Stationary Phase Sorption-Desorption

Once it adheres to (e.g., partitions into) the stationary phase, a solute molecule takes a step backward with respect to the onmoving zone. When desorbed, it moves faster than the partially retarded zone, thus taking a step forward. Sorption and desorption occur randomly, making this process resemble a random walk. 

Consider a sorbed molecule. The desorption of the molecule takes a finite time, as do all kinetic processes. The mean lifetime before desorption occurs may be denoted by td. This means that on average a sorbed molecule will stay with the stationary phase a time td9 but in individual cases the time 

During the time td, the zone center moves forward at velocity Rv and thus travels a distance of Rvtd. Since the zone center is the frame of reference, this means that a molecule sorbed for a time td is taking a step backwards of length 

The total number of sorptions plus desorptions may be taken to approximate the number of steps, n, in the random walk. Since each sorption process is followed by a desorption, the two occur in equal numbers and the total number of steps may be estimated as twice the number of desorptions, a quantity evaluated below. 

Any given zone requires a time L/Rv to migrate distance L to the end of the column. This time is the retention time tr (see Eq. 10.2). Molecules spend a fraction 1 — R of tr thus a time of 

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Hsm is due to the stagnant mobile phase in particle pores, Hs is due to the stationary-phase sorption-desorption. 

From a quantitative point of view vis-a-vis the chromatographic operation, if the plate height is controlled hy pore diffusion (in the stagnant mobile phase, equation (7.1.107h)) and /or stationary-phase sorption-desorption, the plate height H oc d v. Since N = [L/H), for a fixed L,... 

The mixed mode of sorption of the dye l,l -dioctadecyl-3,3,3, 3 -tetramethylindocar-bocyanine pechlorate (Dil) at the interface of an ODS stationary phase and ACN-water mobile phase was studied by single-molecule resolution and fluorescence imaging techniques. The measurements indicated that minimally four types of adsorption sites are present on the surface of the ODS stationary phase. The desorption times of the dye are different at the different adsorption sites resulting in a deformed peak shape [152],... 

Chromatography is essentially a physical method of separation in trtiich the components to be separated are distributed between two phases one of which is stationary (stationeury phase) while the other (the mobile phase) percolates through it in a definite direction. The chroaatographic process occurs as a result of repeated sorption/desorption acts during the movement of the sample components along the stationary bed, and the separation is due to differences in the distribution constants of the Individual sample components. 

If the rate constants for the sorption-desorption processes are small equilibrium between phases need not be achieved instantaneously. This effect is often called resistance-to-mass transfer, and thus transport of solute from one phase to another can be assumed diffusional in nature. As the solute migrates through the column it is sorbed from the mobile phase into the stationary phase. Flow is through the void volume of the solid particles with the result that the solute molecules diffuse through the interstices to reach surface of stationary phase. Likewise, the solute has to diffuse from the interior of the stationary phase to get back into the mobile phase. 

Utilization of traps with a suitable solid sorbent Purgeand Trap - PT), (Closed Loop Strippinganalysis -CLSA), - or a stationary phase on a support - packed PDMS trap -sorption tubes, denuders, passive dosimeters - combined with thermal desorption -TD at the release stage. 

Generally, SPE consists of four steps (Figure 2.42) column preparation, or prewash, sample loading (retention or sorption), column postwash, and sample desorption (elution or desorption), although some of the recent advances in sorbent technology reduce or eliminate column preparation procedures. The prewash step is used to condition the stationary phase if necessary, and the optional column postwash is used to remove undesirable contaminants. Usually, the compounds of interest are retained on the sorbent while interferences are washed away. Analytes are recovered via an elution solvent. 

Sorption-desorption. An important irregularity in molecular migration is caused by sorption and desorption. Each time a molecule is sorbed by the stationary phase, its downstream motion ceases. When desorbed, it proceeds again. The processes of sorption and desorption occur randomly, thus making this stop-and-go sequence an extremely erratic one. 

