Efficiency of Chromatographic Separations

One of the key parameters that characterizes a preparative chromatographic system is the plate number N, which is also designated as column efficiency. 

The plate number as well as the corresponding height of an equivalent theoretical plate (HETP) is well known in chromatography and chemical engineering as a common measure to quantify mass transfer effects and to capture deviations from ideal behavior within pacdced beds (Equation 2.30)  

The plate height lumps together the contributions of fluid dynamic nonidealities (e.g., axial dispersion), mass transfer resistances, and finite adsorption and desorption rates, which all contribute to undesired band broadening. It can be defined as the rate of the local gradient of the width of a Gaussian peak (Equation 2.31)  

For uniformly packed columns and incompressible eluents this expression can be integrated, providing Equation 2.32  

In practice, peak profiles are not measured over axial coordinates but detected at the column outlets over time. Therefore, a corresponding standard deviation of the time-dependent concentration profile Ot can be used (Equation 2.33)  

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Some comparisons of the efficiencies of chromatographic separations are facilitated by defining so-called reduced parameters, which correct for... 

Analytical reaction GC is characterized by specific experimental techniques, a particular, area of application and distinctive design features of the instruments used. It should be emphasized that when chemical methods are used in GC, the efficiency of chromatographic separation, sensitivity and other characteristics of the detector remain virtually the same. However, as a result of chemical reactions, or transformations of the sample mixture, newly formed compounds are subjected to determination or separation, and the separation factors and detection sensitivity can be varied in a controlled manner. It should also be noted that the chemical transformation method is applicable in other fields of analytical chemistry (e.g., spectroscopy, electrochemistry). 

The efficiency of chromatographic separations can be determined by calculating the plate heights. For the SDC system, the theoretical plate height (HETP) can be calculated using Eq. 3 ... 

A chromatographic column provides a location for physically retaining the stationary phase. The column s construction also influences the amount of sample that can be handled, the efficiency of the separation, the number of analytes that can be easily separated, and the amount of time required for the separation. Both packed and capillary columns are used in gas chromatography. 

Successful application of chromatographic techniques relies on resolution, or the resolving power of the particular technique used. Resolution is defined by the relation of selectivity and efficiency of the chromatographic gel media (i). Selectivity is a function of the mode of separation of the gel (i.e., gel filtration, ion exchange, etc.) and efficiency is a function of the support matrix (Le., particle shape, size distribution, mechanical stability, density of interactive chemical groups, etc.). Each of the various modes of chromatographic separation have unique advantages that dictate where and when in a purification process these techniques should be used. 

For the analysis of semivolatile organics, a column performance test for base/neutral and acid fractions must be performed to test the efficiency of chromatographic column for separation of the analytes. For the base/neutral fraction, inject 100 ng of benzidine and determine the benzidine tailing factor which must be less than three. Similarly, for acid fraction, inject 50 ng of pentachlorophenol and calculate its tailing factor which must be less than five. 

The extent of separation can be quantified in terms of the resolution obtained between two consecutive chromatographic peaks. This resolution can be expressed in terms of three elemental characteristics of chromatographic separation retention, selectivity and efficiency. The influence of each of these three factors on resolution will be discussed. 

Determine that the system is functioning properly inject into the chromatograph a suitable number of samples of the standard to ensure that the resolution factor, R, defining the efficiency of the separation between methyl stearate and methyl oleate is 0.9 or greater. Calculate R by the equation... 

As shown in Fig. 9.2.4, most chromatographic analyses involve the separation of more than one component. The efficiency of the separation is given by the separation factor, a, which is a measure of the relative position of each eluted component. 

In common with other application areas of chromatographic separation, a considerable amount of effort has been expended recently on the development of different elution conditions and types of stationary phases for peptide separations in attempts to maximize column selectivities without adversely affecting column efficiences. Peptide retention will invariably be mediated by the participation of electrostatic, hydrogen bonding, and hydrophobic interactions in the distribution phenomenon. The nature of the predominant distribution mechanism will be dependent on the physical and chemical characteristics of the stationary phase as well as the nature of the molecular forces which hold the solute molecules within the mobile and stationary zones. The retention of the solute in all HPLC modes can be described by the equation... 

A computational procedure for the modeling of chromatographic separation of racemic Co(acac)3 into enantiomers on a dinitrobis(arginine)cobalt(III) complex as a chiral selector was described. Predicted elution order calculated from the differences in total energy of interaction for A and A selectands were found to be in agreement with the experimental results. The predictive power of the method and its possible practical applications in designing efficient chiral stationary phases was demonstrated . ... 

Fig. 1.18. Examples of chromatographic separation of a ihree-componcnt sample mixture and possible ways lo improve the separation during HPLC melhtxl development. tA) Satisfactory separation. (B) Unsatisfactory separation — ttw low retention. The elution strength of the mobile phase should be decreased. (C) Good resolution, but too long time of separation. The elution strength of the mobile phase should be increased. (D) Unsatisfactory separation — too low column efficiency. The plate number should be increased by using finer packing panicles or a longer column. (E) Unsatisfactory separation — gixxl retention and column efficiency, but too low separation selectivity. The components of the mobile phase can be changed, a ternary or a quaternary mobile phase, selective mobile phase additives, or another type of the stationary phase can be used.

Fig. 2 Examples of chromatographic separation of a three- component sample. (A) Satisfactory separation. (B) Unsatisfac-g tory separation, too low retention. (C) Good resolution, hut tor long time of separation. (D) Unsatisfactory separation, too lowg column efficiency. (E) Unsatisfactory separation, good retentioi and column efficiency, hut too low separation selectivity. >...

By chemical modification of the silica surface it has become possible to design new highly-selective adsorbents and catalysts, active polymer fillers, efficient thickeners of dispersive media. Interest in the modified silicas, in particular, in the activated matrices based on functional organosilicas has quickened in the past few years as a result of the favorable prospects for their application for various kinds of chromatographic separation, preparation of grafted metal complex catalysts, immobilized enzymes and other biologically active compounds [1]. 

Fast exchange processes and thus, a high chromatographic efficiency of the separator column are ensured by the small size of the latex beads. 

From the usual definition of equilibrium-staged separations, the plate height is calculated as // = L/N. The efficiency of chromatographic columns increases as the number of plates increases and as the plate height decreases. Both of these values are used widely in the literature as measures of column performance. 

D5. de Ligny, C. L., and Remijnse, A. G., Efficiency of chromatographic procedures. IV. The efficiency of chromatography on various kinds of Whatman paper and of thin-layer chromatography on various kinds of cellulose powder for the separation of amino acids. Rec. Trav. Chim. Pays-Bas 86, 421-435 (1967). 

A few of the many types of chelating resins that have been synthesized are listed in Table 7.8. Resins containing the iminodiacetic acid (IDA) functional group have received particular attention. The material known as Chelex 100 has been available for many years but is not very efficient for chromatographic separation of metal ions. More modern IDA resins are more satisfactory. The IDA group forms chelates with a considerable number of metal ions and also provides good selectivity for metal ions that are not complexed. However, some problems can occur. 

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