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Double-focusing mass spectrometer including a Wien filter with inhomogeneous field as mass analyzer
International Journal ofMass Spectrometry and ion Processes, 54 (1983) 333-336 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
333
Short Communication DOUBLE-FOCUSING MASS SPECTROMETER INCLUDING A WIEN FILTER WITH INHOMOGENEOUS FIELD AS MASS ANALYZER
C. CUNA
and D. IOANOVICIU
Institute for Isotopic
and Molecular
Technology,
Cly’- Napoca,
P.O. Box 700 (Romania)
(Firs!. received 24 March 1983; in final form 8 June 1983)
Wien filters have been operated
as single-focusing mass spectrometers, good transmission at low and medium resolutions being demonstrated [l-3]. A double-focusing mass spectrometer combining the homogeneous fields of a Wien filter and a magnet was built by Jordan [4-61. More recently the construction and performance of a triple-focusing mass spectrometer with a Wien filter and a spherical condenser were reported [7]. Here we are dealing with a double-focusing instrument with an inhomogeneous Wien filter and an electric deflector with an adjustable field index. GEOMETRY
OF THE APPARATUS
The design of our instrument is based on the ion-optical calculations of double-focusing systems with inhomogeneous Wien filters and electric toroidal deflectors given in ref. 8. We constructed a double-focusing instrument of “reversed geometry”: the ions traverse the Wien filter before the electric deflector. As a function of the applied fields, the inhomogeneous Wien filter may have a focusing action in the median plane (convergent regime) or may have a defocusing action (divergent regime) [9]. The electric deflector, having Matsuda plates [lo], may work with a variable field index. The mass spectrometer, presented in Fig. 1, has the compact geometry allowed by the divergent-convergent ion optics. Compared to conventional instruments, our choice has the advantage of easier adjustment (only one
Wren filter Ekctric deflector
Fig. 1. The geometry of the mass spectrometer. 0168-l
176/83/$03.00
0 1983 Elsevier Science Publishers B.V.
334
ion-optical element with a curved axis) and the possibility to compensate electrically for small geometrical imperfections, keeping the main path at ground potential {compare to ref. 1 I). To study spontaneous or collision-induced transitions the instrument may be switched from MIKE to IKE mode without breaking the vacuum. ION OPTICAL
PARAMETERS
Using the first-order matrix elements from refs. 9 and 12, after matrix multiplication we obtain the radial magnification M and the mass dispersion coefficient D of the double-focusing spectrometer A4 = M,M, D = M,(l
= a, - cos(k2)
- k sin( kZ)
. (all;’ -t a2)
- M,)/(2k2p)
where M, and M2 are the Wien filter and electrostatic tions, respectively
(1) (2) analyzer’s magnifica-
(3)
The terms Zi’ and /$ are the distances from the filter to the virtual intermediate image, and from this image to the electric deflector, respectively; p depends on the electrostatic field inhomogeneity in the deflector [12]; p is radius of curvature of the ion trajectory on the filter axis in the magnetic field acting alone; re and r,,., are the radii of curvature of the equipotential and magnetic field line of force on the filter axis, respectively. Other symbols are given in Fig. 1. To obtain formulae (1) and (2), the angular and energy focusing conditions were satisfied simultaneously. The following first-order double-focusing design parameters were selected: p = 12.25 cm; ,; = 40.2 cm; /y = 34.4 cm; k = 0.03873i cm-‘; i = fl; D=20.4 cm; Z=30 cm; R,=20 cm; A-8 cm; M,=O.25; M2= -1; +, = 31.8O. RESULTS
The effects of the fringing fields in the filter and the electric deflector were reduced by Herzog shields. The field-free spaces were shielded by p-metal strips.
335
mas Fig. 2. The Nz -C2H$
scale
I mass
writs)
mass doublet record.
