Chromatograph mobile phase

Figure 1 Chromatogram of a neutral compound (toluene) with watenacetonitrile mobile phase. Chromatographic conditions — column 30 cm x 3.9 mm p-Bondapak C18 (10-pm particle size) mobile phase watenacetonitrile (50 50) flow rate 1.5 ml/min column temperature ambient detector wavelength 254 nm.

The separation of compounds by their differential partition between two immiscible phases is the basis for partition chromatography. The system consists of a stationary liquid phase coated on an inert solid support, and an immiscible mobile phase. Chromatographic separations are based on the different equilibrium distributions of the samples between these two phases. The greater the quantity of substance in the stationary phase at equilibrium the dower is the migration. For analyses, this equilibrium must remain constant over a suitable concentration range. Thus an increase in the concentration of solute results in a linear increase in the concentration of solute in the mobile and stationary phase, respectively. Under these conditions, the retention time, tR, is independent of the amount of sample chromatographed and a symmetrical peak (gaussian band) is observed. 

Figure 4-63. Chromatograms of a mixture of beta blockers with different inorganic anions in the mobile phase. Chromatographic conditions Column Zorbax Echpse XDB-C18 (150 X 4.6mm). Mobile phase Aqueous (pH 3.0)-acetonitrile (70 30) flow rate, 1 mL/min. Detection UV at 225 nm. (Reprinted from reference 156, with permission.)...

Internal porosity. The total porosity, e or ej, is the volume fraction of the column that is available to the mobile phase. The external porosity, e, is the volume fraction of the column that is available to the mobile phase percolating through the bed while the internal or particle porosity, characterizes the volume available to the stagnant mobile phase. Chromatographers define the internal porosity of the column as the difference between the total and the external porosities. So, for chromatographers. 

Fig. 8.2. Change of capacity factor k ) of ergotamine with salt concentration in the mobile phase. Chromatographic conditions column, pBondapak Cjg (S pm) (300 mm X 4.7 mm I.D.) mobile phase, methanol-water (60 40) containing S.O mM sodium heptanesulphonate and 1.0% acetic acid flow rate, 2 ml/min temperature, 20 °C detection, UV at 254 nm or 245 nm (0.1 aufs). O, NaCl , MgS04 a, (NH4)2S04 a, U2SO4 , Na2S04. Reproduced from Low et al. (1983), with permission.

Fig. 9.7. Separation of a mixture of enantiomeric amino acids on a reversed phase column with a chiral agent added to the mobile phase. Chromatographic conditions column, Spherisorb/LC-18 (5 pm, 150x4.6 mm I.D.) mobile phase, water chiral additive, Cu(jV,A -di- r-propyl-L-alanine) flow rate, 0.2 ml/min temperature, ambient.

Chromatographic separations are accomplished by continuously passing one sample-free phase, called a mobile phase, over a second sample-free phase that remains fixed, or stationary. The sample is injected, or placed, into the mobile phase. As it moves with the mobile phase, the sample s components partition themselves between the mobile and stationary phases. Those components whose distribution ratio favors the stationary phase require a longer time to pass through the system. Given sufficient time, and sufficient stationary and mobile phase, solutes with similar distribution ratios can be separated. 

Analytical separations may be classified in three ways by the physical state of the mobile phase and stationary phase by the method of contact between the mobile phase and stationary phase or by the chemical or physical mechanism responsible for separating the sample s constituents. The mobile phase is usually a liquid or a gas, and the stationary phase, when present, is a solid or a liquid film coated on a solid surface. Chromatographic techniques are often named by listing the type of mobile phase, followed by the type of stationary phase. Thus, in gas-liquid chromatography the mobile phase is a gas and the stationary phase is a liquid. If only one phase is indicated, as in gas chromatography, it is assumed to be the mobile phase. 

A chromatographic peak may be characterized in many ways, two of which are shown in Figure 12.7. The retention time, is the elapsed time from the introduction of the solute to the peak maximum. The retention time also can be measured indirectly as the volume of mobile phase eluting between the solute s introduction and the appearance of the solute s peak maximum. This is known as the retention volume, Vr. Dividing the retention volume by the mobile phase s flow rate, u, gives the retention time. 

In their original theoretical model of chromatography, Martin and Synge treated the chromatographic column as though it consists of discrete sections at which partitioning of the solute between the stationary and mobile phases occurs. They called each section a theoretical plate and defined column efficiency in terms of the number of theoretical plates, N, or the height of a theoretical plate, H where... 

Nonideal asymmetrical chromatographic bands showing (a) fronting and (b) tailing. Also depicted are the corresponding sorption isotherms showing the relationship between the concentration of solute in the stationary phase as a function of its concentration in the mobile phase. 

Use of column selectivity to improve chromatographic resolution showing (a) the variation in retention time with mobile phase pH, and (b) the resulting change in alpha with mobile phase pH. 

To determine how the height of a theoretical plate can be decreased, it is necessary to understand the experimental factors contributing to the broadening of a solute s chromatographic band. Several theoretical treatments of band broadening have been proposed. We will consider one approach in which the height of a theoretical plate is determined by four contributions multiple paths, longitudinal diffusion, mass transfer in the stationary phase, and mass transfer in the mobile phase. 

In gas chromatography (GC) the sample, which may be a gas or liquid, is injected into a stream of an inert gaseous mobile phase (often called the carrier gas). The sample is carried through a packed or capillary column where the sample s components separate based on their ability to distribute themselves between the mobile and stationary phases. A schematic diagram of a typical gas chromatograph is shown in Figure 12.16. 

A chromatographic technique in which the mobile phase is a gas and the stationary phase is a liquid coated either on a solid packing material or on the column s walls. 

A chromatographic technique in which the mobile phase is a liquid. 

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