Transcript Mass Spectrometry - Home - Wilkes Department of Chemistry

Mass Spectrometry
Sources – making ions can be hard or soft
Referenced MS timeline can be found at:
http://masspec.scripps.edu/MSHistory/timeline.php
“Hard” vs. “Soft” Ionization methods,
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ICP sources
Spark Sources
Glow Discharge
Secondary Ion/Neutral Mass Spec
fast atom bombardment (FAB) sources
desorption sources
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field ionization (FI)
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field desorption,
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laser desorption . . . (especially MALDI)
plasma desoprtion sources
electron impact (EI)
chemical ionization (CI)
electrospray ionization sources (ESI)
“no-prep” atmospheric sources
ICP sources
gas, liquid or solid
sample is introduced
into hot plasma
 an efficient source
of positively charged
analyte ions
 Ar plasma is
generated and maintained at the end of the glass torch located
inside the loops of a water cooled copper load coil.
 RF potential applied to the coil produces an electromagnetic
field in the part of the torch located within its loops.
 electrons are accelerated and collide with Ar atoms in the Ar
gas flowing through the torch producing Ar+ ions and free
electrons - a plasma.
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ICP sources
the ions have to be extracted
from the high temperature
(~ 6000K or more), atmospheric
pressure (760 torr) environment
of an often chemically corrosive
Ar plasma into a mass spectrometer
operating in a high vacuum (10-5 torr) at room temperature.
 interface region contains two successive cones (mm orifices)
 ions in the center of the plasma are sampled into the region
between two cones held at a pressure of about 1-3 torr
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At this stage, most of the Ar atoms are removed by a vacuum pump.
ion beam is further extracted through the skimmer cone orifice
into the front section of the mass spectrometer (pressure of
about 10-3 - 10-4 torr)
Spark Ionization Sources
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samples are physically incorporated into two conductive
electrodes (usually either carbon or silver)
a high-voltage arc is produced, ionizing the material
semiquantitative trace element technique for solids and liquids
 samples: conducting, semiconducting and insulating solids,
powders, crystals, liquids, organometallics, ash from organics,
unknowns and many other sample forms.
detection capabilities encompass the periodic table (Li – U)
 has the ability to determine impurity levels from the sub-ppm
level to 0.1%. SSMS
 total simultaneous elemental coverage
 low detection limits
 high res. capabilities - eliminates many spectral interferences.
Glow Discharge MS
analytical technique for the
bulk elemental analysis of
inorganic solid samples.
 capable of analyzing,
conducting, semi-conducting
and insulating samples.
 amenable to solids, powders,
crystals, wafers, and many other sample forms.
 elemental coverage encompasses Li – U
 determine impurity levels from the sub-ppb to percent level
 advantages include
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high precision and low detection limits,
quantitative accuracy (+/- 25% on average), without the use of standards
high resolution capabilities eliminate most spectral interferences.
Secondary Ion/Neutral Mass Spec
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a primary, high-energy beam
of ions (usually oxygen,
argon, or cesium) is aimed
at a small area of a sample,
such as a mineral grain.
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the primary ions have
energies of ~ 10 keV
the primary ions sputter
away the sample by causing
the ejection of atoms and
ions (called secondary
neutrals and ions)
these secondary ions
(approximately 1% of the
sputtered material) are
accelerated into a mass
spectrometer to reveal the
elemental and isotopic
characteristics of the sample.
Fast Atom Bombardment (FAB)
material to be analyzed is
mixed with a non-volatile
chemical protection
environment called
a matrix
 This is bombarded under
vacuum with a high energy
(4 – 10 keV) beam of atoms.
 atoms are typically an inert gas (Ar or Xe)
 common matricies include glycerol, thioglycerol,
3-nitrobenzyl alcohol (3-NBA), 18-Crown-6 ether,
2-nitrophenyloctyl ether, sulfolane, diethanolamine,
and triethanolamine.