When a volatile compound is introduced into the carrier gas and carried into the column, it is partitioned between the gas and stationary phases by a dynamic countercurrent distribution process. The compound is carried down the column by the carrier gas, retarded to a greater or lesser extent by sorption and desorption in the stationary phase. The elution of the compound is characterized by the partition ratio, k, a dimensionless quantity also called the capacity factor. It is equivalent to the ratio of the time required for the compound to flow through the column (the retention time) to the retention time of a nonretarded compound. The value of the capacity factor depends on the chemical nature of the compound the nature, amount, and surface area of the liquid phase and the column temperature. Under a specified set of experimental conditions, a characteristic capacity factor exists for every compound. Separation by gas chromatography occurs only if the compounds concerned have different capacity factors. 

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. 

The conventional inverse gas chromatography (IGC) is based on equations that assume equilibrium is established during the course o the chromatograph. Consequently, those stationary phases that exhibit marked hysteresis in sorption/desorption give IGC sorption data at considerable variance with long-term gravimetric methods. A modified frontal procedure was developed that avoids the assumption of equilibrium to enable studies of interaction kinetics of gas phase components with a stationary phase, such as a biopolymer, having entropic as well as enthalpic relations affected by concentration shifts and time dependent parameters. 

Methods for determining sorption isotherms by gas chromatography have been published by various authorsQ,-, ). The methods used have been elution and frontal chromatography. The first combines sorption and desorption so that any hysteresis in the equilibrium transport from gas to stationary phase and back to the gas phase can produce corresponding errors. The Kiselev-Yashin equation, as shown in Figure 1,... 

The evaluation of the RPLC chromatograms of nonionizable analytes shows that the number of sorption events varies between n = 13000 and 20000 on a 150 X 3.9-mm column and that it is not affected strongly by the retention time of the analytes [107]. Fly-times in the mobile phase between a desorption and the subsequent adsorption vary roughly between Tm = 3 to 5 ms. During that fly-time, the mobile phase travels a distance that is 1.5 to 2.3-times the particle diameter. The sojourn time in the stationary phase, however, strongly correlates with the retention of the analytes, so the retention of analytes and the selectivity of their separation are mainly due to the variations of Tg, between 8.4 ms (for k = 1.75) and 47 ms (for fc = 12.7). 

In equation (6) is the width at the base of the peak, measiued in the same units as tj.. The theoretical plate model assumes the column to be made of a series of plates. The distribution of the analyte between the mobile and stationary phase occurs at each plate. Therefore, the higher the number of plates, the better the separation since more sorption-desorption cycles occur. Coliunn efficiency can also be expressed in terms of Height Equivalent to a Theoretical Plate value (HETP or H-value) ... 

The stationary phase is a solid on which the sample components are adsorbed. The mobile phase may be a liquid liquid—solid chromatography) or a gas gas-solid chromatography) the components distribute between the two phases through a combination of sorption and desorption processes. Thin-layer chromatography (TLC) is a special example of adsorption chromatography in which the stationary phase is a plane, in the form of a solid supported on an inert plate, and the mobile phase is a liquid. 

This involves partitioning of molecules between the surface of a solid stationary phase and a liquid mobile phase. The d5mamic equilibrium of solutes as they switch between the stationary and mobile phases (the processes of sorption and desorption, respectively) is specific for each molecule and is affected by competition that exists between solutes and solvent for sites on the stationary phase. This is a purely physical process involving the formation of no... 

Figure 21.1 demonstrates the chromatographic process. A small volume of sample solution is injected at the column inlet (Fig. 21.1 A). The mobile solvent phase moves the sample through the column packing (Fig. 21.IB). The individual components undergo sorption and desorption on the packing, thereby slowing their motion in varying amounts depending on their affinity for the packing. Each component X is distributed between the stationary phase (s) and the mobile phase (m) as it passes down the column. According to...

Let us consider that the analyte is composed of one compound. The analyte interacts in a specific way with both the stationary phase (s) and the mobile phase (m). The interactions are usually weak (solvation, adsorption, etc.) without formation of chemical bonds. An electrostatic interaction occurs in specific cases only. According to its structure, an analyte X interacts better with the stationary phase by sorption or mobile phase by desorption. Equilibrium processes between (1) the analyte and stationary phase and (2) the analyte and mobile phase take place. These processes are represented in a simpler way by a single equilibrium process ... 

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