Skewed peaks

When the adsorption equihbrium is nonlinear, skewed peaks are obtained, even when N is large. For a constant separation-factor isotherm with R < 1 (favorable), the leading edge of the chromatographic peak is steeper than the trailing edge. Wmen R > 1 (unfavorable), the opposite is true. 

FIGURE 3 Typical sawtooth skewed peaks when the mobility of the analyte and probe differ. The signal was inverted to obtain positive peaks. 

Errors in estimation of the retention volumes from the apexes of skewed peaks (peak skewing depends on the flow rate)... 

Yau derived characteristical properties of the EMG skewed peak model and proposed a new method to extract band broadening parameters from a skewed and noisy peak. 

J. P. Foley and J. G. Dorsey, Equations for Calculation of Chromatographic Figures of Merit for Ideal and Skewed Peaks, Anal. Chem. 1983,55, 730 ... 

Some of the present liner designs can be used for on-column injection by placing the head of the column about 1/8 inch from the septum. The syringe needle can thereby extend at least 2 inches into the column. The injection block temperature should be set at or below the temperature limit of the liquid phase in the column. Greater injector temperatures should be avoided because the liquid phase will be stripped off or decomposed at the front of the column and result in baseline drift or large, skewed peaks. 

Foley and Dorsey have recently derived a simple manual method for the calculation of plate counts that corrects for the asymmetry of skewed peaks (9 ). This equation has been used in all of our micellar efficiency calculations and is ... 

Foley, J.P. and Dorsey, J.G. (1983) Equations for calculation of chromatographic figures of merit for ideal and skewed peaks. Anal. Chem. 55, 730-737. 

Usually the retention volume is obtained using peak maxima to define the retention times. In this treatment, since bulk absorption only is assumed, band broadening effects and the existence of a non-linear sorption isotherm are not considered, as these usually reflect some surface adsorption, resulting in skewed peaks. 

Measurement Procedure. IGC measurements were started after the thermal and flow equilibrium in the column were stable (2 to 3 h). To facilitate rapid vaporization of the probe (0.01 yL), the injector temperature was kept 30°C above the boiling point of the probe. Measurements were made at five carrier gas flow rates. The retention volumes of six injections for each probe and twenty injections of the marker (H2) at a given flow rate were averaged. The values obtained were extrapolated to zero flow rate to eliminate the flow rate dependence of the retention data. The net retention time (tR) is defined as the time difference between the first statistical moment of the solvent peak and that of the marker gas. Thus, tR was calculated by an on-line computer statistical peak analysis rather than the retention time at the peak maximum (tp,maY). This eliminated inaccuracies arising from slight peak asymmetry, which occurs even for inert and well-coated supports. The specific retention volumes (Vg°) derived from tR and tR max differed by as much as 5% for small retention times and slightly skewed peaks (11,12). 

These considerations are particularly important for non-linear or concentration dependent relations and non-equilibrium conditions, such as those found in chromatographic systems showing markedly skewed peaks (6.). As these authors have shown, there is no identifiable solution to the problem of the thermodynamic properties of the highly skewed chromatogram peak. Thus, the elution method is only valid for equilibrium chromatography. 

The concept of temporal variations in concentration at the flow-through detector explains why pronounced skewed peaks are often observed in flow analysis, especially with loop-based sample introduction. Taylor assumed that dispersion is symmetric in relation to an observer located at the dispersing zone [55,56], but in practice the recorded peaks are usually characterised by a rise time much shorter than the fall time (see also Fig. 1.3e). This skew effect is explained by the fact that the front and trailing portions of the flowing sample, which relate to the rise time and the fall time, respectively, have different residence times in the manifold and are therefore subjected to different extents of dispersion. 

These peaks are inherent to flow injection analysers and other flow systems characterised by large sample dispersion. They tend towards a Gaussian shape, although skewed peaks are often reported. 

The most accurate manual method to determine the area of a skewed peak is to construct a trapezoid. This is easy if you use a template (included), and harder if you do not. Refer to Figure 20-55. Determine the height as shown. You need two widths, one at 15% and the other at 85% of the height. 

Under these conditions the retention time is independent of the sample size. When the distribution coefficient, as often happens, is not independent of sample concentration, abnormal behaviour of the column results. Skewed peaks are the result of non-linear distribution isotherms, as shown in Figure 7.6(b) and Figure 7.6(c), and the retention time will vary with sample concentration. 

A remarkable result is that in this particular case the optimum integration interval is independent of the time constant and of the variance of the noise. Of course, the resulting estimation error depends on both parameters. The theory can be extended to skewed peaks with known or unknown shape, other kinds of noise, etc. 

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