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Field Ionization
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Field ionization (FI) is the generation of M+ ions by removal
of electrons, primarily from gas sample molecules, using a
high electric field.
This generally occurs at a sharp edge or tip that is biased to a
high electrical potential
Field Desorption
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Field desorption (FD) is a method for emitting ions into the
gas phase.
Sample spread on an emitter is heated while a high electric
field is applied.
Ions are then emitted by the tunneling, ion-molecule reactions,
thermal fusion effects,
and other phenomenon
occurring on the emitter
surface and around the
whisker ends.
The ionization phase depends strongly on the sample material
and the spread condition.
Plasma Desorption
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Plasma desorption ionization mass spectrometry (PDMS; also
called fission fragment ionization) is a mass spectrometry
technique in which ionization of material in a solid sample by
bombarding it with ionic or neutral atoms formed as a result of
the nuclear fission of a suitable nuclide, typically the
Californium isotope 252Cf
Science, Vol 191, Issue 4230, 920-925
Copyright © 1976 by American Association for the Advancement of Science
Californium-252 plasma desorption mass spectroscopy
RD Macfarlane and DF Torgerson
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We have shown that 252Cf-PDMS is capable of producing mass spectra of
quasi-molecular ions for a wide variety of compounds, including amino
acids, moderately large peptides, nucleotides, and natural products. Positive
and negative ion mass spectra can be obtained, and in many cases quasimolecular ions are observed in both. The method is nondestructive, as only
a relatively few molecules are used and samples can be recovered after the
measurement is made. Fragmentation patterns are obtained which can yield
structure information. The present sensitivity of the method is at the
nanogram level and there are possibilities for reducing this to picograms.
The mass resolution is sufficient to give elemental identification up to mass
500. This may be extended to higher masses with improved time-of-flight
techniques. There are indications that 252Cf-PDMS may extend the mass
range of molecules that can be studied to as high as 3000 or more.
laser desorption . . .
Especially MALDI
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Matrix-assisted laser desorption ionization (MALDI)
Ionization method using matrix-assisted laser desorption.
a soft ionization technique
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analysis of biomolecules
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proteins,
Peptides
sugars)
large organic molecules
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polymers,
dendrimers
other macromolecules
tend to be fragile and
fragment when ionized
by other methods.
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MALDI cont’d
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identity of suitable matrix compounds is determined using
specific molecular design considerations
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matrix solution is mixed with the analyte (e.g. protein-sample)
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fairly low molecular weight (to allow facile vaporization)
large enough (with a high enough vapor pressure) not to evaporate
during sample preparation or while standing in the spectrometer
are acidic / act as a proton source to encourage ionization of the analyte
have strong absorption in the UV so they rapidly and efficiently absorb
the laser irradiation
functionalized with polar groups - allowing use in aqueous solutions
organic solvent allows hydrophobic molecules to dissolve
water allows for hydrophilic molecules to do the same
solution is spotted onto a MALDI plate
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solvents vaporize, leaving only the recrystallized matrix
analyte molecules spread throughout the crystals in co-crystallized
MALDI spot
laser desorption . . .
UV MALDI Matrix List
l (nm)
Applications
2,5-dihydroxy benzoic
acid
acetonitrile, water,
methanol, acetone,
chloroform
337, 355,
266
peptides,
nucleotides,
oligonucleotides,
oligosaccharides
3,5-dimethoxy-4hydroxycinnamic acid
acetonitrile, water,
acetone, chloroform
337, 355,
266
peptides, proteins,
lipids
4-hydroxy-3methoxycinnamic
acid
acetonitrile, water,
propanol
337, 355,
266
proteins
α-cyano-4hydroxycinnamic acid
acetonitrile, water,
ethanol, acetone
337, 355
peptides, lipids,
nucleotides
Picolinic acid
Ethanol
266
oligonucleotides
3-hydroxy picolinic
acid
Ethanol
337, 355
oligonucleotides
Compound
Solvent
Electron Ionization (EI)
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EI (formerly known as electron impact) is an ionization
technique widely used in mass spectrometry, particularly
for organic molecules.
