An introduction to the time-of-flight technique

In the last two decades, several new types of ion sources have been introduced in organic mass spectrometry like bombardment with heavy ions, PDMS, and laser light, MALDI. Hand in hand with the new ionization techniques followed also a renaissance for the 50 year old time-of-flight technique for the mass analyses. In this paper the basics of the ToF technique is described in a tutorial non-theoretical way for the beginner together with some practical hints. The electrostatic mirror, the delayed extraction technique as well as some recent technical developments are also included.

Authorship

Per Håkansson

The Ångström Laboratory, Uppsala, SwedenThe Ångström LaboratorySwedenUppsala, SwedenThe Ångström Laboratory, Uppsala, Sweden

SCIMAGO INSTITUTIONS RANKINGS

The Ångström Laboratory, Uppsala, SwedenThe Ångström LaboratorySwedenUppsala, SwedenThe Ångström Laboratory, Uppsala, Sweden

Figures

Figures (4)

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Figure 1.
Working principle of a straight time-of-flight mass spectrometer. Molecules are desorbed and ionized when fast heavy ions from an accelerator hit the target at high voltage. The secondary ions are accelerated through a grid and enter a field free drift tube. At the end, the ions are stopped in micro channel plate detector which gives a stop pulse to the digital clock. The start pulse comes from a burst of electrons that are generated when the beam passes a thin foil. The time difference between start and stop pulses are proportional to the square root of the ion mass. The main principle is the same for bombardment with fission fragments from ²⁵²Cf source (PDMS) or keV ions (SIMS) or laser light (MALDI).

Thumbnail

Figure 2.
The typical detector in a time-of-flight spectrometer consists of two micro channel plates coupled in tandem. The total gain is about 10⁶. The plates can be coupled either with the anode plate grounded (a) or at positive high voltage (b).

Thumbnail

Figure 3.
A time-of-flight spectrometer equipped with an electrostatic reflector or ion mirror for improved mass resolving power. The mirror is built up by high precision stainless steel rings separated with high precision ceramic balls. A resistor chain distribute the mirror voltage over the rings uniformly. A stop detector at the rear of the mirror gives the possibility to study metastable decays by coincidence measurements between that detector and the detector for the reflected ions. The spectrometer has also an einzel lens to focus the secondary ion beam and a set of deflection plates for steering the beam correctly into the mirror.

Thumbnail

Figure 4.
In delayed extraction or time lag focusing an extra grid is mounted between the target and grounded acceleration grid. In the desorption moment, t=0, the target and first grid are on the same potential U2. After a certain delay, t_delay the target potential is quickly switched from U2 to U1 and the ions are extracted from the ion source. An ion with high initial energy will be less accelerated than an ion with low initial energy. By choosing the delay time and the distances correctly a net focusing effect can be obtained at the stop detector plane.

Figure 1. Working principle of a straight time-of-flight mass spectrometer. Molecules are desorbed and ionized when fast heavy ions from an accelerator hit the target at high voltage. The secondary ions are accelerated through a grid and enter a field free drift tube. At the end, the ions are stopped in micro channel plate detector which gives a stop pulse to the digital clock. The start pulse comes from a burst of electrons that are generated when the beam passes a thin foil. The time difference between start and stop pulses are proportional to the square root of the ion mass. The main principle is the same for bombardment with fission fragments from ²⁵²Cf source (PDMS) or keV ions (SIMS) or laser light (MALDI).

Figure 2. The typical detector in a time-of-flight spectrometer consists of two micro channel plates coupled in tandem. The total gain is about 10⁶. The plates can be coupled either with the anode plate grounded (a) or at positive high voltage (b).

Figure 3. A time-of-flight spectrometer equipped with an electrostatic reflector or ion mirror for improved mass resolving power. The mirror is built up by high precision stainless steel rings separated with high precision ceramic balls. A resistor chain distribute the mirror voltage over the rings uniformly. A stop detector at the rear of the mirror gives the possibility to study metastable decays by coincidence measurements between that detector and the detector for the reflected ions. The spectrometer has also an einzel lens to focus the secondary ion beam and a set of deflection plates for steering the beam correctly into the mirror.

Figure 4. In delayed extraction or time lag focusing an extra grid is mounted between the target and grounded acceleration grid. In the desorption moment, t=0, the target and first grid are on the same potential U2. After a certain delay, t_delay the target potential is quickly switched from U2 to U1 and the ions are extracted from the ion source. An ion with high initial energy will be less accelerated than an ion with low initial energy. By choosing the delay time and the distances correctly a net focusing effect can be obtained at the stop detector plane.

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An introduction to the time-of-flight technique

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