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Purpose of Mass Spectrometry

Produces spectra of masses from the molecules in a sample of material, and fragments of the molecules.
Used to determine

the elemental composition of a sample
the masses of particles and of molecules
potential chemical structures of molecules by analyzing the fragments
the identity of unknown compounds by determining mass and matching to known spectra
the isotopic composition of elements in a molecule

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Stages

The ionizer converts some of the sample into ions.

Mass analyzers separate the ions according to their mass-to-charge ratio.

The detector records either the charge induced or the current produced when an ion passes by or hits a surface

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Mass Spectrometry Methods

Ionization �method	Typical �Analytes	Sample �Introduction	Mass �Range
Electron Impact (EI)	Relatively �small �volatile	GC or �liquid/solid �probe	to �1,000 �Daltons
Chemical Ionization (CI)	Relatively �small �volatile	GC or �liquid/solid �probe	to �1,000 �Daltons
Electrospray (ESI)	Peptides �Proteins �nonvolatile	Liquid �Chromatography �or syringe	to �200,000 �Daltons
Fast Atom Bombardment (FAB)	Carbohydrates �Organometallics �Peptides �nonvolatile	Sample mixed �in viscous �matrix	to �6,000 �Daltons
Matrix Assisted Laser Desorption �(MALDI)	Peptides �Proteins �Nucleotides	Sample mixed �in solid �matrix	to �500,000 �Daltons

http://www.chem.arizona.edu/massspec/intro_html/intro.html

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Mass Spectrum of CO₂

Molecular ion peak

[CO₂]⁺ = 44

[CO]⁺ = 28

[O]⁺ = 16

[C]⁺ = 12

Fragment Peaks

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Mass Spectrum of Bromine, Br₂

Bromine has two isotopes: 50.69% ⁷⁹Br and 49.31% ⁸¹Br

⁷⁹Br⁺

⁸¹Br⁺

[⁷⁹Br⁸¹Br]⁺

[⁷⁹Br⁷⁹Br]⁺

[⁸¹Br⁸¹Br]⁺

Molecular Ion Peaks

Fragments

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Practice: Methyl Bromide, CH₃Br

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Answers: Methyl Bromide, CH₃Br

[CH₃⁸¹Br]⁺

[CH₂⁸¹Br]⁺

[C⁸¹Br]⁺ and [CH₂⁷⁹Br]⁺

[CH⁸¹Br]⁺ and [CH₃⁷⁹Br]⁺

[CH⁷⁹Br]⁺

[CH₃]⁺

[C⁷⁹Br]⁺

[⁸¹Br]⁺

[⁷⁹Br]⁺

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Practice: Methylene Chloride (CH₂Cl₂)

Chlorine is 75.77% ³⁵Cl and 24.23% ³⁷Cl

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Practice: Vinyl Chloride (CH₂CHCl)

Chlorine is 75.77% ³⁵Cl and 24.23% ³⁷Cl

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Spectra of Larger Molecules

Spectra of large molecules have many fragments, and the interpretation of their spectra is beyond the scope of this course.

Codeine, C₁₈H₂₁NO₃

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Toluene ( C₇H₈) Fragmentation

**Note that the molecular ion peak is NOT the dominant peak in the spectrum

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Mass Spectrometry in Forensics

Mass spectrometry is used to confirm the identify of unknowns, such as illegal drugs
Unknowns are often not pure, and must be separated from a mixture
Gas chromatography is used to separate the components of the mixture
Mass spectrometry “fingerprints” the components, so that they can be matched to existing known spectra

Gas

Chromatograph

Mixture

Mass

Spectrometer