Corrected retention time

The corrected retention time (t r) is the time elapsed between the dead point and the peak maximum. 

An eluted solute was originally identified from its corrected retention volume which was calculated from its corrected retention time. It follows that the accuracy of the measurement depended on the measurement and constancy of the mobile phase flow rate. To eliminate the errors involved in flow rate measurement, particularly for mobile phases that were compressible, the capacity ratio of a solute (k ) was introduced. The capacity ratio of a solute is defined as the ratio of its distribution coefficient to the phase ratio (a) of the column, where... 

In a similar manner the elapsed time between the elution of the unretained solute and the peak maximum is called the corrected retention time (t r). The volume of mobile phase that passes through... 

The corrected retention volume and corrected retention time can be used for solute identification but more appropriate measurements for this purpose will be discussed later. 

It is seen that the separation ratio is independent of the phase ratios of the two columns and the flow rates employed. It follows that the separation ratio of a solute can be used more reliably as a means of solute identification. Again, if the data is being processed by a computer, the corrected retention times will be used to calculate the separation ratios. In practice, a standard substance is often added to a mixture and the separation ratio of the substance of interest to the standard is used for identification. 

Commercially available GC-MS systems present major differences in their detection and recording system. Many quadrupole instruments use SIM for the determination of analytes at trace levels. With this type of instrumentation, more than 1-10 ng of the analyte is required to record a full-scan mass spectrum. In contrast, instruments based on ion-trap technology can record a full-scan mass spectrum on an analyte at pg level. With SIM, a limited number of ions are monitored during a selected time interval of the chromatogram. The presence of the analyte is determined by the presence of these diagnostic ions at the correct retention time and in the correct abundance ratio (33). 

When the coulometric detector was turned on, both leuco forms were completely oxidized to their nonfluorescing chromatic forms and thus vanished from the fluorescence channel. This disappearance was balanced by the arrearance of their chromatic forms in the diode array channel. The confirmation of malachite green, gentian violet, and tlieir leuco analogs in catfish and trout tissue could be based, therefore, on the correct retention times, the observation of the natural fluorescence of the leuco forms when the coulometric detector was turned off, the absence of the leuco form peaks in the 588 nm channel when the coulometric detector was off, the disappearance of the fluorescence of the leuco forms when the coulometric detector was on, the appearance of peaks of parent drugs formed by oxidation of the leuco forms in tlie. S88-nm diode array channel, and the correct ultraviolet-visible spectra maxima for all four peaks. 

To determine the constants that are related to a given stationary phase, a series of n-alkanes are injected on the column in the isothermal mode. Under these conditions, the logarithm of the corrected retention times /R(n)(tR(n) = carbon atoms present in the alKane (Fig. 2.14). A graph of carbon number n versus log R(n) is usually a straight line ... 

A method based on the corrected retention times of two alkanes that bracket compound X in the chromatogram (see Fig. 2.15) and equation (2.7) ... 

Figure 2.15—Graphical measurement of Kovats index (/ = lOO/jj) on a column in the isothermal mode.The equivalent carbon number nx is obtained using the logarithm of the corrected retention time tLX). When using a temperature program, a linear relationship can be obtained using a corrected formula. However, this is achieved with a lower precision.

The partition ratio (k) is defined as the amount of solute in the stationary phase compared to the amount in the mobile phase. The value of k is calculated for each component from the corrected retention time (t r) and the gas holdup time (tj by the equation ... 

The elution point of an unretained substance occurs at the dead time (t ) and the volume of mobile phase that has passed through the column between the injection point and the dead point is the dead volume (VJ. The dead volume is given by (Q tj, where (Q) is the flow rate of mobile phase through the column. The volume of mobile phase that passes through the column between the injection point and the peak maximum is called the retention volume (V,), which is given by (Q f.) where (k) is the time that has elapsed between the injection point and the peak maximum. The difference between the retention volume and the dead time - VJ is called the corrected retention volume which is also equal to the product of the corrected retention time (t ) and the flow rate (Q). 

Chromatographic Considerations. The experimental problem is now reduced to the far from trivial one of determining the ratio of the retention volumes of the two isotopically substituted molecules. Consider a typical chromatogram for two difficultly resolvable isomers such as is sketched in Figure 1. Here to is the time from injection that a completely inert nonadsorbed material is eluted t2 and ti are the corrected retention times P2/P1 = ti/t2) (21) t is the average residence time in the column and tv is the peak width. 

Corrected Retention Time and Corrected Retention Volume... 

If the mobile phase is compressible, the simple product of the corrected retention time and flow rate will be incorrect, and the corrected retention volume must be taken as the product of the corrected retention time and the mean flow rate. The true corrected retention volume has been shown to be given by [1]... 

The interpretation of the retention time as the first moment is more comprehensive than the use of the peak time (fmax). mi indicates the position of the peak center on the time scale that, in the case of unsymmetrical peaks, may deviate significantly from the position of the peak maximum. The thermodynamically correct retention time is actually derived from the position of the Gaussian component in a deconvoluted total peak... 

An unquestionable benefit of GC analysis is the possibility to correct retention time shifts easily by calculating retention time indices (Kovats index) or even by reanalyzing analytes on a second stationary phase resulting in a second confirmative index. 

Since the stochastic models gives the corrected retention time in the form of = UTs and the retention time as tp = fo + we can write the second central moment as... 

Corrected retention time used to calculate Kovats index... 

Retention volume and retention time are measured from the time the sample is introduced into the chromatograph to when the component(s) are eluted from the column no allowance is made for the volume of mobile phase in the system nor the time the mobile phase takes to pass from the injector to the detector. A correction therefore has to be made to obtain a more accurate representation of the retention of a component by the stationary phase. Fr, the corrected retention volume, accounts for the volume of mobile phase in the system, and tR, the corrected retention time, takes into account the time the mobile phase takes to pass through the system. and are often referred to as the dead volume and dead time, respectively (Figure 2.2). 

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