Retention time example

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 ... 

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... 

Identification of the pesticides is based on retention time on at least two dissimilar glc columns. A nonpolar and a relatively polar packing are generaUy used, for example, OV-17 and a mixture of QE-1 and DC-200. 

Peracid Classification. Peracids can be broadly classified into organic and inorganic peracids, based on standard nomenclature. The limited number of inorganic peracids has required no subclassification scheme (4). However, the tremendous number of new organic peracids developed (85) has resulted in proposals for classification. Eor example, a classification scheme based on Hquid chromatography retention times and critical miceUization constants (CMC) of the parent acids has been proposed (89). The parent acids are used because of the instabiHty of the peracids under chromatographic and miceUization measurement conditions. This classification scheme is shown in Table 1. 

Furthermore, in the example given, the peaks were considered to be truly Gaussian in shape. Asymmetric peaks can distort the position or the peak maximum of the envelope to an even greater extent. In general, the retention time of a composite peak should never be assumed to have a specific relationship with those of the unresolved pair. 

The results obtained were probably as accurate and precise as any available and, consequently, were unique at the time of publication and probably unique even today. Data were reported for different columns, different mobile phases, packings of different particle size and for different solutes. Consequently, such data can be used in many ways to evaluate existing equations and also any developed in the future. For this reason, the full data are reproduced in Tables 1 and 2 in Appendix 1. It should be noted that in the curve fitting procedure, the true linear velocity calculated using the retention time of the totally excluded solute was employed. An example of an HETP curve obtained for benzyl acetate using 4.86%v/v ethyl acetate in hexane as the mobile phase and fitted to the Van Deemter equation is shown in Figure 1. 

When analytes lack the selectivity in the new polar organic mode or reversed-phase mode, typical normal phase (hexane with ethanol or isopropanol) can also be tested. Normally, 20 % ethanol will give a reasonable retention time for most analytes on vancomycin and teicoplanin, while 40 % ethanol is more appropriate for ristocetin A CSP. The hexane/alcohol composition is favored on many occasions (preparative scale, for example) and offers better selectivity for some less polar compounds. Those compounds with a carbonyl group in the a or (3 position to the chiral center have an excellent chance to be resolved in this mode. The simplified method development protocols are illustrated in Fig. 2-6. The optimization will be discussed in detail later in this chapter. 

For example, only those dihydropyrimidines that contained a hydrogen-bonding donor at position 3 next to the chiral center were separated. Remarkably, dihydropyrimidines with non-substituted nitrogen atoms at positions 1 and 3 resulted in separations with longer retention times and decreased separation factors a. Increas-... 

Another example of the use of a C8 column for the separation of some benzodiazepines is shown in figure 8. The column used was 25 cm long, 4.6 mm in diameter packed with silica based, C8 reverse phase packing particle size 5 p. The mobile phase consisted of 26.5% v/v of methanol, 16.5%v/v acetonitrile and 57.05v/v of 0.1M ammonium acetate adjusted to a pH of 6.0 with glacial acetic acid and the flow-rate was 2 ml/min. The approximate column efficiency available at the optimum velocity would be about 15,000 theoretical plates. The retention time of the last peak is about 12 minutes giving a retention volume of 24 ml. 

An example of a separation primarily based on polar interactions using silica gel as the stationary phase is shown in figure 10. The macro-cyclic tricothecane derivatives are secondary metabolites of the soil fungi Myrothecium Verrucaia. They exhibit antibiotic, antifungal and cytostatic activity and, consequently, their analysis is of interest to the pharmaceutical industry. The column used was 25 cm long, 4.6 mm in diameter and packed with silica gel particles 5 p in diameter which should give approximately 25,000 theoretical plates if operated at the optimum velocity. The flow rate was 1.5 ml/min, and as the retention time of the last peak was about 40 minutes, the retention volume of the last peak would be about 60 ml. 

There are a number of features worthy of note in this figure. For example, there is a difference in retention times, determined by the two detectors, of ca. 0.32 min, and this reflects the fact that they are used in series, i.e. the column effluent passes through the UV detector on its way to the mass spectrometer. 

By way of graphical example of the various algebraic and geometrical concepts that are introduced in this chapter, we will make use of a measurement table adapted from Walczak etal.[ ]. Table 31.2 describes 23 substituted chalcones in terms of eight chromatographic retention times. Chalcone molecules are constituted of two phenyl rings joined by a chain of three-carbon atoms which carries a double bond and a ketone function. Substitutions have been made on each of the phenyl rings at the para-positions with respect to the chain. The substituents are CFj, F, H, methyl, ethyl, i-propyl, t-butyl, methoxy, dimethylamine, phenyl and NO2. Not all combinations two-by-two of these substituents are represented in the... 

These retention times may vary from system to system. An example chromatogram is shown in Figure 2. 

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