Calculated exact mass

As we have already seen, the isotopic mass also is the exact mass of an isotope. The isotopic mass is very close but not equal to the nominal mass of that isotope (Table 3.1). Accordingly, the calculated exact mass of a molecule or of a mono-isotopic ion equals its monoisotopic mass (Chap. 3.1.4). The isotope C represents the only exception from non-integer isotopic masses, because the unified atomic mass [u] is defined as of the mass of one atom of nuclide C. 

As a consequence of these individual non-integer isotopic masses, almost no combination of elements in an empirical formula has the same calculated exact mass, or simply exact mass as it is often referred to, as another one. [25] In other words, at infinite mass accuracy it is possible to identify the empirical formula by mass spectrometry alone. 

The mass accuracy is defined as the difference between measured accurate mass and calculated exact mass. The mass accuracy can be stated as absolute units of u (or mmu) or as relative mass accuracy in ppm, i.e., absolute mass accuracy divided by the mass it is determined for. As mass spectrometers tend to have similar absolute mass accuracies over a comparatively wide range, absolute mass accuracy represents a more meaningful way of stating mass accuracies than the more trendy use of ppm. 

Example The [M-Cl]" ion, [CHCl2], represents the base peak in the El spectrum of chloroform. The results of three subsequent determinations for the major peaks of the isotopic pattern are listed below (Fig. 3.15). The typical printout of a mass spectrometer data system provides experimental accurate mass and relative intensity of the signal and an error as compared to the calculated exact mass of possible compositions. For the [ CH Cl2] ion, the experimental accurate mass values yield an average of 82.9442 0.0006 u. The comparatively small absolute error of 0.6 mmu corresponds to a relative error of 7.5 ppm. 

Figure 5. Electron impact mass spectrum of tetraethyleneglycol-di(2-ethylhex-anoate), compound 82. The elemental compositions of m/e 127 and 171 were established by high resolution mass spectrometry the A values indicate the error (in millimass units) between measured and calculated exact masses.

Molecular Formula C24H48NO4S Calculated Exact Mass 446.3310 Observed Exact Mass 446.3310... 

Table 9 Calculated exact masses for CyD innermolecular products3...

Mass accuracy is the difference (AAT) between the measured accurate mass M and the calculated exact mass. It can be stated as absolute units of mass (differences of so many millimass units, mmu, 10-3 u) or as a relative mass accuracy in parts per million. 

The mass accuracy of single compound measurements is below 0.5 ppm, which means that the measured mass of a molecule of 1000000 Da would differ by 0.5 amu or an ion mass of 500 Da would have an uncertainty of 0.00025 amu compared to the calculated exact mass. 

The electron mass (0.000549 amu) is substracted or added depending on the char state when calculating exact mass. Exact mass is calculated by the authors of this chapter. 

From other information concerning the molecular structure, such as isotope peak calculations, exact mass measurements, or spectral or chani-cal evidence, the presence or absence of nitrogen may be known. A compound known to be a steroid, for example, must have an even molecular weight. On the other hand, a substance that becomes water soluble under acidic conditions must contain at least one basic nitrogen function in this case, the molecular weight may be odd or even, depending on the number of nitrogen atoms actually present. 

Role of the Electron Mass When Calculating Exact Mass... 

The absolute mass accuracy, Am/z, is defined as the difference between measured accurate mass and calculated exact mass ... 

Although they are related, the measured accurate mass should be distinguished from the calculated exact mass, which is the mass determined by summing the exact isotope masses of the elements present in a particular ion. For example, the calculated exact mass of (acetone molecular ion) is 58.0419 Da, and... 

The deviation of the measured accurate mass from the calculated exact mass of an ion. It can be expressed as an absolute value in milliDaltons (mDa) or as a relative value in parts-per-million (ppm) error. The absolute accuracy is calculated as accurate mass - exact mass. 

---