Topics in..
. Chemical Instrumentation
Edifed by 5. 2. LEWIN, New York University, New York 3, N. Y.
conditions must be standardized for both the known compounds and mixtures. I t is not essential to calculate the sensitivity coefficients for every mass number because the pattern of a compound relates the various peak heights to each other. As a simple example consider a mixture of n-hexanc. 2-methvl ~ e n t a n e and 3-methyl pentsne being measured a t masses 27, 43, and 57. The equations
These articles, most of which are to be conlribuled by guest authors, are intended to serve the readers of this JOURNAL by calling attention lo new developments i n the theory, design, or availability of chemical laboratory instrumenlation, o i by presenting useful insights and explanations of topics that are of practical importance to those who use, ,or teach the use of, modern instrumentation and instrumental techniws.
XIII. Mass Spectrometry
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Part Two
Stephen E. Wiberley and David A. Aikens, Department of Chemistry, Renrselaer Polytechnic Institute, Troy, New Yark Applications of Mass Spectrometry
The principal applications of mass spectrometry iue qualitative and quantitative snalysis of simple gases and organic compounds. Some specific material pertinent to qualitative sndysis can be found in the discussion of the effect of molecular structure an fragmentation. Hence, only information of a general nature will be included here. Qualitative Analysis and Structure Determination
Normally each compound has a characteristic fragmentation pattern that can be used to identify the compound. There are a few exceptions t o this rule however. For example the spectra of ortho-, me&, and para xylene are virtually identical and these compounds cannot be distinguished by mess spectrometry. Extensive tabulations of mass spectra are available. Perhaps the beat known is that published by the American Petroleum Institute, Project 44 (8). A compilatian containing spectra of 279 eompounds for qualitative identification has been compiled by S. M. Rock (9). Peak intensities are indicated only as being in one of the classes &2%, 2 2 0 % 20-50'3'0, or 50-100% of the base or most intense peak. McLafferty (10) has tabulated the empirical and structurel formulas of ions that might hs found a t a particular mass peak in a mass spectrum. In addition molecular structure often can he established by detailed interpretation of mass spectra in terms of organic reaction mechrtnisms. This mechanistic approach was discussed previously in relation to the influence of molecular struotnre on mass apectrn and that section should be consulted for details and examples. Quantifative Analysis
Quantitative analysis by mass spectrometry is based on the fact that the
peak heights a t various mass numbers w e proportional to the pressure of the compound yielding these peaks. In sddition the mass spectrum of a mixture is a. linear superposition of the m a s spectrum of each of the individual components. To determine the contribution of each compound a t a given mass, the mass spectrum of the pure compound is measured in order to find the peak height obtained for s given partial pressurc of the compound. Thus, for any compound the sensitivity coefficient equals the relative output (i.e., peak height) nt a specific mass number divided by the pressure of the gas in the instrument.
Se?p,
+ S'np2 + S"sipr
=
Hsi
where p h , p., and p, represent the partial pressures of n-hexane, 2-methyl pentane and 3-methyl pentane respectively. Sn, and Ssi represent the sensitivity coefficients a t masses 27, 43, and 57 for n-hexane, while S'W, S'U, S'W,and S"rr, S"O, and S"S represent the eorresponding sensitivity coefficients a t the designated mass numbers for %methyl pentane and 3-methyl pentane. Hn, H
