Mass Spectrometry Lecture 2

1
Mass SpectrometryLecture 2

• Dr Kevin Welham
• F229

2
Mass Spectrometry

• Mass Analysers
• These are the devices used to separate the ions
  produced in the ion source into their individual
  m/z ratios and focus them on the detector.
• A number of different mass analysers exist in
  modern mass spectrometry and we will consider
  each in turn.

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Mass Spectrometry

• Mass Analysers
• Magnetic sector single focussing
• - double focussing
• Quadrupole analysers
• Ion Trap (Quistor) devices
• Fourier Transform Ion Cyclotron Resonance (FTICR
  ) analysers
• Time-of-Flight (TOF) analysers

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Mass Spectrometry

• Magnetic sectors single focussing
• Generally bulky and expensive.
• Earliest type of analyser and still popular.
• High accelerating potential especially in
  comparison with other methods (usually 4-10kV).
• Magnets wedge shaped and must provide homogeneous
  fields.
• Ion beam enters and exits at exactly 90.
• Focuses ions according to their momentum.

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Mass Spectrometry
Magnetic sector analysers
Ion kinetic energy m v2/2 zeV (1) Deflecting
force BzeV centrifugal force
mv2/r (2) Combining these m/z B2r2e/2V (3)
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Mass Spectrometry

• Write down the three parameters which can be
  varied to provide separation of m/z values.

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Mass Spectrometry

• The number of charges, z is usually equal to
  one, so equation (3) shows that a spectrum of
  masses can be obtained by changing one of the
  three variable B, r or V, as illustrated in
  Figure. Commercial instruments have been produced
  for each of these modes of scanning
• Magnetic scanning, B Most commonly used
  provides wide mass range without loss of
  performance at high mass, as in the case of
  voltage scanning (see below). The disadvantages
  of hysteresis and slow scanning associated with
  electromagnets have largely been overcome with
  modern laminated and/or air core magnet designs.
• Change of radius, r Not possible with a fixed
  detector, although special applications such as
  accurate isotope ratio measurements use two or
  more fixed detectors. This method was used
  historically with photoplate detection, and has
  been revived with microchannel plate photodiode
  array detectors this can improve signal-to-noise
  ratio as all the ions of interest can be
  monitored simultaneously and hence continuously.
• Voltage scanning, V No theoretical limit to the
  mass range as the mass is inversely proportional
  to the accelerating potential. However in
  practice the extraction efficiency of ions from
  the source depends upon V in addition the ions
  rely on the kinetic energy imparted by the
  potential for effective transmission though the
  instrument, and hence the performance
  deteriorates at high mass. Advantages are the
  relatively low cost of permanent magnets (which
  do not, of course, require electronic circuits),
  fast scanning and the reproducibility of the scan
  function as there is no hysteresis.

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Mass Spectrometry

• Magnetic sectors double focussing
• Adds a second electric sector to provide energy
  focussing of ions independent of mass.
• Can be forward Geometry with electric sector
  before magnetic sector (Nier-Johnson geometry)
  or reverse geometry with magnetic sector before
  electric sector.
• Double focussing means that both the energy and
  momentum focus is designed to coincide at the
  collector slit.
• Very high mass resolution can be achieved by this
  arrangement.
• Very bulky and expensive but high performance.

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Mass Spectrometry
Schematic of double focussing magnetic sector
mass spectrometer
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Mass Spectrometry

• Mass Resolution
• Elements have non-integer masses e.g. H1.007825,
  C12.00000, N14.00307, O15.99492.
• Resolution in mass spectrometry is defined as the
  ability to separate an ion of mass M from another
  with mass M?M.
• The resolution is defined as R M
• ?M
• Two peaks of equal height are said to be resolved
  if the the valley between them does not exceed
  10 of the peak height (10 valley definition),
  which for symmetrical peaks corresponds to
  defining the resolution relative to the
  peak-width at 5 of the height (see fig).

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Mass Spectrometry

• Fig shows 10 valley definition of mass
  resolution.
• Other definitions exist such as 50 peak height,
  Full width at half maximum height (FWHM)

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Mass Spectrometry

• Quadrupole analysers
• True mass analyser, unaffected by changes in ion
  velocity.
• Cheaper, lower performance.
• More flexible, simpler, easier to interface to GC
  and HPLC.
• Faster scanning and stepping between masses.
• Easier to interface to computers and easier
  calibration due to linear scan law.

