introduction of stable isotope ratio mass spectrometry

1. Jitesh Kumar
MSc (F) Forensic Science

2. • Isotope-ratio mass spectrometry (IRMS) is a
specialization of mass spectrometry, in which mass
spectrometric methods are used to measure the relative
abundance of isotopes in a given sample
• Isotope Ratio Mass Spectrometry (IRMS) is a
specialized technique used to provide information about
the geographic, chemical, and biological origins of
substances.
• The ability to determine the source of an organic
substance stems from the relative isotopic abundances
of the elements which comprise the material.

3. • Mass spectrometry is a powerful analytical technique
used to quantify known materials, to identify unknown
compounds within a sample, and to elucidate the
structure and chemical properties of different
molecules.
• The complete process involves the conversion of the
sample into gaseous ions, with or without
fragmentation, which are then characterized by their
mass to charge ratios (m/z) and relative abundances.

4. • Mass spectrometry is an instrumental technique in
which sample is converted to rapidly moving positive
ions by electron bombardment and charged particles are
separated according to their masses.

5. Introduction to Mass Spectrometry
Sample
introduction
Ionization
Minimize collisions, interferences
Separate
masses
Count ions
Collect results

6. • To measure relative molecular mass
• To know the fragmentation of the molecules
• Comparison of mass spectra with known compounds.

7. • Organic molecule are bombarded with electron
• Converted into highly energetic positively charged ions
(molecular ions or parent ions)
• Further break up into smaller ions (fragment ions or
daughter ions)
• The formed ions are separated by deflection in
magnetic field according to their mass and charge
• MASS SPECTRUM

8. • A static gas mass spectrometer is one in which a
gaseous sample for analysis is fed into the source of the
instrument and then left in the source without further
supply or pumping throughout the analysis.
• This method can be used for 'stable isotope' analysis of
light gases , but it is particularly used in the isotopic
analysis of noble gases (rare or inert gases)
for radiometric dating or isotope geochemistry.
• Important examples are argon–argon dating and helium
isotope analysis.

9. Spectra of Nobel gases

10. A mass spectrometer generates multiple ions
then separates them according to their specific mass-to-
charge ratio (m/z)
then records the relative abundance of each ion type
Ions provide information concerning the nature and the
structure of their precursor(a chemical rxn that
produces another compound) molecule
In the spectrum of a pure compound, the molecular ion,
if present, appears at the highest value of m/z and
gives the molecular mass of the compound

11. • The first step in the mass spectrometric analysis of compounds
is the production of gas phase ions of the compound, basically
by electron ionization. This molecular ion undergoes
fragmentation. Each primary product ion derived from the
molecular ion, in turn, undergoes fragmentation, and so on.
The ions are separated in the mass spectrometer according to
their mass-to-charge ratio, and are detected in proportion to
their abundance. A mass spectrum of the molecule is thus
produced. It displays the result in the form of a plot of ion
abundance versus mass-to-charge ratio. Ions provide
information concerning the nature and the structure of their
precursor molecule. In the spectrum of a pure compound, the
molecular ion, if present, appears at the highest value of m/z
(followed by ions containing heavier isotopes) and gives the
molecular mass of the compound

12. • The gas source mass spectrometer includes three fundamental parts,
• (1) a "source" of positively charged ions or molecular ions,
• (2) a magnetic analyzer, and
• (3) ion collectors.
• The source is a low-pressure chamber (~10-5 torr) into which sample or
standard gas is ionized, either by an electron beam produced by a hot
filament, or by a strong electrostatic field.
• Once formed, the ions are accelerated and focused by charged plates into a
beam that enters a flight tube.
• The flight tube has a bend that coincides with an electromagnet that alters
the path of the ions according to their mass/charge ratio, thus several beams
leave the magnetic sector.
• Multiple ion detectors are arranged to collect the ion beams of interest.
These collectors measure each beam as a current that can be amplified and
determined with high precision.

14. • Moving wire IRMS is useful for analyzing Carbon-
13 ratios of compounds in a solution, such as after
purification by liquid chromatography. The solution (or
outflow from the chromatography) is dried onto a nickel
or stainless steel wire. After the residue is deposited on
the wire, it enters a furnace where the sample is converted
to CO2 and water by combustion. The gas stream finally
enters a capillary, is dried, ionized, and analyzed. This
process allows a mixture of compounds to be purified and
analyzed continuously, which can decrease the analysis
time by a factor of four. Moving wire IRMS is quite
sensitive, and samples containing as little as 1 nano-
mole of Carbon can yield precise (within 1‰) results.

