Electron Impact (EI)Upon bombardmen

Electron Impact (EI)
Upon bombardment with high energy electrons, molecules will lose an electron to become an ion.
Electron removal generally happens in the following order:
1) lone pair electrons,
2) pi bond electrons,
3) sigma bond electrons.
M+ is the molecular (or parent) ion.
Usually, the molecular ion retains excess energy after bombardment and fragments into smaller ions, whose masses depend on functional groups present

Advantages and disadvantages of electron impact ionization:
Advantages:
1) Good sensitivity
2) The fragmentation pattern allows for unambiguous identification of an analyte.
Disadvantages:
1) Samples must be volatilized, so the technique is not useful for high boiling or thermally unstable compounds.
2) Extensive fragmentation can cause the disappearance of the M peak  poor for measuring MW of compound
3) base peak m/z « M+

Desorption Ion Sources:
• Soft ionization techniques that usually result in spectra that consist of only M or M+1 peaks.
• Commonly used for biological samples (i.e. proteins & DNA) or thermally unstable molecules and can measure molecular weights that exceed 10,000 amu.
1. Electrospray Ionization (ESI)
• The most common ionization technique used to analyze biomolecules.
• Can be used to analyze biological macromolecules > 100,000 amu.
2. Fast Atom Bombardment (FAB)
• Used primarily for high molecular weight polar compounds.

LC-MS interface: Electrospray Ionization (ESI)
The sample is pumped through a needle surrounded by several kV of potential.
The charged spray of ultra-fine droplets of sample then passes into a capillary.
While in the capillary the solvent evaporates and the charge is attached to the analyte

Fast Atom Bombardment (FAB)
The sample is prepared in a glycerol matrix and bombarded with high velocity argon or xenon atoms.
Analyte anions and cations sputter off the sample, but only cations enter the mass analyzer due to a negatively charged accelerator/repeller plate at the analyzer inlet.
The matrix reduces fragmentation of the analyte by absorbing most of the vibrational energy imparted by the fast atom stream.

Chemical Ionization (CI)
• Same setup as EI except the ionization chamber is pressurized with a reagent gas.
• The reagent gas is present in a 103 to 104 excess over the analyte.
• The reagent gas, usually methane, is preferentially ionized.
CH4 + e-  CH4•+ + 2e-
• The primary ions CH4•+ and CH3+ form, which go on to give secondary ions.
CH4•+ + CH4  CH5+ + CH3•
CH3+ + CH4  C2H5+ + H2
CH4 + C2H5+  C3H5+ + 2H2
MH+ is a protonated parent ion or quasi-molecular ion

Mass Analyzers
Resolution -measurement of a mass analyzers ability to separate masses that are very close together
R = resolution
m = average mass of two adjacent peaks
Dm= mass difference of two adjacent peaks
Two peaks are considered resolved if the height of the valley between the peaks is < 10% the height of the shortest peak.

High Resolution Mass Spectrometry
If we have a mass analyzer capable of high resolving powers (>~5000 for low m/z

Results (Indonesian) 1:

Electron Impact (EI)Upon bombardment with high energy electrons, molecules will lose an electron to become an ion. Electron removal generally happens in the following order: 1) lone pair electrons, 2) pi bond electrons, 3) sigma bond electrons. M+ is the molecular (or parent) ion. Usually, the molecular ion retains excess energy after bombardment and fragments into smaller ions, whose masses depend on functional groups presentAdvantages and disadvantages of electron impact ionization:Advantages: 1) Good sensitivity 2) The fragmentation pattern allows for unambiguous identification of an analyte.Disadvantages: 1) Samples must be volatilized, so the technique is not useful for high boiling or thermally unstable compounds. 2) Extensive fragmentation can cause the disappearance of the M peak  poor for measuring MW of compound 3) base peak m/z « M+Desorption Ion Sources:• Soft ionization techniques that usually result in spectra that consist of only M or M+1 peaks.• Commonly used for biological samples (i.e. proteins & DNA) or thermally unstable molecules and can measure molecular weights that exceed 10,000 amu.1. Electrospray Ionization (ESI)• The most common ionization technique used to analyze biomolecules. • Can be used to analyze biological macromolecules > 100,000 amu. 2. Fast Atom Bombardment (FAB)• Used primarily for high molecular weight polar compounds. LC-MS interface: Electrospray Ionization (ESI) The sample is pumped through a needle surrounded by several kV of potential.The charged spray of ultra-fine droplets of sample then passes into a capillary.While in the capillary the solvent evaporates and the charge is attached to the analyteFast Atom Bombardment (FAB)The sample is prepared in a glycerol matrix and bombarded with high velocity argon or xenon atoms.Analyte anions and cations sputter off the sample, but only cations enter the mass analyzer due to a negatively charged accelerator/repeller plate at the analyzer inlet.The matrix reduces fragmentation of the analyte by absorbing most of the vibrational energy imparted by the fast atom stream.Chemical Ionization (CI)• Same setup as EI except the ionization chamber is pressurized with a reagent gas. • The reagent gas is present in a 103 to 104 excess over the analyte. • The reagent gas, usually methane, is preferentially ionized. CH4 + e-  CH4•+ + 2e-• The primary ions CH4•+ and CH3+ form, which go on to give secondary ions. CH4•+ + CH4  CH5+ + CH3•CH3+ + CH4  C2H5+ + H2CH4 + C2H5+  C3H5+ + 2H2MH+ is a protonated parent ion or quasi-molecular ionMass AnalyzersResolution -measurement of a mass analyzers ability to separate masses that are very close together R = resolution m = average mass of two adjacent peaks Dm= mass difference of two adjacent peaksTwo peaks are considered resolved if the height of the valley between the peaks is < 10% the height of the shortest peak.High Resolution Mass Spectrometry If we have a mass analyzer capable of high resolving powers (>~5000 for low m/z <100),  separate ions of the same nominal mass. The exact mass for C5H8O+ is 84.05696 Da. The exact mass for C6H12+ is 84.09335 Da.Some Common Mass Analyzers1. Magnetic Sector Mass AnalyzerSelect mass based on centrifugal force in a magnetic field and the velocity of the molecular ion in the magnetic field.Curvature of molecular ions flight path depends on the mass of the ion, velocity of the ion, and the magnetic field strengthMass is selected by varying either the strength of the magnetic field or the velocity of the ions.The velocity of the ions can be varied by adjusting the potential difference between the repeller plate and the accelerator plates in an EI ionizer.Behaviour of an ion in the mass analyzerKinetic energy of an accelerated ion:KE = ½ m×  2 = e ×V = velocity of the ione= the charge on the ion V= accelerating voltage applied in the ionization chamberIn the presence of magnetic field a charge particle will move in a curved light path. The radius of curvature: r= m/eHH= magnetic field strengthCombined m H2r2 e 2VQuadrapole Filter• A direct voltage and a radiofrequency are applied to the four metal rods  oscillating electrostatic fieldAt a given oscillating electrostatic field, only ions of the proper m/z will pass through the quadrapole and reach the detector slit.Quadrapole filters have very fast scan times (< 1 s) and are often used as detectors for gas chromatography or liquid chromatography systems.Detectors Electron Multipliers Discrete DynodeMolecular ions cause the release of electrons from an ion sensitive dynode.Electrons cascade through a series of discrete dynodes to amplify the signal ~107 times.