Asymmetry of peaks

The asymmetry of peak shape is preserved in anthraxolite heated to 1200°C. showing that turbostratic disorder persists in spite of a general enhancement of ordering. The band is also sharper and narrower. This may be interpreted to mean either that fewer class intervals are represented in the crystallite size distribution or that increased ordering of aromatic lamellae has reached the point where graphite (hid) planes are more common. Diffraction peaks of both (100) and (101) fall with the 2-A. band. 

Absorption, 6, 8, 40 Accuracy, definition of, 99 Activity coefficient, 109-110 Adjusted retention volume, 10 Adsorption, 6, 8, 40 Affinity chromatography, 224-225 Area, measurement of, 102 Area normalization, 104 Asymmetry of peaks, 14, 43... 

A more severe test mixture, including acids and bases, is shown in Figure 8.7. Note the asymmetry of peak 6 and the TZ numbers for the members of the homologous series of esters—peaks 9, 11, and 12. 

In this section, we will deal with the major contributions to the asymmetry of peaks. We will first deal with asymmetry due to overloading, and then we will deal with asymmetry due to secondary interactions. 

Taking into account the asymmetry of peaks the formula for resolution (Eq. 16) needs to be modified. Let an L denote leading edges and a T denote tailing edges in Fig. 6. Therefore, the modified equation for resolution is... 

Equation (4.10) applies only to symmetrical peaks. Asymmetrical peaks show that infinite dilution has not been attained and the more general eqn (4.8) should be used. The asymmetry originating in kinetic factors or those not depending on the column may not be api)lied in any theory and therefore it should be avoided, for instance by utilizing low flow rates. In the following is assumed that the asymmetry of peaks is not due to these factors. 

Band Asymmetry. The peak asymmetry factor AF is often defined as the ratio of peak half-widths at 10% of peak height, that is, the ratio b/a, as shown in Fig. 11.2. When the asymmetry ratio lies outside the range 0.95-1.15 for a peak of k =2, the effective plate number should be calculated from the expression... 

In practice, the calculation of peak skew for highly tailing peaks is rendered difficult by basehne errors in the calculation of third moments. The peak asymmetry factor, A, = b/a, at 10 percent of peak height (see Fig. 16-32) is thus frequently used. An approximate relationship between peak skew and A, for taihng peaks, based on data in Yau et al. is Peak skew= [0.51 -t- 0.19/(A, — 1)] . Values of A, < 1.25... 

FIG. 16-32 Exponentially modified Gaussian peak with Xq/Gq = 1.5. The graph also shows the definition of the peak asymmetry factor at 10 percent of peak height. 

Figure 4. The Effect of Peak Asymmetry on the Apparent Composition of Closely Eluting Solutes...

There are a number of causes of peak asymmetry in both gas and liquid chromatography, including heat of adsorption, high activity sites on the support or absorbent, and nonlinear adsorption isotherms. Assuming that good quality supports and adsorbents are used, and the column is well thermostatted, the major factor causing peak asymmetry appears to result from nonlinear adsorption isotherms. 

The major cause of peak asymmetry in GC is sample overload and this occurs mostly in preparative and semi-preparative separations. There are two forms of sample overload, volume overload and mass overload. 

Figure 8-38 shows the residenee time distributions of some eom-mereial and fixed bed reaetors. These shapes ean be eompared with some statistieal distributions, namely the Gamma (or Erlang) and the Gaussian distribution funetions. However, these distributions are represented by limited parameters that define the asymmetry, the peak. 

In practice, it is probable that both of the effects discussed contribute to the overall peak asymmetry. Unfortunately, peak asymmetry varies in extent from the very obvious to the barely noticeable and because of this, peak asymmetry is often dismissed as the normal shape of a single solute peak. Such an assumption can cause serious errors in both qualitative and quantitative analysis. 

The area of a peak is the integration of the peak height (concentration) with respect to time (volume flow of mobile phase) and thus is proportional to the total mass of solute eluted. Measurement of peak area accommodates peak asymmetry and even peak tailing without compromising the simple relationship between peak area and mass. Consequently, peak area measurements give more accurate results under conditions where the chromatography is not perfect and the peak profiles not truly Gaussian or Poisson. 

Having chosen the test mixture and mobile diase composition, the chromatogram is run, usually at a fairly fast chart speed to reduce errors associated with the measurement of peak widths, etc.. Figure 4.10. The parameters calculated from the chromatogram are the retention volume and capacity factor of each component, the plate count for the unretained peak and at least one of the retained peaks, the peak asymmetry factor for each component, and the separation factor for at least one pair of solutes. The pressure drop for the column at the optimum test flow rate should also be noted. This data is then used to determine two types of performance criteria. These are kinetic parameters, which indicate how well the column is physically packed, and thermodynamic parameters, which indicate whether the column packing material meets the manufacturer s specifications. Examples of such thermodynamic parameters are whether the percentage oi bonded... 

A more sophisticated means of quantitating peak asymmetry is through the theory of peak moments.1314 Briefly, the zeroth moment is the peak area, the first moment is the average retention time... 

Peak asymmetry factor AF Ratio of peak half-widths at 10% peak height... 

The retention and the peak asymmetry of benzoic acid also indicate the inertness of the bonded phase. If basic compounds remain on the surface or are used as reagents, the peak asymmetry of benzoic acid is poor. The peak height is lower than that of the same quantity of o-toluic acid.3,4 This phenomenon is observed if the basic catalyst that was used in the synthesis process has not been completely washed off the stationary phase or if active amino groups remain. This type of column is not suitable for the separation of acidic compounds. 

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