The gas phase reaction producing electron ionization is:
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M + e-  M+ + 2elow energies (around 20 eV), the interactions between the
electrons and the analyte molecules do not transfer enough
energy to cause ionization
at around 70 eV, the de Broglie wavelength of the electrons
matches the length of typical bonds in organic molecules (about
0.14 nm), and energy transfer to organic analyte molecules is
maximized, leading to the strongest possible ionization and
fragmentation
Electron Ionization (EI)
Chemical Ionization (CI)
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Chemical ionization (CI) is an ionization technique used in
mass spectrometry
ionization is achieved by interaction of its molecules with
reagent ions
the analyte is ionized by chemical ion-molecule reactions
during collisions in the source
the process may involve transfer of an electron, a proton or
other charged species between the reactants.
a less energetic procedure than electron ionization and the ions
produced are, for example, protonated molecules: [M + H]+.
These ions are often relatively stable, tending not to fragment
as readily as ions produced by electron ionization.
Chemical Ionization (CI)
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typical reagent gases (ex. CH4, isobutane, or NH3) are present
in a millionfold excess with respect to the analyte.
analyte is ionized by ion-molecule chemical reactions:
 Primary Ion Formation:
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CH4 + e-  CH4+ + 2e-
Secondary Reagent Ions:
CH4 + CH4+  CH5+ + CH3
CH4 + CH3+  C2H5+ + H2
Product Ion Formation:
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M + CH5+  CH4 + [M + H] +
(protonation)
AH + CH3+  CH4 + A+
(H− abstraction)
M + CH5+  [M+ CH5] + (adduct formation)
A + CH4+  CH4 + A+
(charge exchange)
electrospray ionization sources (ESI)
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A soft ionization
technique
Solvated drop of analyte
is ionized
Solvent is removed in
vacuuo
Charged analyte left for
MS analysis
High m/z analytes easily
examined
Electrospray
results
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Figure 2. Total ion chromatogram
(TIC) and the full scan mass spectra
of GTI-2040 and major metabolites
(M1-M5)
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(A) shows the TIC of GTI-2040 and
M1 to M5 metabolites;
(B) shows the mass spectrum of M1,
the putative 3′N-1 metabolite with
retention time (RT) of 14.8, which
contains an ion envelope including
[M-6H],6- [M-5H],5- and [M-4H]4ions;
(C) shows the mass spectrum of M2,
the putative 3′N-2 metabolite at RT
14.2 minutes, which contains the most
abundant ion of [M-3H]3-; and
(D) shows the mass spectrum of M3,
the putative 3′N-3 metabolite at RT
12.9 minutes containing the most
abundant ion of [M-3H].3-
No-prep MS (DART)
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DART is a mass spectrometry system that:
can analyze samples in the gas, liquid, or solid phase
 operates at 0 potential
 operates in the open air (atmospheric pressure)
 does not require solvents
 obtains mass spectra from sample
on the surface of anything imaginable;
i.e. commodities, ball caps, glass rods,
plastics, adhesive tape, etc.
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It is a form of
chemical ionization
that takes place
at atmospheric
pressure.
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No-prep MS (DART)
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an electrical potential is applied to a gas (ex. N2 or He),
generates a plasma, & interacts with sample and atm.
different ionization mechanisms can be favored by
changing operating conditions
H+ transfer is dominant mode of positive ionization.
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electrons can be formed if the carrier gas can form
metastable species with high enough internal energy.
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metastable He atoms react with H2O to produce ionized water
clusters that can protonate the sample molecule, forming MH+
He reacts with atmospheric H2O forming negative-ion clusters
that react with analytes to form negatively charged ions.
Forensic note:
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in negative-ionization mode, nitrate and nitrite ions are not
produced because plasma formation by the carrier gas is isolated
from air. Those ions interfere with the detection of nitrogen-based
explosives and reduce the sensitivity of anion detection.