Resolving Power and Resolution of a Mass Spectrometer

The resolving power of a mass spectrometer is defined as its ability to separate ions of two different m/z values. Numerically, the resolution is equal to the mass of one singly charged ion, M, divided by the difference in mass between M and the next m/z value that can be separated. For example, to separate two singly charged ions of 999 and 1001 Da requires a resolving power of 999/(1001 — 999) = 500. That is  

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 

Resolving power for commercial mass spectrometers depends on the instrument design, and can range from 500 to more than 1 x 10 . In general, the higher the resolving power, the greater the complexity and cost of the MS instrument. 

Resolution is the value of AM at a given M and is often expressed in ppm. For the spectrometer with a resolving power of 600 earlier described, the resolution would be 1 part in 600 parts. To distinguish between with an exact mass of 28.0061 Da and with an exact mass of 27.9949 Da, we would need a resolution of 

All mass spectrometers require a sample input system, an ionization source, a mass analyzer, and a detector. All of the components with the exception of some sample input systems or ion source volumes are under vacuum (10 -10 torr for that portion where ions are separated by mass, i.e., the analyzer, or 10 -10 torr in some ion sources, where the ions are initially formed), so vacuum pumps of various types are required. Modern mass spectrometers have all of the components under computer control, with a computer-based data acquisition and processing system. A block diagram of a typical mass spectrometer is shown in Fig. 9.4. 

9949 Da, we would need a resolution of (28.0061 - 27.9949) = 0.0112 Da in 28 Da. To convert this resolution to ppm, divide 0.0112 by 28 and multiply by 1 x 10 a resolution of 400 ppm is required. The resolving power needed would be 27.9949/0.00112 = 2500. Mass accuracy describes how well the top of the MS peak can be defined and kept stable and its position identified on the mass axis. Mass axis calibration is usually established by a separate portion of the tuning algorithm, which identifies the centroids of several mass peaks of precisely known mass in the spectrum of a suitable calibration compound and adjusts tuning parameters to place these at the appropriate positions on the mass axis. Like resolution, it can be expressed as a percentage or ppm value of the calibration mass values. 

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