Peak width, definition

Peak width definition (threshold) Lets the computer know how to distinguish between true peaks and detector spikes. 

Figure 3.15 Demonstration of two definitions of mass resolution (R = m/4m) 10 % valley definition and peak width definition.

Values derived using the 10% peak width definition within the single ion mode (see Section 5.1.1.1.1) iQ cmjq ratios less than 100. 

Peedc-to-peak resolution in SEC can be calculated by the ratio of peak separation at the peak maxiaut to the sum of the baseline peak widths. This general definition of resolution is less useful in SEC, where a measure of the ability of the column to separate solutes of different molecular weight is required. For this purposes, we define a new term, the specific resolution factor, R, which relates peak resolution to sample molecular weight, assuming all measurements are made within the linear region of the molecular weight calibration curve, equation (4.41)... 

Constant peak width is recognized to give more peak capacity in LC and the same is true in 2DLC. The use of this equation is interesting in that the number of theoretical plates is not constant across a chromatogram of constant peak width. This is easily shown by recalling the definition of the number of plates as... 

Two neighboring peaks are assumed to be sufficiently separated when the valley separating their maxima has decreased to 10 % of their intensity. Hence, this is known as 10 % valley definition of resolution, Rio%- The 10 % valley conditions are fulfilled if the peak width at 5 % relative height equals the mass difference of the corresponding ions, because then the 5 % contribution of each peak to the same point of the m/z axis adds up to 10 % (Fig. 3.16). 

Peak Width. Peak width depends on the mass resolution. A resolution of 1 mass unit is sufficient to distinguish ions in most qualitative/quantitative small molecule applications. A typical definition of unit resolution is when the peak width at half-height is about 0.6 to 0.8 mass unit. The profile scan of ions on a typical benchtop LC-MS has a bandwidth of about 1 mass unit (Figure 13.1). 

FWHM Full width at half-maximum. Mass resolution is often difficult to determine at or near the base of a peak due to baseline noise and peak overlap. It is more common to measure the width of the peak halfway to the peak maximum, where a clean measurement is possible. The most common alternative to FWHM was the 10% valley definition, in which the peak width at 10% of height was examined. This latter definition is common in the literature, especially for magnetic sector mass spectrometers, but is currently used much less frequently than FWHM. The choice of FWHM or 10% valley has an impact on the calculation of resolving power. 

For our purposes, the 5 a width is more useful for determining Vw. It is determined by drawing tangents to both sides of the peak and measuring the distance between the intersection of these with the base line. Using this definition of peak width, the calculation of N equals 16 times the square of VB/VW-Different peaks in a mixture will give different efficiency values. 

The suggestion given before that in an ideal programmed analysis the peak width should be the same for all solutes (see also figure 6.1c) corresponds to the assumption that kg is equal for all peaks. For a definition of linear solvent strength (LSS) gradients see section 5.4. 

Perhaps it is obvious that the meaning of Eq. (1) is that peak width would depend on retention time fR, since retention time expresses the total time (or number of opportunities) available for zone broadening. This interpretation of L can be shown by combining a few basic equations. The definition of H is... 

The spread of data about the mean is usually measured with the standard deviation ct, but another common parameter is the range. (In our discussion of Gaussian chromatographic peaks, we have used peak width.) By definition,... 

The resolving power can also be determined with an isolated peak. Indeed, the resolving power is also defined using the peak width Am at x % of the peak height. Often x is taken to be 50 % and Am is designated as full width at half maximum (FWHM). The relationship between the two definitions is obvious for two peaks with equal intensities. The resolution full width at x % of the peak height is equal to the resolution at 2x % for the valley (Figure 2.2). 

A more convenient definition, commonly used with trapping and ToF analyzers, pertains to a single well-resolved peak where AMX is the peak width at a specified height x, usually half-maximum height (full-width at half-maximum height, FWHM) (Figure 5(b)). It should be noted that this FWHM definition of resolution equates to about twice that calculated from the 10% valley definition. 

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