Mass Spectrometry
Download
Report
Transcript Mass Spectrometry
Mass Spectrometry
Mass Spectrometry
Used to identify organic compounds
mainly through molar mass and
identification of major fragments.
A mass spectrometer is often used as
the “detector” for a gas
chromatograph (GC-MS)
A mixture is separated and a total ion
chromatogram (TIC) obtained. A TIC is the
GC-MS equivalent of a gas chromatogram.
The MS then allows identification of the
components without having to rely on a
comparison of retention times.
Mass Spectrometers
Mass Spectrometry
Analysis time once the GC has been set
up (column chosen, gas flow set, oven
program finalized) is about 20
minutes.
Not too expensive
Our benchtop model was $30K.
Compounds that can be analyzed are
those that can be analyzed on a GC:
Low to moderate boiling liquids
(and, of course, gases)
Mass Spectrometers
analyze particles…cations!, not
photons.
Impact from a high-energy electron
knocks an electron from the organic
compound and creates the Molecular
ion (aka parent ion)
a cation with an unpaired electron M+•
It has the m/z of the molecule, so the
value of m/z in most cases is the
value of the molar mass (provided z,
the charge of the cation, is +1)
Mass Spectrometers
analyze cations, not photons
Impact from a high-energy electron
also causes fragmentation.
Base peak
is the most intense m/z peak
is the most stable fragment
Other fragments form as well, some
from complex rearrangements
Mass Spectrometers
The MS detects the m/z of each
fragment by directing the ionized
particles through a magnetic
quadrupole
The trajectory of the ions in the magnetic
field bends as a function of the mass of the
particle and its charge:
Mass Spectrometers
The magnetic quadrupole thus acts as
a mass filter.
For a given magnetic field strength, light
particles are bent more and heavier
particles less.
By changing the magnetic field strength,
m/z can be focused on the detector (an
electron multiplier) in succession and yield
a mass spectrum
A plot of signal vs m/z
OCCC’s GC-MS
Shimadzu
GC
GC-17A version 3
Restek Rtx-XLB 30 meter fused silica
capillary column with 0.25 mm inner
diameter and a 0.25μm coating (the
liquid phase is low polarity…and
proprietary) for a column volume of
1.5 mL
Column operates from 30° - 340°C
OCCC’s GC-MS
Shimadzu
MS parameters
GCMS-QP5000
requires an operating pressure of
approximately 2 Pa
quadruple mass filter
electron multiplier detector
turns on after 2 min to clear solvent
set up for 2 scans of 35-300 m/z per
second
Fragmentation Patterns Benzene Ring
Peaks at 51 and 77
are very typical of
the benzene ring.
Odd m/z suggests N
may be present.
Fragmentation Patterns - the
benzylic cation, the allylic cation
Extraneous peaks
happen.
Fragmentation Patterns Amines
Fragmentation Patterns alcohols
Alcohols can
fragment so
easily that the
M+ peak may
be missing
altogether.
Fragmentation Patterns aldehydes and ketones
Fragmentation Patterns halogens
Chlorine has 3:1 ratio of
35Cl
:
37Cl
Look for peaks at 35 and 37
Look for a gap of 35.
Look for M:M+2 ratio of 3:1.
Bromine has 1:1 ratio of
79Br
Look for peaks at 79 and 81
Look for a gap of 79.
Look for M:M+2 ratio of 1:1.
Iodine
Look for peak at 127.
Look for a gap of 127.
:
81Br
Rule of Thirteen
Once you have the molecular weight
(MW)…
Divide MW by 13 and express the result
as an integer (n) and a remainder (r).
If your compound is a hydrocarbon, its
formula is approximated as CnHn+r.
You may then find elements of
unsaturation from the molecular
formula.
Rule of Thirteen
If you have hetero atoms in your
compound, adjust the formula to
accommodate:
For O, add O and subtract CH4.
For N, add N and subtract CH2.
For Cl, add Cl and subtract either
C2H11 or C3H-1.
Again, calculate elements of
unsaturation from the formula.
How to Analyze a Mass
Spectrum
Identify the molecular ion peak, if present.
Note if the MW is odd.
Mark the base peak and show the fragment
which gives rise to it.
Mark significant fragment peaks and
identify the fragments giving rise to them.
Mark peaks or gaps that indicate the
halogens, Cl (M : M+2 peaks = 3:1), Br (M :
M+2 peaks = 1:1), I (M or gap = 127).
The mass of the compound with Br will be
M+1.