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Mass Spectrometry

• Consists of four parallel rods coupled in pairs.
• One pair (x) has positive DC potential(U) and
  other pair (y) has negative DC potential (-U)
  applied.
• An AC potential Vcos?t which is 180 out of phase
  is between each pair of rods is superimposed on
  the DC potential.

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Mass Spectrometry

• Peak value of the AC voltage is greater than the
  DC voltage, so the "positive" pair are sometimes
  negative, and vice versa, as shown above.
• Low kinetic energy ions (20eV) are introduced
  and under the influence of the oscillating field
  follow a spiral path through the analyzer between
  the rods (along the z axis).

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Mass Spectrometry

• Ion motion on the x and y axes is defined by the
  Mathieu equations, which are of the form shown
  below for x, with an identical equation for y.

Where
a 8eU/ mr2?2 and q 4eV/ mr2?2 where r
radius from rod surface to center line, thus a/q
2U/V
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Mass Spectrometry

• If the AC frequency is kept constant, m is
  linearly proportional to U and V. In scanning the
  instrument the ratio a/q is kept constant by
  maintaining a fixed ratio between U and V, which
  are scanned simultaneously to give a spectrum
  which is linear in mass. The behaviour of the
  ions is defined by the stability diagram shown
  above.

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Mass Spectrometry

• Quadrupole Ion Trap (QUISTOR)
• Essentially a 3D quadrupole, ions move in x and y
  directions but no z axis so ions move around a
  central point.
• Device for storing (trapping) ions, quadrupole
  ion storage trap (QUISTOR).
• Ions can be formed by a variety of techniques
  either externally or internally within the trap.
• Constructed from a doughnut shaped ring electrode
  with two end caps, all having cylindrically
  symmetric hyperbolic surfaces.

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Mass Spectrometry

• Radio frequency AC voltage of about 1kV at 1.1
  MHz is applied to the ring electrode, which
  induces the oscillatory motion of the ions.
• The equations presented for the quadrupole
  analyzer also apply here, and in the AC-only mode
  there is no mass discrimination.
• In the trap there are stability criteria for the
  ion trajectories in a manner analogous to the
  quadrupole, and the application of the AC signal
  stores ions of all masses down to a low cut-off
  value of m/z.

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Mass Spectrometry

• Increasing the AC voltage causes the ion
  trajectories to increase and to approach the end
  caps.
• As this voltage is scanned upwards the ions are
  ejected from the trap in order of increasing m/z
  and those that emerge through apertures in the
  lower end cap are detected by the electron
  multiplier.
• Helium is used as a buffer gas in the trap at
  about 10-3 Torr to dampen out the more violent
  motions and to give improved spectral stability.
• The storage time can be increased by delaying the
  start of the voltage ramp.

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Mass Spectrometry

• Fourier transform ion cyclotron resonance mass
  spectrometers (FT-ICR)
• Have become increasingly popular with the use of
  Fourier transform (FT) techniques, the
  development of superconducting magnets and the
  importance of pulsed desorption ionization
  methods and are now in the forefront of mass
  spectrometer development.
• Mass analyser consists of a cubic cell made up of
  six isolated plates, 2.5 cm square, contained in
  a homogenous magnetic field generated by a
  superconducting magnet.
• Ions may be formed within the cell or externally
  and injected into the analyser cell.

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Mass Spectrometry
Schematic diagram of an FTICR mass analyser.
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Mass Spectrometry

• A small potential applied to two opposite plates
  (trapping plates) parallel to the axis of the
  magnetic field traps the ions within the field.
  Any thermal energy they have will cause them to
  spiral close to the centre of the cube with
  cyclotron frequencies ?c and velocities v
  inversely proportional to m/z
• ?c v/r zeb/m
• The cyclotron frequencies range form MHz down to
  kHz.
• The application of a burst of low amplitude RF
  power to the side plates (excitation plates) will
  excite the ions into larger orbits. They will now
  pass close to the top and bottom plates (receiver
  plates), and so will induce an electrical signal
  in the circuit connecting these plates.
• This signal is amplified, digitised and stored in
  a computer for subsequent processing. This
  involves an analysis of the constituent
  frequencies of the complex waveform that arises
  from having a complete range of m/z-values, and
  is carried out by Fourier transformation.
• From this the mass spectrum is derived a key
  feature of this means of detection is that,
  unlike all other mass spectrometers, it is
  nondestructive.

