Exact mass definition

Other numerical values are exact by definition. For example, the atomic mass scale was established by fixing the mass of one atom of 12C as 12.000 Ou. As many more zeros could be added as desired. Other examples include the definition of the inch (1 in = 2.5400 cm) and the calorie (1 cal = 4.184 00 J). 

With respect to mass spectral matching, the criteria for identification vary depending on the technique used for mass spectral data acquisition (see summary of requirements in Table 8). It is interesting to note that while the FDA does not rule out the use of exact mass measurements, it views these data as problematical as there are no generally accepted specific standards for their use. The problem here is that it is difficult to be definitive about the resolving power required, particularly, when analytes have masses greater than m/z 500. Clearly the resolving power and accuracy must be sufficient to exclude all reasonable alternative elemental compositions and they recommend that if exact mass measurements are to be used then multiple structurally specific ions should be measured. 

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]. 

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... 

E.29 The king of Zirconia is naturally fond of the element zirconium and has established an independent definition of the mole based on zirconium. The mass of one zirconium-90 atom is 1.4929 X 10 22 g. If zirconium were the standard used for molar mass (instead of carbon-12), 1 mol would be defined as the amount of substance that contains the same number of entities as there are atoms in exactly 90 g of zirconium-90. In that case, what would be (a) the molar mass of carbon-12 (b) the (average) molar mass of gold ... 

Note that the number of significant figures in the result is determined by the precision of the mass of 202Hg, because the mass of12C is established by definition as an exact number. 

Example Cesium iodide is frequently used for mass calibration in fast atom bombardment (FAB) mass spectrometry (Chap. 9) because it yields cluster ions of the general formula [Cs(CsI)n] in positive-ion and [I(CsI)J in negative-ion mode. For the [Cs(CsI)io] cluster ion, m/z 2730.9 is calculated instead of the correct value m/z 2731.00405 by using only one decimal place instead of the exact values Mi33Cs = 132.905447 and M1271 = 126.904468. T e error of 0.104 u is acceptable for LR work, but definitely not acceptable if accurate mass measurements have to be performed. 

Spectral analysis shows quite clearly that the various types of atoms are exactly the same on Earth as in the sky, in my own hand or in the hand of Orion. Stars are material objects, in the baryonic sense of the term. All astrophysical objects, apart from a noteworthy fraction of the dark-matter haloes, all stars and gaseous clouds are undoubtedly composed of atoms. However, the relative proportions of these atoms vary from one place to another. The term abundance is traditionally used to describe the quantity of a particular element relative to the quantity of hydrogen. Apart from this purely astronomical definition, the global criterion of metallicity has been defined with a view to chemical differentiation of various media. Astronomers abuse the term metaT by applying it to all elements heavier than helium. They reserve the letter Z for the mass fraction of elements above helium in a given sample, i.e. the percentage of metals by mass contained in 1 g of the matter under consideration. (Note that the same symbol is used for the atomic number, i.e. the number of protons in the nucleus. The context should distinguish which is intended.)... 

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