Monoisotopic

Natural Isotopic Abundances. The relative abundances of natural isotopes produce peaks one or more mass units larger than the parent ion (Table 7.75a). For a compound C H O N, a formula allows one to calculate the percent of the heavy isotope contributions from a monoisotopic peak, Pto the Pm + 1 peak ... 

Recently, it has become possible to create isotopes that do not exist naturally. These are the artificial isotopes, and all are radioactive. For example, 13 artificially created isotopes of iodine are known, as well as its naturally occurring monoisotopic form of mass 127. Mass spectrometry is able to measure m/z values for both natural and artificial isotopes. 

For organometailic compounds, the situation becomes even more complicated because the presence of elements such as platinum, iron, and copper introduces more complex isotopic patterns. In a very general sense, for inorganic chemistry, as atomic number increases, the number of isotopes occurring naturally for any one element can increase considerably. An element of small atomic number, lithium, has only two natural isotopes, but tin has ten, xenon has nine, and mercury has seven isotopes. This general phenomenon should be approached with caution because, for example, yttrium of atomic mass 89 is monoisotopic, and iridium has just two natural isotopes at masses 191 and 193. Nevertheless, the occurrence and variation in patterns of multi-isotopic elements often make their mass spectrometric identification easy, as depicted for the cases of dimethylmercury and dimethylplatinum in Figure 47.4. 

Phosphorus and rhodium are unusual among the elements in that they consist of atoms that naturally contain only one ratio of protons to neutrons and therefore have only one mass 31 (15 protons plus 16 neutrons) for phosphorus and 103 (45 protons and 58 neutrons) for rhodium. Such elements are called monoisotopic — each of their atoms has one (and only one) mass in each case. 

For any one element, the abundances (relative amounts) of isotopes can be described in percentage terms. Thus, fluorine is monoisotopic viz., it contains only nuclei of atomic mass 19, and phosphorus has 100% abundance of atoms with atomic mass 31. For carbon, the first two isotopes occur in the proportions of 98.882 to 1.108. 

Isotope peaks can be very informative in GC/MS analysis. Generally for interpretation, one focuses on the monoisotopic peak. The monoisotopic... 

Monoisotopic mass The mass of an ion calculated using the exact mass of the most abundant isotope of each element in the formula (e.g., C = 12.0000, O = 15.9949). 

X 12 = 72 (Multiply 6 carbon atoms by monoisotopic mass of carbon.)... 

Showing the monoisotopic masses of fragments MH+ of /i-lactoglobulin A cysteine residues are shown in bold script, i.e. cysteine-C aromatic residues are underlined, i.e. phenylalanine-F, tyrosine-F, and tryptophan- mIz expected for singly charged species. 

Thr28-Gin27-Vai28-Gin2 -Asp8°-Giy -Lys32 Monoisotopic mass = 774.39 Da... 

Monoisotopic molecular weight The molecular weight of an analyte, calculated by using the masses of the more/most abundant isotopes of each of the elements present. 

Table 6.6 Some frequently encountered atoms with their monoisotopic and average atomic masses...

Atom Isotope Natural abundance Monoisotopic Average mass mass ... 

Table 6.6 presents a list of some of the most commonly encountered atoms in polymer/additive analysis, together with their monoisotopic and average masses. For the same nominal mass, different exact masses (elemental compositions) do exist. Knowledge of the exact mass of an unknown substance allows its atomic composition to be established. The exact mass of an ion proves the presence of a particular species (compound in a mixture). 

Most mass spectrometers will resolve ions with unit resolution up to at least 2000 Da, and so monoisotopic atomic masses are used in these cases. Above 2000 Da, the resolution should be checked and, if it is insufficient to resolve adjacent isotopes, then average atomic masses can be used in calculations. 

High mass accuracy (<5 ppm selectivity and elemental composition based on monoisotopic masses)... 

Da of the monoisotopic masses, high sensitivity (down to 20fmolp,L 1), and the absence of any fragmentation, are important advantages for a L-ToF system. The main characteristics of MALDI-ToFMS, as applied directly to polymer/additive dissolutions, are summarised in Table 9.7. 

Figure 8.1 Heme (iron protoporphyrin IX) structure. The most frequently observed cleavages in LDMS of the intact species are denoted. Heme elemental composition is C34H32N404Fe monoisotopic molecular mass is 616.176 Da average molecular mass is 616.487 Da.

Senko, M. W. Beu, S. C. McLafferty, F. W. Determination of monoisotopic masses and ion populations for large biomolecules from resolved isotopic distributions. J. Am. Soc. Mass Spectrom. 1995, 6,229-233. 

A comparison between the theoretical isotope pattern of CgsH OsgLi, for the lithiated 18-mer of 3 (see inset in Figure 6), with that experimentally obtained indicates that they are very similar. Furthermore, the experimentally observed monoisotopic peak at m/z 1907.9 is very similar to that expected (m/z 1908.0) for this oligomer. 

Table 2.5 Natural isotopic abundances and monoisotopic masses of common elements...

The chemistry of rare earth elements makes them particularly useful in studies of marine geochemistry [637]. But the determination of rare earths in seawater at ultratrace levels has always been a difficult task. Of the various methods applied, instrumental neutron activation analysis and isotope dilution mass spectrometry were the main techniques used for the determination of rare earths in seawater. However, sample preparation is tedious and large amounts of water are required in neutron activation analysis. In addition, the method can only offer relatively low sample throughputs and some rare earths cannot be determined. The main drawbacks of isotopic dilution mass spectrometry are that it is time-consuming and expensive, and monoisotopic elements cannot be determined as well. 

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