Peak shape height/width/area

Figure 25-3 indicates how precision is affected by peak size and shape with two manual measurement techniques for noise-free peaks. By height-width measurements the precision for peak areas improves with increasing area in proportion to the square root of the area, and by planimetry, in proportion to the power of the area. 

Inverse gas chromatography is typically considered to be a singlesolute chromatographic technique. The shape, height, width, and total area of the eluting peak are used to determine the absorption and desorption characteristics of the probe or binding component with the solid absorbent. 

Example The influence of relative slit width on peak shape and resolution is demonstrated on the second isotopic peak of toluene molecular ion, m/z 94 (Fig. 4.25). With the entrance slit at 50 pm and the exit slit at 500 pm the peak is flat-topped (left), because a narrow beam from the entrance sweeps over the wide open detector slit keeping the intensity constant as the scan proceeds until the beam passes over the other edge of the slit. Closing the exit slit to 100 pm increases resolution to 2000 without affecting the peak height (middle), but reduces the peak area by a factor of 4 in accordance with an increase in resolution by the same factor. Further reduction of the exit slit width to 30 pm improves... 

Height-width is the preferred manual area technique assuming reasonable peak shapes. 

Figure 5.1 shows a portion of a chromatogram. Three parameters are illustrated, the peak width, W, the noise, N, and the peak height, S. Peak height is measured in any convenient units from the base of the peak to its maximum. The width of the peak (in seconds) is defined in various ways (a) the peak width at half the height (illustrated), (b) the base of the triangle which most closely matches the shape of the peak, (c) a multiple of the variance, or second moment, of the peak shape, (d) the ratio (in consistent units) 2A/S where A is the area of the peak. The first method is used here because it is the easiest to measure. 

Figure 24.1 Peak shapes as obtained with different column diameters and separation performances (as a function of particle diameter at a given column length). Columns 1 and 3 are packed with a coarse stationary phase, columns 2 and 4 with a fine one. The packing quality, defined as reduced plate height h, is the same in all four cases. Separation performance is independent of column inner diameter therefore peaks 1 and 3 as well as 2 and 4, respectively, are ofthe same width. The peak height in the eluate can be calculated from equation (15) it is higher when the column is thinner and the separation performance is better. Peak areas cannot be compared although the same amount is injected in any case if a concentration-sensitive detector is used optimum flow rate depends on particle size and hence also the residence time in the detector.

There is always a linear relationship between sample injected volume and the area of the recorded peak. Regarding the use of peak height or peak width, three main scenarios can be considered, each one associated with a different influence of the injected sample volume on the recorded peak shape. 

To illustrate the influence of the filter width on the distortion of the peak shape, the error of evaluating the height and area by polynomial smoothing of Gaussian and Lorentz peaks is shown in Figure 3.3 (cf. Figure 3.4). 

The Mossbauer parameters are derived from the peak parameters (base line parameters, peak position, peak width, and peak area/height) via the fitting process by computer evaluation of spectra in the case of the so-called model-dependent evaluation. In this case, an exact a priori knowledge about the spectrum decomposition (peak-shape function, number, and type of subspectra corresponding to the interactions assumed for each microenvironment in the model) is inevitably necessary. (Incorrectly chosen number of peaks renders the analysis itself incorrect.)... 

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