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Chapter 8Chapter 8Mass SpectrometryMass Spectrometry
Mass spectrometry can analyze gases, liquids, and solid samples.
For a compound to be analyzed in a mass spectrometer, it must be in the gaseous state.
Gases and liquids are introduced directly into the vacuum chamber through a syringe.
Solid samples can be placed on the tip of a rod, which is then inserted into the vacuum chamber through a vacuum tight seal. The solid is sublimed or vaporized in the presence of heat..
If a sample is impure, it must be separated into its individual components prior to analysis by mass spectrometry.
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Separation is necessary since a mixture will generate many overlapping peaks, and a pure compound may generate many peaks.
Most commonly, the mixture is separated by gas chromatography:
A sample is injected into the gas chromatograph.
A chromatogram is generated that shows the individual components.
An individual component is then selected for analysis with the mass spectrometer.
For a compound to be analyzed in a mass spectrometer, it must be in the gaseous state.
Solids and liquids are vaporized with heat.
The vapors are then bombarded with a beam of electrons.
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In the electron ionization mass spectrometer, positive ions are accelerated through the accelerating plates.
The magnetic field separates the ions by their mass-to-charge ratio (m/z).
Since the charge (z) is usually +1, a direct measure of the particle is obtained.
By varying the magnetic field, the abundance of each mass is detected.
A detector records the mass-to-charge ration and this informatio is given as a mass spectrum.
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The compound may be ionized in several ways:The compound may be ionized in several ways:(i) Electron ionization:Electron ionization: bombardment with electrons.
(ii) Chemical ionization:Chemical ionization: a reagent gas (e.g., CH4, NH3, isobutane) is ionized by electron impact. These ions react with the sample molecules to produce ions.
(iii) ElectrosprayElectrospray ionization:ionization: generates ions directly from solution. A spray of charged droplets is created by the application of a high potential difference.
(iv) The atmospheric pressure chemical ionization:The atmospheric pressure chemical ionization: contains a heated vaporizer that aids in the vaporization of the compound.
(v) MALDI: matrixMALDI: matrix--assisted laser assisted laser desorptiondesorption ionization.ionization.The compound is dissolved in a solution containing an excess of the matrix compound which absorbs at a laser wavelength.The solution is placed on the laser target.A pulse of UV radiation of this mixture produces a plasma from the vaporization of both the matrix and the compound.
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(vi)(vi) Fast atom bombardment:Fast atom bombardment:The compound is dissolved in a liquid matrix such as glycerol or m-nitrobenzyl alcohol.The mixture is placed on a probe and bombarded with a high-energy beam of cesium ions or xenon atoms that desorbs molecular ions and fragments from the compound.
(vii)vii) Inductively coupled plasma:Inductively coupled plasma:The sample is injected into gaseous ions that are produced by the inductive coupling of high energy to argon.The sample droplets are vaporized and ionized.
(viii)(viii) Other types of mass analyzers:Other types of mass analyzers:Double-focusing mass spectrometer.Quadrupole mass filter.Quadrupole ion storage.Time-of-flight mass analyzer.Fourier transform-ion cyclotron resonance.Tandem mass spectrometer.
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Cleavage ReactionCleavage ReactionWhen an electron strikes a neutral molecule, that molecule may be ionized by the loss of one electron.
The molecular ion, , is formed when the original molecule loses one electron and becomes positive.
The molecular ion is also a radical cation.
After its formation, the molecular ion can undergo fragmentation and rearrangement.
These processes are extremely rapid (10-10-10-6 sec). The longer-lived particles can be detected by the ion collector, but a shorter-lived particle may not have sufficient lifetime to reach the ion collector.
In some cases, the molecular ion is too short-lived to be detected.
•+
M
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Example:Example:
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In a mass spectrum, the tallest peak is the base peak.
The base peak is usually not the same as the molecular ion.
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Isotopes in Mass SpectraIsotopes in Mass Spectra
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Fragmentation in Mass SpectrumFragmentation in Mass SpectrumA molecule hit with the electron either becomes a molecular ion or breaks apart into fragments.
Some of these fragmentation patterns are fairly simple.
Others involve additional reactions or rearrangements.
Cleavage is formed at branch sites that lead to more substitutedcarbocations and radicals.
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Example:Example: Hexane
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Example:Example: 1-bromobutane
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Formation of propyl cationis not highly favored since the peak at m/z 43 is very
small.
For peaks containing
bromide, M+2 peaks is
approximately the same size as the M peak.
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Example:Example: 2-chloro-2-methylpropane
The molecular ion peak is not seen in the spectrum.
(short(short--lived)lived)
M+2 peak is ~1/3 as large as the M peak
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Example:Example: p-xylene
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AlcoholsAlcohols can undergo dehydration to yield an alkene
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AlcoholsAlcohols can also undergo cleavage to produce a carbocation, which rearranges to a more stable species.
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Example: Example: 3-methyl-1-butanol
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The molecular ion peak (m/z 88) is not visible since it dehydrates so easily
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Both aminesamines and ethersethers undergo a similar type of fragmentation reaction.
If fragmentation occurs next to a nitrogen or oxygen, then electron rearrangement may occur to form either an iminiumion or an oxonium ion.
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Additional stabilized cations may be formed from α−cleavage
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Example:Example:Diethylamine (m/z 73) can fragment between the nitrogen and carbon.
α-cleavage
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Compounds containing carbonyl groupscarbonyl groups may fragment next to the carbonyl. Electron delocalization occurs to yield on acylium ion.
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Carboxylic acid derivativesCarboxylic acid derivatives
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Example:Example: Butanal (m/z 72) can fragment to form methyl radical and the propanal cation (m/z 57)
Butanal can also fragment to yield an ethyl cation (m/z 29) and ethanalradical or an ethyl radical and ethanal cation (m/z 43).
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McLaffertyMcLafferty rearrangementrearrangement occurs in carbonyl compounds only if the γ carbon contains a hydrogen.In the mass spectrum peaks from the alkene and enol would occur.
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Since butanal contains a γ hydrogen, Mclaffertyrearrangement produces two products: ethylene (m/z 28) radical cation and the enol radical cation (m/z 44).
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THE ENDTHE END
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