Mass resolution peak width definition

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

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. 

Figure 5 There are two definitions of mass resolution. These are based on either two overlapping peaks of equal intensity separated by AM (a) or a single well-defined peak with AM defined as the full-width at half-maximum height (FWHM) (b).

A common question asked of the mass spectrometrist is how close in miz can two peaks at mJz = 1 000 or mIz = 20,000 be and still be distinguished with confidence The detailed answer is not as simple as one would like because it is dependent on the sample and instrument resolution. First, there are different definitions that exist for the resolving power of a mass spectrometer. A common definition, R = ml Am at mass m, is used in this chapter. The value of Amk is the measurement of the mass peak at full width at half maximum (FWHM). If the FWHM for the C monoisotopic peak for a 1000-amu peptide is 0.5 amu, then R = 2000, and for a 20,000-amu protein with the same FWHM, R = 40,000. For the preceding example, the isotope peaks would be distinguishable. In other words, for miz up to 10,000 amu, where z = 1, it is possible to obtain monoisotopic information with a mass spectrometer capable of resolution between 20,000 to 30,000. At high mass (>10,000 amu), however, most mass spectrometers can only acquire average molecular weight data, not monoisotopic data. The Fourier transform mass spectrometer is capable ofR= 1,000,000 at miz = 1000. 

If the peak shape is approximately Gaussian, the resolution can be obtained by a single peak. In fact, as shown by Fig. 2.1, the mass difference, AM, is equal to the peak width at 5% of its height and, accordingly to the gaussian definition, it is about two times the fwhm. Consequently, with this approach it is possible to estimate the resolution of a mass analyzer simply by looking at a single peak, without introduction of two isobaric species of different accurate mass. 

A different definition uses the peak width at half height as Am (50 % definition) for the calculation (see Fig. 1). This approach is used often for Fourier transform mass spectroscopy (FTMS) and time-of-flight (TOF) instruments. In modem mass spectrometers, the resolution is calculated directly by the computer from the peak width. 

Figure 2.4 Mass resolution, (a) Calculation of the resolution offered by a mass spectrometer on the basis of the width of a spectral peak at 5% peak height (Am) at mass m. (b) 10% valley definition calculation of the resolution required to resolve the peaks of two ions showing a mass difference of m2 - m. ...

The display of line spectra on screen must be considered completely separately from the resolution of the analyser. By definition, a mass peak with unit mass resolution has a base width of one mass unit or 1000 mDa. On the other hand, the position of the top of the mass peak (centroid) can be calculated exactly. Data sometimes given with one or several digits of a mass unit lead to the false impression of a resolution higher than unit mass resolution. Components appearing at the same time at the ion source with signals of the same nominal mass, but slightly differing exact mass, which can naturally occur in GC-MS (as a result of co-eluates, the matrix, column bleed, etc.) cannot be separated at unit mass... 

The term resolution essentially describes the same the small difference being that its definition is based on the resulting signals. The resolution R is defined as the ratio of the mass of interest, m, to the difference in mass. Am, as defined by the width of a peak at a specific fraction of the peak height. [1]... 

With the advent of linear quadrupole analyzers the full width at half maximum (FWHM) definition of resolution became widespread especially among instruments manufacturers. It is also commonly used for time-of-flight and quadrupole ion trap mass analyzers. With Gaussian peak shapes, the ratio of / fwhm to Rio% is 1.8. The practical consequences of resolution for a pair of peaks at different m/z are illustrated below (Fig. 3.17). 

Resolution the ratio m/Sm where m and m + Sm are the relative masses of the two ions that yield neighbouring peaks with a valley depth x % of the weakest peak s intensity. In the commercial description of mass spectrometers, x = 50 is normally used. However, 10 % valley has been largely used in the past. Another definition entails using for Sm the width of an isolated peak at x % of its maximum. 

Figure 16.14 Resolving power. Left, definition of this parameter in the case of an isolated peak. Depending upon the manufacturer the width of the peak is measured either at 50 per cent or at 5 per cent of its height. Right, example of a low-resolution spectrum of a sample of lead. The value of R found depends very much on the compound and the mass chosen and of the slit width of the instrument.

Generally, the higher the resolution the better is the separation. But when are two peaks considered as being separated This is a question of definition and depends on the analyser. For TOF, Am is deflned as the full width at half maximum (FWHM), i.e. the width of the peak at half its height (Fig. 4.5). With this definition, it is possible to read Rs out of a single peak. Typical resolutions obtained for TOF instruments are Rs = 15,000 (FWHM). For other mass analysers, other definitions like the 70 % valley or 50 % valley are used (Fig. 4.6). For the 50 % valley definition, two peaks are considered separated if the minimum between them (the valley) is not more than 50 % of the peak height whereas for the 10% valley, the minimum between two peaks must not be more than 10% of the peak height. 

What is resolution The resolution of one mass from another and the sensitivity of ion detection are arguably the two most important performance parameters of amass spectrometer. Resolution is a measure of the ability of a mass analyzer to separate ions with different m/z values. Resolution is determined experimentally from the measured width of a single peak at a defined percentage height of that peak and then calculated as mlAm, where m equals mass and Am is the width of the peak. The full width of the peak at half its maximum height (FWHM) is the definition of resolution used most commonly (Figure 1.8). For example, when an ion of mass 600 has... 

Figure 3.14 Illustration of the two most common definitions of resolution in mass spectrometry (a) full width at half maximum (FWHM) and (b) 10% valley definition. Note that the two partially overlapping peaks in (b) have been chosen to have the same heights but this definition would apply equally well to two peaks with different heights.

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