Mass analyzers resolving power

Mass analyzer Resolving power (xlO3) Mass accuracy (ppm) m z range (upper limit) (xlO3) Acquisition speed (Hz) Linear dynamic range Price... 

An added consideration is that the TOF instruments are easily and quickly calibrated. As the mass range increases again (m/z 5,000-50,000), magnetic-sector instruments (with added electric sector) and ion cyclotron resonance instruments are very effective, but their prices tend to match the increases in resolving powers. At the top end of these ranges, masses of several million have been analyzed by using Fourier-transform ion cyclotron resonance (FTICR) instruments, but such measurements tend to be isolated rather than targets that can be achieved in everyday use. 

Accurate mass measurement requires high resolving power. The difference in degrees of difficulty between measuring an m/z of 28 and one of 28.000 is likely to be large. Table 39.3 shows the broad mass ranges achievable with various analyzers. 

Resolution or resolving power is the ability of a mass spectrometer, and in particular of its analyzer system, to separate ions with different m/z ratios. An example of mass spectra obtained at different resolutions is reported in Figure 2.1 by increasing the resolution the peak shape becomes more and more narrow thus allowing the separation of ions with their m/z values differing in decimals (10 1—10 3). 

The m/z values of peptide ions are mathematically derived from the sine wave profile by the performance of a fast Fourier transform operation. Thus, the detection of ions by FTICR is distinct from results from other MS approaches because the peptide ions are detected by their oscillation near the detection plate rather than by collision with a detector. Consequently, masses are resolved only by cyclotron frequency and not in space (sector instruments) or time (TOF analyzers). The magnetic field strength measured in Tesla correlates with the performance properties of FTICR. The instruments are very powerful and provide exquisitely high mass accuracy, mass resolution, and sensitivity—desirable properties in the analysis of complex protein mixtures. FTICR instruments are especially compatible with ESI29 but may also be used with MALDI as an ionization source.30 FTICR requires sophisticated expertise. Nevertheless, this technique is increasingly employed successfully in proteomics studies. 

A word of caution from the authors The peak height and peak area are not interchangeable quantities. Consider for example how the height-to-area relationship depends on the resolving power of the mass analyzer or the response time of the detector. 

The molecular ions produced in the MALDI process have relatively high initial velocities, which can cause reduction in mass resolving power and transmission, primarily for TOF analyzers with axial ion extraction (see Section 2.2.1). Hence, the MALDI-MS mass resolving power depends strongly on laser fluence and is highest when the laser fluence is close to the threshold level. 

A mass calibration for FTICR analyzers with superconducting magnets is very stable and is valid for many days for normal applications. Mass accuracy < 1 ppm can be obtained over a fairly wide mass range. Unique elemental composition can be determined for masses over 800 Da [262]. Recently, 0.1 ppm mass accuracy, which required a mass resolving power >300,000, has been achieved for several thousand peaks by a 14.5 T instrument [263] and commercial instruments with mass accuracy <0.2 ppm are available. As with the orbitrap (see Section 2.2.5) the frequency is... 

The FTICR analyzer is relatively slow. In a low resolution mode (<25,000 FWHM) scans can be performed in substantially less than a second. A high resolution scan is more time demanding, and more than 1 s is often required for mass resolving powers of 100,000 or more. 

Mass analyzer Representative resolving power values... 

Resolution (or resolving power) plays an important role in mass spectrometry for applications requiring the characterization of very similar chemical species. The ability to detect and accurately measure the m/z ratio of a particular ion depends directly on the resolving power of the mass analyzer. For example, if a sample contains two isobaric compounds (i.e., having the same nominal molecular mass but different elemental formulae) the difference in the exact masses of the molecular ions will be much less than 1 m/z unit. Any mass analyzer possessing a nominal resolving power (e.g., RP< 1000) will register only one peak in the mass spectrum of such a binary mixture. Attempts to measure the... 

Therefore, a mass analyzer capable of resolving powers of 26,000 or better is required to reveal the presence of two ions (representing two distinct compounds). If an analyzer with RP< 26,000 is used, the presence of a second compound would be overlooked. Additionally, attempts to measure the exact m/z value of the peak will not provide a true value, but rather some average value (e.g., m/z 937.5072, assuming the compounds produce signals of comparable intensity) that will be of little use in identifying either unknown compound. Ambiguous results such as these only complicate qualitative analysis efforts. 

Achieving high resolving power and high m/z measurement accuracy is one way of decreasing uncertainty when the determination of unknown analyte identity is the object of an experiment. But like many techniques, an increase in experimental or interpretive confidence does not come without some cost (e.g., instrument size, complexity, price, etc.). However, exact m/z measurements (and their associated elemental formula information) are but one type of information that can be derived from mass spectrometers. In the sections that follow, a variety of mass analyzers will be described in terms of their basic principles, functionality and applications. 

Perhaps the simplest mass analyzer of all, the TOF mass spectrometer [46] has experienced a reemergence in the past several years. Like the 3D quadrupole ion trap, the TOF analyzer has come to commercial prominence several decades after its initial introduction. The limitations of electronic components in the 1960s constrained the capabilities of the instrument, limiting its mass range and resolving power. The TOF analyzer operates in a pulsed mode, requiring either a pulsed ion... 

What mass analyzer exhibits the highest achievable mass resolving power (ICR spectrometer, a.k.a. FTMS). 

Although the above mass spectrometric tools have mass ranges and resolving powers adequate for chemical analysis, mass spectral characterization and structural analysis of biopolymers generally demand efficient detection of ions over a wide mass range, accurate mass measurements, and high mass resolution. The FT-ICR analyzer is able to combine high resolution and MS" capabilities. ... 

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