Column Efficiency in

The discussion on band broadening in Section 2603 is generally applicable to I.C. Here, we illustrate the important effect of stationary-phase particle size and describe two additional sources of zone spreading that are sometimes of considerable importance in LC 

FIGURE 28-2 Effect of particle size of packing and flow rate on plate height II In LC. Column dimensions 30 cm x 2,4 mm. Solute Al./V -diethyi-p-aminoazobenzene. Mobile phase mixture of hexane, methylene chloride, isopropyl alcohol. (From R. E. Majors, J. Chromatogr. 

It has been shown that the contribution of cxtracol-umn effects to the total pl.iie height is given by  

Lxtracolumn broadening can become quite. serious when small-bore columns are used. I lerc, the radius of the extracolumn components should be reduced to 

010 inch or less, and the length of extraculumn tubing made as small as feasible to minimize this source of broadening, 

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In their original theoretical model of chromatography, Martin and Synge treated the chromatographic column as though it consists of discrete sections at which partitioning of the solute between the stationary and mobile phases occurs. They called each section a theoretical plate and defined column efficiency in terms of the number of theoretical plates, N, or the height of a theoretical plate, H where... 

Equation (22) allows the maximum sample volume that can be used without seriously denigrating the performance of the column to be calculated from the retention volume of the solute and the column efficiency. In any separation, there will be one pair of solutes that are eluted closest together (which, as will be seen in Part 3 of this book, is defined as the critical pair) and it is the retention volume of the first of these that is usually employed in equation (22) to calculate the maximum acceptable sample volume. 

FIGURE 9.6 Theoretical ( ) and normally found ( ) column efficiencies in SEC columns with packings of different particle size. 

Improvement of column efficiency in terms of the number of theoretical plates realized by increasing column length often yields marginal increases in resolution, with a corresponding increase of analysis time to unacceptable levels. This... 

Warren, Jr., F. V. and Bidlingmeyer, B. A., Influence of temperature on column efficiency in reversed phase liquid chromatography, Anal. Chem., 60, 2821, 1988. 

As in interactive modes of chromatography, reduction in particle diameter reduces mass transfer effects and improves column efficiency in SEC. Column packings with particle diameters of 10 to 12 pm are available for less demanding applications, whereas SEC packings with particle diameters of 4 to 5 pm can be used for applications requiring higher resolution. 

Mechanical and chemical stability of novel stationary phases are basic requirements concerning their application. A lack in stability generally causes a loss in resolution and thus reduces column efficiency. In addition, the reproducibility of retention times, being important for qualitative analysis, may be affected. Evaluation of the mechanical stability of polymeric stationary phases is usually accomplished by the determination of the pressure drop across the column, when employing solvents of different polarity within a wide range of flow rates. A stationary phase can be considered as mechanically stable if a linear relationship between applied flow rate and resulting back pressure is obtained. 

Indicate which of the following parameters can decrease or increase column efficiency in liquid chromatography. 

Column efficiency in theoretical plates. 2/ Analysis time. 

Lipkowski, J., Pawlowska, M., Sybilska, D., Experimental-study of selectivity and column efficiency in clathrate chromatography using Werner complexes as clathrate host components. 1. Relationship between selectivity of Ni(Ncs)2(4-methylpyridine)4. Guest clathrate sorbents and composition of the mobile phase. J Chromatogr. 1979,176,43-53. 

On the other hand, the lack of internal pore structure with micropellicular sorbents is of distinct advantage in the analytical HPLC of biological macromolecules because undesirable steric effects can significantly reduce the efficiency of columns packed with porous sorbents and also result in poor recovery. Furthermore, the micropellicular stationary phases which have a solid, fluid-impervious core, are generally more stable at elevated temperature than conventional porous supports. At elevated column temperature the viscosity of the mobile phase decreases with concomitant increase in solute diffusivity and improvement of sorption kinetics. From these considerations, it follows that columns packed with micropellicular stationary phases offer the possibility of significant improvements in the speed and column efficiency in the analysis of proteins, peptides and other biopolymers over those obtained with conventional porous stationary phases. In this paper, we describe selected examples for the use of micropellicular reversed phase... 

Fields reported that continuous silica xerogels prepared from potassium silicate solutions could be used as highly permeable support media, and exhibit reasonable chromatographic efficiency in HPLC [23]. Minakuchi et al. reported the preparation and evaluation of continuous porous-silica columns that provide a much higher column efficiency in HPLC than do conventional columns packed with particles [13-16,18], The monolithic columns prepared in a capillary can also be used in CEC. 

High Column Efficiency in Gas Liquid Chromatography at Inlet Pressures to 2500 psi, M. N. Myers and J. C. Giddings, Anal. Chem., 37, 1453 (1965). 

Fair et al. (203) investigated scaleup of Oldershaw column data to commercial columns. Over the region of practical interest (50 to 35 percent of flood), the commercial point efficiency was either equal to or slightly higher than the Oldershaw column efficiency. Fair et al. concluded that the Oldershaw column efficiency la essentially a point efficiency measurement, and recommend this point efficiency for the design of commercial columns. A mixing model can be used to convert the point efficiency to overall column efficiency. This will enhance the commercial column efficiency. In a later paper (145), Chan and Fair include an additional correction for weir height this correction will also enhance the commercial column efficiency. A conservative approach proposed by Fair et al. is to apply the Oldershaw cal urn n effi-... 

Under the proper conditions, column efficiency in MECC is outstanding. In one separation of purine compounds we obtained over 600,000 plates/m for theophylline. However, the effects of parameters such as column dimensions, applied voltage, and the concentration of the buffer and surfactant on efficiency can be very dramatic. A brief discussion of how these parameters influence efficiency follows. 

The process of band broadening (Figure 2.1) is measured by the column efficiency or the number of theoretical plates N, equation (2.24)), which is equal to the square of the ratio of the retention time to the standard deviation of the peak. In theory, the value of N for packed columns has only a small dependency on k and may be considered to be a constant for a particular column. Column efficiency in open-tubular systems decreases markedly with increased retention. For this reason open-tubular liquid chromatography systems must be operated at relatively low kf values (see section 2.5.S.2). 

TaMe 2.1 Relationship between flow rate (Fy), linear velocity (u), analysis time and column efficiency in liquid chromatography using a conventional packed column (5 pm, 100 x 5 mm i.d.)... 

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