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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 9 Issue 2 (2004) pp 62-66

Factor Analysis of Numeric Isotope Patterns of Compounds Containing Small Numbers of Chlorine and Bromine Atoms

Ray A. Gross, Jr.

Department of Physical Sciences, Prince George’s Community College, Largo, MD 20774,
Received September 16, 2003. Accepted December 9, 2003.

Published online: 18 February 2004

Abstract. The mass spectra of aromatic compounds containing m bromine and n chlorine atoms display m + n + 1 characteristic molecular-ion peaks at intervals of two amu. For a given compound, the relative intensity of these peaks may be represented by a numeric isotope pattern predicated on a 3:1 ratio of chlorine isotopes and 1:1 ratio of bromine isotopes. For example, the theoretical numeric pattern for any bromodichlorobenzene is 9:15:7:1. It is shown that numeric patterns may be factored, leading to the discovery that the pattern’s lead number identifies the chlorine pattern within the overall pattern and hence n; when the lead number is 9, the chlorine pattern is 9:6:1 and n equals 2. The value of m follows from the number of terms in the pattern. Real patterns follow those predicted closely enough for both n and m to be found for an unknown directly from a normalized numeric isotope pattern (i.e., one in which all terms in the pattern are relative to that of the highest-mass term taken as one). The method exemplified for analyzing patterns should prove useful in courses where numeric isotope patterns are studied.

Key Words: In the Classroom; instrumental analysis; analytic chemistry

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Issue date: April 1, 2004

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