Columns comparison

These values are practically temperature independant, and they are very close to those found for the Apiezon L column. Comparison with the values of a series of alkybenzenes shows that the 5-position of thiazole possesses behavior analogous to that of a benzenic position in gas-liquid chromatography. 

TABLE 22.1 Column Comparison among Four Molecular Weight Separation Techniques"... 

The conceptual idea of a theoretical plate can be used in SEC to measure column efficiency and to compare the performance of packed coluians. For column comparisons it is usually measured with small molecules, such as toluene, acetone or benzyl alcohol, which can explore all of the pores of the packing (K jc - 1). Plate counts measured in this way produce HETP values lower than the actual values measured with monodisperse polymers and proteins. The plate count in this case can be expressed by equation (4.40)... 

It is important, first, to realize that efficiency is not a function solely of the column. Bad extracolumn parameters, such as detector cell volume or tubing diameters, can make the best column in the world look terrible. Second, efficiency measurements are very poor ways of comparing or purchasing columns unless all other parameters are constant. Many columns are bought and sold because they have a higher plate count than someone else s column. The efficiency calculations could have been made with different equations, on different compounds, on different machines, at different flow rates, all of which will have a profound effect on efficiency. The only valid use of plate counts that I have found is in column comparisons where all other variables are equal, or in following column aging over a period of days or months. 

Figure 5. Axial dispersion in the liquid phase of a slurry column. Comparison of data of Kara et al [70] with correlations.

Figure 16.17 Simultaneous breakthrough profiles of 2-butanol and tert-amyl alcohol on a carbon column. Comparison of experimental (symbols) and calculated (hnes) profiles, (a) Column length, 82 cm Cq,i = Cq = 1 mg/mL. Calculated profiles with solid lines, kf2 = 1-95 X 10 cm/s dashed lines, fcy2 = 1-68 x 10 cm/s. (b) Column length 41 cm, Cq,i = Cq,2 = 1 mg/mL. Calculated profiles with solid lines. Dpi = 7.40 x 10 cm /s dotted lines. Dpi = 7.77 x 10 cm /s dashed lines. Dpi = 7.03 x 10 cm /s. Reproduced with permission from A.l. Liapis and D.W.T. Rippin, Chem. Eng. Sci., 33 (1978) 593 (Figs. 2 and 5).

GRAMS OF CARBON DIOXIDE PASSED THROUGH COLUMN Comparison of calculated and experimental desorp-... 

Figure 6.24 Concentration profile of a membrane column, comparison between theory and experiment.13...

Zhang, Z., Mazzotti, M., and Morbidelli, M. (2004a) Continuous chromatographic processes with a small number of columns comparison of simulated moving bed with Varicol, PowerFeed and ModiCon. Korean J. Chem. Eng., 21, 454 64. 

Methods for predicting efficiency also parallel those for tray columns comparison against a similar installation, use of empirical methods, direct scaleup from laboratory or pilot plant, and use of theoretically derived models. Approaches by vendors of packing usually center on comparisons with similar installations (the so-called vendor experience ) and empirical approximations. Direct scaleup from small column studies is difficult with packed columns because of the unknown effects of geometrical factors and the variations of liquid distribution that are required for practical reasons. Theoretical or semitheoretical models are difficult to validate because of the flow effects on interfacial area. It may be concluded that there is no veiy good way to predict packed column efficiency, at least for the random type packings. 

Baur R., Taylor R. and Krishna R. (2003). Bifurcation analysis for TAME synthesis in a reactive distillation column Comparison of pseudo-homogeneous and heterogeneous reaction kinetics models. 

Column Comparison, Comparison Criteria Similarity of Selectivities... 

Cramers, C.A. Vermeer, E.A. Direct sample introduction of high boiling compounds onto glass capillary columns. Comparison of manual and automatic sampling. Chromatographia 1975, 8 (9), 479-481. 

A discussion of column evaluation may seem inappropriate to biochemists who have traditionally packed gel SEC columns themselves and run them without testing. Such columns would eventually be unpacked when the performance degraded, and the packing material would be cleaned or discarded. The quality of the column was not significant in relationship to the time necessary to evaluate it. HPSEC columns are usually more expensive than their classical counterparts, and consequently, users are concerned about quality assurance, column longevity, sample recovery, and extension of column lifetime. Because there are major differences in columns from different manufacturers, users must have a qualitative basis for column comparison. Columns should also be evaluated periodically during their use so that loss of efficiency can be monitored and columns cleaned, when necessary, to restore resolution. 

Stir bar has to be removed, introduced into a glass tube, and transferred to thermal desorption instrument. After desorption and cryofocusing within a cooled programmed temperature vaporization (PTV) injector, the volatiles were transferred onto the analytical GC column. Comparison of SPME and the above-mentioned stir bar sorptive extraction (SBSE) technique using identical phases for both techniques exhibited striking differences in the recoveries, which has been attributed to ca. 100 times higher phase ratio in SBSE than in SPME. A comprehensive treatment of SBSE, discussion of the principle, the extraction procedure, and numerous applications was recently been published by David and Sandra (2007). 

A column-comparison fimction based on values of H, S, etc. for columns 1 and 2 has been derived [12] ... 

An imderstanding of column selectivity is important for various reasons, but mainly when it is necessary to replace one column by another with either similar or very different selectivity. Column selectivity can be accurately characteri2 ed by five measurable properties of the column H, Hydrophobidty S, Steric resistance to insertion of bulky solute molecules into the stationary phase A, column hydrogen-bond Acidity B, column hydrogen-bond Basicity C, column Cation-exchange activity. Values of these column parameters (H, S, etc.) have now been measured for a large number of different RP LC columns. Given values of H, S, etc., it is possible to compare two columns in terms of selectivity by means of a column-comparison function F. Commercial software is available (Column Match Rheodyne LLC, Rohnert Park, CA, USA) that facilitates the convenient comparison of any two columns in terms of selectivity the software also includes values of H, S, etc. for about 300 different columns. 

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