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Delayed extraction

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Delayed extraction with laser desorption ionization. Top: the laser fires at t=0 forming a fast "red" ion and slow "blue" ion of the same m/z, Middle: the red ion takes the lead since it has a greater velocity (and kinetic energy) but since it is farther from the acceleration plate at voltage +V, it gains less energy than the blue ion. Bottom: The blue ion has gained sufficient energy such that it arrives at the detector at the same time as the red ion.

Delayed extraction is a method used with a time-of-flight mass spectrometer in which the accelerating voltage pulse is applied after a time delay following pulsed desorption ionization from a flat surface of target plate or pulsed ion production during electron ionization or laser ionization in some narrow space between two plates of the ion extraction system. The extraction delay can produce energy focusing and improve mass resolution.[1][2][3]

Implementation

Resolution can be improved in time-of-flight mass spectrometer with ions produced at high vacuum conditions (better than few microTorr) by allowing the initial packet ions to spread in space due to their translational energy before being accelerated into the flight tube. With electron ionization ions produced from atoms or molecules from a rarefied gas, this is referred to as "time-lag focusing"[4]. With ions produced by laser desorption/ionization or MALDI[1] from a conductive surface of target plate, this is referred as "delayed extraction."

With delayed extraction, the mass resolution is improved due to the correlation between ion velocity and position after the ions have been ejected from the surface of a sample placed at the target plate (electrode) or produced from neutrals between the two extraction plates. Ions produced with greater kinetic energy have a higher velocity and during the delay time move closer to the extraction electrode before the accelerating voltage is applied across the target or pulsed electrode. The slower ions with less kinetic energy stay closer to a surface of the target electrode or pulsed electrode when the accelerating voltage is applied and therefore start being accelerated at a greater potential compared to the ions farther from the target electrode. With the proper delay time, the slower ions will receive enough extra potential energy to match the faster ions. Ions of the same mass will then drift through the flight tube with close to the same velocity and will be spatially focused at the detector.

See also

References

  1. ^ a b Brown RS, Lennon JJ (1995). "Mass resolution improvement by incorporation of pulsed ion extraction in a matrix-assisted laser desorption/ionization linear time-of-flight mass spectrometer". Anal. Chem. 67 (13): 1998–2003. doi:10.1021/ac00109a015. PMID 8694246. {{cite journal}}: Unknown parameter |month= ignored (help)
  2. ^ Colby, Steven M. (1994), "Improving the resolution of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry by exploiting the correlation between ion position and velocity", Rapid Communications in Mass Spectrometry, 8: 865, doi:10.1002/rcm.1290081102
  3. ^ Guilhaus, M. (1997), "Perfect Timing: Time-of-flight Mass Spectrometry", Rapid Communications in Mass Spectrometry, 11: 951, doi:10.1002/(SICI)1097-0231(19970615)11:9<951::AID-RCM785>3.0.CO;2-H
  4. ^ Wiley, W. C. (1955), "Time-of-Flight Mass Spectrometer with Improved Resolution", Review of Scientific Instruments, 26: 1150, doi:10.1063/1.1715212
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