How to Solve Mass Spectral Problems

In this section, we will use EI-MS data to determine the structure of two unknown compounds. Even if the compound you are analyzing is not in searchable MS databases available to you, it is still possible to determine the structure of the compound with a few key pieces of data. If the molecular formula is available, either from an exact mass determination (Section 3.6) or Rule of Thirteen analysis (Section 1.5) on the molecular ion, the process is much simpler. Furthermore, knowing the main functional group(s) in the compound will assist in analyzing the fragmentation pattern. Information from an infrared spectrum and/or NMR spectra are useful in this regard. 

An unknown compound has the mass spectrum shown. The infrared spectrum of the unknown shows significant peaks at 

There is also a band from aliphatic C—H stretching from 2879 to 2979 cm  

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The molecular ion appears at an m/z value of 134. Applying the Rule of Thirteen gives the following possible molecular formulas  

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An example of how information from fragmentation patterns can be used to solve structural problems is given in Worked Example 12.1. This example is a simple one, but the principles used are broadly applicable for organic structure determination by mass spectrometry. We ll see in the next section and in later chapters that specific functional groups, such as alcohols, ketones, aldehydes, and amines, show specific kinds of mass spectral fragmentations that can be interpreted to provide structural information. 

This approach is particularly effective with IR and NMR spectroscopy, and with mass spectrometry. Practical rules are given to help students see what information is available in the spectrum and what spectral characteristics usually correspond to what structural features. Sample problems and Study Problems located throughout the text show how the clues from various spectra are combined to propose a structure. The emphasis is on helping students develop an intuitive feel for using spectroscopy to solve structural problems. A comprehensive list of the spectroscopy problems found in each chapter is available online a www.pearsonhighered.com. 

Mass spectrum interpretation is essential to solve one or more of the following problems establishment of molecular weight and of empirical formula detection of functional groups and other substituents determination of molecular skeleton (atom connectivity) elucidation of precise structure and, even in favorable cases, certain stereochemical features. As discussed in the previous chapters, electrospray (ESI) and atmospheric pressure chemical ionization (APCI) are two of the most effective and successful interfaces for the liquid chromatography—mass spectrometry (LC—MS) that have been developed. Thus, we will focus on how to interpret the mass spectral data generated by either ESI or APCI in this section. 

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