Stationary phase separating compounds

The inside of a monolithic electrochromatography column contains silicate fingers" coated with stationary phase. Aromatic compounds were separated with a mean plate number of 80 000 in a 50-cm column with 15-kV applied voltage. [From J. D. Hayes and A. Malik, Sol-Gel Monolithic Columns with Reversed Eleclroosmotic How for Capillary Eleclrochromolography" Anal. Chem. 2000, 72.4090. Photo courtesy A. Malik, University of South Honda.]... 

Gas chromatography is based on the distribution of a compound between two phases. In gas-solid chromatography (GSC) the phases are gas and solid, the injected compound is carried by the gas through a column filled with solid phase, and pmrtitioning occurs via the sorption-desorption of the compound (probe) as it travels past the solid. Superimposed upon the forward velocity is radial notion of the probe molecules caused by random diffusion through the stationary phase. Separation of two or more components injected simultaneously occurs as a result of differing affinities for the stationary phase. In gas-liquid chromatography (GLC), the stationary phase is a liquid coated onto a solid suppx>rt. The mathematical treatment is equivalent for GLC and CSC. 

Gas chromatography is a technique for separating mixtures of compounds by partitioning the compounds between a flowing gas (mobile phase) and a nonvolatile liquid phase (stationary phase). Separation is achieved by a combination of factors such as boiling point, polarity and compound affinity. Identification of compounds is achieved by measuring the time a compound takes to elute off the column. This retention time is characteristic of a compound under given criteria and column, and identification can be achieved by comparison with known substances. 

A computer program for the optimisation of ID GC separations [30] has been modified to include the additional variables required for GCxGC [31]. The estimation is also based on Equation (6), but the use of the distribution constant K instead of the retention factor k allows the extension of its application to different column geometries and to mixed stationary phases. The compounds in the Grob test mixture were used in the validation. The retention times of these compounds, obtained using two different temperature programs were the only experimental data required. Results were good except for the retention of a few compounds in the D column. 

Chromatography is a popular and widely used method of purification. Chromatography is based on the differences in the adsorption and the solubility of the compounds to be separated. It involves two phases — a stationary phase and a moving (mobile) phase. The moving phase actually carries the compounds along the stationary phase. Depending on the interactions between the compounds and the stationary phase, the compounds get held up or slowed down by the stationary phase. 

A nonpolar mobile phase passing through a packed column that contains a polar stationary phase defines normal-phase HPLC (NP-HPLC). For example, if -hexane comprises the mobile-phase and silica gel is used for the stationary phase, separations of nonpolar organic analytes as shown in Fig. 4.1 is accomplished. With respect to neutral organic compounds, the polar and ionic domains cannot be reached by NP- HPLC. NP-HPLC was the first high-pressure form of liquid chromatography to be developed. If the stationary phase could be made hydrophobic by chemical treatment and the mobile phase made more polar, a reversal of mobile/stationary-phase polarities could be achieved. Like it or not, we are stuck with this nomenclature RP-HPLC has certainly extended the range of analyte polarity that... 

Adsorption chromatography The stationary phase is a polar adsorbent, in most cases silica. The mobile phase is nonpolar (usually a solvent with polarity within the range from hexane to esters). It competes with the sample molecules for adsorption at the active sites of the stationary phase. Nonpolar compounds are eluted first, followed by solutes of increasing polarity. Steric properties of the sample compounds can play an important role and therefore adsorption chromatography is the method of choice for the separation of many classes of isomers. 

HILIC is a strongly upcoming LC technique during the last years and was recently reviewed several times [23-25]. Briefly, it is understood as aqueous chromatography on normal phase materials and especially suitable for the separation of very polar, that is, hydrophilic compounds. As column materials, silica, modified silica, or functionalized polymer particles are available, which commonly provide a water-enriched liquid layer within the stationary phase. Separation is achieved by partitioning of polar analytes from the eluent into the hydrophilic environment. 

Figure 6.45 Gradient elution of various aliphatic quaternary ammonium compounds on a polar-embedded stationary phase. Separator column Acclaim Surfactant Plus, 3 pm column dimensions 150mmx3mm Ld. column temperature 25 °C eluent 5 mmol/L formic acid/ MeCN (95 5 v/v) and (B) 5 mmol/L formic add/ MeCN (60 40 v/v) gradient linear, 100% A to 100% B in 12 min flow rate 0.5 mL/min ...

In a gas chromatograph-mass spectrometer, a mixture of compounds is first separated based on their boiling points and affinity for the stationary phase. Each compound is then analyzed individually. 

The nonmodified sorbents discussed so far show polar surface characteristics. But there are many chromatographic separation problems that can be solved using hydrophobic interactions of the stationary phase with compounds of appropriate molecular structure. Sorbents that are suitable for this task are the so-called reversed-phase materials (RP phases). In this connection, reversed phase means that the relative polarities of the stationary and mobile phase are reversed compared with the already described situation in adsorption chromatography i.e., the RP stationary phase is less polar... 

Table 3 Volatile Compounds Present in the Aroma Extracts of Boiled Pork, Using Solid-Phase Microextraction with Two Different Stationary Phases, Separately and Combined...

GPC on Sephadex GIO. The fraction containing low-molecular-weight compounds below 700 Da was analyzed by RP-HPLC but could not be separated because of insufficient retention of most of the components on the hydrophobic stationary phase. Separation and characterization were achieved by HILIC-ESI-MS, as shown in Fig. 9. Apart from free amino acids such as lysine and arginine, which showed large peaks, the glutamyl dipeptides Glu-Val, Glu-Gly, Glu-Ser, Glu-His, Glu-Glu, Glu-Lys, Glu-... 

Twenty-eight chiral compounds were separated from their enantiomers by HPLC on a teicoplanin chiral stationary phase. Figure 8-12 shows some of the structures contained in the data set. This is a very complex stationary phase and modeling of the possible interactions with the analytes is impracticable. In such a situation, learning from known examples seemed more appropriate, and the chirality code looked quite appealing for representing such data. 

Chromatography (Section 13 22) A method for separation and analysis of mixtures based on the different rates at which different compounds are removed from a stationary phase by a moving phase... 

Analytically, the inclusion phenomenon has been used in chromatography both for the separation of ions and molecules, in Hquid and gas phase (1,79,170,171). Peralkylated cyclodextrins enjoy high popularity as the active component of hplc and gc stationary phases efficient in the optical separation of chiral compounds (57,172). Chromatographic isotope separations have also been shown to occur with the help of Werner clathrates and crown complexes (79,173). 

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