Mass spectrum resolution

Assuming that the mass spectrometer has sufficient mass resolution, the computer can prepare accurate ma.ss data on the m/z values from an unknown substance. To prepare that data, the system must acquire the mass spectrum of a known reference substance for which accurate masses for its ions are already known, and the computer must have a stored table of these reference masses. The computer is programmed first to inspect the newly acquired data from the reference compound in comparison with its stored reference spectrum if all is well, the system then acquires data from the unknown substance. By comparison and interpolation techniques using the known reference... 

Resolution 10% valley definition, m/Am. Let two peaks of equal height in a mass spectrum at masses m and m/Am be separated by a valley that 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 height. Then the resolution (10% valley definition) is m/Am. It is usually a function of m therefore, m/Am should be given for a number of values of m. 

The mass spectrum of pyridazine is simple and high resolution measurements have shown that the ion at m/e 52 is composed of both (73.5%) and C3H2N (26.5%) ions ... 

In quadrupole-based SIMS instruments, mass separation is achieved by passing the secondary ions down a path surrounded by four rods excited with various AC and DC voltages. Different sets of AC and DC conditions are used to direct the flight path of the selected secondary ions into the detector. The primary advantage of this kind of spectrometer is the high speed at which they can switch from peak to peak and their ability to perform analysis of dielectric thin films and bulk insulators. The ability of the quadrupole to switch rapidly between mass peaks enables acquisition of depth profiles with more data points per depth, which improves depth resolution. Additionally, most quadrupole-based SIMS instruments are equipped with enhanced vacuum systems, reducing the detrimental contribution of residual atmospheric species to the mass spectrum. 

Mass spectrum obtained from the NIST Hasteloy Ni-basad standard alloy, using electron-gas SNMSd (Laybold INA-3). The sputtering energy was 1250 V, increasing the sputtered atom flux at the expense of depth resolution. Matrix ion currents ware about 10 cps, yielding background limHed detection at about 2 ppm. 

Qualitatively, the spark source mass spectrum is relatively simple and easy to interpret. Most instrumentation has been designed to operate with a mass resolution Al/dM of about 1500. For example, at mass M= 60 a difference of 0.04 amu can be resolved. This is sufficient for the separation of most hydrocarbons from metals of the same nominal mass and for precise mass determinations to identify most species. Each exposure, as described earlier and shown in Figure 2, covers the mass range from Be to U, with the elemental isotopic patterns clearly resolved for positive identification. 

Structure of Oxy-F Compound F is extremely unstable and is difficult to obtain at a level of purity suitable for NMR studies. However, an oxidation product, Oxy-F, formed when F is left standing at — 20° C, is considerably more stable than F and can be purified to a sufficiently high level of purity. Oxy-F is nonfluorescent and shows absorption maxima at 237 nm and 275 nm (shoulder). The high-resolution FAB mass spectrum indicated the molecular formula of Oxy-F to be C33H3809N4Na2 [m/z 703.2363 (M + Na)+ and 681.2483 (M + H)"1"]. The H and 13C NMR data allowed the assignment of structure 7 to oxy-F (Fig. 3.2.6 Nakamura et al., 1988). 

Resolution (10% valley definition) Let two peaks of equal height in a mass spectrum at masses m and (m - Am) be separated by a valley that at its lowest point is 10% of the height of the peaks. 

Figure 4.13 Theoretical molecular-ion region of the mass spectrum of a molecule of composition C35H48NgOnS at a mass spectrometer resolution of 1500.

If, however, we consider a protein of modest size, such as aprotin with a molecular formula of C284H432N84O79S7 at a similar mass spectrometer resolution, the molecular-ion region of its mass spectrum, shown in Figure 4.14, does not show the individual isotopic contributions, a resolution around 5000 being required for these to be evident (Figure 4.15). 

Figure 4.17 Theoretical mass spectrum of the 7+ charge state of aprotin (molecular weight, 6507) at a mass spectrometer resolution of 5000.

The kinetics study [38] utilized a Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer to measure the pathway branching ratios. The ability to eject selected masses and the extremely high mass resolution of this technique ensured that the observed CD3CH2 was in fact a primary product of the reaction. Temporal profiles from this reaction are shown in Fig. 1. Noticeably absent from the mass spectrum are the cations C2D2H3 and... 

Low resolution MS yields specificity comparable to that of high resolution MS, if a relatively pure sample is delivered to the ion source. Either high resolution GC or additional sample purification is required. To obtain sufficient specificity, it is necessary to demonstrate that the intensities of the major peaks in the mass spectrum are in the correct proportions. Usually 10 to 50 ng of sample is required to establish identity unambiguously. Use of preparative GC for purification of nitrosamines detected by the TEA ( ) is readily adaptable to any nitrosamine present in a complex mixture and requires a minimum of analytical method development when new types of samples are examined. 

Figure 7a demonstrates that FTMS can simultaneously detect ions over a broad mass range. This is the actual high resolution mass spectrum, not a stick plot. This spectrum was obtained when a mixture of CO,, C H., and acetone was leaked into the chamber and ionized by the electron beam. The total sample pressure was... 

C0H4, and acetone, (b) High resolution FTM mass spectrum of the mass 28 region showing CO, N , and C2H4. Reproduced with permission from Ref. 18. Copyright 1985, North-Holland Physics. 

Principles and Characteristics Mass spectrometry can provide the accurate mass determination in a direct measurement mode. For a properly calibrated mass spectrometer the mass accuracy should be expected to be good to at least 0.1 Da. Accurate mass measurements can be made at any resolution (resolution matters only when separating masses). For polymer/additive deformulation the nominal molecular weight of an analyte, as determined with an accuracy of 0.1 Da from the mass spectrum, is generally insufficient to characterise the sample, in view of the small mass differences in commercial additives. With the thousands of additives, it is obvious that the same nominal mass often corresponds to quite a number of possible additive types, e.g. NPG dibenzoate, Tinuvin 312, Uvistat 247, Flexricin P-1, isobutylpalmitate and fumaric acid for m = 312 Da see also Table 6.7 for m = 268 Da. Accurate mass measurements are most often made in El mode, since the sensitivity is high, and reference mass peaks are readily available (using various fluorinated reference materials). Accurate mass measurements can also be made in Cl... 

Meyer-Dulheuer [55] has analysed the pure additives (phenolic antioxidants, benzotriazole UV stabilisers and HALS compounds) of Table 9.8 in THF solutions by means of MALDI-ToFMS. As it turns out, polar molecules in the mass range of below 800 Da, which have a high absorption coefficient at the laser wavelength used, can often be measured without any matrix [55,56]. In this case, there is no matrix-assisted laser desorption and ionisation (MALDI) process any more. It is a simple laser desorption/ionisation (LDI) process. The advantage of this method is a matrix-free mass spectrum with the same mass resolution as in the MALDI case,... 

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