Retention time calculation

PREPARATION OF KETONE DERIVATIVE. For additional evidence of the presence of amphetamine, a ketone derivative is prepared by adding 0.5 cm3 of acetone to the urine extract and evaporating at 60° to a volume of about 50 yl. Some unreacted amphetamine remains. A 5-yl aliquot of the mixture is chromatographed and the relative retention times calculated. The retention time of diphenylamine will not change, as it does not form a ketone derivative. 

One method for the quantitative determination of the concentration of constituents in a sample analyzed by gas chromatography is area normalization. Here, complete elution of all the sample constituents is necessary. The area of each peak is then measured and corrected for differences in detector response to the different eluates. This correction involves dividing the area by an empirically determined correction factor. The concentration of the analyte is found from the ratio of its corrected area to the total corrected area of all peaks. For a chromatogram containing three peaks, the relative areas were found to be 16.4, 45.2, and 30.2, in order of increasing retention time. Calculate the percentage of each compound if the relative detector responses were 0.60, 0.78, and 0.88, respectively. 

As we have shown, the heights of the two concentration discontinuities decrease constantly during their migration, as soon as the plateau at the feed concentration has vanished. The velocity associated with the maximum concentration of the band on the continuous (rear) side is always larger than the shock velocity. This is why the shock erodes continuously. The trajectory of the second shock can be obtained and its retention time calculated. It is longer than the retention time of the same shock in the case of a wide rectangular injection because the shock is less high, hence its velocity is lower (Eq. 8.15). 

Figure 6.26 (a) Results for isotherm determination forTrdger s base on Chiralpak AD for pure components and mixtures (symbols, experimental data lines, retention times calculated by Equations 6.189 and 6.190... 

The retention-time calculations are based on the volume between the perforated plate and the weir plate. The cross-sectional area available to the phases (oil, water (or liquid), and gas) is calculated and the mean plug velocity and retention times are given. The cross-sectional area below a level /j in a horizontal cylinder is generally given by the geometric relation ... 

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. 

Using the data from Problem 1, calculate the resolution and selectivity factors for each pair of adjacent compounds. For resolution, use both equations 12.1 and 12.21, and compare your results. Discuss how you might improve the resolution between compounds B and C. The retention time for an unretained solute is 1.19 min. 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

For estimating purposes for direct-heat drying applications, it can be assumed that the average exit-gas temperature leaving the sohds bed wih approach the final solids discharge temperature on an ordi-naiy unit carrying a 5- to 15-cm-deep bed. Calculation of the heat load and selec tion of an inlet-air temperature and superficial velocity (Table 12-32) will then permit approximate sizing, provided an approximation of the minimum required retention time can be made. 

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... 

To aid in solute identification, a standard substance is usually added to a mixture and the separation ratio of the solutes of interest to the standard is used for identification purposes. In practice, separation ratios are calculated as the ratio of the distances in centimeters between the dead point and the maximum of each peak. If the flow rate is sufficiently constant and data processing is employed, then the corresponding retention times can be used. 

If the mobile phase is a liquid, and can be considered incompressible, then the volume of the mobile phase eluted from the column, between the injection and the peak maximum, can be easily obtained from the product of the flow rate and the retention time. For more precise measurements, the volume of eluent can be directly measured volumetrically by means of a burette or other suitable volume measuring vessel that is placed at the end of the column. If the mobile phase is compressible, however, the volume of mobile phase that passes through the column, measured at the exit, will no longer represent the true retention volume, as the volume flow will increase continuously along the column as the pressure falls. This problem was solved by James and Martin [3], who derived a correction factor that allowed the actual retention volume to be calculated from the retention volume measured at the column outlet at atmospheric pressure, and a function of the inlet/outlet pressure ratio. This correction factor can be derived as follows. 

Thus, rearranging equation (5), the retention time at a particular program rate (a) can be calculated as that time (tr) nota bena tr=nt) that allows the following equation to be satisfied. 

In addition, if the program is run for the second enantiomer, then the ratio of the retention times of the two isomers can be calculated (the separation ratio) for a range of different solvent program rates. 

---