Chromatographic selectivity enhancing

Yan, C. and Martire, D.E., Molecular theory of chromatographic selectivity enhancement for blocklike solutes in anisotropic stationary phases and its application. Anal Chem., 64, 1246, 1992. 

Selectivity is dependent on the nature of the stationary phase (i.e., CIS, C8, phenyl, cyano, etc.) and the mobile phase composition. The selectivity effects of the mobile phase can be skillfully exploited by experienced chromatographers to enhance separation of key analytes in the sample. 

Uf course, the enhancement of chromatographic selectivity by secondary chemical equilibria is neither new nor confined to reversed-phase systems. Most widespread probably has been the exploitation of protonic equilibria by appropriately ati usting the pH of the eluent so that the degree of ionization of the eluite is altered. Generally the ionized and neutral forms of an eluite are retained differently (2( 7. 208). Formation of metal complexes of certain eluites has also been utilized for modulating retention behavior for higher selectivity. 

Historically, alumina used to be one of the standard adsorbents for LSC. Snyder (ref. [350], chapter 11) has compared the chromatographic selectivity of silica and alumina surfaces extensively. Alumina may offer some advantages over silica, especially for separations that can be enhanced at high pH values. In recent years therefore, there has been a revival of interest in alumina and its applications in LC [364]. 

The LC methods discussed before were based mainly on physico-chemical interactions between the solute on the one hand and the two chromatographic phases on the other. Although we have seen that in RPLC the degree of ionization of weakly acidic or basic solutes may be a major factor in the control of retention and selectivity, the ionic species themselves were not exploited purposefully to realize or enhance the separation. In fact, in a typical RPLC system all fully ionized solutes will show little retention and therefore little resolution can be achieved between different ions. The methods described in this section make positive use of the ionic character of solutes to create a chromatographically selective system. 

Lor a particular analytical separation, each biosolute will have an optimal k] value for maximum resolution with a designated column, flow rate, and mobile phase composition. Similar criteria apply in preparative (overload) chromatography with multicomponent mixtures, where resolution is similarly enhanced following optimization of chromatographic selectivity and zone bandwidth. The conventional approach to process purification with low molecular weight solutes has frequently been based on linear scale-up of the performance of an analytical column system. In these cases, high-resolution separations can be achieved often without the burden of conformational or... 

Variation of the mobile-phase pH is a powerful parameter to enhance the chromatographic selectivity and retention for mixtures of basic, acidic, and neutral compounds. Figure 4-22 shows that for neutral analytes (benzamide and llavone) the mobile-phase pH has no effect on the chromatographic retention [57]. However, for the organic acids (hydroxyisophthaUc acid and feno-profen, 4.5) at pH 2, which is at least 2 pH units below the acid analyte pKa values, maximum retention is obtained, while at pH 7 and 12 there is a... 

Chromatographic processes eliminate high temperatures and even allow the complete deterpenation of citms oils. Additionally they allow the enrichment of non-volatile compounds. Selectively enhanced fractions of coumarins and tocopherols, natural flavonoids and carotenoids are well suited for usage in special applications which feature excellent stability and capture the desired aroma profile. 

Selectivity enhancement for chiral analysis can be done by derivatization of racemic mixtures with a chiral agent. The resulting diastereomers can then be separated using conventional chromatographic methods (26) such as reversed-phase HPLC or GC, obviating the need to use chromatographic systems with an expensive chiral stationary phase. 

Lorenzelli, L., Benvenuto, A., Adami, A. et al. (2005) Development of a gas chromatography silicon-based microsystem in clinical diagnostics. Biosens Bioelectron, 20 (10), 1968-1976. Zampolli, S., Ehni, I., Stiirmann J. et al. (2005) Selectivity enhancement of metal oxide gas sensors using a micromachined gas chromatographic column. Sens Actual B, 105 (2), 400-406. Bessoth, F.G., Naji, O.P., Eijkel, J.C.T. and Manz, A. (2002) Towards an on-chip gas chromatograph the development of a gas injector and a dc plasma emission detector. J Anal Atom Spectrom, 17 (8), 794-799. 

In si)me instances, it is useful to convert the components of a sample to a derivative before, or sometimes after, chromatographic separation. Such treatment may be desirable (1) to reduce the polarity of the species so that partition rather than adsorption or ion-exchange columns can be used (2) to increase (he detector response, and thus sensitivity, for all of the sample components and (3) to selectively enhance the detector response to certain components of the sample. 

Zampolli S., Elmi I., Sturmann J., Nicoletti S., Dori L., and Cardinal G. C., Selectivity enhancement of metal oxide gas sensors using a micromachined gas chromatographic column, Sens. Actuators B, 105(2), 400, 2005. 

Anigbogu et al. [158] studied the effects of methanol, r-butyl alcohol (TBA), and cyclopentanol (CP) on anthracene and pyrene retention on a C)8 column (A = 255 nm) using 50% to 70% methanol in water containing 3 mM ) -cyclodextrin and 1% TBA or CP. On the basis of retention effects, the authors speculate that TBA and CP assist in the formation of a cyclodextrin/pyrene complex and conclude that TBA and CP may be effectively used as mobile phase modifiers in these fiised-ring systems. Schuette and Warner [159] conducted a similar study on the effects of N pentanol on or y-cyclodextrin/PAH complexes. A solution of 0.1 M 1-pentanol with 5mM y- or j -cyclodextrin increased the fluorescence emission intensity markedly in Ihe 430-490 nm range. Chromatographic selectivity and efficiency were enhanced and the detection limits were lowered by nearly an order of magnitude. 

In the course of mixture separation, the composition and properties of both mobile phase (MP) and stationary phase (SP) are purposefully altered by means of introduction of some active components into the MP, which are absorbed by it and then sorbed by the SP (e.g. on a silica gel layer). This procedure enables a new principle of control over chromatographic process to be implemented, which enhances the selectivity of separation. As a possible way of controlling the chromatographic system s properties in TLC, the pH of the mobile phase and sorbent surface may be changed by means of partial air replacement by ammonia (a basic gaseous component) or carbon dioxide (an acidic one). 

Chlorophenoxy acids are relatively polar pesticides which are usually determined by LC because volatile derivatives have to be prepared for GC analysis. This group of herbicides can be detected by multiresidue methods combined with automated procedures for sample clean-up, although selectivity and sensitivity can be enhanced by coupled-column chromatographic techniques (52). The experimental conditions for Such analyses are shown in Table 13.1. 

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