Comparison of Columns

In the literature, the hydrophobicity is often used as a criterion for comparison of stationary phases. For this purpose, In k values k = retention factor) of typical RP analytes on different stationary phases are compared, on similar hydrophobic phases the analyte will be equally strongly retained, leading to similar In k values. This practice should be critically questioned. Without going into details about 

From a practical perspective, the retention behavior (a measure of the hydrophobicity) is only of fringe interest. The reason is that in comparative tests the user is more interested in finding columns that show similarly good separation for certain analytes rather than in finding columns on which the analytes elute after a similar retention time. And a measure of the separating efficiency happens to be the selectivity (separation factor, a). Similarly, strong interactions - and thus similar retention factors - do not at all mean that the respective phases also show similar selectivity. 

Zorbax SB Cl 8 Luna Nova-Pak Fluofix (EW Prodigy YMCC18 Hypersil BDS Prontosil Cl 8 Inertsil CDS 2 Prontosil AQ Repro-Sil CDS MP-Gel YMC AQ Purospher Nucleosil HD Discovery Cl 8 Repro-Sil AQ Supelcosil ABZ plus Jupiter Hypersil Elite Toso Haas TSK Inertsil QDS 3 Fluofix INW Ultrasep ES Discovery Cl 6 Nucleosil 50 Zorbax SB C8 Superspher LiChrosorb Superspher Select B Bondapak Nucleosil 100 LiChrosher Select B Supersorb QDS 2 LiChrospher Platinum Cl 8 Gromsil AB VYDAC Superisorb QDS 1 Platinum EPS Hypersil QDS Nucleosil Protect 1 Zorbax QDS 

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The precision of SEC must be established before a comparison of columns and calibration standards can be made. Consistency in flow rate or elution time is the first requirement to obtain precision in SEC. Consistency in flow rate or elution time can be monitored by the elution time of the PEO standards, which are run before and after the samples. Elution time or flow rate can be considered consistent if the elution times of the PEO standards before and after the samples agree within 0.1 min. 

Figure 3-37. Comparison of columns of various liquids to register 43.3 psig on pressure gauge at bottom of column.

The values of x in column 4 were obtained by the Ethyl Corporation by a chemical method, for which the estimated precision is 0.02 ml of tetraethyllead fluid per gallon. Comparison of columns 4 and 5 shows agreement within these limits for all samples except B62M-3 the reason for the considerably greater discrepancy here is unknown. The precision of the x-ray work is better than was expected. (The precision is sufficiently great to warrant consideration of the difference in the x-ray absorption of the base stocks, samples AOT-1 and B62M-1.) Further-... 

Peter, A. et al.. Comparison of column performances in direct high performance hquid chromatographic enantioseparation of 1- or 3-methyl-substituted tetrahy-droisoquinohne analogs. Application of direct and indirect methods, Biomed. Chromatogr, 19, 459, 2005. 

Comparison of columns 3 and 7 of Table II shows, as expected, that for all systems except the first, the sample deviations of total pressure are reduced after temperature smoothing. At this point in time no clearcut explanation can be given for the unexpected result for the potassium nitrate system (8). However, the paper reveals that an Othmer still was used to give x and y values only, and these data were subjected to smoothing. The boiling points of synthetic liquid measures were determined separately in a three-necked flask under total reflux. With the other systems of Table II all data were measured in a modified Othmer still. 

A comparison of columns 4 and 8 reveals no clear pattern, which is perhaps of greater significance. The use of raw data yields smaller values of the vapor composition sample deviations in four out of six cases, but the effects are small and could be masked by errors in the vapor compositions themselves. It seems likely that the greatest source of error lies in determination of vapor composition. Thus there is very little difference in using raw or smoothed data. A typical example of the fit is shown in Figure 2. The optimum smoothing parameters used in run 1 were found to be the same as required for run 2, and are listed in columns 11 and 12 of Table II. 

