Sanja Risticevic Chem 323 Poster Presentation

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Sanja RisticevicChem 323 Poster Presentation

• Quadrupole Ion Trap Mass Spectrometry

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Introduction

• Ion trap mass spectrometry has recently developed
  very rapidly
• It is a high performance technique and one of the
  leading tools in the chemistry and biochemistry
  fields
• It can be used for measurements of very high
  mass/charge ratios
• Ion trap mass spectrometry has high resolution
  capabilities and also excellent non-destructive
  broad-band Fourier transform capabilities

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• Advantages of using ion trap
• High sensitivity
• Capable of high performance
• Compactness and mechanical simplicity
• Ion/Molecule reactions can be studied for
  mass-selected ions and the reaction time can be
  varied in the ion trap. Therefore, the kinetics
  and equilibrium of ion-molecule reactions can be
  studied
• High resolution for slow scans
• The resonance experiments are applicable in the
  study of ions that have high m/z ratios
• Fourier transform techniques provide
  non-destructive detection
• MS/MS experiments are possible (multiple stage
  mass spectrometry). In these experiments,
  individual ions can be examined in a mixture of
  ions. The ions of interest are isolated by their
  characteristic m/z values and they dissociate.
  The product ions are then analyzed in a second
  mass measurement step.

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• Ions are subjected to stabilizing and
  destabilizing forces applied by the field. The
  forces occur in three dimensions.
• To record the mass spectra, the quadrupole ion
  trap can be operated in the mass selective
  instability scan mode which is the usual mode of
  operation
• Ions of a given m/z value undergo stable motion
  in the trap
• A helium buffer gas is used to remove the kinetic
  energy from ions and cause them to occupy the
  central region of the trap
• The ion trap can hold up to 105-106 ions before
  columbic repulsions reduce the mass resolution
• Ions are trapped in the system consisting of
  three electrodes which have hyperbolic surfaces
• The central electrode is the rotationally
  symmetrical ring electrode and it is located
  between two end-cap electrodes of the same
  cross-section

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• The diagram which illustrates the ion trap
  instrumentation. r0 is the internal radius of
  the ring electrode and z0 is the closest distance
  from the center to the end-cap electrodes.

The electrodes are aligned and isolated using
ceramic posts.
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Potential for trapping ions

• The quadrupole potential surface is saddle-shaped
  when the phase of the voltage signal is positive
• The ion shown is on a potential downhill in the
  z-direction and is accelerated from the center of
  the device

Potential used for trapping ions in the radial
direction. The ion is accelerated away from the
trapping center in the axial direction.
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• When the voltage field changes sign, this ion is
  accelerated toward the center of the trap. The
  ions are trapped in both the r and z directions.

Potential used for trapping ions in the axial
direction. The ion is accelerated towards the
trap center.
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• Trapped ions have characteristic frequencies of
  oscillation known as secular frequencies. The
  frequency in the r-direction is half the
  frequency in the z-direction.
• An additional potential of frequency equal to the
  secular frequency causes ions to absorb more
  kinetic energy. Ions are activated in the
  z-direction when the signal is applied between
  the two end-cap electrodes. When the signal is
  strong, it is possible for the ions to be ejected
  from the trap in the z-direction.
• The population of ions in the trap can also be
  controlled since particular ions can be excited
  so that they dissociate or get ejected

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• Ions move along the qz axis until qz 0.908.
  Here the ions become unstable in the normal
  mass-selective instability mode of operation and
  reach the boundary. If the supplementary
  resonance frequency can be varied, qz becomes a
  variable. Thus, qz can be lowered by modulating
  the ion motion at a chosen frequency. For this
  purpose, a dipolar electric field is applied
  across the end-cap electrodes. This is called
  the resonance ejection experiment which can be
  used to increase m/z. The ions of a particular
  m/z value pick up translational energy and exit
  the trap through a hole in the end-cap electrode.
  These ions exit the ion trap in the sequence of
  m/z values and reach an external detector.

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• The resonance experiment can also be used to
  allow the fragmentation of the specified ions
  only.
• Resonance experiments are usually performed by
  applying a mixture of frequencies. Thus, the
  ions of different m/z values are manipulated
  simultaneously. The technique used for this is
  called SWIFT (stored waveform inverse Fourier
  transform).
• This technique is also used for the ion
  population control. For example, in trace level
  analysis, the trap can be filled with the analyte
  ions only. Thus, the ions can be stored
  selectively which is very applicable in the
  ultra-trace-level analysis of volatile organic
  compounds. This analysis can be performed at
  levels as low as parts-per-quadrillion (pg/L).
• A Brief Word on Non-Destructive Ion Detection
• The population of a single ion can be measured
  multiple times
• Achieved by impulsive excitation of a group of
  trapped ions of different m/z values
• The ion image currents are induced on a small
  detector electrode. This electrode is isolated
  from the end-cap electrodes.
• The image currents are measured using a
  differential preamplifier, filter and amplifier.
  The image currents are then Fourier analyzed and
  the broad-band spectra can be obtained.

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Mass Selective Instability Scan

• Ions of different m/z values arrive at the
  detector at different times
• When the voltage is increased across the ring
  electrode, the ions of high m/z are ejected.
  When the voltage is changed too fast, the loss of
  resolution can result. Therefore, the rate of
  voltage change should be slow to ensure the high
  resolution.
• A zoom-scan mode can be applied in these
  circumstances to provide a better study of the
  ions which have m/zlt10 dalton/charge
• The high resolution of the instrument helps to
  resolve the isotopic forms of the multiply
  -charged ions

The figure shows the zoom scan of the 4 charged
state of rat interleukin-8. There is one-forth
m/z unit difference between carbon isotopes.
These carbon isotopes are different in mass by
one dalton. This is electrospray ionization mass
spectrum, but it shows how the zoom-scan mode of
the ion trap can be useful in the isotopic study.

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MS/MS Experiment

• The additional sequence of operations in the scan
  function is used. The ionization is followed by
  the selection of a parent ion. Thus, all other
  ions are ejected from the trap. The parent ion
  then undergoes a translational excitation.
  Excited ions collide with a helium buffer gas and
  dissociate. The resulting product ions are
  recorded by scanning the voltage. Thus, a second
  massanalysis scan is performed.
• Applications
• The enriched specificity is useful in the
  distinction of isomers, sequencing of biopolymers
  and analysis of complex mixtures
• The structural elucidation of complex molecules
  in the presence of mixtures. A compound can be
  fragmented and the resulting fragments can be
  further analyzed.
• Conclusions
• The sensitivity and resolution of the ion trap
  are outstanding. Thus, an increasing number of
  analyses will be performed using ion trap mass
  spectrometry.
• The small size of the instrument, the low cost
  and the reasonable pressure requirements make
  this device one of the most powerful tools in the
  chemical analysis.

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References

• 1 http//www.currentseparations.com/issues
• 16-3/cs16-3c.pdf
• 2 Schalley,C.A.Modern Mass Spectrometry, volume
  225, Springer, New York, 2003.