Desorption Electrospray Ionization

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Desorption Electrospray Ionization

• Colin Wynne
• Biological Mass Spectrommetry
• 4/14/07

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New Mass Spectrommetry Technique

• Ambient Ionization method
• No vacuum needed
• Sample fully accessible
• usually no sample prep needed

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Applications

• Study of compounds seperated on Thin Layer
  Chromatography plate
• Study of pharmaceuticals directly from tablets.
• Study of in vivo biological tissue.
• Study of explosives, chemical warfare, and
  biological warfare agents on a variety of
  surfaces.

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How does it work?

• Solid sample is placed on a surface.
• Liquid sample is dried on a surface.
• No other sample preparation needed.
• 5050 alcohol-water mix nebulized (made into
  small particles) and directed at the sample.
• Sample exterior bombarded with the nebulized mix
  is ionized and transported to the optics of the
  mass spectrometer.
• Sprayer set at 60O angle from surface.
• Inlet to Mass Spectrometer set at 10O angle.

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• DESI spectra also are similar to ESI spectra.
• Multiple charge states
• Sodium adducts present in spectra.

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How are those surface molecules ionized?

• This is the major question of this paper.
• Four competing theories.
• Droplet pick-up
• Chemical sputtering
• Gas phase ionization
• Shockwave model.

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Ionization theories

• Pick-up droplet method
• Material taken up by upward moving droplet after
  collision with surface.
• Ionization occurs via same process as ESI.
• Field desorption model
• Charge residue model
• Chemical sputtering
• Compounds with a high vapor pressure react with
  free ions above sample or with droplet incoming
  or outgoing the surface.

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How is ionization method determined?

• Droplet sizes and velocities both before and
  after impact are measured.
• Uses crossing laser beams to measure size and
  velocity of droplets.
• Droplets have to have a diameter of at least
  0.5µm to be detected.
• Same method (Phase Doppler Particle Analysis) has
  been used to measure droplets of different ESI
  configurations.

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Initial Measurements

• 5050 methanol/water mix was sprayed at 2µl/min
  using a 1130 kPa supply pressure.
• Droplet diameters and velocities were measured at
  1mm, 2mm, 5mm, and 10 mm.
• Diameters were distributed across populations
  centered at 2, 4, and 6µm at 5 mm from the
  sprayer tip.

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Droplet before impact

• Droplet velocities decrease with distance from
  the sprayer due to drag forces.
• 120 m/s at 2 mm from sprayer
• 40 m/s at 10 mm from sprayer
• Kinetic energy per impacting water molecule was
  calculated to be 0.6meV.

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Droplet After Impact

• Velocities decreased with increasing distance
  away from plane of surface.
• However, velocities increased with greater
  distance from direct impact site.
• Authors assumed this to be due to droplets moving
  closer to surface after impact, then lifting off
  to the minimum measurable height at greater
  distances.

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Measurements under changing conditions

• Used Rhodamine B and Melittin as samples.
• Ion intensities were analyzed under differing
  conditions.
• Spray Voltage
• Gas Line pressure
• Solvent flow rate
• Distance of sample sprayer from surface (for
  melittin).
• Drop velocities and diameters were also analyzed
  under these conditions.

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Optimum Conditions

• For Rhodamine B, the highest ion intensity
  occurred at
• High spray voltage (5-6kV)
• 100 psi gas line pressure
• Varying solvent flow rate resulted in no
  significant change in intensity.
• For Melittin, highest ion intensity occurred at
• High spray voltage
• High pressure
• 2-4 mm sprayer to surface distance.
• Solvent flow rate only significantly increased
  beyond 5µl/min.

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Rhodamine B
Melittin
Optimum pressure
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Conclusions

• Sputtering by inelastic nuclear collisions or
  conventional momentum transfer not likely as
  ionization method.
• Needs higher cluster energy per nucleon than the
  0.6meV calculated for water molecule impacting at
  less than 200 m/s.
• Shockwave theory also disproven.
• As shown in last slide, solvent molecule would
  not break the sound barrier at impact with
  analyte.

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Conclusions

• So, which theories are still viable.
• Chemical sputtering
• Droplet pick-up
• Chemical sputtering not able to be studied by the
  method in the paper.
• Authors theorize that Droplet pick-up is the
  major process leading to ionization.
• Due to theory that scattered droplets have higher
  velocities when scattered close to plane of
  surface.

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Reference

• Kebarle, P., Tiang, L. From Ions in solution to
  ions in the gas phase The Mechanism of
  ESI-MS. J. Anal. Chem. 2003, 65, 972A-986A.
• Takats, Z., Wiseman, JM., Cooks, RG., Ambient
  mass spectrometry using desorption electrospray
  ionizationinstrumentation, mechanisms, and
  applications in forensics, chemistry, and
  biology. J. Mass Spec. 2005, 40 (10),
  1261-1275.
• Venter, A., Sojka, P., and Cooks, R.J. Droplet
  Dynamics and Ionization Mechanisms in Desorption
  Electrospray Ionization Mass Spectrometry. J.
  Anal. Chem. 2006, 78, 8549-8555.