Tangential skim

The perpendicular drop should only be used if (1) the peaks are symmetrical and about the same height and width, (2) the valley between the peaks is no more than 5% of the peak height, (3) the baseline is flat, and (4) noise does not obscure the accurate placement of the baseline. If the peaks are asymmetrical, a perpendicular drop will overestimate the area of the smaller peak. Under those circumstances, a tangential skim may be a better approach. 

Figure 7.18 Integration of incompletely resolved peaks by means of a tangential skim.

Chromatographic data analysis systems generally employ three methods for determining baselines in overlapping peaks perpendicular drop, linear tangential skim, and exponential skim (see Fig. 1). In order to choose the most appropriate method, the analyst must understand the assumptions and weaknesses of the three methods. 

The peak area errors for the two most studied de-convolution methods (i.e., perpendicular drop and linear tangential skim) are dependent on a complex combination of resolution, relative peak width, relative peak height, and asymmetry ratio [1]. Exponential skimming assumes that the tailing of the first peak can be described by an exponential decay and that the peaks are sufficiently resolved to determine the decay parameters. Nonetheless, some broad generalizations can be made ... 

For cases when exponential skimming is not possible or appropriate, due to extremely poor resolution or tailing, the tangential skim method should be used when the peak widths of the unresolved peaks vary significantly (>2 1) [2]. 

A peak can be skimmed from a mother peak by a tangential line (tangential skim). 

The vertical separation method ( Vertical skim, drop ), separation of the tangential ( skim Tangential ), the valley-to-valley separation method, the Gauss separation method ( Gaussian skim ), or the exponential removal ( Exponential skim ) ... 

Three different milk samples were analysed by P solid-state NMR spectroscopy a milk powder sample ( PL60 obtained from a tangential microfiltration of skim milk, Triballat, France) rehydrated at 68% (32% PL60 w/w in D2O, pH=6.37), a lipoproteic matrix as a model cheese sample and a semi-hard cheese. 

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