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Mass Spectrometry

• The instrument can be operated in a number of
  different modes. The excited ions will gradually
  lose kinetic energy and will fall back towards
  the centre of the cell.
• They can be ejected (quenched) by applying a
  voltage pulse to the trapping plates before new
  ions are produced, or they can be re-excited to
  produce a new signal that allows spectrum
  averaging in order to obtain an enhanced
  signal-to-noise ratio.
• Alternatively, by application of only certain
  frequencies, some ions can be excited to such
  large orbits that they strike the sides of the
  cube and are destroyed. This will leave only
  selected ions for further study, e.g. in tandem
  mass spectrometry experiments.
• It is possible to vary the time between ion
  formation and excitation for CI or to study the
  kinetics of unimolecular fragmentation or
  ion/molecule reactions.

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Mass Spectrometry

• FTMS instruments can operate at very high
  resolving power, and the measurement of the
  cyclotron frequency can be very precise.
• This allows very accurate measurement of m/z and
  very precise separation of adjacent peaks.
• In a 4.7 tesla magnetic field at 10-8 Pa, the
  time domain signal measured for m/z 18 over 51
  seconds has been recorded to give a resolving
  power of M/? M 100,000,000.
• In FTMS the cell can contain both positive and
  negative ions simultaneously, and Cl can be
  resolved from Cl-, a difference of two electrons
  (ca 0.001 Da).
• Resolving power for the FTMS is inversely
  proportional to mass, and so is lower for high
  mass ions.

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Mass Spectrometry

• The mass range for FTMS instruments depends upon
  the strength of the magnetic field.
• With large superconducting magnets 10,000 Da for
  singly charged ions is well within reach in this
  respect performance is comparable to large sector
  instruments.
• The dynamic range is poor compared with sector
  instruments as there is a space-charge limit to
  the number of ions that can be stored
  simultaneously, i.e. ions repel each other and
  are lost. The dynamic range achievable with a
  sector instrument can be greater than 1061,
  whereas FTMS gives only 1031.
• The pressure in the FTMS cell must remain low as
  ions will collide with residual gas molecules,
  leading to a randomisation of their motion. This
  limits the ionization methods available unless
  the ions are prepared remotely and injected into
  the cell.

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Mass Spectrometry

• Time-of-Flight Mass Spectrometry
• Have recently enjoyed a considerable resurgence
  with the introduction of desorption ionisation
  techniques such as laser desorption.

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Mass Spectrometry

• Ions formed are pulsed into the TOF analyser.
• Ions are accelerated by a high voltage pulse (up
  to 3kV in early instruments).
• Ions acquire the same (fixed) kinetic energy.
• zeV mv2 /2
• Thus m/z 8 1/ v2 8 t2

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Mass Spectrometry

• Ions travel down a 1m drift tube and their time
  of arrival at the detector is recorded.
• Masses calculated from flight times.
• Output can respond very rapidly to changes in
  sample composition, spectra recorded in
  milliseconds.
• In theory mass range is infinite.
• Early instruments limited by poor electronics and
  limited resolution.
• More recently MALDI technique has made use of the
  unrestricted mass range.

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Mass Spectrometry
Schematic of a modern reflectron TOF mass
spectrometer.
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Mass Spectrometry

• Resolution has been enhanced by
• Modern electronics allow nanosecond precision in
  time measurements.
• Pulsed nature of ionisation techniques such as
  MALDI. All ions formed in very short time
  interval, so all travel simultaneously down tube.
  Enhanced by time delay gating.
• Development of reflectron instruments allows
  compensation for thermal energy spread and
  kinetic energy spread.
• These improvements mean that resolutions of
  around 20,000 are now obtainable.