-PA imprinted stationary phase used in Fig. 6-5. Mobile phase MeCN TFA (0.01 %) FI O (2.5 %). The tendency for fronting and the increase in retention with sample load is attributed in part to saturation of the mobile phase modifier.

It has been previously noted (see Section 4.2 above) that use of the moving-belt interface allows El spectra to be obtained from compounds that do not yield spectra when analysis is attempted using a conventional El probe. The same is true when the dynamic-EAB probe is used in that spectra may be obtained from compounds that do not yield spectra when a static-FAB probe is used. This has been attributed to the presence of the mobile phase. 

The most important feature of any interface which is capable of allowing an El spectrum to be produced is that the mobile phase is totally removed so that the spectra obtained may be attributed solely to the analyte. Whether or not this is accomplished depends upon the composition of the mobile phase, its flow rate and the conditions employed within the interface, i.e. temperature, nebulizing gas flow, etc. 

There are two notable features of the quantitative performance of this type of interface. It has been found that non-linear responses are often obtained at low analyte concentrations. This has been attributed to the formation of smaller particles than at higher concentrations and their more easy removal by the jet separator. Signal enhancement has been observed due to the presence of (a) coeluting compounds (including any isotopically labelled internal standard that may be used), and (b) mobile-phase additives such as ammonium acetate. It has been suggested that ion-molecule aggregates are formed and these cause larger particles to be produced in the desolvation chamber. Such particles are transferred to the mass spectrometer more efficiently. It was found, however, that the particle size distribution after addition of ammonium acetate, when enhancement was observed, was little different to that in the absence of ammonium acetate when no enhancement was observed. 

The difference in movement rates of various compounds through a column is attributed to differential migration in HPLC. This can be related to the equilibrium distribution of different compounds such as X, Y, and Z between the stationary phase and the flowing solvent(s), or mobile phase. The speed with which each compound moves through the column (ux) is determined by the number of molecules of that compound in the moving phase, at any moment, since sample molecules do not move through the column while they are in the stationary phase. The molecules of the solvent or mobile phase move at the fastest possible rate except in size exclusion chromatography, where molecular... 

Reverse phase HPLC describes methods that utilize a polar mobile phase in combination with a nonpolar stationary phase. As stated above, the nonpolar stationary phase structure is a bonded phase—a structure that is chemically bonded to the silica particles. Here, typical column names often have the carbon number designation indicating the length of a carbon chain to which the nonpolar nature is attributed. Typical designations are C8, C18 (or ODS, meaning octadecyl silane), etc. Common mobile phase liquids are water, methanol, acetonitrile (CH3CN), and acetic acid buffered solutions. 

In general terms, the variation from laboratory to laboratory (between-laboratory) was greater than that attributed to the analytical error displayed within laboratories (intralaboratory). There are many reasons for the interlaboratory variation that can be attributed to operational parameters such as mobile phase flowrate, mobile phase and buffer composition, vaporiser temperature, tip temperature and source temperature. 

Since the separation process in CEC has a number of attributes similar to those of HPLC, the most important variables affecting the separation are the same for both of these techniques. However, in HPLC mobile phase, flow and separation are independent variables. Therefore, the most important operational variables are the analyte-sorbent interactions that can be modulated by the chemistry of the packing, composition of the mobile phase, and temperature. In contrast, the CEC column has a dual role as it serves as both (i) a flow driving device and (ii) separation unit at the same time. Although the set of variables typical of HPLC is also effective in CEC, their changes may affect in one way or another both column functions. Therefore, optimization of the separation process in CEC is more complex than in HPLC. 

Peak purity tests are used to demonstrate that an observed chromatographic peak is attributable to a single component. Mass spectrometry is the most sensitive and accurate technique to use for peak purity evaluation because of the specific information derived from the analysis. However, a good number of HPLC methods use mobile phase conditions that are incompatible with mass spectrometry detection. In this case, PDA spectrophotometers using peak purity algorithms may be used to support the specificity of the method. Almost all commercially available diode array detectors are equipped with proprietary software that will perform these calculations. Although this technique is more universal in application to HPLC methods, the data provided is neither particularly... 

The most commonly used LC/MS interfaces in pharmaceutical analysis are ESI and APCI. An ESI interface on the majority of commercial mass spectrometers utilizes both heat and nebulization to achieve conditions in favor of solvent evaporation over analyte decomposition. While ionization in APCI occurs in the gas phase, ionization using ESI occurs in solution. Attributes of a mobile phase such as surface tension, conductivity, viscosity, dielectric constant, flow rate and pFi, all determine the ionization efficiency. They therefore need to be taken into consideration and controlled. 

The concept that coals can be usefully considered to consist of two distinct phases of constituents is not new and has been advanced in different ways since the early part of this century. Its revival in recent times is largely attributable to the efforts of the late Peter Given, to whom is owed the now wide use of the terms mobile phase and macromolecular network (1-3. ... 

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