Mass resolving power definition

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). 

Although there is no clear definition of a high-resolution mass spectrometer, it is generally accepted that S-MS, FT-MS, and TOF-MS devices and the related hybrid mass spectrometers are high-resolution instruments. For example, the maximum mass resolving power of a double-focusing S-MS system is roughly 60 000, and in a TOF-MS... 

Mass resolving power The ratio of the mass of an ion to the full-width-at-half-maximum (FWHM) of its spectral feature in a mass spectrum, or m/Am. Notably, the definitions of resolving power and resolution were reversed for historical reasons. Since in modern MS, one usually reports only m/Am, in this book we use the terms mass resolving power and resolution to refer to the same properties of mass spectra. For details please refer to Section 5.3.1. 

According to its definition in Equation 3.35, the mass resolving power is inversely proportional to the resolution. Am. Thus, if Am = 0.01 Da for a peak with a maximum located at 100 Da, then the resolving power would be 10. However, this definition is not universally used and in some definitions the resolving power and the resolution are essentially one and the same quantity. This is highly confusing and so in this book we will stick to the above definitions of resolving power as m/Am and resolution as Am, where Am will be considered to be the FWHM of an isolated peak in the mass spectrum. 

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. 

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. 

No discussion of MS data or comparison of mass analyzers would be complete without including some definition of the data quality, particularly, the accuracy of the data and the resolving power at which they were obtained. 

With respect to mass spectral matching, the criteria for identification vary depending on the technique used for mass spectral data acquisition (see summary of requirements in Table 8). It is interesting to note that while the FDA does not rule out the use of exact mass measurements, it views these data as problematical as there are no generally accepted specific standards for their use. The problem here is that it is difficult to be definitive about the resolving power required, particularly, when analytes have masses greater than m/z 500. Clearly the resolving power and accuracy must be sufficient to exclude all reasonable alternative elemental compositions and they recommend that if exact mass measurements are to be used then multiple structurally specific ions should be measured. 

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. 

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.

The resolving power is determined by actual measurement of the mass spectral peaks obtained. The method for calculating AM must also be specified. Two methods are commonly used to indicate the separation between peaks and these are shown in Fig. 9.3. One definition is that the overlap between the peaks is 10% or less for two peaks of equal height that is, the height of the overlap should not be more than 10% of... 

It is important to understand how resolution and resolving power are used in reporting mass spectrometric data. The terms resolution and resolving power are defined in some pubhshed lists as they are above, while in others the definitions are reversed. On occasion, the two terms are used interchangeably. Having multiple definitions can be confusing. 

By definition, the mass resolution of a mass spectrometer is its ability to distinguish between two neighboring ions that differ only slightly in their mass (Am). Mathematically, it is the inverse of resolving power (RP), given as... 

Figure 3.1. Resolving power 10% valley definition (depicted by separation of two ions of mass m and m2) and FWHM definition hH is half the height of the peak).

Another important equation regards the resolving power in lEF, expressed in A(pl) units, i.e., in the minimum difference of surface charge between two adjacent proteins that the lEF technique is able to resolve. If two adjacent zones of equal mass have a peak-to-peak distance three times larger than the distance from peak to inflection point, there will be a concentration minimum approximating the two outer inflection points. Taking this criterion for resolved adjacent proteins, Rilbe has derived the following equation for minimally but definitely resolved zones ... 

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