Lecture Notes of Mass Spectrometry |, Study notes of Organic Chemistry

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Organic Chemistry II

Chapter 13

January 20, 2011 Priyantha Sugathapala

• (^) Mass spectrometry is a technique used for measuring the molecular weight and determining the molecular formula of an organic compound.
• (^) In a mass spectrometer, a molecule is vaporized and ionized by bombardment with a beam of high-energy electrons.
• (^) The energy of the electrons is ~ 1600 kcal (or 70 eV).
• (^) Since it takes ~ 100 kcal of energy to cleave a typical s bond, 1600 kcal is an enormous amount of energy to come into contact with a molecule.
• (^) The electron beam ionizes the molecule by causing it to eject an electron.

Overview of Mass Spectrometry

• (^) When the electron beam ionizes the molecule, the species that is formed is called a radical cation, and symbolized as M +• .
• (^) The radical cation M+•^ is called the molecular ion or parent ion.
• (^) The mass of M+•^ represents the molecular weight of M.
• (^) Because M is unstable, it decomposes to form fragments of radicals and cations that have a lower molecular weight than M +• .

Function of a Mass Spectrometer

• (^) The mass spectrometer analyzes the masses of cations.
• (^) A mass spectrum is a plot of the amount of each cation (its relative abundance) versus its mass-to-charge ratio ( m/z , where m is mass, and z is charge).
• (^) Since z is almost always +1, m/z actually measures the mass ( m ) of the individual ions.

Function of a Mass Spectrometer

• (^) The mass spectrum of CH 4 consists of more peaks than just the M peak.
• (^) Since the molecular ion is unstable, it fragments into other cations and radical cations containing one, two, three, or four fewer hydrogen atoms than methane itself.
• (^) Thus, the peaks at m/z 15, 14, 13, and 12 are due to these lower molecular weight fragments.

Peaks in a Mass Spectrum

Figure 13.

Mass Spectrum of Hexane

• (^) The molecular ion for hexane is at m/z = 86.
• (^) A small M + 1 peak occurs at m/z = 87.
• (^) The base peak occurs at m/z = 57 (C 4

H

9

).

• (^) Major fragment peaks also occur at 43 (C 3 H 7 + ) and 29 (C 2 H 5 + ).

• (^) Most elements have one major isotope.
• (^) Chlorine has two common isotopes, 35 Cl and 37 Cl, which occur naturally in a 3:1 ratio. - (^) Thus, there are two peaks in a 3:1 ratio for the molecular ion of an alkyl chloride. - (^) The larger peak, the M peak, corresponds to the compound containing the 35 Cl. The smaller peak, the M + 2 peak, corresponds to the compound containing 37 Cl. - (^) When the molecular ion consists of two peaks (M and M + 2) in a 3:1 ratio, a Cl atom is present.
• (^) Br has two common isotopes, 79 Br and 81 Br, in a ratio of ~ 1:1.
  • (^) When the molecular ion consists of two peaks (M and M + 2) in a 1:1 ratio, a Br atom is present.

Alkyl Halides and the M + 2 Peak

Figure 13.

Mass Spectrum of 2-Chloropropane

Fragmentation Patterns

• (^) Cleavage of C − C bonds forms lower molecular weight fragments that correspond to lines in the mass spectrum. Figure 13.

Some Common Fragmentation Patterns

Carbonyls

Alcohols

• (^) A molecule having a molecular ion at m/z = 60 using a low- resolution mass spectrometer could have any one of the following molecular formulas.
• (^) A high-resolution mass spectrometer would differentiate between these to give only one possible formula.

Exact Mass in High-Res Mass Spectra

• (^) Mass spectrometry can be combined with gas chromatography into a single instrument used to analyze mixtures of compounds.
• (^) A gas chromatograph (GC) consists of a thin capillary column containing a viscous high-boiling liquid, all housed in an oven.
• (^) When the sample is injected into the GC, it is vaporized and swept by an inert gas through the column.
• (^) The components travel through the column at different rates, often separated by boiling point, with lower boiling compounds exiting the column before higher boiling compounds.

Gas Chromatography—Mass Spectrometry

Figure 13.

Function of GC-MS

• (^) To analyze a urine sample for tetrahydrocannabinol (THC), the principal psychoactive component of marijuana, the organic compounds are extracted from urine, purified, concentrated, and injected into the GC-MS.
• (^) THC appears as a GC peak, whose mass spectrum gives a molecular ion at 314, its molecular weight. Figure 13.

Analysis Using GC-MS