Isotopic pattern average molecular mass

Mass spectra of isotopic patterns of two alkanes having the molecular formulae C20H42 and C100H202, respectively. The monoisotopic mass is the lighter mass of the isotopic pattern whereas the average mass, used by chemists in stoichiometric calculations, is the balanced mean value of all the observed masses. 

From the abundance generating functions that are used to calculate the isotopic pattern, also the average molar mass of the molecular can be calculated. If the abundance generating function, e.g., for carbon tetrachloride, is as given in Eq. (12.12), then the average molar mass is given as... 

The monoisotopic mass is meaningful for low-mass compounds because the elemental composition can be determined from a well-defined isotopic pattern of the molecular ion. The nominal and monoisotopic masses can both be correlated with the most abundant peak in the isotopic cluster. As the mass of a compound, however, increases, the isotopic pattern becomes more synunetrical and extends over many masses [21]. Also, the monoisotopic peak becomes difficult to identify. For high-mass compounds (e.g., proteins and oligonucleotides), the molecular ion profile measured coalesces and becomes a single asymmetric peak. For such compounds, the average mass value is accepted as the molecular mass. 

Although a spectral feature reveals the mass of an ion, the value is not necessarily accurate enough for rehable identification of the corresponding molecule. In low-resolution mass spectra, the isotopic pattern of an ion may be buried within a single spectral feature. The measurement of m/z may result in an observation closer to the average mass than the exact mass of the ion (as discussed in Section 5.3.2). Even in the case of a well-resolved isotopic pattern, mass measurement may still be affected by the width and shape of the spectral feature. Deconvolution of spectral features contaminated with multiple ions of similar mass is important for accurate molecular identification but beyond the scope of this book. Here we will focus on the determination of accurate masses of single-component samples. 

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