Mass Resolution and Resolving Power

One of the challenges in measuring isotope ratios accurately is to achieve acceptable mass resolution to separate atomic ions from polyatomic ions, largely consisting of argide, hydride, and oxide ions. For example, in the measurement of Fe isotopes ( Fe, Fe, Fe, and Fe), common spectral interferences occur as a result of the presence of ArN+, ArO+, and ArOH ions at masses 54, 56, 57, 

The first generation of MC-ICP-MS instruments employed the relatively low mass resolution ion optics found in thermal ionization instruments. As users 

and hydrides of Ar and N interfere with almost every isotope of Ca (Table 3.1). 

Isotope Natural abundance (%) Interferences Resolution required 

The practical mass resolution of this configuration is called the resolving power and calculated as shown in the following equation  

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GC-MS of the fluid inclusion oils can be performed on benchtop quadrupole, triple quadrupole, or sector instruments. The main criterion is sensitivity because such low amounts of oil are recovered from fluid inclusions, sometime lower-sensitivity instruments such as benchtop quadrupole instruments do not give sufficient quality data to be reliably interpreted. Further subordinate criteria are mass resolution and resolving power it helps to separate certain compounds using better than unit mass resolution and/or tandem mass spectrometry... 

Mass Resolution and Resolving Power. Thermo Fisher Scientific,... 

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. ... 

It is important to realize the difference between accurate mass determination and resolving power. Accurate mass can be determined at a quite low resolution but requires an isolated peak, and thus applies safely only on a high-quality spectrum of one pure compound. Risks of having mixed peaks, that is peaks of compounds having the same nominal mass but different elemental compositions, with pure compounds still exist for the... 

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. 

Resolving Power and Resolution In Mass Spectrometry The resolving power of a mass spectrometer describes the smallest mass differences which can be separated by the mass analyser (Webb, 2004b). Resolution and resolving power in MS today are defined differently depending on the analyser or instrument type, and is often stated without the indication of the definition employed. The new fUPAC definitions of terms used in MS provide a precise definition. 

ESI-FTMS is well-suited for accurate mass measurement of large molecules. The superb mass accuracy and resolving power allow the determination of the molecular weights of large proteins. Unit mass resolution has been demonstrated for the protein chondroitinase I... 

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). 

Orbital trapping mass spectrometers achieve resolutions of up to 105 and would be the next choice after ToF mass spectrometers if resolving powers above 104 are required. In addition to mass resolution, the selectivity of an MS can be critical to distinguish between co-eluting and not mass-resolved compounds. For example, typical triple-quad mass spectrometers usually cannot achieve better than unit-mass resolution. However, special operation modes like neutral loss scans and precursor ion scans can filter out compounds of interest even if neither LC separations nor MS scans would be sufficient to resolve these compounds (note that this is a filtering step). 

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. 

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... 

Mass Spectrometer Tuning. A three-step process was used to tune the quadrupole mass spectrometer prior to its use as a detector for SFC. In the first step, perfluorotributylamine (Pfaltz Bauer Inc., Stamford, Conn.) was ionized by electron ionization and used to calibrate the mass axis. In the second step, methane was introduced into the Cl source and the reagent ion profiles were optimized. In the third step, the mass resolution was adjusted for improved sensitivity. This was accomplished by introducing a volatile brominated compound, such as 2-bromopentane, into the Cl source. The mass spectrometer s resolving power was reduced such that the peaks... 

Basic characteristics of the quality of mass analyzer are resolving power (RP), which has been used to specify the ability of mass analyzers performing high-resolution mass analysis and it is defined and/or calculated by using the mass (M) and the full-width-at-half-maximum (FWHM) of a mass spectral peak, and mass accuracy (MA) is the measured error in m/q divided by the accurate m/q [105]. 

Example I The low-resolution (LR) positive-ion ESI mass spectrum and the charge-deconvoluted molecular weight spectrum inset) of bovine serum albumin (BSA) as obtained from a quadrupole ion trap instrument are compared below (Fig. 12.24). In case of such high-mass ions the resolving power of a quadrupole ion trap is insufficient to separate different cationization products of equal charge state. Nonetheless, charge deconvolution reveals that ion series A belongs to the noncovalent BSA dimer, while series B results from the monomer [26]. 

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