Time-of-flight
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The Time of flight (TOF) method of measuring particle mass-to-charge ratio is done as follows. An ion of known electrical charge and unknown mass enters a mass spectrometer and is accelerated by an electrical field of known strength. This acceleration results in any given ion having the same kinetic energy as any other ion given that they all have the same charge. The velocity of the ion will depend however on the mass-to-charge ratio.
The time that it subsequently takes for the particle to reach a detector at a known distance is measured. This time will depend on the mass-to-charge ratio of the particle (heavier particles reach lower speeds). From this time and the known experimental parameters one can find the mass-to-charge ratio of the particle. This method of analysis is a powerful tool for finding the mass-to-charge ratio of charged particles, atoms and molecules.
In near infrared spectroscopy Time of Flight method is used to estimate the wavelength dependent optical pathlength.
In kinematics, TOF is the duration in which a projectile is travelling through the air. Given the initial velocity u of the particle, the downward (ie., gravitational) acceleration a, and the projectile's angle of projection θ (measured relative to the horizontal), then a simple rearrangement of the SUVAT equation s=ut+1/2at² results in this equation for the time of flight of a projectile: t=2u(Sin θ)/ | a | .
[edit] Anatomy
Usually the tube is praised for simplicity, but for precision measurements of charged low energy particles the electric and the magnetic field in the flight tube has to be controlled within 10 mV and 1 nT respectively.
The work function homogenity of the tube can be controlled by a Kelvin probe. The magnetic field can be measured by a fluxgate compass. High frequencies are passively shielded and damped by radar absorbent material. To generate arbitrary low frequencies field the screen is parted into plates (overlapping and connected by capicators) with bias voltage on each plate and a bias current on coil behind plate whose flux is closed by an outer core. In this way the tube can be configured to act as a weak achromatic quadrupole lens with an aperture with a grid and a delay line detector in the diffraction plane to do angle resolved measurements. Changing the field the angle of the field of view can be changed and a deflecting bias can be superimposed to scan through all angles.
When no delay line detector is used focusing the ions onto a detector can be accomplished through the use of two or three einzel lenses placed in the vacuum tube located between the ion source and the detector.
The sample should be immersed into the tube with holes and apertures for and against stray light to do magnetic experiments and to control the electrons from their start.
[edit] External links
- ABRF MALDI TOF Tutorial
- IFR/JIC TOF MS Tutorial
- FAST ComTec TOF Application Note
- Jordan TOF Products TOF Mass Spectrometer Tutorial
- University of Bristol TOF-MS Tutorial
[edit] References
- Mamyrin, B. A.; Karataev, V. I.; Shmikk, D. V.; Zagulin, V. A. The mass-reflectron, a new nonmagnetic time-of-flight mass spectrometer with high resolution Sov. Phys. JETP, 1973, 37, 45.
- Stephens, W. E., A Pulsed Mass Spectrometer with Time Dispersion Phys. Rev., 1946, 69, 691.
- Wiley, W. C.; MacLaren, I. H., Time-of-Flight Spectrometer with Improved ResolutionRev. Sci. Instr., 1955, 26, 1150.