The best vacuum attained was 5 X 10B7 Torr, the pressure rising to values between 10m6 and 10m5 Torr during sample introduction. A modified Nier-type electron bombardment ion source produced a beam of 5 keV maximum energy. The plates of the filter and deflector were fed from separate, highly stabilized supplies. The coil located on the central part of the magnet yoke ensured a magnetic field intensity of 8000 Oe on the filter’s axis. With flat plates in the Wien filter, a maximum resolution of 5600 ‘at peak half height was obtained on the Ar+ peak with a final source slit of 0.1 mm width. In Fig. 2 the N2+ - C,HI,+ mass doublet record is given. With this instrument, metastable transitions and the energies then released were studied for hydrogen, benzene and toluene ions. CONCLUSION
We consider that the experiments conducted with this instrument are a first step on the way to building versatile double-focusing mass spectrometers with variable ion-optical parameters by using a variable-index electric field deflector associated with a Wien filter (a) with Matsuda plates, or (b) with interchangeable plates with various le radii (which are relatively easy to manufacture).
336 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
H. Ewald and S. C&be, Phys. Verh., 5 (1954) 104. L. Hohnlid, Int. J. Mass Spectrom. Ion Phys., 17 (1975) 403. D. Ioanoviciu and C. Cuna, Int. J. Mass Spectrom. Ion Phys., 25 (1977) 117. E.B. Jordan, Phys. Rev., 57 (1940) 1072. E.B. Jordan, Phys. Rev., 58 (1940) 1009. E.B. Jordan, Phys. Rev., 60 (1941) 710. S. Taya, K. Tokiguchi, I. Kanomata and H. Matsuda, Nucl. Instrum. Methods, 150 (1978) 165. D. Ioanoviciu and C. Cuna, Int. J. Mass Spectrom. Ion Phys., 15 (1974) 79. D. Ioanoviciu, Int. J. Mass Spectrom. Ion Phys., 11 (1973) 169. H. Matsuda and Y. Fujita, Int. J. Mass Spectrom. Ion Phys., 16 (1975) 395. T. Matsuo and H. Wollnik, Int. J. Mass Spectrom. Ion Phys., 18 (1975) 205. H. Wollnik, in A. Septier (Ed.), Focusing of Charged Particles, Vol. 2, Academic Press, New York, 1967, p_ 163.
333
Short Communication DOUBLE-FOCUSING MASS SPECTROMETER INCLUDING A WIEN FILTER WITH INHOMOGENEOUS FIELD AS MASS ANALYZER
C. CUNA
and D. IOANOVICIU
Institute for Isotopic
and Molecular
Technology,
Cly’- Napoca,
P.O. Box 700 (Romania)
(Firs!. received 24 March 1983; in final form 8 June 1983)
Wien filters have been operated
as single-focusing mass spectrometers, good transmission at low and medium resolutions being demonstrated [l-3]. A double-focusing mass spectrometer combining the homogeneous fields of a Wien filter and a magnet was built by Jordan [4-61. More recently the construction and performance of a triple-focusing mass spectrometer with a Wien filter and a spherical condenser were reported [7]. Here we are dealing with a double-focusing instrument with an inhomogeneous Wien filter and an electric deflector with an adjustable field index. GEOMETRY
OF THE APPARATUS
The design of our instrument is based on the ion-optical calculations of double-focusing systems with inhomogeneous Wien filters and electric toroidal deflectors given in ref. 8. We constructed a double-focusing instrument of “reversed geometry”: the ions traverse the Wien filter before the electric deflector. As a function of the applied fields, the inhomogeneous Wien filter may have a focusing action in the median plane (convergent regime) or may have a defocusing action (divergent regime) [9]. The electric deflector, having Matsuda plates [lo], may work with a variable field index. The mass spectrometer, presented in Fig. 1, has the compact geometry allowed by the divergent-convergent ion optics. Compared to conventional instruments, our choice has the advantage of easier adjustment (only one
Wren filter Ekctric deflector
Fig. 1. The geometry of the mass spectrometer. 0168-l
176/83/$03.00
0 1983 Elsevier Science Publishers B.V.
