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Mass Spectrometry. Can determine information about the molecular weight and molecular structure. Electron impact ionization. An electron is accelerated ...
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Mass Spectrometry Can determine information about the molecular weight and molecular structure Electron impact ionization An electron is accelerated toward the sample Upon collision, an electron is expelled from the molecule The sample thus becomes charged
When a charged species passes through a magnetic field the flight path is bent The charged particle is attracted to one “pole” in the presence of a magnetic filed, hence the path is bent The radius of curvature is m/z Therefore lighter species bend more than heavier species with a given magnetic field
The parent ion is called the molecular ion peak (M+) m/z 72 Can find molecular ion, but what are the other peaks?
The molecular ion peak can fragment Due to the high energy of the radical/cation generated, this species can fragment Remember only the charged species will be detected (the radical species will not be affected by the magnetic field) The probability of obtaining a given fragement is due to the STABILITY of the cations produced
The ratio of these peaks is diagnostic for which atoms are present The natural abundance of isotopes is well known M M+1 M+ H 100% C 98.9% 1.1% S 95% 0.8% 4.2% Cl 75.5% 24.5% Br 50.5% 49.5% I 100%
Can distinguish atoms by the ratio of peaks above the molecular ion Especially useful to distinguish which halogen is present Cl Br I m/z 78 M/M+2 = 3 ~ 3/ m/z 122 M/M+2 = 1 m/z 170
Fragmentation Behavior of Common Functional Groups Alkenes With an alkene the common fragmentation is to create an allylic carbocation m/z 70 m/z^55
Alcohols Two common effects OH m/z 74 m/z 56 H 2 O
McLafferty Rearrangement Any ketone containing a γ-hydrogen can rearrange to the enol form in a MS H O m/z 100 O H m/z 58
Ketones Ketones can also do α-cleavage similar to alcohols O m/z 100 O (^) O CH 2 CH 2 CH 2 CH 3 m/z 43
Differentiating Structures Using HRMS Many structures may have the same integer value molecular weight, but different molecular formulas These structures can be differentiated with HRMS For example:
