Samenvatting - Mass Spectrometry in Forensics

MASS SPECTROMETRY IN
FORENSICS
LECTURE 1: INTRODUCTION TO MASS SPECTROMETRY
- A method to determine the m/z of a charged molecule or a charged fragment of
that molecule.
- The mass spectrometer measures the m/z and the relative abundances of ions.
(relative abundances = this fragment u will see the most)
 m  atomic mass units.
 z  number of elemental charged (z = 1, 2, 3, …)

Principle of Mass Spectrometry
- a moving current is deflected in a magnetic field.




3 essential parts
- Ion source  where the molecule is introduced. (ionization)
- Mass selection  (analyzer)
- Ion detection  (detector)
Ms operates in high vacuum.

Advantage of Each System
- GC/MS  definitive answer, accepted, spectral library searching.
- LC/MS  analytical/prep, MW answer.
- CE/MS  rapid, high sensitivity analysis.
- SFC/MS  ease of sample recovery.
- LC/MS/MS  structural information, trace metabolites ID.

Overview of a MS

- A current (+ or -) with velocity v entering a
perpendicular genetic field will be deflected
according to the so-called right-hand-rule.
- Low mass takes a hair pin, High mass goes right on.

,GC-MS IONISATION TECHNIQUES
GC-MS is suited for small molecules. (Volatile, small organic compounds, not proteins RNA
ect)
- Ionization modes:
 Electron ionization (electron impact ionization)
 Chemical ionization.
- Fragmentation reactions

EI-ionization
- Destroy into fragments and reconstruct the molecule.




 The EI source
- Molecular ion is radical (one electron unpaired, odd number of electrons).
- Many fragmentation.

EI source reaction
 M + e- (70 eV)  M+° + 2e-
- Results in:
 Molecular ion is radical (odd number of electrons)
 Many fragments
 Molecular ion; sometimes NOT present in the mass spectrum.
 Fragments give structural information (like a jigsaw puzzle of a molecule)

Electron Ionization (EI)
- The fragmentation with Electron Ionization (EI) is reproducible.
 The same fragmentation pattern is found in all EI MS equipment (with 70 eV).
- An EI MS spectrum and be compared with a library of MS spectra (database).
 The most likely molecular formula will be presented.
- EI mass spectrum can be regarded as a fingerprint of a compound.



CI source:
 Ionise a gass, form complex [M +H]+ or [M +NH4]+
- [M +H]+ is a Pseudomolecular ion (Quasi molecular ion)
 One mass unit higher.
 Number of electrons = as in the original molecule (even number of
electrons)
- Less fragmentation

, - Gives information on the molecular mass.
- Reagent gass (~1 mbar)
 Methane, ammonia, isobutane.
 Energized by electrons (100-200 eV)

- Proton transfer (that’s the ionization here)
- M + CH5+  M + CH5+ becomes M + CH5+  [M +H]+ CH4
- Fragmentation between EI and CI is totally different (pagina met de 2
chramotogrammen)

GC-MS ANALYZER TYPES

Quadrupole mass analyzer
- Creates a magnetic field,
compound has to reach the
detector. If a compound is not
selected it will leave the
magnetic field. Only one
compound (mass) will reach the
detector.
- A quadrupole acts as a mass
filter by generating a magnetic
field.
- A DC and a RF potential is
applied on the rods.



Time-of-flight MS (TOF)
- Ion will go through the tube
with certain velocity.
- Fragments with a lower
mass will reach the
detector first.
- Suitable for large
compounds.
- The reflector/ mirror device
to give all ions the kinetic energy + focus to detector.



GC-MS DETECTOR TYPE

MS ion Detector
- Ions will come into the cone
and it will multiply the
electrons and the signal will
be stronger because of the
cone.