Short communication
A TOF mass spectrometer with higher resolution and sensitivity via elimination of chromatic TOF aberrations of higher orders

https://doi.org/10.1016/j.ijms.2014.11.007Get rights and content

Highlights

  • A new ion source for the time-of-flight mass spectrometer is proposed.

  • Expressions determining the time-of-flight of ions in such a source were determined.

  • Conditions that enable us to eliminate time-of-flight chromatic aberrations in such a source were determined.

  • Ion sources with rotational and two-dimensional symmetry were calculated.

  • Schemes of time-of-flight mass spectrometers with new ion sources are proposed.

Abstract

The expressions describing the time-of-flight (TOF) and conditions of TOF focusing of ion packets by energies in the ion source with two accelerating gaps have been obtained. The first gap is the ionization region with a uniform electrostatic field, whereas the second gap is the immersion objective with a non-uniform electrostatic field, where the role of the “cathode” (emitting surface) is played by the exit window of the ionization region.

Numerical calculations were used to obtain the ratios between geometric and electrical parameters of three-electrode immersion objectives with two-dimensional and rotational symmetry, determining conditions of four orders of TOF focusing by energy simultaneously with spatial focusing of ion packets. The schemes of time-of-flight mass spectrometers of high resolution and high sensitivity with direct and orthogonal ion injection are presented.

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]:ΔzfεΦ0,where f is the focal distance from the ion source, is the initial energy spread of ions, 0 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 conditionE=Φu1+3U0εis fulfilled, the first-order TOF chromatic aberration coefficient is equal to zero (Dtε(1)=0).

If the condition z=zT(1) or z=zT(2) is fulfilled, it follows from the Eqs. (11), (13) that the TOF chromatic aberration coefficient of the second-order (Dtε(2)=0) or the fourth-order (Dtε(4)=0) is, respectively, equal to zero.

Both coefficients are equal to zero simultaneously if the conditionz=zT(1)=zT(2)is 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 conditionz=zT(1)=zT(2)=zFis 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 equationzF=zpp,where p = p(z) is a partial solution of the paraxial equationΦp''+12Φ'p'+Qp

Time-of-flight dispersion and mass resolution

The plane z=zT(1) 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 followsT=T01υ0(zzT(1)),whereT0=1υ0(zT(1)zT(0))is the time of flight of the central ion from the point of its emission z = zu to the main reference plane of TOF focusing z=zT(1). 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.

References (7)

  • S.B. Bimurzaev et al.

    Nucl. Instr. Methods A

    (2011)
  • W.C. Wiley et al.

    Rev. Sci. Instrum.

    (1955)
  • E.K. Zavoisky et al.

    Rep. USSR Acad. Sci.

    (1956)
There are more references available in the full text version of this article.

Cited by (5)

  • Analytical Methods for the Calculation and Simulation of New Schemes of Static and Time-of-Flight Mass Spectrometers

    2016, Advances in Imaging and Electron Physics
    Citation Excerpt :

    Along with acceleration, such fields carry out high-quality TOF and spatial focusing of ionic packages on the entrance plane of the detector of ions. So, some studies (i.e., Yakushev et al., 2014; Bimurzaev, 2015) considered a simple TOF mass analyzer (TOF MA) on the basis of the nonhomogeneous accelerating field with axial symmetry. Ions are extracted from the source by an impulse start of the electric field of the accelerator.

View full text