MagneticSector Mass Spectrometry

1
Mass Spectroscopy Separation of Ions on the
Basis of Mass to Charge Ratio by Kymberly
Forrest, Olivia Gilbert, Nicole Griglione, and
Mary Jones
2
Introduction

• Mass spectroscopy is an effective method for
  determining the weight and structure of mixtures
  of compounds.
• It is advantageous to other methods of analysis
  in its ability to separate and analyze mixtures
  of compounds.
• It is an extremely sensitive technique with
  current levels of accuracy within the range of
  one mass unit.

3
Applications

• General analysis of inorganic, organic, and
  bio-organic chemicals
• Geological Sample dating
• Drug analysis and research
• Process studies in the petroleum, chemical, and
  pharmaceutical industries
• Surface analysis
• Structural resolution of proteins and genetic
  material

4
General Setup

• Common to all mass spectrometers are
• Sample Inlet (2) Ionization Source (3) Mass
  Analyzer
• (4) Ion Detector (5) Vacuum System

5
Methods of Separation

• This presentation examines different types of
  Mass Analyzers, which differ in the physical
  properties they manipulate.
• Magnetic, which depends on Voltage
• Time of Flight, which depends on Mass
• Electric Quadrupole, which depends on AC/DC
  Currents
• Common to these Mass Analyzers is the separation
  of particles on the basis of mass to charge
  ratios (m/q).

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Magnetic-Sector Mass Spectrometry
7
Magnetic-Sector Mass Spectrometry
THEORY The ion source accelerates ions to a
kinetic energy given by KE ½ mv2 qV Where m
is the mass of the ion, v is its velocity, q is
the charge on the ion, and V is the applied
voltage of the ion optics.
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Magnetic-Sector Mass Spectrometry

• The ions enter the flight tube and are deflected
  by the magnetic field, B.
• Only ions of mass-to-charge ratio that have equal
  centripetal and centrifugal forces pass through
  the flight tube
• mv2 /r BqV, where r is the radius of curvature

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Magnetic-Sector Mass Spectrometry

• mv2 /r BqV
• By rearranging the equation and eliminating the
  velocity term using the previous equations, r
  mv/qB 1/B(2Vm/q)1/2
• Therefore, m/q B2r2/(2V)
• This equation shows that the m/q ratio of the
  ions that reach the detector can be varied by
  changing either the magnetic field (B) or the
  applied voltage of the ion optics (V).

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Magnetic-Sector Mass Spectrometry
In summary, by varying the voltage or magnetic
field of the magnetic-sector analyzer, the
individual ion beams are separated spatially and
each has a unique radius of curvature according
to its mass/charge ratio.
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Time of Flight Mass Spectrometry (TOF-MS)
INTRODUCTION

• Separates ions based on flight time
• Operates in pulsed mode
• Ions accelerated by an electric field
• Lighter ions reach the detector first

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Time of Flight Mass Spectrometry (TOF-MS)
THEORY

• KEqV when electrons are accelerated through an
  electric field
• KE of ion is ½mv2, so qV ½mv2 and velocity is
  inversely proportional to mass
• Transit time (t) is L/v, where L is drift tube
  length and v is velocity
• So tL/(2V/m/q)½ can be solved for charge-mass
  ratio

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Time of Flight Mass Spectrometry (TOF-MS)
HOW ITS DONE

• Reflectron is series of rings or grids that
  serves to focus ions to improve resolution
• Exact values of L and V do not need to be known
  if two or more ions of known mass are used as
  mass calibration points
• Produces a mass spectrum as a function of time
  (can be measured every 10 nsec)

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Time of Flight Mass Spectrometry (TOF-MS)
ADVANTAGES

• Good for kinetic studies of fast reactions and
  for use with gas chromatography to analyze peaks
  from chromatograph
• Can register molecular ions that decompose in the
  flight tube

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Quadrupole Mass Analyzers
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Background InfoQuadrupole Mass Analyzers

• been in use since the 1950s
• most commonly used mass spec today
• sometimes referred to as mass filters because
  ions of only a single mass to charge (m/q) ratio
  pass through the apparatus
• separate ions based on oscillations in an
  electric field (the quadrupole field) using AC
  and DC currents

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Benefits Quadrupole Mass Analyzers
easy to use simple construction fast low
cost can achieve unit to 0.1 m/q resolution
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More BenefitsQuadrupole Mass Analyzers

• tolerant of relatively poor vacuums (5 x
  10-5torr), which make them well suited to
  electrospray ionization (because these ions are
  produced under atmospheric conditions)
• quadrupoles are now capable of routinely
  analyzing up to a m/q ratio of 3000, which is
  useful in electrospray ionization of
  biomolecules, which commonly produces a charge
  distribution below m/z 3000

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Basis of Quadrupole Mass Filter

• consists of 4 parallel metal rods, or electrodes
• opposite electrodes have potentials of the same
  sign
• one set of opposite electrodes has applied
  potential of UVcos(?t)
• other set has potential of
• - UVcos?t
• U DC voltage, VAC voltage, ? angular velocity
  of alternating voltage

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Operation of Quadrupole Mass Filter

• voltages applied to electrodes affect trajectory
  of ions with the m/q ratio of interest as they
  travel down the center of the four rods
• these ions pass through the electrode system
• ions with other m/z ratios are thrown out of
  their original path
• these ions are filtered out or lost to the walls
  of the quadrupole, and then ejected as waste by a
  vacuum system
• in this manner the ions of interest are separated

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Obtaining A Spectrum Quadrupole Mass Analyzers

• a mass spectrum is obtained by varying the
  voltages on the rods and monitoring which ions
  pass through the filter
• two methods for varying rod voltages
• vary ? while holding U and V constant
• vary U and V but keep the ratio U/V fixed
• remember U DC voltage, VAC voltage, ? angular
  velocity of alternating voltage
• The resolution is determined by the magnitude of
  U/V ratio
• beam currents for individual m/q ratios may be as
  low as 10-13 A and so electron multipliers are
  usually used

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ApplicationsQuadrupole Mass Analyzers

• partial pressure analyzers
• GC/MS
• upper atmosphere and space research
• separation of proteins and other biomolecules
  with electrospray
• plasma diagnostics
• multielement and isotopic analyses

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Summary

• Magnetic Mass Spectroscopy shows the
  relationship between voltage and mass to charge
  ratio
• m/q B2r2/(2V)
• Time of Flight Mass Spectroscopy shows the
  relationship between tube time and mass to charge
  ratio
• m/q 2Vt2/L2
• Electric Quadrupole shows the relationship
  between AC/DC currents and mass to ratios.

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Sources
Dean, John A., et al. Instrumental Methods of
Analysis. Belmont, CA Wadsworth Company, 1988.
465-485. De Laeter, John R. Toronto John Wiley
and Sons, Inc., 2001. 68-70 Sparkman, O. David.
Mass Spec Desk Reference. Pittsburg Global
View, 2000. 47-48. http//elchem.kaist.ac.kr/vt/c
hem-ed/ms/ms-intro.htm http//masspec.scripps.edu/
information/intro/ http//ms.mc.vanderbilt.edu/tut
orials/ms/ms.htm http//www.abrf.org/ABRFNews/1996
/September1996/sep96iontrap.html http//www.chem.a
rizona.edu/massspec/intro_html/intro.html