Resolving power, definition

Equation (19) shows that the resolving power of the column (using the definition proposed by Giddings) is directly proportional to the square root of (Ne), the number... 

The power of a microscope to reveal details depends less on the magnification and more on the clarity or sharpness of the image produced by the objective. A simple definition of the resolving power of a microscope is the smallest distance between two points in the object such that the two points can be distinguished in the image. 

Let two peaks of equal height in a mass spectrum at masses m and m, Am, be separated by a valley which at its lowest point is just 10% of the height of either peak. For similar peaks at a mass exceeding m, let the height of the valley at its lowest point be more (by any amount) than 10% of either peak. Then the resolution (10% valley definition) is m/Am. The ratio m/Am should be given for a number of values of m [4], Comment. This is a typical example of the confusion regarding the definition of the term resolution. Here resolution is used instead of the more appropriate phrase mass resolving power (which is the inverse of resolution). 

Mass spectrometry. (+)Fast atom bombardment (FAB) mass spectrometry was carried out with a JEOL JMS-SX/SX102A mass spectrometer. Dried samples were dissolved in methanol-water, mixed with (thio-) glycerol, and applied to a direct insertion probe. During the high resolution FAB-MS measurements, a resolving power of 10,000 (10% valley definition) was used. Cesium iodide, glycerol, or polyethylene oxide (MWav = 600) was used to calibrate the mass spectrometer. 

Resolving Power or Resolution is the ability of a mass spectrometer to distinguish between ions of different mass-to-charge ratios such that greater resolution corresponds directly to the increased ability to differentiate ions. For example, a mass spectrometer with a resolution of 500 can distinguish between ions of m/z = 500 and 501. The most common definition of resolution is given by the following equation ... 

Because slits in the range 0.1 to 1mm width are used in mass spectrometers, the separated ion beams have a defined breath b close to the slits. The capability of the mass spectrometer for separating ion beams with different masses m and m+Ara is characterized by the mass resolution R (or resolving power) using the following definition ... 

Resolving Power (RP) A measurement of how effectively a mass analyzer can distinguish between two peaks at different, but similar m/z. Mathematically, the formula M/ AM is used, where M is the m/z value for one of the peaks and AM is the spacing, in unified atomic mass units, between the peaks. Most commonly, AM is the mass resolution, either via the 10% valley or FWHM definitions (see below). (Note that the definition used will affect the resolving power calculated.) Resolving power of 500-1000 approximately corresponds to unit resolution (e.g., at m/z 700 and FWHM resolution of 0.7, RP = 1000). 

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. 

The quadrupole/time-of-flight analyzer (QToF) has become a key option in the qualitative and quantitative analytical arena. Instruments with resolving power of 20000 (50% valley definition) can provide <5ppm mass accuracy for parent and product ion identification and for 20mda mass selection windows quantitation. While the triple quadra-pole retains the lead in sensitivity for quantification, the QToF has a decided edge on specificity (Micromass, 1999) and qualitative analysis. 

From a strict biochemical point of view a clear-cut definition of the role of the liver in the biosynthesis of any particular plasma protein can be made only when the particular protein has been clearly and cleanly isolated, as in the case of fibrinogen. The practical difficulties of effecting such isolations on a small scale from isotopic labeling studies of the plasma proteins, such as we have described, seriously militate against such a detailed demonstration at present. The use of fractionation techniques with greater resolving power such as acrylamide gel electrophoresis already show some promise in our laboratory toward affording a more definitive picture of the biosynthetic role of the liver and the nonhepatic tissues in plasma protein production. 

Last but not least, there is the characteristic of a mass analyser concerning the resolution or its resolving power. Resolution or resolving power is the ability of a mass analyser to yield distinct signals for two ions with a small m/z difference (Figure 2.1). The exact definition of these terms is one of the more confusing subjects of mass spectrometry terminology that continues to be debated. We will use here the definitions proposed by Marshall [1], This will be described in more details in Chapter 6. 

Two peaks are considered to be resolved if the valley between them is equal to 10 % of the weaker peak intensity when using magnetic or ion cyclotron resonance (ICR) instruments and 50 % when using quadrupoles, ion trap, TOF, and so on. If Am is the smallest mass difference for which two peaks with masses m and m + Am are resolved, the definition of the resolving power R is R = ml Am. Therefore, a greater resolving power corresponds to the increased ability to distinguish ions with a smaller mass difference. 

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). 

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