Time-of-flight mass spectrometry

They are often abbreviated as TOF-MS, TOF -MS or TOF only. Systems with a reflectron are also called RTOF ( for reflectron time-of -flight ).

Operation

In -flight mass spectrometer to determine the mass-to- charge ratio takes place by measuring the time of flight. For this purpose, the ions are accelerated in an electric field and then go through a route. The ions of the charge have, after passing through an electric power absorbed energy, which is then present as a kinetic energy.

Because of the context, the flight time is proportional to the square root of the mass - to-charge ratio:

The time of arrival of the ions at the end of the route is detected by a detector, usually secondary electron multiplier. The signal may be digitized by a high-speed A / D converter or TDC and a spectrum shown in Fig. Currently, mass measurements with an accuracy of about 2 ppm possible. In addition, a relatively high mass resolution can be achieved.

Unlike many other types of mass spectrometer flight mass spectrometer are no filters, that is, all ions are measured simultaneously. This eliminates the need to scan through the mass range, and allows very fast measurements without limitation in the mass range. The mass range is only limited by the electronics. With flight mass spectrometers up to 100,000 complete mass spectra can be recorded per second.

Ion mirror and reflectron

With the use of an ion mirror at the end of the flight path, an electric field is applied, which is opposite to the accelerating voltage. Thus, the ions are decelerated and then accelerated again in the opposite direction. Ions such as light behave in a mirror, that is, by changing the orientation of the field, the ions can be steered around the corner. A gradient is applied, instead of a single field, the energy distribution of ions can be reduced and thus the mass resolution is increased. This arrangement then called reflectron. The field is applied by a plurality of annular electrodes to the flight path of the ions. Depending on the mathematical function, follow the gradations of tension, a distinction between linear and non-linear reflectron. The non-linear reflectron are again divided into those with a square function, and those having a circular path function.

Applications

The main advantages of flight mass spectrometers are in the broad mass range, which is why they are often used along with soft ionization methods in biological analyzes ( see, eg, MALDI).

Flight mass spectrometer are also suitable for the analysis of fast processes such as in the GC x GC -TOF and the IMS -TOF.

In addition, it can be determined with the time-of the element, molecular and isotopic composition of the Earth's atmosphere and ionosphere.

In the comet probe Rosetta European Space Agency ESA a flight mass spectrometer is used. The probe flies in the direction of the comet Churyumov- Gerasimenko, which it will reach in 2014 since 2004. In this comet traces of prehistory of the solar system have received. Since the formation of our solar system 4.6 billion years ago, the chemical composition and the isotopic ratio has hardly changed because the comet very far away during its existence from the sun, and was exposed to little heat. By means of flight mass spectrometry of parts of the comet to be examined, which composition had the accretion disk from which the solar system was formed.