Nominal mass definition

There is a definite possibility that even the nominal mass number is in error, because of the very large number of spectra examined and the "uncertified" nature of these. This is especially true, naturally, at higher mass numbers, where resolution is decreased and mass markers become less reliable. However, many of these errors were eliminated because of the correlative nature of this study, and in doubtful cases, the peak was put in the "unclassified" section. The author welcomes correspondence on any errors which are foundby readers, and hopes that such helpful criticism, as well as the increased availability of high resolution spectra — e.g., (3) — will largely eliminate such errors in later editions of this table. 

Let us briefly repeat i) the isotopic mass is also the exact mass of an isotope it) the isotopic mass is very close but not equal to the nominal mass of that isotope Hi) accordingly, the calculated exact mass of a molecule or of a mono-isotopic ion equals its monoisotopic mass iv) due to the definition of our mass scale, the isotope represents the only exception from non-integer isotopic mass. As a consequence of these individual non-integer isotopic masses, almost no combination of elements in a molecular or ionic formula has the same calculated exact mass, or simply exact mass as it is often referred to, as any other one [36]. 

Definition The molecular ion has the same formula as the corresponding neutral molecule. The neutral and its molecular ion only differ by one (or more) electron(s). A singly charged molecular ion can either be a positive radical ion, IVE, or a negative radical ion, hT (not in the case of El). ITie nominal mass of this ion corresponds to the sum of the masses of the most abundant isotopes of the various atoms that make up the molecule [5,6]. 

The display of line spectra on screen must be considered completely separately from the resolution of the analyser. By definition, a mass peak with unit mass resolution has a base width of one mass unit or 1000 mDa. On the other hand, the position of the top of the mass peak (centroid) can be calculated exactly. Data sometimes given with one or several digits of a mass unit lead to the false impression of a resolution higher than unit mass resolution. Components appearing at the same time at the ion source with signals of the same nominal mass, but slightly differing exact mass, which can naturally occur in GC-MS (as a result of co-eluates, the matrix, column bleed, etc.) cannot be separated at unit mass... 

The detection and quantification of polychlorinated dibenzo-p-dioxins, and in particular the 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (2,3,7,8-TCDD), is a major application for double-focusing magnetic-sector mass spectrometers. Despite extensive clean-up procedures, samples still contain compounds such as polychlorinated biphenyls and benzyl phenyl ethers, which have the same nominal masses as the compounds of interest. The sample is spiked with a known amount of the isotope labelled form of 2,3,7,8-TCDD, introduced via gas chromatography and recorded by high-resolution mass spectrometry. The measurement is quantified by comparison of the native dioxin response to that from the C-labelled form, and verified by confirmation of the ratio of the major isotopes of both the native and the C-labelled dioxins. At 10 000 resolving power (10% valley definition) the detection level for 2,3,7,8-TCDD is about 1 femtogram, or 3 attomole (Figure 6). 

Na and Nb are the numbers of atoms in the two groups A and B and S is the switching function. With the group-based switching function, it is necessary to define the distance between the two groups (i.e. the two points Ta and Tb). There is no definitive way to do this. As with cutoffs, a special marker atom can be nominated within each residue, or the centre of mass, centre of geometry or centre of charge may be used. 

The law of definite proportions was a crucial step in the development of modern chemistry, and by 1808, Proust s conclusions had become widely accepted. We now recognize that this law is not strictly true in all cases. Although all gaseous compounds obey Proust s law, certain solids exist with a small range of compositions and are called nonstoichiometric compounds. An example is wiistite, which has the nominal chemical formula FeO (with 77.73% iron by mass), but the composition of which, in fact, ranges continuously from Feo.950 (with 76.8% iron) down to Fco.siO (74.8% iron), depending on the method of preparation. Such compounds are called berthollides, in honor of Berthollet. We now know, on the atomic level, why they are nonstoichiometric (see the discussion in Section 21.6). 

FIGURE 13.1 Definition of FWHM and comparison of mass spectral data simulated for two nominally isobaric metabolites (500 Th) but with a decimal mass difference of 36.4 mDa using FWHM resolutions of 1,000, 5,000,16,667, and 50,000 (top to bottom). 

Improving the mass resolution (definition is covered in Section 5.1.1.1.1) reduces this m/q spread, thereby allowing for the separation of secondary ion signals of similar (nominal) m/q ratios. Minimal spread is noted in the FT-ICR mass spectra shown in Figure 5.2(c) where an extremely High Mass Resolution (HMR) was used (Smith et al. 2013). 

The reaction proces.ses can be described by a combination of results from leach tests and SIMS. The nominal composition and leach rates (g in - d" ) of a glass formulation, PNL 76-68 (Pacific North-West Laboratories), used as the. standard high-level nuclear waste glass matrix [4], are shown in Table 4. The definition of leach rate u.sed normalizes the loss (in g) to the proportion of that element in the nominal bulk composition. It will be seen that the leach rates for Cs, Na, Mo, Si and B exceeded the overall rate of mass loss from the gla.ss surface, whereas the elements Fe, Zn and Ti did not show significant loss to solution under these conditions. There were also several elements, e.g.. Ca. Ba, Cd and Sr, that were neither rapidly leached nor apparently retained in the surface layers. Studies described in the above-mentioned reviews, using XPS, FTIR, SIMS, SEM and dissolution rates, have established clearly that the primary reaction occurring in solution is the bond-breaking attack by OH at Si (or Al and... 

A common problem in the allometric modeling of IH toxicity has been how to best statistically treat non-independent parameters (see Section 9.2.1). Often, both regressor and predictor terms are dependent on the species body mass (M). A solution has been developed [eqn (9.6)] that is both faithful to the definition of a nominal IH dose [eqn (9.5)] and eliminates non-independent parameters. 

---