Vacancy chromatography

Vacancy chromatography is a special method of development that can provide both negative and positive peaks in the chromatogram. It is not commonly used, although... 

Vacancy chromatography has some quite unique properties and a number of potentially useful applications. Vacancy chromatography can be theoretically investigated using the equations derived from the plate theory for the elution of... 

Although, even today, vacancy chromatography is rarely used in analytical laboratories generally, there are a number of applications where it appears it might be very useful. The technique, that was suggested by Zhukhovitski for quality control is a particularly interesting application. Consider a pharmaceutical product that contains... 

Several variants of separation methods based on dialysis, ultrafiltration, and size exclusion chromatography have been developed that work under equilibrium conditions. Size exclusion chromatography especially has become the method of choice for binding measurements. The Hummel-Dreyer method, the vacancy peak method, and frontal analysis are variants that also apply to capillary electrophoresis. In comparison to chromatographic methods, capillary electrophoresis is faster, needs only minimal amounts of substances, and contains no stationary phase that may absorb parts of the equilibrium mixture or must be pre-equilibrated. 

Vacancy chromatography has a number of applications areas in practice, none of which appear to have been extensively exploited. One particularly interesting application is that of quality control. If a particular product has a number of components present, and their relative composition must be kept constant as in, for example, a pharmaceutical product, Vacancy Chromatography can provide a particularly simple analytical procedure for quality control. The mobile phase is made up containing the components of the product in the specified proportions, but at a low concentration suitable for LC analysis. A sample of the product is dissolved in some pure mobile phase at the same total mass concentration as the standards in the mobile phase. A sample is then injected on the column. If the product contains the components in the specified proportion, no peaks will appear on the chromatogram as the sample and mobile phase will have the same composition. If any component is in excess, it will show a positive peak. If any component is present below specifications, it will show a negative peak. The size of the peak will provide an accurate measure of the difference between the sample and that of the required standard. 

The desire to use a universal detector like a conductance detector is the reason for the use of a second, suppressor, column. Alternative detector systems have been used without a suppressor some ions absorb in the UV and can be used with non-UV absorbing counterions in the mobile phase some work has been done with the universal refractive index detector and, finally, vacancy chromatography (discussed more fully later in this chapter) has been applied whereby the mobile phase... 

Vacancy Chromatography. One way to use a UV detector for analytes that do not absorb is to include a UV absorber as part of the mobile phase. When the non-UV absorbing analytes enter the detector cell, they cause a decrease in the baseline absorbance, appear as negative peaks, and can be used for routine analysis. In effect, they produce a vacancy hence the name for this method of detection. 

Vacancy chromatography, 206 Van Deemter equation, 3, 28, 37 Van der Waals forces, 43, 44 Viscosity of water mixtures, 198... 

It is well known that drugs bind to plasma proteins, particularly to serum albumin and a-acid glycoprotein, and that only the unbound, or free, fraction is responsible for any pharmacological effect. For protein-drug binding studies size-exclusion chromatography in one of three variants—namely, the Hum-mel-Dreyer method (1962), the vacancy peak method (Sebille, et al., 1979), and frontal analysis (Cooper and Wood, 1968)—is the traditional method of... 

Gennaro, M.C. Ghost, vacancy, dip, or system peaks A contribution in investigations about injection and system peaks in liquid chromatography. J. Liq. Chromatogr. Rel. Technol. 1987, 10, 3347-3375. 

Concentration pulse chromatography (also called elution on a plateau, step and pulse method, system peak method, or perturbation chromatography) is experimentally much simpler [100,103,104]. The same experimental procedure is used as for the determination of smgle-component isotherms. First, the column is equilibrated with a solution of the multicomponent mixture of interest in the mobile phase. When the eluent has reached the composition of the feed to the column, a small pulse of the pure mobile phase (vacancy) or of a solution having a composition different from that of the plateau concentration (see end of this section) is... 

A general mathematical treatment of system peaks and of the closely related method of vacancy chromatography was given by Helfferich and Klein [8]. This work includes a detailed analysis of the phenomena that take place upon injection of a sample into a chromatographic column. It is based on the use of the solution of the ideal model of chromatography for multicomponent systems, with competitive Langmuir isotherms (see Chapters 8 and 9), and of the ft-transform. 

Other conditions can be written for other problems. For example, in vacancy chromatography there is no sample, and the initial condition (Eq. 13.5) and the boxmdary conditions (Eq. 13.7) with C — — 0 are applied to all the addi-... 

Figure 13.9 Vacancy chromatography. Calculation of the chromatogram obtained with a binary mixture dissolved in the mobile phase. Low additive concentrations Cl = C2 = 0.0001 M. Injected amount 4.9 imol of pure solvent. Competitive Langmuir isotherms qi = ajCj)/ l+ J bjCj), with fli = 12, = 1.5, qs = 2, bj = Uj/qs- Reproduced with permission from S. Golshan- Shi-razi and G. Guiochon, Anal. Chem., 62 (1990) 923 (Fig. 8). (S)1990 American Chemical Society.

Vacancy Chromatography A chromatographic process in which a sample solution is used as the mobile phase and a sample of pure solvent is injected periodically. A vacancy of each component migrates along the column, instead of a positive peak in conventional elution. 

Traditionally, the detectors for the older ion exchange chromatography were refractive index, conductivity, and uv/vis spectroscopy in isolated cases. Today a wider variety is available. There are three types of electroconductivity detectors (d.c. conductivity, d.c. amperometry, pulsed amperometry) direct uv/vis with postcolumn reactants, "vacancy" uv/vis, and fluorescence. 

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