Molecular Mass Spectrometry

Flashcards covering the basics of Mass Spectrometry

What does Molecular Mass Spectrometry measure?

Relative abundance vs m/z (mass/charge)

What happens when a molecule of ethyl benzene is hit with a stream of electrons?

It knocks off one of the molecules' stating electrons, forming a molecular ion (M*)

What is the significance of the dot in M* (molecular ion)?

It indicates that it is a radical ion.

What happens to the excited state of a molecule after collision with an electron?

It often fragments into pieces to get rid of the energy.

What is the role of the negatively charged lens in mass spectrometry?

All positive ions are attracted to a negatively charged lens that gets ions moving into the mass spectrometer.

What does the mass spectrometer sort ions by?

Mass/charge ratio (m/z)

What is the base peak in mass spectrometry?

The largest peak, arbitrarily assigned a value of 100.

What is the starting point in mass spectrometry?

Creation of gaseous ions.

What does the appearance of the mass spectrum depend on?

Method used to get ions.

What are the two major categories of ion sources?

Gas phase sources and desorption sources.

What is the difference between gas phase and desorption sources?

Gas phase sources start with the molecule already in gas, while desorption sources convert liquid or solid samples to gas.

What are the two common gas-phase sources in commercial mass spectrometry?

Electron Impact (EI) and Chemical Ionization (CI).

What distinguishes hard and soft sources?

Hard sources give the molecular ion enough energy that it tends to fragment. Soft sources cause little fragmentation.

What are the advantages of Electron Impact (EI) sources?

Easy to use, gives a comparatively large number of ions, lots of fragmentation to use pattern to identify compound.

What are the disadvantages of Electron Impact (EI) sources?

Lots of fragmentation can sometimes obscure the molecular ion, sample must be a stable gas, sometimes thermal decomposition occurs before ionization, limited to MW in 103 range.

How are ions formed in Chemical Ionization (CI)?

By colliding the molecule with ions of an excess reagent gas.

What is a typical reagent gas used in Chemical Ionization (CI)?

Methane (CH4).

What type of molecular ion is commonly derived by Chemical Ionization (CI)?

Molecular ion +/- 1

How are ions formed in Field Ionization (FI)?

Under the influence of a large electric field (108 V/cm).

What are the three Gas Phase ion sources?

Electron Impact Ionization (EI), Chemical Ionization (CI), and Field Ionization (FI).

What are the two Desorption Ionization sources?

Matrix Assisted Laser Desorption Ionization (MALDI) and Electrospray Ionization (ESI).

For what type of molecules is MALDI best suited?

Biopolymers in the 1,000 - 100,000's range.

What is used to sublime the sample in MALDI?

A laser pulse.

What conditions are used for Electro Spray Ionization ESI?

Ambient pressure and T

What type of chromatography is best suited for ESI?

HPLC/MS

What is the relationship between the resolution of HPLC and Mass Spectrometry?

HPLC:
R = 2(t₂ - t1) / (W₂ + W₁)
Mass Spectrometry:
R = M / ΔM

Name five types of Mass Analyzers.

Magnetic Sector Mass Analyzer, Quadrupole Mass Analyzer, Time of Flight (TOF) Mass Analyzer, Ion Trap Mass Analyzer, Orbitrap Mass Analyzer

What are some of the applications of Molecular Mass Spectrometry?

Elucidation of the structure of organic and biological molecules, Determination of the molecular mass of peptides, proteins, and oligonucleotides, Identification of components in thin-layer and paper chromatograms, Determination of amino acid sequences in sample of polypeptides and proteins, Detection and identification of species separated by chromatography and capillary electrophoresis, Identification of drugs of abuse and metabolites of drugs of abuse in blood, urine, and saliva, Monitoring gases in patient's breath during surgery, Testing for the presence of drugs in blood in race horses and in Olympic athletes, Dating archaeological specimens, Analyses of aerosol particles, Determination of pesticide residues in food, Monitoring volatile organic species in water supplies