Separation selectivity tuning

Columns of different polarities may be coupled in series under the conditions of dual column chromatography, two-dimensional chromatography, or comprehensive two-dimensional chromatography. In this part, only the separation selectivity tuning in dual column chromatography will be illustrated with some examples. 

The separation selectivity tuning in dual column chromatography may be illustrated by HPLC separation of the enantiomers of 2/-3,5-dinitrobenzoyl derivatives of some amino acids in two chiral P-cyclodextrin columns of opposite separation selectivity coupled in series. Fig. 3 shows... 

Figure 4.4 Flow diagram for choosing the appropriate neat ionic liquid or immobilized ionic liquid composition for a particular analyte separation. Note that the most important characteristics for choosing the appropriate stationary phase are separation selectivity and thermal stability. Both of these properties can be effectively tuned and optimized by controlling the cation and anion combination.

Further research on mixed IL stationary phases will allow for the chroma-tographer to tune the stationary phase composition to provide enhanced control over the separation selectivity and analyte elution order, particularly for complicated analyte mixtures. The development of models that correlate analyte retention with the IL composition will prove useful for multidimensional GC. Micellar GC utilizing IL solvents presents an exciting class of highly selective stationary phases. The development of CSPs will likely mature as more chiral ILs are synthesized and evaluated from the chiral pool. 

The protic organic modifier methanol is superior for selectivity tuning if acidic analytes (proton donors) are separated on stationary phases with proton acceptor capabilities (N-containing functional groups). 

If selectivity tuning of certain separations is possible by changing pH or changing temperature, the question arises what is the preferred way. It is obviously more convenient to change the temperature. This can be easily set at the instrument and does not require further mobile phase preparation work. Moreover, there is limited flexibility for pH changes with a given buffer type, especially if the buffer capacity has to be maintained. These considerations clearly promote the temperature as the first choice, and this optimization experiment can easily be automated. 

Thus, the separation selectivity of the chromatographic system can be tuned by any parameter, which influences the interaction of an analyte with stationary and mobile... 

The separation selectivity may be tuned by the modification of both enthalpic and entropic terms ... 

Separation selectivity in column chromatography can be tuned in (i) one column, and (ii) two or more columns coupled in series. 

For a given sample and separation mechanism, the separation selectivity of the chromatographic system depends on the nature of both mobile as well as stationary phase. Interactions of solute with mobile and stationary phases may include a mixture of non-polar dispersive and various polar forces. The terms polar and non-polar have been commonly used to describe a property of both the solute, as well as mobile and stationary phases. These terms, however, should not be confused with selectivity. The separation selectivity of a stationary phase can be changed discontinuously by selection of a proper column packing (by its polarity or other parameters), or tuned continuously, synthesizing tailor-made phases or mixing stationary phases differing in polarity. ... 

Separation selectivity, in principle, may be tuned by thermodynamic parameters (temperature and pressure) and/or by polarity of stationary and mobile phase. The separation selectivity for a given sample is usually optimized by a trial-and-error approach on a column chosen by chromatographer. 

The change of temperature under the isocratic, as well as programing, conditions is often used to tune separation selectivity in gas chromatographic separations. Temperature has not been very actively utilized in HPLC, mainly because of reported stability problems of the most commonly used stationary phases. However, more interest in the application of temperature for retention control has come nowadays because of the trend of miniaturization in chromatography and the availability of temperature-stable stationary phases.""... 

The separation selectivity of dual column HPLC, GC, and SFC coupled in series may be tuned by changing thermodynamic parameters and/or contribution of individual column polarities." " In analytical praxis, it is convenient to keep constant the entire column parameters... 

From Eqs. (12-15), it follows that the separation selectivity of a column series can, for example, be tuned by changing the mobile-phase flows in individual columns. 

Benicka, E. Krupcik, J. Lehotay, J. Sandra, P. Armstrong, D.W. Selectivity tuning in a HPLC multicomponent separation. J. Liquid Chromatogr. Relat. Technol. 2005, 28 (10), 1453-1471. 

The functionalities in those phases are the methyl, phenyl, cyano, trifluoro, and hydroxy-ether group, respectively. The similarity with HPLC phases is remarkable. Based on the above, Sandra et al. [22] proposed five basic phases for capillary GC. The formulas are given in Figure 12. For the majority of high-resolution separations, these stationary phases provide more than adequate performance. For some applications, the functionalities are combined on the same siloxane backbone, or columns are coupled (selectivity tuning). Functionalities can also be modified to provide specific interactions. Optical phases and liquid crystals complete the set of preferred CGC phases. These tailor-made phases are discussed in Section ] 1.3.4.3.3. 

Figure 50. Multidimensional CGC analysis of pepermint oil, two column system constant pressure - constant pressure A) Separation on MeSi B) Analysis on MeSi after heartcnt C) Analysis of heartcul fraction on column B with intermediate trapping D) Analysis of heartcul fraction on column B without intermediate trapping (selectivity tuning)...

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