Peaks mass resolution

High mass resolution techniques are used to separate peaks at the same nominal mass by the very small mass differences between them. As an example, a combination of Si and H to form the molecular ion Si H , severely degrades the detection limit of phosphorous ( P) in a silicon sample. The exact mass of phosphorous ( P) is 31.9738 amu while the real masses of the interfering Si H and Si H2 molecules are 31.9816 amu and 31.9921 amu, respectively. Figure 8 shows a mass... 

SiiH2 are completely separated from the peak. Quadrupole instruments are not usually capable of such high mass resolution. 

Figure 3.5 shows the positive SSIMS spectrum from a silicon wafer, illustrating both the allocation of peaks and potential isobaric problems. SSIMS reveals many impurities on the surface, particularly hydrocarbons, for which it is especially sensitive. The spectrum also demonstrates reduction of isobaric interference by high-mass resolution. For reasons discussed in Sect. 3.1.3, the peak heights cannot be taken to be directly proportional to the concentrations on the surface, and standards must be used to quantify trace elements. 

The results obtained by Brewis et al. demonstrate an important feature of TOF-SIMS and that is the ultra-high mass resolution of the teehnique. Without sueh high speetral resolution, it would be impossible to resolve peaks sueh as those shown in Fig. 50 into eomponents and a great deal of information about the oxidation proeess would be unavailable. 

Resolution Quadrupole instruments are not capable of achieving the high resolution that is common with double-focusing magnetic-sector instruments. In GC/MS analyses, a compromise is struck between sensitivity (ion transmission) and mass resolution. In the quadrupole instrument, the resolution is set to the lowest possible value commensurate with resolving peaks differing by 1 Dalton (unit resolution). 

Figure 6.1 Sections (mlz 5000-10,000) from unpublished spectra—standard (a) and blind-coded (b)—obtained from Enterobacter cloaca from work reported in Holland et al.17 showing the very low mass resolution, additional peaks, and three consistent signals used to identify this organism in the first reported whole-cell MALDI experiments in 1996.

Mass resolution describes the capability of an MS to distinguish ions with different m/z values. It is defined by the M/AM equation in which M is the m/z ratio of a mass peak and AM is the full width of a peak at half its maximum height. The mass resolution of an instrument often correlates with its accuracy. Mass range indicates the m/z range at which the mass analyzer best functions. For example, quadrupole mass analyzers exhibit a mass range of up to 4000 m/z, while the mass ranges of TOF extend up to 100,000. The operating principles of common MS instruments are discussed below. 

Mass Resolution The smallest mass difference Am (Am in Da or Am/m in, e.g., ppm) between two equal magnitude peaks such that the valley between them is a specified fraction of the peak height [3]. 

The FT-ICR/MS is an ideal instrument for studying ion-molecule reactions over an extended time scale due to the excellent trapping of ions in the cell and the unmatched mass resolution and mass accuracy. Mass resolution is defined as the mass divided by the peak width at half height... 

Example ESI selectively ionizes the basic compounds, i.e., only a small fraction of the entire chemical composition, in a sample of South American cmde oil. Nevertheless, the positive-ion ESI-FT-ICR mass spectrum exhibits more than 11,100 resolved peaks, of which >75 % may be assigned to a unique elemental composition (CcHhOoNnSJ. Such a separation in mass is possible because the average mass resolution in the m/z 225-1000 broadband spectrum is approximately 350,000 (Fig. 12.12). This demonstrates the current upper limit for the number of chemically distinct components resolved and identified in a single step. [86]... 

For example for an ion measured at m/z 552 with a peak width of 0.5 m/z units (FWHM) the mass resolution would be 0.5, while the mass resolving power... 

Fig. 1.4 Illustration of the mass resolution using two peaks of equal intensities (Amg) and a single peak (Amt).

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