Column retentivity capacity factor

Metal ions sorption by 2-pyridine carboxaldehyde phe-nylhydrazone supported by chemical binding on a silica surface were confirmed to the Langmuir isotherm. The modified phase was used and applied as a metal-ion extractant for determination of trace amounts of iron, cobalt, nickel, and copper. The relative orders of the Langmuir constants K and the column retention capacity factors K for the four transition metal ions are the same as the natural order of the stabihty constants for their metal chelates Fe(II) < Co(II) < Ni(II) < Cu(II). The structure is given in Scheme 5. 

Elution volume. The amount of solvent required to elute a certain component from the column. See Capacity factor and Retention volume. 

The objective of all chromatographic separation is resolution. This experiment illustrates resolution and the factors that affect it. As discussed in Chapters 1 and 3 resolution cannot occur if the components are not partially retained or slowed down (retarded) by the column. Therefore, before calculating resolution, it is important to use the results of the experiment to calculate the fundamental chromatographic parameters of retention, capacity factor, selectivity, and efficiency. 

A solute s capacity factor can be determined from a chromatogram by measuring the column s void time, f, and the solute s retention time, (see Figure 12.7). The mobile phase s average linear velocity, m, is equal to the length of the column, L, divided by the time required to elute a nonretained solute. 

In a chromatographic analysis of low-molecular-weight acids, butyric acid elutes with a retention time of 7.63 min. The column s void time is 0.31 min. Calculate the capacity factor for butyric acid. 

Now that we have defined capacity factor, selectivity, and column efficiency we consider their relationship to chromatographic resolution. Since we are only interested in the resolution between solutes eluting with similar retention times, it is safe to assume that the peak widths for the two solutes are approximately the same. Equation 12.1, therefore, is written as... 

The time taken for an analyte to elute from a chromatographic column with a particular mobile phase is termed its retention time, fan- Since this will vary with column length and mobile phase flow rate, it is more useful to use the capacity factor, k. This relates the retention time of an analyte to the time taken by an unretained compound, i.e. one which passes through the column without interacting with the stationary phase, to elute from the column under identical conditions (to). This is represented mathematically by the following equation ... 

Often, the retention time is used but, as discussed above in Section 2.3, this absolute parameter changes with column length and flow rate and this precludes the use of reference data obtained in other laboratories. To make use of these reference data, the capacity factor (k ), which removes such variability, must be employed. 

Temperature has an influence on the retention and consequently on the capacity factors of carotenoids in HPLC columns. Usually, as the column temperature increases, the retention decreases however, in a polymeric C30 column, after an initial decrease of the t values of cis isomers of carotenoids, the retention of cis isomers actually increases at temperatures above 35°C. This different behavior can be explained by the increased order and rigidity of the C30 stationary phase at lower temperatures that in turn induce preferential retention of long, narrow solutes as the trans isomer and partial exclusion of bent and bulky cis isomers. The greater chain mobihty and less rigid conformation of the C30 at higher temperatures may increase the contact area available for interaction with the cis isomers and also may lower... 

Having chosen the test mixture and mobile diase composition, the chromatogram is run, usually at a fairly fast chart speed to reduce errors associated with the measurement of peak widths, etc.. Figure 4.10. The parameters calculated from the chromatogram are the retention volume and capacity factor of each component, the plate count for the unretained peak and at least one of the retained peaks, the peak asymmetry factor for each component, and the separation factor for at least one pair of solutes. The pressure drop for the column at the optimum test flow rate should also be noted. This data is then used to determine two types of performance criteria. These are kinetic parameters, which indicate how well the column is physically packed, and thermodynamic parameters, which indicate whether the column packing material meets the manufacturer s specifications. Examples of such thermodynamic parameters are whether the percentage oi bonded... 

Figure 8.25 Schematic arrangement ot a two-column system for separation of a sample comprised of co xinents spanning a wide rage of capacity factors (A) and heartcutting of a group of analytes of similar retention to an analytical column for separation (B).

Establish control charts of instrumental performance. Day-to-day variations in pump flow rate, relative response factors, absolute response to a standard, column plate counts, and standard retention times or capacity factors are all useful monitors of the performance of a system. By requiring that operators maintain control charts, troubleshooting is made much easier. The maintenance of control charts should be limited to a few minutes per day. 

Therefore, a 4a separation (R = 1), in which peak retention times differ by four times the width at half-height, corresponds to a 2% area overlap between peaks.1 The maximum number of peaks that could be separated in a given time period assuming a given value of R, is defined as the peak capacity.1 The peak capacity must be greater — usually much greater — than the number of components in the mixture for a separation to succeed. The resolution of two compounds can also be written in terms of the number of plates of a column, N, the selectivity, a, and the capacity factors, k, and k j, as12... 

Samples are taken directly from 10 mM DMSO-solvated stocks in 96-well plates. Typically, 2 pL of sample is aspirated, and injected on to the column - this represents about 8 pg of sample with MW = 400. The retention time (R,) of peaks eluting from the column is related to the capacity factor log k by Eq. (8) ... 

Ten columns of the 24 available in a cartridge were employed to analyze all compounds in duplicate. Uracil, was employed as a dead volume marker (tO) needed for the evaluation of retention factor [k = (tr - t0)/t0]. Two additional columns were used for simultaneous analysis of the unknown. Values for the log of the capacity factor k were calculated for every compound at each percent organic content of the mobile phase log k = log [(tr - t0)/t0. For each compound, a plot of log k versus percent acetonitrile was used to calculate log k w (log k at 0% acetonitrile). 

It is a great deal of work to actually determine a true equilibrium constant and most chemical separation methods speak in terms of values which are proportional to the actual equilibrium constant. At constant flow, the time that a given type of molecule is retained is related to the time for the void volume to pass after the sample is placed in a column or on a plate with the addition of the time for the net retention volume. If the flow remains constant, the temperature of the separation remains constant and no stationary phase is gained or lost, one can attempt qualitative identification using retention times. It is more reasonable to calculate the ratio of net retention volume to the void volume and call the result partition factor or capacity factor, k. ... 

The separation of substituted benzene derivatives on a reversed-phase C-18 column has been examined [78]. The correlations between the logarithm of the capacity factor and several descriptors for the molecular size and shape and the physical properties of a solute were determined. The results indicated that hydrophobicity is the dominant factor to control the retention of substituted benzenes. Their retention in reversed-phase HPLC can be predicted with the help of the equations derived by multicombination of the parameters. 

In order to determine the applicability of retention indices, based on the alkyl arylketone scale, as the basis of a reproducible method of reporting retentions, the separation of 10 barbiturates and a set of column test compounds were examined on an octadecylsilyl bonded silica (ODS-Hypersil) column with methanol-buffer of pH 8.5 as eluent [100]. The effects on the capacity factors and retention indices, on changing the eluent composition, pH, ionic strengthened temperature, showed that the retention indices of the barbiturates were much less susceptible to minor changes in the eluent than the capacity factors. 

The capacity factor is a measure of the retention of a component per column volume, since the retention time is referred to the time for the unretained component. The greater the capacity factor, the longer that component is retained and the better the chances for good resolution. An optimum range for k values is between 2 and 6. 

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