Short communicationA TOF mass spectrometer with higher resolution and sensitivity via elimination of chromatic TOF aberrations of higher orders
Graphical abstract
Introduction
It is well known that the TOF mass spectrometer of simple structure [1] consisting of an ion source, a field-free drift space and an ion detector has a relatively low resolution and sensitivity. The main factor limiting its resolution is the initial energy spread of ions in the packet generated by the ion source. A low sensitivity is caused by the fact that a uniform electric field in the accelerating gap of the ion source formed by flat fine-structure grids cannot provide spatial focusing of ion packets. The presence of fine-structure grids also reduces sensitivity.
To improve the resolution of TOF mass spectrometer, the authors [1] used the ion source providing TOF focusing of ions by energy in the plane coinciding with the plane of the detector. This method enabled them to eliminate some terms in the expansion of the total time-of-flight of ions in powers of the initial energy spread. However, in this case the first-order TOF chromatic aberration typical of any emission system remains unchanged [2]. It is this aberration that determines the width (in the direction of movement) of the ion packet in the detector plane [3]:where f is the focal distance from the ion source, qɛ is the initial energy spread of ions, qΦ0 is the drift energy of ions, and q is the ion charge. To reduce the influence of the width on the resolution in the TOF mass reflectron [3] a temporary primary focus is created near the source. Then, in the image plane of the detector, the ion reflector creates an image of the ion packet of a width close to its own width in the plane of the temporary primary focus.
This paper considers a possibility of creation of a simple-circuit TOF mass spectrometer (without an ion reflector) with high resolution and sensitivity. To solve this problem, the ion source must have two accelerating gaps – the ionization region with a uniform electrostatic field and a system of electrodes forming a non-uniform electrostatic field. This field is directly adjacent to the exit window of the ionization region, forming the immersion objective [4], in which the role of the “cathode” (emitting surface) is played by the exit window of the ionization region. Only such mutual arrangement of the accelerating gaps provides elimination of the first-order TOF chromatic aberration of the ionization region. In addition, the non-uniform field of the immersion objective enables us to get high-quality TOF focusing of ion packets in the plane of the detector simultaneously with spatial focusing.
TOF chromatic aberrations play an important role in TOF focusing of charged particle beams. In electron-optical systems with a straight optical axis TOF geometrical aberrations are effectively reduced by simple diaphragming, i.e. using rather narrow paraxial beams. TOF chromatic aberrations remain unchanged and impose principal limitations on the quality of TOF focusing.
Section snippets
Time-of-flight
Let us consider an ion source consisting of an ionization region with a uniform electric field and an accelerating gap in the form of an immersion objective with a non-uniform electric field. In order to study TOF chromatic aberrations of the ion source it is sufficient to consider the motion of particles along its main optical axis z and to determine the dependence of the time of flight of particles on their initial energies.
Let us first determine the time of flight of ions in the ionization
Conditions for elimination of TOF chromatic aberrations
As it is seen from (23), if the conditionis fulfilled, the first-order TOF chromatic aberration coefficient is equal to zero .
If the condition or is fulfilled, it follows from the Eqs. (11), (13) that the TOF chromatic aberration coefficient of the second-order or the fourth-order is, respectively, equal to zero.
Both coefficients are equal to zero simultaneously if the conditionis fulfilled.
In case of the flat surface of
Spatial-time-of-flight focusing
If the focal plane z = zF of the immersion objective coincides with reference planes of the TOF focusing, i.e. if conditionis fulfilled in the plane of the detector coinciding with the focal plane z = zF of the immersion objective, the TOF focusing of ions by energy to the fourth order inclusively is achieved simultaneously with spatial focusing.
The location of plane z = zF is determined from the equationwhere p = p(z) is a partial solution of the paraxial equation
Time-of-flight dispersion and mass resolution
The plane is said to be the main reference plane of the TOF focusing of the immersion objective. Let us rewrite the Eq. (8) taking into account (25) as followswhereis the time of flight of the central ion from the point of its emission z = zu to the main reference plane of TOF focusing . This time is called the time interval of focusing. The dependence of the time interval of focusing on the ion mass determines the value of TOF dispersion by
Calculations of time-of-flight mass spectrometers
Three-electrode immersion objectives with two types of symmetry: rotational and two-dimensional have been studied. In the immersion objective with rotational symmetry the accelerating non-uniform electrostatic field is created by two coaxial cylinders of equal diameter, whereas in the immersion objective with two-dimensional symmetry it is created by two pairs of flat plates. In both cases, the role of the “cathode” is played by the exit window of the ionization region.
The ratio between
Conclusions
In conclusion it should be noted that the results of this research lay a physical basis for creation of a simple TOF mass spectrometer (without an ion reflector) whose resolution and sensitivity are not worse than the same parameters of TOF mass reflectrons. Such high resolution is achieved due to possibility of eliminating of the first-order TOF chromatic aberration and high-quality TOF focusing of ion packets by energy in such a device. In addition, the non-uniform electrostatic field of the
Acknowledgments
This work was supported in part by the Ministry of Education and Science of the Republic of Kazakhstan, grant no. 5 IPS GF3.
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