Time-of-flight mass spectrometer

Time-of-flight mass spectrometer (ESI-TOF).

Flight mass spectrometer ( English time-of-flight mass spectrometer ) are a subclass of the mass spectrometer .

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

functionality

In the time-of-flight mass spectrometer, the mass-to-charge ratio is determined by measuring the time of flight. To do this, the ions are accelerated in an electrical field and then travel through a flight path . After passing through an electrical voltage, the ions of the charge have absorbed the energy , which is then available as kinetic energy. ${\ displaystyle d}$ ${\ displaystyle q}$ ${\ displaystyle U}$ ${\ displaystyle E_ {p} = U \ cdot q}$

Because of the relationship , the flight time is proportional to the square root of the mass-charge ratio: ${\ displaystyle E = {\ tfrac {1} {2}} mv ^ {2}}$ ${\ displaystyle T_ {f}}$

{\ displaystyle T_ {f} \ propto {\ sqrt {\ frac {m} {q}}}}

.

The time at which the ions arrive at the end of the flight path is detected by a detector, usually a secondary electron multiplier. Its signal can be digitized by a fast A / D converter or TDC and displayed in a spectrum. Mass determinations with an accuracy of about 2 ppm are currently possible. In addition, a very high mass resolution can also be achieved.

In contrast to many other types of mass spectrometers, time-of-flight mass spectrometers are not filters, which means that all ions are measured simultaneously. This eliminates the need to scan through the mass domain and allows very fast measurements with no restriction in the mass domain. The mass range is practically only limited by the electronics. Time-of-flight mass spectrometers can record up to 100,000 complete mass spectra per second.

Ion mirror and reflectron

Trajectory of ions in reflector mode

When using an ion mirror, an electric field is applied at the end of the flight path, which is opposite to the acceleration voltage. This slows down the ions and then accelerates them again in the opposite direction. The ions behave like light on a mirror , which means that by changing the orientation of the field, the ions can be deflected around the corner. If a gradient is applied instead of a simple field, the energy distribution of the ions can be reduced and thus the mass resolution can be increased. This arrangement is called a reflectron . The field is created by several ring-shaped electrodes around the flight path of the ions. Depending on the mathematical function followed by the gradations of the voltages, a distinction is made between linear and non-linear reflectrons. The non-linear reflectrons are subdivided into those with a quadratic function and those with a circular path function.

Applications

The main advantages of time-of-flight mass spectrometers are in the large mass range, which is why they are often used together with soft ionization methods in biological analyzes (see e.g. MALDI ).

Time-of-flight mass spectrometers are also suitable for analyzing fast processes such as the GCxGC-TOF and the IMS -TOF.

In addition, time-of-flight mass spectrometry can be used to determine the element , molecule and isotope composition in the earth's atmosphere and the ionosphere .

A time-of-flight mass spectrometer is used in the Rosetta comet probe operated by the European Space Agency . The probe reached the Churyumov-Gerasimenko comet in 2014 . Traces of the prehistory of the solar system have been preserved in this comet . Since the formation of our solar system 4.6 billion years ago, the chemical composition and the isotope ratio have hardly changed because the comet was very far from the sun and exposed to little heat during its existence. Using time-of-flight mass spectrometry of parts of the comet, it is intended to investigate the composition of the accretion disk from which the solar system emerged.

Web links

Matthias Schrod: New methods for the synthesis and analysis of phenol-formaldehyde resins urn : nbn: de: tuda-tuprints-2879 , page 43, Figure 3.2: Scheme of a MALDI-TOF mass spectrometer with reflectron geometry