15. • Accelerator mass spectrometry (AMS) is a form of mass
spectrometry that accelerates ions to extraordinarily
high kinetic energies before mass analysis.
• AMS is performed by converting the atoms in the sample into a
beam of fast moving ions (charged atoms). The mass of these
ions is then measured by the application of magnetic and
electric fields.
• The special strength of AMS among the mass spectrometric
methods is its power to separate a rare isotope from an
abundant neighboring mass ("abundance sensitivity", e.g. 14C
from 12C).
• The method suppresses molecular isobars completely and in
many cases can separate atomic isobars (e.g. 14N from 14C)
also.

17. AMS

19. a) ION SOURCE
generates negative
carbon ions
by Cs sputtering
b) INJECTOR MAGNET
separates ions by mass,
masses 12, 13, and 14 injected
c) ACCELERATOR
generates 2.5 million volts,
accelerates C- ions
d) TERMINAL
C- ions interact with
‘stripper’ gas Ar,
become C+ ions,
molecular species CH
destroyed
e) ELECTROSTATIC DEFLECTOR
specific charge of ions selected (3+)
f) MAGNETIC SEPARATION
13C steered into cup, 14C
passes through to solid detector
g) Si BARRIER DETECTOR
pulse produced is proportional to the energy of ion,
can differentiate b/t 14C and other ions count rate
for modern sample = 100cps

20. A) The ion source produces a beam of ions (atoms that carry
an electrical charge) from a few milligrams of solid
material. The element is first chemically extracted from the
sample (for example, a rock, rain water, a meteorite) then it
is loaded into a copper holder and inserted into the ion
source through a vacuum lock.
• Atoms are sputtered from the sample by cesium ions which
are produced on a hot spherical ionizer and focused to a
small spot on the sample. Negative ions produced on the
surface of the sample are extracted from the ion source and
sent down the evacuated beam line towards the first magnet.
At this point the beam is about 10 microamps which
corresponds to 1013 ions per second (mostly the stable
isotopes).

21. B) The injector magnet bends the negative ion beam by
90° to select the mass of interest, a radioisotope of the
element inserted in the sample holder, and reject the
much-more-intense neighboring stable isotopes.
• Several vacuum pumps remove all the air from the
beam line so the beam particles have a free path.
• There are still lots of molecules and isobars (isotopes
of neighboring elements having the same mass) that
must be removed by more magnets after the accelerator.

22. C) The tandem accelerator consists of two accelerating gaps with a
large positive voltage in the middle.
• Think of it as a bridge that spans the inside of a large pressure vessel
containing CO2 and N2 insulating gas at a pressure of over 10
atmospheres.
• The bridge holds two long vacuum tubes with many glass
(electrically insulating) sections.
• The center of the accelerator, called the terminal, is charged to a
voltage of up to 10 million volts by two rotating chains.
• The negative ions traveling down the beam tube are attracted
(accelerated) towards the positive terminal.
• At the terminal they pass through an electron stripper, either a gas
or a very thin carbon foil, and emerge as positive ions. These are
repelled from the positive terminal, accelerating again to ground
potential at the far end.
• The name tandem accelerator comes from this dual acceleration
concept. The final velocity is a few percent of the speed of light or
about 50 million miles per hour.

25. D) The analyzing and switching magnets select the
mass of the radionuclide of interest, further reducing
the intensity of neighboring stable isotopes.
• In addition, they eliminate molecules completely by
selecting only the highly charged ions that are
produced in the terminal stripper.
• Isotope ratios are measured by alternately selecting
the stable and radioisotopes with the injector and
analyzing magnets.

26. E) The electrostatic analyzer is a pair of metal plates
at high voltage that deflects the beam to the left by
20 degrees. This selects particles based on their
energy and thus removes the ions that happen to
receive the wrong energy from the accelerator.
F) The gas ionization detector counts ions one at a
time as they come down the beam line. The ions are
slowed down and come to rest in propane gas. As
they stop, electrons are knocked off the gas atoms.
These electrons are collected on metal plates,
amplified, and read into the computer.
• For each atom, the computer determines the rate of
energy loss and from that deduces the nuclear charge
(element atomic number) to distinguish interfering
isobars.

27. • The ion extraction from the sample
• The rejection of the primary isotopes
• The beam acceleration
• The rejection of the isobaric ions
• The rare isotope counting
•negative 14N ions not stable
• stripping destroys molecules

28. Accelerator mass spectrometry (AMS)
• Some applications :
• Archeology
• Geology
• Medicine
• Food chemistry
• Radiation protection
• Ecology
• Radioecology
• Aerosol science
• Microdosing

29. • carbon isotopic ratio of soil(EA-IRMS)
• two samples had identical physical or chemical
properties
• to determine whether or not a product’s actual
contents
• Isotope ratio analysis can be used to establish
whether or not the product contains natural
sweeteners (from the original food source), or
artificial sweeteners

30. Thank You