Peaks retention behavior

We can illustrate the various peaks retention behavior as a function of temperature by making a plot such as shown in Figure 4.9, which presents the log of the retention factor as a function of the reciprocal of the (absolute) column temperature from 75 to 130°C. This type of relationship can be characterized by two constants as shown in the following equation ... 

Some optimization system models rely on predetermined libraries of compounds already calibrated on several common stationary phases. If a peak of interest is in such a library and the analyst is using one of the characterized phases, then no additional calibration may be necessary as long as (1) the calibration column and the experimental column dimensions are known with good accuracy, (2) the calibration gas chromatograph s oven temperature and experimental oven temperatures are standardized, and (3) the pressure drops and ambient pressures for the calibration and experimental systems are known accurately. If not, then the simulations will be less accurate. However, small errors in these areas will not distort simulated results so much that peak elution order and relative retention will be meaningless. Even when not exact down to the second, simulations provide a wealth of useful information about peak retention behavior under a range of test conditions. 

Retention Behavior. On a chromatogram the distance on the time axis from the point of sample injection to the peak of an eluted component is called the uncorrected retention time The corresponding retention volume is the product of retention time and flow rate, expressed as volume of mobile phase per unit time ... 

Optimization Criteria for Interpretive Methods. As noted earlier in our discussion of the simplex methods, there are many chromatographic response functions (CRFs) for the evaluation and comparison of chromatograms during an optimization process. Here we discuss two CRFs that we employed successfully with this interpretive method of optimization. Since the retention behavior of every solute must be modeled prior to optimization, the number of sample components is known beforehand it is thus unnecessary to include the number of peaks in these CRFs as was done in CRF-3 (equation 8) for the simplex. 

As is evident from the preceding discussion, the retention behavior of a polypeptide or protein P- expressed in terms of the capacity factor k is governed by thermodynamic considerations. Peak dispersion, on the other hand, arises from time-dependent kinetic phenomena, which are most conveniently expressed in terms of the reduced plate height he, . When no secondary effects, i.e., when no temperature effects, conformational changes, slow chemical equilibrium, pH effects, etc. occur as part of the chromatographic distribution process, then the resolution Rs, that can be achieved between adjacent components separated under these equilibrium or nearequilibrium conditions can be expressed as... 

Small organic (e.g., formic and acetic) acids are effective volatile IPRs. They impact the retention behaviors of pH-sensitive compounds, changing their charge status and providing pairing anions that may easily interact with protonated solutes. Many chromatographic separations benefit in terms of retention, resolution, and peak shape under acidic conditions due to suppression of silanol activity. Furthermore, the acidity of these IPRs facilitates the formation of the protonated molecular ion [M + H] measured by mass spectrometry in the usual positive ion mode. 

A whole-column detection system fabricated with parts from a typical A4 size optical scanner was used to monitor peak crossover that occurs when a solute moves slower than another one, then during gradient elution, migrates faster and reaches the column outlet earlier. A 0.3-mm spatial resolution and a 3.6 ms temporal resolution were obtained and proved adequate for directly monitoring solute retention behavior in a liquid chromatography column under IPC conditions [57]. On line radiochemical detection was also explored [58]. 

Preparation of the reference mixture in the same matrix as the wastewater samples, namely the wastewater itself, was investigated for minimizing sample differences and their effect on peak retention-time behavior. The same series of 17 reference compounds was added to a sample of Oxy-6 gas condensate. A sufficient amount of each reference was added to swamp any nearby wastewater headspace peak. Triplicate analysis were run, and the computed retention indices for the reference compounds and the unknown wastewater headspace peaks were stastically compared. The results (Table 1, column V Fig. 2, peaks with asterisks) show a substantially improved correlation. Fourteen of the 17 reference compounds, including all the methyl- and ethyl-pyridine isomers, were correlated with wastewater peaks. Since 7 of the correlated peaks were among the 22 peaks proviously shown (see Section 32) to be common to the 8 wastewater samples (Table 1), these compounds may be presumed to be present in all the waters 2- and 3-methyl- 2-, 3-, and 4-ethyl- 2,4-dimethyl- and 2,6-dimethyl-pyridines. Furthermore, the 37 peaks cotmnon to Oxy-6 gas condensate and Oxy-7 and 8 gas condensate (see Section 32) included 11 of the... 

Only one research group has illustrated the reversed phase retention behavior of solutes on zirconia-silica modified surfaces. In the study by Melo et al. on PMOS gamma-irradiated modified surfaces, the reversed phase retention behavior of a test mixture containing acetone, benzonitrile, benzene, toluene, and naphthalene was evaluated. The authors illustrated good resolution of the five-component text mixture as shown in Fig. 7. Retention, however, decreased after the stationary phase was washed with 5000 column volumes of base (pH 10). Despite the base washing, uniform peak shape was maintained, with only a slight reduction in resolution as discussed in Alkaline Stability above. 

The silica ion exchanger manufactured by Wescan is based on a macroporous substrate with a pore width of 300 A. The big differences in the retention behavior of monovalent and divalent anions are characteristic for the respective chromatogram depicted in Fig. 3-28. The negative signal appearing at about 20 minutes is the system peak (see Section 3.3.4.3), which is inevitable upon employing phthalates as eluents. 

The addition of even minute amounts of sodium carbonate has a particularly strong effect on the retention behavior of multivalent anions. These comprise, for example, the two iron cyanide complexes Fe(CN)63 and Fe(CN)64, whose separation is obtained with an eluent containing only 3 10 4 mol/L sodium carbonate (see Fig. 5-9), apart from tetrabutylammonium hydroxide and acetonitrile. Lowering the acetonitrile content in favor of sodium carbonate, the resolution between both signals will decrease drastically, although the peak shape of the iron(II) complex will be distinctly improved. 

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