MCAT Organic Chemistry Review by Princeton Review

Author:Princeton Review [Review, The Princeton]
Language: eng
Format: epub, pdf
ISBN: 978-1-101-88242-9
Publisher: Random House Children's Books
Published: 2015-03-16T16:00:00+00:00

The M/e label on the x-axis represents the ratio of mass (M) to charge (e). In most cases e = +1, so peaks can simply be viewed as molecular mass. The y-axis represents the relative abundance of each species of a particular mass detected in the sample. Masses, though generally not labeled as such, are measured in amu.

Two aspects of the above spectrum may be puzzling: 1) If the molecular weight of nonane is 128 g/mol, why are there peaks greater than this value, and 2) why are there significant peaks in the sample with masses lower than 128?

Remember, atoms can come in a number of different isotopes. For example, the most prevalent mass of hydrogen in nature is 1, but deuterium has an extra neutron and weighs 2 (natural abundance = .015%). Likewise, the most abundant isotope of carbon is 12C, but 13C exists as 1.1% of all carbon atoms. So, the small peaks with masses larger than the main peak represent molecules that have one or more of these less abundant isotopes.

The masses lower than 128 in the above scan represent the masses of molecular fragments. The high energy beam of electrons used to ionize molecules in the mass spectrometer can cause the molecule to break into smaller parts. The figure below shows where n-nonane might have been broken to produce peaks with the masses found above. The outer, curved brackets represent a fragment which has lost the terminal CH3 group and hence is 15 less than the peak at 128. The inner, square brackets show a fragment weighing 99, having lost CH2CH3.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.