Retention times

Apolar stationary phases having no dipolar moments, that is their center of gravities of their positive and negative electric charges coincide. With this type of compound, the components elute as a function of their increasing boiiing points. The time difference between the moment of injection and the moment the component leaves the column is called the retention time. 

A correlation between retention times and boiling points is established by calibration with a known mixture of hydrocarbons, usually normal paraffins, whose boiling points are known (see Figure 2.2). From this information, the distribution of boiling points of the sample mixture is obtained. 

The foregoing is an equilibrium analysis, yet some transient effects are probably important to film resilience. Rayleigh [182] noted that surface freshly formed by some insult to the film would have a greater than equilibrium surface tension (note Fig. 11-15). A recent analysis [222] of the effect of surface elasticity on foam stability relates the nonequilibrium surfactant surface coverage to the foam retention time or time for a bubble to pass through a wet foam. The adsorption process is important in a new means of obtaining a foam by supplying vapor phase surfactants [223]. 

It is important to realize that many important processes, such as retention times in a given chromatographic column, are not just a simple aspect of a molecule. These are actually statistical averages of all possible interactions of that molecule and another. These sorts of processes can only be modeled on a molecular level by obtaining many results and then using a statistical distribution of those results. In some cases, group additivities or QSPR methods may be substituted. 

Recovery factor Reduced column length Reduced plate height Reduced velocity Relative retention ratio Retardation factor d Retention time Retention volume Selectivity coefficient Separation factor... 

Retention Behavior. On a chromatogram the distance on the time axis from the point of sample injection to the peak of an eluted component is called the uncorrected retention time The corresponding retention volume is the product of retention time and flow rate, expressed as volume of mobile phase per unit time ... 

Time of Analysis. The retention time required to perform a separation is given by... 

Typical chromatogram of detector response as a function of retention time. 

A chromatographic peak may be characterized in many ways, two of which are shown in Figure 12.7. The retention time, is the elapsed time from the introduction of the solute to the peak maximum. The retention time also can be measured indirectly as the volume of mobile phase eluting between the solute s introduction and the appearance of the solute s peak maximum. This is known as the retention volume, Vr. Dividing the retention volume by the mobile phase s flow rate, u, gives the retention time. 

The second important parameter is the chromatographic peak s width at the baseline, w. As shown in Figure 12.7, baseline width is determined by the intersection with the baseline of tangent lines drawn through the inflection points on either side of the chromatographic peak. Baseline width is measured in units of time or volume, depending on whether the retention time or retention volume is of interest. 

Measurement of the column s void time, and the retention time, and baseline width, w, for a solute. 

In a chromatographic analysis of lemon oil a peak for limonene has a retention time of 8.36 min with a baseline width of 0.96 min. y-Terpinene elutes at 9.54 min, with a baseline width of 0.64 min. What is the resolution between the two peaks ... 

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. 

The difference between a solute s retention time and column s void time (t/). 

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. 

The identities of the solutes are defined such that solute A always has the smaller retention time. Accordingly, the selectivity factor is equal to 1 when the solutes elute with identical retention times, and is greater than 1 when is greater than fr A-... 

In the same chromatographic analysis for low-molecular-weight acids considered in Example 12.2, the retention time for isobutyric acid is 5.98 min. What is the selectivity factor for isobutyric acid and butyric acid ... 

A chromatographic analysis for the chlorinated pesticide Dieldrin gives a peak with a retention time of 8.68 min and a baseline width of 0.29 min. How many theoretical plates are involved in this separation Given that the column used in this analysis is 2.0 meters long, what is the height of a theoretical plate ... 

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 retention times for solutes A and B are replaced with their respective capacity factors by rearranging equation 12.10... 

Effect of a change in kg on resolution and retention time. The original value of kg is assumed to be 1. 

Any improvement in resolution obtained by increasing ki generally comes at the expense of a longer analysis time. This is also indicated in Figure 12.11, which shows the relative change in retention time as a function of the new capacity factor. Note that a minimum in the retention time curve occurs when b is equal to 2, and that retention time increases in either direction. Increasing b from 2 to 10, for example, approximately doubles solute B s retention time. 

Adjusting the capacity factor to improve resolution between one pair of solutes may lead to an unacceptably long retention time for other solutes. For example, improving resolution for solutes with short retention times by increasing... 

Use of column selectivity to improve chromatographic resolution showing (a) the variation in retention time with mobile phase pH, and (b) the resulting change in alpha with mobile phase pH. 

With conventional nonspectroscopic detectors, other methods must be used to identify the solutes. One approach is to spike the sample by adding an aliquot of a suspected analyte and looking for an increase in peak height. Retention times also can be compared with values measured for standards, provided that the operating conditions are identical. Because of the difficulty of exactly matching such conditions, tables of retention times are of limited utility. 

A means for normalizing retention times by comparing a solute s retention time with those for normal alkanes. 

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