Analyte between mobile and stationary

Theoretical plate In plate theory, the chromatographic column is viewed as a series of narrow layers, known as theoretical plates, within each of which equilibration of the analyte between mobile and stationary phases occurs. 

N column efficiency defined as the number of theoretical plates in the column, i.e., in Plate Theory N describes the number of effective equilibrations of the analyte between mobile and stationary phases N = 4.(Vj/AVj) =4.(tj/Ati) = 5.545.(tf/Atp, where AV and At are the chromatographic peak widths in terms of elution volume and elution time, respectively, and subscripts i and j refer to widths measured at the peak inflection points and at half peak height, respectively. 

Plate Theory is based upon a approximation of discrete sequential partitioning equilibria of analyte between mobile and stationary phases (Eigure 3.3), so it is convenient to define a partition coefficient for analyte A as ... 

Almost 30 years ago, Colin and Guiochon mentioned in an excellent review [10] that there are essentially three possible ways to model separation mechanism. The first one is analyte partitioning between mobile and stationary phases, the second one is the adsorption of the analyte on the surface of nonpolar adsorbent, and the third one has been suggested by Knox and Pryde [11], where they assume the preferential adsorption of the organic mobile-phase modifier on the adsorbent surface followed by the analyte partitioning into this adsorbed layer. 

As a first example of an applicable model traditional partitioning mechanism will be considered. In this mechanism the analyte is distributed between the mobile and stationary phases, and phenomenological description of this process is given in Section 2.1. The Vm and Vs are the volume of the mobile and the volume of the stationary phases in the column, respectively. Instant equilibrium of the analyte distribution between mobile and stationary phases is assumed. 

This equation has been derived for the model of the analyte distribution between mobile and stationary phases and is the same as expression (2-30) in Section 2.6. To be able to use this equation, we need to dehne (or independently determine) the volumes of these phases. The question of the determination or definition of the volume of stationary phase is the subject of significant controversy in scientihc literature, especially as it is related to the reversed-phase HPLC process [19]. 

Since the late 1950s, analytical chromatographic reactors have been applied to studies of reactions (Coca et al., 1993), determination of reaction kinetics (Bassett and Habgood, 1960), characterization of stationary phases (Jeng and Langer, 1989), or examination of interactions between mobile and stationary phase (Coca et al., 1989). 

Since there is no interaction between the analyte and the packing, the solute distribution between mobile and stationary phase is controlled by entropy alone (3) ... 

Figure 4.20 Plot of the logarithm of an analyte s chromatographic distribution constant between mobile and stationary phases and column temperature for two different chemical compounds.

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.

Every chromatographic process is controlled by the equilibrium distribution of the solute between the mobile and stationary phases. The retention volume V describing the volume of mobile phase that is required to elute the analyte from the column, is given by Equation 17.4 ... 

Chromatography Analytical technique to separate chemical species by continuous partitioning between a mobile and stationary phase. 

In recent years, for analytical purposes the direct approach has become the most popular. Therefore, only this approach will be discussed in the next sections. With the direct approach, the enantiomers are placed in a chiral environment, since only chiral molecules can distinguish between enantiomers. The separation of the enantiomers is based on the complex formation of labile diastereoisomers between the enantiomers and a chiral auxiliary, the so-called chiral selector. The separation can only be accomplished if the complexes possess different stability constants. The chiral selectors can be either chiral molecules that are bound to the chromatographic sorbent and thus form a CSP, or chiral molecules that are added to the mobile phase, called chiral mobile phase additives (CMPA). The combination of several chiral selectors in the mobile phase, and of chiral mobile and stationary phases is also feasible. 

For liquid chromatography separation to take place, samples or sample extracts must be miscible with the mobile phase. The separation is possible due to specific interactions between the analyte molecules and the mobile and stationary phases. To decrease the time of analysis, gradient elution is applied, with the polarity of the mobile phase solvent increasing through analysis time. Up to four solvents of different polarities may be mixed to achieve the fastest and the most complete separation. 

In the RP CEC of neutral species selectivity is provided primarily by differences in the partition of the analytes between the hydrophobic stationary phase and the more polar mobile phase. There are also contributions from interactions with the silica support, the major one being polar interactions with ionised silanol groups. This is identical to the process in LC, albeit with the advantages of higher efficiencies in CEC resulting from the plug-flow profile. Additional selectivity is introduced in the case of charged species in CEC due to differences in the analytes electromobilities. 

A special application of LLC is ion pair partition chromatography. In this procedure, the ionic form of the solute (analyte) is paired with an appropriate counter ion of decreased polarity, e.g. tetra-tertiary-butyl amine. This ion pair is then partitioned between selected mobile and stationary phases to achieve the desired separation. In practice, ion pair chromatography is commonly conducted by utilizing a mobile phase comprised of a miscible aqueous/organic mixture containing a relatively high concentration of counter ion. The technique is applicable to analysis of many types of ionic compounds (10). 

The and constants are related, respectively, to the mass transfer resistance between the stationary and mobile phase as weU as between the mobile and stationary phase, i.e., to the time needed for the analyte molecules to equilibrate between the mobile and stationary phases. If the resistance is high and the time scale of the... 

In the above definition the presence of two different phases is stated and consequently there is an interface between them. One of these phases provides the analyte transport and is usually referred to as the mobile phase, and the other phase is immobile and is typically referred to as the stationary phase. A mixture of components, usually called analytes, are dispersed in the mobile phase at the molecular level allowing for their uniform transport and interactions with the mobile and stationary phases. 

As a very rough first approximation the chromatographic retention process could be described on the basis of simple single equilibria of the analyte distribution between the mobile and stationary phases. The equilibrium constant of this process is proportional to the analyte retention factor... 

Partitioning is the first and probably the simplest model of the retention mechanism. It assumes the existence of two different phases (mobile and stationary) and instant equilibrium of the analyte partitioning between these phases. Simple phenomenological interpretation of the dynamic partitioning process was also introduced at about the same time. Probably, the most consistent and understandable description of this theory is given by C. Cramers, A. Keulemans, and H. McNair in 1961 in their chapter Techniques of Gas Chromatography [12]. The analyte partition coefficient is defined as... 

Later these mnemonic derivations appear in all textbooks dealing with gas and liquid chromatography. Equation (2-30) describes the retention of the analyte, which undergoes only one process of ideal partitioning between well-defined mobile and stationary phases. 

The total amount of the analyte in the column cross section dx is distributed between the mobile and stationary phases and could be written in the following form ... 

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