2nd/3rd Year Undergraduate Experiments in Physical Chemistry
7.13 Time-of-flight Mass Spectrometry
Introduction || Location || Safety || Web links || Instructions
This experiment illustrates the basic principles and applications of mass spectrometry, including mass determination, interpretation of cracking patterns and use of mass spectra as molecular fingerprints. The technique is widely used to identify compounds and to determine their molecular structure in fields such as fundamental research, the chemical industry, medicine and forensic science.
This mass spectrometer uses the ion time-of-flight method to separate masses. A pulse of electrons ionises gas molecules; immediately afterwards a pulsed electric field accelerates all ions of the same charge to the same energy, after which they fly at different velocities to a distant detector. The kinetic energy given to the ions is equal to the product of their charge q and the applied voltage V.
For a flight distance D, the flight time t is
It transpires that the flight time is determined by the mass divided by the charge; spectra are always shown on a scale for ions of a single charge, so doubly (or triply) charged ions appear at half (one third) of their real masses.
One of the most useful capabilities of mass spectrometers is to determine molecular masses. In high resolution instruments masses are determined with such precision that molecular formulae can be determined, because the fractional part of the exact mass is different for each atomic (isotopic) composition at a single nominal mass number. The same thing can be done in favourable cases even with a low resolution instrument; you will do it in this practical.
The experiment is located in the Lower Teaching Laboratory.
Safety glasses should be worn throughout, because the glass inlet tubes are evacuated and the samples are eye irritants. The samples should be handled in the fume cupboard at all times except when brought to and taken from the apparatus. Many of them are highly flammable. The flight tube uses very high voltages: its connectors must not be disturbed in any way. Precautions must be taken when handling liquid nitrogen; take care not to spill it on any tight-fitting clothing. If a spill onto clothing in contact with the skin occurs, remove the garment immediately to avoid potentially unpleasant burns. For your own safety and the safety of the apparatus please do not change the settings of any controls apart from those specifically indicated.
MSDS data for chemicals used in this experiment:
Instructions for this experiment are available for reference in a folder in the Lower Teaching Laboratory. You may obtain a personal copy from the technician in that laboratory, or download instructions (pdf, 14 pages, 120k, updated October 22, 2004).
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Updated October 22, 2004.