Liquid chromatography equilibration

By application of first-order, kinetic equations, B. Anderson and Degn claimed that an equilibrated (25°) aqueous solution of D-fructose contains 31.56% of jS-D-fructofuranose and 68.44% of -D-fructopyranose. N.m.r. studies, however, showed that, at equilibrium, a solution of D-fructose contains /3-D-fructopyranose, -D-fructofuranose, a-D-fructofuranose, and a trace of a-D-fructopyranose the distribution of these isomers was shown by gas-liquid chromatography to be 76,19.5, and 4%, respectively. Based on Anderson and Degn s result, Shallenberger reasoned that, as 0.68 X 1.8 = 1.22 (which approximates the reported sweetness of mutarotated D-fructose ), the furanose form(s) must possess very little sweetness. 

The most common approaches to sulfonylurea determinations involve high-performance liquid chromatography (HPLC). The earliest reported methods utilized normal-phase liquid chromatography (LC) with photoconductivity detection this type of detector demonstrated undesirably long equilibration times and is no longer... 

The popularity of reversed-phase liquid chromatography (RPC) is easily explained by its unmatched simplicity, versatility and scope [15,22,50,52,71,149,288-290]. Neutral and ionic solutes can be separated simultaneously and the rapid equilibration of the stationary phase with changes in mobile phase composition allows gradient elution techniques to be used routinely. Secondary chemical equilibria, such as ion suppression, ion-pair formation, metal complexatlon, and micelle formation are easily exploited in RPC to optimize separation selectivity and to augment changes availaple from varying the mobile phase solvent composition. Retention in RPC, at least in the accepted ideal sense, occurs by non-specific hydrophobic interactions of the solute with the... 

Figure 4.17 General phenonenaloglcal retention model for a solute that participates in a secondary chemical equilibrium in liquid chromatography. A - solute, X - equilibrant, AX analyte-equilibrant coeplex, Kjq - secondary chemical equilibrium constant, and and are the primary distribution constants for A and AX, respectively, between the mobile and stationary phases.

Based on high performance liquid chromatography (HPLC) studies regarding the equilibration of isomeric fractions of P-carotene isomers at 45°C, a model consisting of two reversible concurrent isomerization reactions was developed by Pesek et al. 1990. Under dark storage conditions at 45°C, a P-carotene solution reached an equilibrium after 4-6 days yielding approximately 66% aW-trans-, 8% of 9-cis-, and 25% of 13-d.s-P-carotene. The observed rate constant (k) for the formation of the 13-d.v- isomer was faster than that of the 9-d.s-p-carotene isomer, and the back rate constants toward the all -trans- isomer were intrinsically faster as compared to the formation of d,v-isomcrs of P-carotene (Chart 12.1). 

Pulsed ultrafiltration MS (PUF-MS) represents an inline high throughput affinity screening method with a variety of potential uses in the discovery and development of pharmaceuticals [22]. The in-line combination of solution-phase equilibration, ultrafiltration, and electrospray liquid chromatography mass spectrometry (LC-ESI-MS) facilitates the identification of high affinity target-specific... 

Methods. Adsorption isotherms were run at constant feed molar ratio of C oS0 /Ci3eS0.. The feed solutions had a pH of 4.25 and a NaCl concentration of 0.15 M. Ten ml of feed solution was added to 0.5 g alumina in a screw top centrifuge tube and centrifuged at 700 RPM for 45 minutes at room temperature. The tube was then placed in a water bath at 30°C for four days, the liquid decanted from the mineral and analyzed. The surfactant concentrations were analyzed using high performance liquid chromatography with a conductivity detector. The solution pH after equilibration was determined using pH electrodes. The equilibrium pH increased to 6.8 at equilibrium because the PZC of alumina is approximately 9. 

ENTHALPY AND ENTROPY OF VAPORIZATION. The first accurate thermodynamic quantities obtained using GC were enthalpies of vaporization. In 1960 Mackle, Mayrick, and Rooney, (12) measured the heats of vaporization of 2-thiobutane, 3-thiobutane, and 4-thio-heptane by using gas-liquid chromatography coupled with a bypass sampling system. In this system the sample is placed in a sample tube, connected to the apparatus, and cooled with dry ice. Then the sample is warmed to the specified temperature and equilibrated in one arm of the apparatus. The liquid is then isolated from the system and carrier gas allowed to pass through the arm, sweeping the vapors onto the gas chromatographic column. 

High Performance Liquid Chromatography. Although chiral mobile phase additives have been used in high performance liquid chromatography (hplc), the laige amounts of solvent, thus chiral mobile phase additive, required to pre-equilibrate the stationary phase renders this approach much less attractive than for dc and is not discussed here. 

In principle, the experimental protocol of fluidized bed adsorption does not deviate from packed bed operations, the main difference being the direction of liquid flow. The standard sequence of frontal chromatography, equilibration, sample application, wash, elution, and cleaning (CIP) is performed with an upward direction of flow as shown in Fig. 3. During equilibration of the matrix the stabilization of the fluidized bed occurs, in case of size and/or density distribution of the adsorbent particles the classification within the bed may be detected by visual observation of the bed. As discussed below, bed stability may... 

LC-MS/MS System Used and Analytical Conditions A Shimadzu liquid chromatography system was used (Shimadzu Scientific Instruments, USA). The system consisted of a Shimadzu SCL-10A VP system controller with four LC-10AD VP pumps. A Perkin-Elmer series 200 autosampler, equipped with a cool-tray (Perkin Elmer, Norwalk, CT, USA) was used for sample injection. Moreover, a six-port switching-valve was implemented in order to switch between an extraction column and the analytical column. Samples were injected onto the extraction column (Oasis HLB column). After a short extraction/equilibration time of 0.3 min, the samples were eluted from the extraction column onto the analytical column (Symmetry C18) (Figure 1). 

High-performance liquid chromatography (HPLC) can be ambiguous in indicating purity a pure spiropyran may thermally equilibrate in a polar mobile phase and the open and closed forms will appear as two peaks. 

Preferably, high pressure liquid chromatography (hplc) is used to separate the active pre- and cis-isomers of vitamin D3 from other isomers and allows theic analysis by comparison with the chromatograph of a sample of pure reference j-vitamin D3, which is equilibrated to a mixture of pre- and cis-isomers (82,84,85). This method is more sensitive and provides information on isomer distribution as well as the active pre- and cis-isomer content of a vitamin D sample. It is applicable to most forms of vitamin D, including the more dilute formulations, ie, multivitamin preparations containing at least 1 lU/g (AOAC Methods 979.24 980.26 981.17 982.29 985.27) (82). The practical problem of isolation of the vitamin material from interfering and extraneous components is the limiting factor in the assay of low level formulations. 

Unlike high-performance liquid chromatography (HPLC), there is no maximum length for capillaries in CEC, but the longer columns mean slower chromatography and equilibration. Generally, columns in CEC are no longer than 60 cm. 

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