Retention times adjusted

In a separation of a mixture of hydrocarbons, the following adjusted retention times were measured. 

The concentrations of benzoic acid, aspartame, caffeine, and saccharin in a variety of beverages are determined in this experiment. A Gig column and a mobile phase of 80% v/v acetic acid (pH = 4.2) and 20% v/v methanol are used to effect the separation. A UV detector set to 254 nm is used to measure the eluent s absorbance. The ability to adjust retention times by changing the mobile phase s pH is also explored. 

The adjusted retention times for octane, toluene, and nonane on a particular GC column are 15.98 min, 17.73 min, and 20.42 min, respectively. What is the retention index for all three compounds ... 

What is the retention index for a compound whose adjusted retention time is 9.36 min ... 

Adjusted retention time (r R) The retention time for a substance (rR) minus that of an unretained substance (fm) t R = tR - fm. 

Adjusted retention time, 327 Adjusted retention volume, 327 Air, 139, 140... 

During their passage through the column, sample molecules spend part of the time in the mobile phase and part in the stationary phase. All molecules spend the same amount of time in the mobile phase. This time is called the column dead tine or holdup time (t.) and is equivalent to the tine required for an unretained solute to reach the detector frsolute retention time (t,) is the time between the instant of saiq>le introduction and when the detector senses the maximum of the retained peak. This value is greater than the column holdup time by the amount of time the solute spends in the stationary phase and is called the adjusted retention time (t, ). These values lead to the fundamental relationship, equation (1.1), describing retention in gas and liquid chromatography. 

Mathematical methods for determining the gas holdup tine are based on the linearity of the plot of adjusted retention time against carbon number for a homologous series of compounds. Large errors in this case can arise from the anomalous behavior of early members of the homologous series (deviation from linearity in the above relationship). The accuracy with which the gas holdup time is determined by using only well retained members of a homologous series can be compromised by instability in the column temperature and carrier gas flow rate [353,357]. The most accurate estimates... 

In cases where a mixture has a large number of components, or pure standards are not available, published retention data must be consulted. The uncorrected retention time, tR (p. 86), is not suitable for this purpose because it cannot be compared with data from different columns and instruments. Valid comparisons can be made using relative retention data which are dependent only on column temperature and type of stationary phase. An adjusted retention time, / R, is first obtained by subtracting from tR the time required to elute a non-retained substance such as air (Figure 4.26)... 

Graph of log adjusted retention times of n-alkanes versus carbon number. The open circles show log retention times of n-alkane standards. The retention of an unknown (full circle) can be determined from the graph as shown. 

The adjusted retention time provides a measure of the strength of intermolecular interaction between the analyte and the stationary phase, with stronger interactions giving a longer time. The gas hold-up time is derived from the flow rate and the column dimensions and is often measured by injecting a non-retained compound. The retention factor, which represents a ratio of the mass of analyte dissolved in the stationary phase to the mass in the mobile phase, can be calculated from the adjusted retention time and the gas hold-up time. 

Using the retention data and the chromatogram shown in Fig. 14.8, tabulate the following for each peak retention time ( r), adjusted retention time (t K), retention factor (k), partition coefficient (Kc) and number of theoretical plates (N). The column phase ratio was 250 and the gas hold up time ( m) was 0.995 min. 

Another parameter often measured is the adjusted retention time, Ur. This is the difference between the retention time of a given component and the retention time of an unretained substance, tM, which is often air for GC and the sample solvent for HPLC. Thus, the adjusted retention time is a measure of the exact time a mixture component spends in the stationary phase. Figure 11.17 shows how this measurement is made. The most important use of this retention time information is in peak identification, or qualitative analysis. This subject will be discussed in more detail in Chapter 12. 

Finally, a parameter known as the capacity factor may be determined. The capacity factor, symbolized k (k-prime) is the adjusted retention time divided by the retention time of an unretained substance, tM, such as air in GC or the sample solvent in HPLC. 

What is the difference between retention time and adjusted retention time ... 

Draw an example of an instrumental chromatogram showing one peak, label the x- and y-axes, and show clearly how the retention time and adjusted retention time are measured. 

As mentioned in Section 11.8.4, the parameters that are most important for a qualitative analysis using most GC detectors are retention time, tR adjusted retention time, t R and selectivity, a. Their definitions were graphically presented in Figures 11.16 and 11.17. Under a given set of conditions (the nature of the stationary phase, the column temperature, the carrier flow rate, the column length and diameter, and the instrument dead volume), the retention time is a particular value for each component. It changes... 

The retention time of a mixture component is the time from when the sample is first injected until the component s peak is at its apex. Adjusted retention time is the difference between the retention time of the mixture component and the retention time of air. See Figure 11.17. 

Figure 19.3. Chromatogram obtained by elution chromatography of a mixture of two solutes. The retention time // is the time taken by a solute to pass through the column. Im is the mobile-phase holdup and is measured as the retention time of a non-sorbed solute. t R is the adjusted retention time, the total time spent by the solute in the stationary phase it is equal to // — Im- tw is the width of a solute band at the baseline, i.e., the distance between the points of intersection of the baseline with tangents at the points of inflexion on the sides of the...

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