Stationary-phase concentration, column

The distribution of the solute between the mobile and the stationary phases is continuous. A differential equation that describes the travel of a zone along the column is composed. Then the band profile is calculated by the integration of the differential mass balance equation under proper initial and boundary conditions. Throughout this chapter, we assume that both the chemistry and the packing density of the stationary phase are radially homogeneous. Thus, the mobile and stationary phase concentrations as well as the flow velocities are radially uniform, and a one-dimensional mass balance equation can be considered. 

Column temperature is another variable that is useful in improving separations since one class of compounds may be affected more than another by change in column temperature. Compounds from two such classes which are not separable at one temperature may be completely resolved if the temperature is changed 20°C. This effect is also helpful in tentative identification of compounds since change of retention index with temperature for various classes of compounds has been the subject of many papers. A more detailed discussion of the effect of stationary phase concentration and temperature on separations has been published (6). 

Eq. (7.22) represents the mathematical description of flow in a chromatographic column for a single-component elution, where C is the mobile-phase concentration, e is the bed porosity, q is the stationary-phase concentration. Dap is the apparent diffusion coefficient, c is the length-independent variable and t is the time-independent variable. 

Co-elution of interfering compound from previous injection. —> Use sample cleanup adjust selectivity by changing mobile or stationary phase. Flush column with strong solvent at end of ran end gradient at higher solvent concentration. 

The matrices and Z2 are n + p) x 1 column vectors. They represent the perturbations caused by the injection of the sample to the compositions of the stationary and the mobile phases at eqiuhbrium, respectively. These perturbations are the changes of the concentrations of the n + p components of the chromatographic system. The matrix is an (n + p) x (n + p) square matrix, and its general element is dqj /dC . is a 1 x (n + p) row vector. Its elements are the n + p partial derivatives of the stationary phase concentration q. of the fcth component of the system (sample components and additives included), by respect to the mobile phase concentrations. All the matrix elements are derived by differentiation of the isotherm equations (Eq. 13.3). They are calculated for the initial composition of the mobile phase, before the perturbation is applied. There are four different blocks in the matrix jS (see Figure 13.2). They can be considered as two square matrices of dimensions nxn and p x p, respectively, and two rectangular matrices... 

This equation is valid everywhere in the column, and particularly at the coliurm exit, hence it can be used to describe the elution chromatogram. A perturbation Aqs of the stationary phase concentration of the additive takes place everywhere a perturbation of the mobile phase concentration of a sample component takes place. These perturbations migrate along the column at the constant component velocity Uz,i = h/(1 -I- Eq. 7.3). When a sample component perturbation... 

Phenol can be detected electrochemically by oxidation at a carbon paste electrode (Wehmeyer et al., 1983). A convenient means of determining a low concentration of phenol in a small volume of sample is by liquid chromatography with electrochemical detection (LCEC). A diagram of the LCEC system is shown in Fig. 2. The sample is injected by means of 20-pl sample loop into a 5-cm column slurry-packed with lO-pm Cjg stationary phase. The column serves to separate the peak for phenol from other assay constituents in order to achieve a better detection limit. The phenol is detected by oxidation in a thin-layer electrochemical cell with a carbon paste working electrode. 

Fig. 9-50. Trace analysis of sodium in excess of ammonium on a crown ether modified stationary phase. - Separator column lonPac CS15 (2-mm) column temperature 40°C eluant 5 mmol/L H2SO4 -acetonitrile (91 9 v/v) flow rate 0.3 mL/min detection suppressed conductivity injection volume lOGOpL solute concentrations 1 pg/L sodium (1), 10 mg/L ammonium (2) and calcium (3, not quantitated).

FIGURE 3.13 Effect of concentration of stationary phase and column temperature on sample resolution (methyl esters of fatty acids). (Reproduced from Reference 20 W. A. Supina, in Modern Practice of Gas Chromatography, 2nd ed., R. L. Grob, ed., copyright 1985, John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.)... 

A relationship between stationary phase concentration and column temperature is depicted in Figure 3.13. Decreasing colunm temperature increases time of analysis in order to have the same analysis time on a heavier loaded packing in an identical column at the same flowrate requires a higher column temperature. 

Substituting the exchange capacity (Cap.) of the column for the stationary phase concentration of the counterion [CS], and k for the ratio of the metal concentration in the stationary phase to the metal concentration in mobile phase [(M" ) mS ] an expression is... 

This somewhat lengthy experiment provides a thorough introduction to the use of GG for the analysis of trace-level environmental pollutants. Sediment samples are extracted by sonicating with 3 X 100-mL portions of 1 1 acetone hexane. The extracts are then filtered and concentrated before bringing to a final volume of 10 mL. Samples are analyzed with a capillary column using a stationary phase of 5% phenylmethyl silicone, a splitless injection, and an EGD detector. 

The total stationary-phase volume required to process a given feed stream is proportional to the inlet concentration and volume of the feed. For example, for a typical inlet concentration of protein of 10 g/L, in a 100 L volume of feed, a column volume of at least 100 L is needed for size-exclusion chromatography. In comparison, an ion-exchange column having an adsorption capacity of 50 g/L would only require 20 L of column volume for the same feed. 

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