334
ion-optical element with a curved axis) and the possibility to compensate electrically for small geometrical imperfections, keeping the main path at ground potential {compare to ref. 1 I). To study spontaneous or collision-induced transitions the instrument may be switched from MIKE to IKE mode without breaking the vacuum. ION OPTICAL
PARAMETERS
Using the first-order matrix elements from refs. 9 and 12, after matrix multiplication we obtain the radial magnification M and the mass dispersion coefficient D of the double-focusing spectrometer A4 = M,M, D = M,(l
= a, - cos(k2)
- k sin( kZ)
. (all;’ -t a2)
- M,)/(2k2p)
where M, and M2 are the Wien filter and electrostatic tions, respectively
(1) (2) analyzer’s magnifica-
(3)
The terms Zi’ and /$ are the distances from the filter to the virtual intermediate image, and from this image to the electric deflector, respectively; p depends on the electrostatic field inhomogeneity in the deflector [12]; p is radius of curvature of the ion trajectory on the filter axis in the magnetic field acting alone; re and r,,., are the radii of curvature of the equipotential and magnetic field line of force on the filter axis, respectively. Other symbols are given in Fig. 1. To obtain formulae (1) and (2), the angular and energy focusing conditions were satisfied simultaneously. The following first-order double-focusing design parameters were selected: p = 12.25 cm; ,; = 40.2 cm; /y = 34.4 cm; k = 0.03873i cm-‘; i = fl; D=20.4 cm; Z=30 cm; R,=20 cm; A-8 cm; M,=O.25; M2= -1; +, = 31.8O. RESULTS
The effects of the fringing fields in the filter and the electric deflector were reduced by Herzog shields. The field-free spaces were shielded by p-metal strips.
335
mas Fig. 2. The Nz -C2H$
scale
I mass
writs)
mass doublet record.
The best vacuum attained was 5 X 10B7 Torr, the pressure rising to values between 10m6 and 10m5 Torr during sample introduction. A modified Nier-type electron bombardment ion source produced a beam of 5 keV maximum energy. The plates of the filter and deflector were fed from separate, highly stabilized supplies. The coil located on the central part of the magnet yoke ensured a magnetic field intensity of 8000 Oe on the filter’s axis. With flat plates in the Wien filter, a maximum resolution of 5600 ‘at peak half height was obtained on the Ar+ peak with a final source slit of 0.1 mm width. In Fig. 2 the N2+ - C,HI,+ mass doublet record is given. With this instrument, metastable transitions and the energies then released were studied for hydrogen, benzene and toluene ions. CONCLUSION
We consider that the experiments conducted with this instrument are a first step on the way to building versatile double-focusing mass spectrometers with variable ion-optical parameters by using a variable-index electric field deflector associated with a Wien filter (a) with Matsuda plates, or (b) with interchangeable plates with various le radii (which are relatively easy to manufacture).
336 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
H. Ewald and S. C&be, Phys. Verh., 5 (1954) 104. L. Hohnlid, Int. J. Mass Spectrom. Ion Phys., 17 (1975) 403. D. Ioanoviciu and C. Cuna, Int. J. Mass Spectrom. Ion Phys., 25 (1977) 117. E.B. Jordan, Phys. Rev., 57 (1940) 1072. E.B. Jordan, Phys. Rev., 58 (1940) 1009. E.B. Jordan, Phys. Rev., 60 (1941) 710. S. Taya, K. Tokiguchi, I. Kanomata and H. Matsuda, Nucl. Instrum. Methods, 150 (1978) 165. D. Ioanoviciu and C. Cuna, Int. J. Mass Spectrom. Ion Phys., 15 (1974) 79. D. Ioanoviciu, Int. J. Mass Spectrom. Ion Phys., 11 (1973) 169. H. Matsuda and Y. Fujita, Int. J. Mass Spectrom. Ion Phys., 16 (1975) 395. T. Matsuo and H. Wollnik, Int. J. Mass Spectrom. Ion Phys., 18 (1975) 205. H. Wollnik, in A. Septier (Ed.), Focusing of Charged Particles, Vol. 2, Academic Press, New York, 1967, p_ 163.