Retention time - composition relationship

In the case of the nitro method, the microstructure of the copolymer seems to have an effect on the retention time-composition relationship. The nitro column composition-retention time relationship is not a straight line for copolymers. Instead the relationship could be said to consist of two straight lines. One of the straight lines covers the composition range of 40 to 78 wt% styrene (Figure 1.5b). 

Figure 1.5 Composition versus retention time relationships for ESI copolymers. Reproduced with permission from J.W. Lyons, D. Poche, F.C-Y. Wong and P.B. Smith, Advanced Materials, 2000,12, 23, 1847. 2000, Wiley [23]...

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. 

Equation (3) merely sums the two peaks to produce a single envelope. Providing retention times can be measured precisely, the data can be used to determine the composition of a mixture of two substances that, although having finite retention differences, are eluted as a single peak. This can be achieved, providing the standard deviation of the measured retention time is small compared with the difference in retention times of the two solutes. Now, there is a direct relationship between retention volume measured in plate volumes and the equivalent times, which is depicted in Figure 6. 

It is seen that the profile of the combined peaks is perfectly symmetrical and displays no hint that there are two solutes present. Obviously an absorption ratio curve from a diode array detector would quickly disclose the presence of the two components, as would an appropriate changes in mobile phase composition. However, there would be a further clue for the analyst to follow that would give warning of the "duplicity" of the peak. The double peak would be very broad and be inconsistent with the change in peak width of the other solute peaks with retention time. The peak width of a solute increases regularly with retention time but, unfortunately, the relationship is not smooth. There are good reasons for this, but they... 

There are two major factors that influehce retention volume measurement and they are temperature and solvent composition. In order to measure retention volume with adequate precision it is necessary to know the relationship between retention time and temperature so that the control limits of the column temperature can be specified. 

The conceptual basis for understanding the connection between isocratic and gradient elution is well established and is called "linear solvent strength theory".22 27 Linear solvent strength theory proposes that, for a given solute, mobile phase, and column, if one measures the retention time of an analyte at two organic component concentrations, it will be possible to predict the retention time with any other mobile phase composition. The k value that would be observed in pure water, kw, is related to the actual k by the relationship... 

However, the calculations required for such an optimization are quite involved. This is caused by the requirement to calculate the retention times of each solute (and the resolutions of each pair of adjacent peaks) from the isocratic retention vs. composition relationships. In order to characterize the response surface, these calculations need to be performed a number of times. Finally, the optimum needs to be found on the response surface. If all four program parameters (initial and final concentration, slope and shape) are considered, the number of calculations would be large, even though the response surface may be simple compared with those encountered in selectivity optimization (see the discussion in section 6.3.2.1). 

PCB retention times increase with chlorination level, and within chlorination levels, with less chlorine substitution in the ortho position (i.e., coplanar PCBs are more strongly retained). These relationships are of theoretical interest but are of less use now that accurate retention time assignments are possible with actual standards. The use of commercial mixtures such as Aroclors as quantitative secondary standards for CQCS PCB analysis is now to be discouraged [4], as detailed studies of congener distributions show significantly different proportions among different lots [7]. In the case of Aroclor 1254, there are actually two different mixtures of radically different composition produced by totally different synthetic processes [9]. 

The kinetic characteristics are determined in GC methods both directly and indirectly. In direct pulse chromatographic methods the reaction rate can be established by the direct determination of the amount (concentration) of the reacting component, whereas in indirect methods this is done on the basis of the variations with time of the chromatographic properties of the reacting system, which are usually determined from the relationship between and the retention times of the non-reacting components and the composition of the reaction mixture used as the stationary phase [58]. Pulse chromatography... 

As when using exact solutions, the first step when using solutions employing numerical Integration Is to determine the relationship between k and solvent composition for each solute using limited Isocratlc data. Next the retention times of each solute must be calculated In turn. First, the time spent by the... 

The choice of the column and the composition of the mobile phase were studied to obtain the best compromise between resolution and analysis time. The Nucleosyl CN reversed phase column was used because C 18 and C 8 retention times were excessive. The eluant composed of acetonitrile (40 %) and a buffer (60 %) (KH2PO4 0.05 M, EDTA 0.1 mM) gave the best separation of polyamines, when the pH was adjusted to 5.7. This pH is very important because changes in pH modify retention times of polyamines. Spermine and spermidine are the most affected by pH. In these conditions, the chromatograms showed satisfactory results and symmetrical peaks. The retention times were 40 minutes for spermine, 57 minutes for putrescine, 68 minutes for internal standard and 75 minutes for spermidine (Fig.l.). They were constant in repeated analyses. A good linear relationship (r = 0.99) existed between polyamine concentration and the peak height over the range 1 pmole to 10 nmole when the derivatization time was carefully controlled (5 min). 

Figure 19.2 Relationship between the rate of degradation and retention time required according to the composition of the substrate.

Fig. 4 shows that benzoic acid depends on the pH of the ammonium acetate/methanol buffer (pH 4.4—5.1), providing effective reduction for a particular separation (retention time decreased from 8.0 to 3.4 min) in addition to decrease in asymmetry (2.32 to 1.81). The main effect on absorption, including that affecting the separation property, is as yet unknown. The probable explanation is that the absorption coefficient tends to decrease during the ionic phase and that the intrinsic relationship is demonstrated by the pH of the medium. The mobile and stationary phases for ideal separation that provide the best characterization of interaction are as yet unknown, mainly because the asymmetry is too large, as can be seen in Fig. 4. It has been assumed that a particular composition of the mobile phase interferes with both the absorption coefficient and the adjusted retention time (k).

Concentrations of moderator at or above that which causes the surface of a stationary phase to be completely covered can only govern the interactions that take place in the mobile phase. It follows that retention can be modified by using different mixtures of solvents as the mobile phase, or in GC by using mixed stationary phases. The theory behind solute retention by mixed stationary phases was first examined by Purnell and, at the time, his discoveries were met with considerable criticism and disbelief. Purnell et al. [5], Laub and Purnell [6] and Laub [7], examined the effect of mixed phases on solute retention and concluded that, for a wide range of binary mixtures, the corrected retention volume of a solute was linearly related to the volume fraction of either one of the two phases. This was quite an unexpected relationship, as at that time it was tentatively (although not rationally) assumed that the retention volume would be some form of the exponent of the stationary phase composition. It was also found that certain mixtures did not obey this rule and these will be discussed later. In terms of an expression for solute retention, the results of Purnell and his co-workers can be given as follows,... 

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