Mass Spectra Structural Determination of Simple Molecules

INTERPRETATION OF MASS SPECTRA STRUCTURAL DETERMINATION OF SIMPLE MOLECULES 

A mass spectrum is a plot or table of the mass-to-charge ratio, m/z, of detected ions vs. their relative abundance (relative concentration). A typical mass spectral plot for a small organic molecule, benzene, is presented in Fig. 10.1. The m/z values are plotted on the x-axis relative abundance is plotted on the y-axis. The most abundant peak in the spectmm is called the base peak. The base peak is assigned an abundance of 100% and the other peak heights are plotted as percentages of that base peak. A tabular form of benzene mass spectral data is given in Table 10.1. The tabular data have the advantage that very low abundance ions can be listed, such as the ions at m/z = 64 and 80 which are too small to be seen on the normalized plot. 

The mass spectra we will smdy have been obtained by electron ionization (El) since other techniques enhance the molecular ion, but greatly reduce fragmentation, making structural analysis difficult. Consequently, the following discussion is concerned mostly with El spectra of pure compounds. The ions that appear in these spectra are all positively charged ions. These are also low-resolution mass spectra the m/z values are measured 

There are two ways to interpret such spectra. The first is to compare the spectrum you have with those in a searchable computerized mass spectral database. The second is to evaluate the spectrum using the interpretation procedure described subsequently. In either case, once an unknown compound has been identified from its mass spectrum, the pure compound should be obtained and analyzed under the same conditions as the sample for conhrmation. Over 10 milhon chemical compounds have been identified. No mass spectral database contains spectra for every possible compound, although mass spectral databases of over 400,000 spectra are available. The mass spectral database from the US National Institute for Standards and Technology (NIST) may be purchased from a number of licensed vendors. Limited mass spectra from NIST are available online in the NIST Chemistry WebBook (http //webbook.nist.gov). Commercial vendors and publishers offer specialized mass spectral hbraries of compounds, such as environmental compounds, pharmaceuticals, natural products, oil industry compounds, and the like. 

In practice, the analyte spectrum is entered into the computer, which compares it to the spectra in the stored database using a search algorithm. There are a number of algorithms currently available, including Probability Based Matching, designed by Professor 

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