Comparison of column performance among the different types of columns is not easy. Resolution and analysis time at a minimum temperature seem to be the best criteria for comparison. Golay (26) suggests the calculation of performance index (P.I.) for intercomparison of columns. 

A comprehensive comparison of columns and mobile phases for use in the separation of carotenoids by reverse-phase HPLC. 

The importance of the amount of carbon load on the column, which varies widely between columns from different manufacturers, has been discussed for a long time. The differences in chromatographic retention and selectivity are a result of the utilization of different silica materials as supports and a variety of reagents and procedures to produce the bonded phases. Several studies have shown that the capacity factor, k, generally increases with increasing carbon content (12). However, sometimes the results show that k values are not always correlated with the differences in carbon content. This may be explained, as Unger (13) illustrates, by the fact that the carbon content alone is often misleading in the comparison of columns because of differences... 

Direct comparison of column efficiencies of the continuous separation medium prepared in 4.5 cm long both capillary and linear channel as measured using non retained marker acetone did not reveal any difference and an identical plate height of 4-5 pm was observed for both at flow velocities of 0.5-1.5 mm/s. This indicates that there is again no substantial difference between beds formed in both formats. Obviously, the channel behaves as a capillary although it does not have the circular cross section. 

Adams, F., Burmester, C., Hue, N. V., and Long, F. L. (1980). Comparison of column-displacement and centrifuge methods for obtaining soil solution. Soil Sci. Soc. Am. J. 44, 733—735. Amoozegar-Fard, A. D., Nielsen, D. R., and Warrick, A. W. (1982). Soil solute concentration distribution for spatially varying pore water velocities and apparent diffusion coefficients. Soil Sci. Soc. Am.J. 46, 3—9. 

Comparison of column efficiency for bubble-cap, sieve, and valve finite-stage contactors. 

Comparisons of columns of different lengths that are also packed with different sized particles may be made by use of the reduced plate height Qi) which is a dimensionless parameter defined by equation (2.29). 

Figure 9. Comparison of column performance at equivalent loading.

The equivalent height of a theoretical plate H, as already defined (expression 1.5), is calculated for reference compounds to permit a comparison of columns of different lengths. H does not behave as a constant, its value depends upon the compound chosen and upon the experimental conditions. 

TABLE 11.4-3 Comparison of Column Crystallization with Standard Separatism Techniques ... 

Figure 21.14. Comparison of columns used in liquid chromatography. A Classical open column chromatography with large porous particle packings B HFLC with pellicular packings C HPLC with microparticulate packings.

Crude oil and bitumen (comparison of column phase configurations)... 

Wen, E., Asiaie, R., and Horvdth, C., Dynamics of capillary electrochromatography n. Comparison of column efficiency parameters in microscale high-performance liquid chromatography and capillary electrochromatography, J. Chromatogr. A, 855, 349,1999. 

Figure 2-57. Graph showing comparison of column types with critical stress.

Information on the number of theoretical plates, N, or plate height, H, can be problematic, as outlined in Section 8.1. Dimensionless values (numbers without units) are much more suitable for absolute comparison of columns, a whole range of different types of columns (long or short, thick and thin, normal or reversed phase, large or microparticles, with various types of packing) being covered. Dimensionless values are also easy to remember. 

The hydrophobic selectivity is the methylene group selectivity of the stationary phase, that is, the selectivity of two compounds differing by a methylene group, which are retained predominantly due to their hydrophobicity. This selectivity is to be used, for example, in the case of neutral, hydrophobic analytes for the comparison of columns and assessment of their similarity, and has a much higher significance than the hydrophobicity treated in Section 4.4.1, and later sections. 

A comprehensive review of diversified tests and visualization tools for the comparison of columns is available in Ref. [8]. In the following, we present three tests that appear to us to be quite informative. For the reasons set out earlier, the selectivity (separation factor) is used as the classification criterion for comparison of columns. 

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