Molecular weight, from mass spectrum

Observed molecular weights taken from mass spectrum in Figure 7.13c. 

Determine a compound s molecular weight from its mass spectrum. 

A modification of the STIRS algorithm (9) makes possible the prediction of the molecular weight from an unknown mass spectrum with 91% reliability (95% for the first and second choices). This program has been extended recently to predict the elemental composition (10) with somewhat lower accuracy. An example of the information supplied by STIRS is given in Table I. 

Note that we have not considered the peak at mjz 55, which has resulted from splitting off a mass of 47. There is not a reasonable fragment of mass 47 which can split off hexan-3-ol. The peak at mjz 55 probably arises after an initial dehydration of hexan-3-ol to hexene. Loss of water (mass 18 Daltons) would give hexene, molecular weight 84. The spectrum in fact shows a small peak at mjz 84. If hexene loses C2H5 (mass 29 Daltons), then we would expect a peak at mjz 55. 

Mass spectrometry (MS) provides the molecular weight and valuable information about the molecular formula, using a very small sample. High-resolution mass spectrometry (HRMS) can provide an accurate molecular formula. The mass spectrum also provides structural information that can confirm a structure derived from NMR and IR spectroscopy. 

Molecular weight of pyrolysis products directly derivable from mass spectrum through (quasi)molecular ions... 

What kinds of information can we get from a mass spectrum Certainly the most obvious information is the molecular weight, which in itself can be invaluable. For example, if we were given samples of hexane (MW = 86), 1-hexene (MW = 84), and 1-hexyne (MW = 82), mass spectrometry would easily distinguish them. 

The mass spectrum produced should provide unambiguous molecular weight information from the wide range of compounds amenable to analysis by HPLC, including biomolecules with molecular weights in excess of 1000 Da. The study of these types of molecule by mass spectrometry may be subject to limitations associated with their ionization and detection and the mass range of the instrument being used. 

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