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A variable frequency supply for a quadrupole mass spectrometer
Short communication received 27 October 1971; accepted 8 November 1971
A variable frequency supply for a quadrupole mass spectrometer A circuit is described which generates potentials of the form +(U÷ Vcos~ot) suitable for operating a quadrupole mass spectrometer in the frequency scanning mode. Most commercial signal generators are sufficiently stable in both frequency and output voltage to form the basis of a variable frequency quadrupole rf generator. Some further voltage amplification is usually necessary along with the provision of identical magnitude outputs which are 180 ° out of phase. Each output also requires the addition of a dc level in such a manner that the ratio rf/dc remains constant over the swept frequency range. The system used is shown in Figure 1. Initial rf signals of 1 Vp.p amplitude in the range 400 k H z ~ 3 MHz were obtained from an Airmec rf Oscillator Type 858. Subsequent amplification (25 Vp.p at 1 MHz) was provided by a Tektronics L Type plug-in Amplifier operated in a Tektronics Type 132 Power Supply. This power supply has identical positive and negative outputs and thus provides the desired out of phase components. The outputs from this unit are decoupled by the condensers C~, C~ and the resistor chain (R = 3.66kf~) Tektronics type 132 power supply
C~
f tt
Cs
~-it-
-T °
+ (U +Vcos wt)
terminates the amplifier and helps maintain a reasonably flat frequency response. Tapping points for the rectifying circuits are also taken from this resistor chain. Provision of the appropriate dc voltages (0-+6V), which were added to the output via the 470 kD resistors, was derived from the two diodes and associated smoothing condensers Cs, C4. These levels could be set identical to one another at the required value by the two 1 Mf~ potentiometers. As the resolution of the quadrupole is critically dependent on the rf/dc ratio it is essential that this remains constant over the operating frequency range. For this reason it is imperative that germanium diodes are used in the rectifying circuits. Measurement of the very small signal characteristics (1=10-7--+10 -5 A) have indicated that the forward voltage drop is a factor of 10 less than that for silicon diodes. A typical value for an OA5 was 0.02 V at 10-6 A. This low forward voltage drop ensures that the ratio rf/dc at the output is effectively constant for wide variations of the rf level. A change of only 0.3 per cent was calculated for a variation of the rf voltage from 36-->18 Vp.p. This circuit has been used with a quadrupole having field radius r0 = 0.41 cm and rod length 25 cm to observe ions from glow discharge source, operating in various helium admixtures. Adequate resolution in the range 4-40 amu was achieved if the rf/dc ratio was set for maximum resolution (5.96:1). In these studies the potential of point A, which is in fact the potential of the quadrupole axis, was at ground but if necessary this can be operated up to 500 V. Some difficulty was experienced for lighter masses ( < 4 ainu) due to high background currents ( ~ 10-11 A) brought about by the fall of the rf voltage at high frequencies. This fall off was due mainly to the high frequency limitation of the Type 132 Power supply and at present investigations are in progress to correct this by the inclusion of small inductors in parallel with the output to provide a wideband resonance at frequencies around 2 MHz. In spite of this limitation the existing arrangement has allowed qualitative observations to be made on hydrogen ions. D J Mellor
Figure L R~, R4 1.5 k.Q; R=, R8 330 ~; Rs, R6 470 k all 2 per cent ¼W. VR1, VR= 1 M ~ lin; (71, C~, C5, C~, 0.1 /~F 500 V, C3, C4 470 pF. D1, D~ matched pair OA5.
Department of Electronic and Electrical Engineering University of Sheffield Mappin Street Sheffield S1 3JD, England
Vacuum/volume 21/number 12. Pergamon Press LtdlPrinted in Great Britain
607
A variable frequency supply for a quadrupole mass spectrometer A circuit is described which generates potentials of the form +(U÷ Vcos~ot) suitable for operating a quadrupole mass spectrometer in the frequency scanning mode. Most commercial signal generators are sufficiently stable in both frequency and output voltage to form the basis of a variable frequency quadrupole rf generator. Some further voltage amplification is usually necessary along with the provision of identical magnitude outputs which are 180 ° out of phase. Each output also requires the addition of a dc level in such a manner that the ratio rf/dc remains constant over the swept frequency range. The system used is shown in Figure 1. Initial rf signals of 1 Vp.p amplitude in the range 400 k H z ~ 3 MHz were obtained from an Airmec rf Oscillator Type 858. Subsequent amplification (25 Vp.p at 1 MHz) was provided by a Tektronics L Type plug-in Amplifier operated in a Tektronics Type 132 Power Supply. This power supply has identical positive and negative outputs and thus provides the desired out of phase components. The outputs from this unit are decoupled by the condensers C~, C~ and the resistor chain (R = 3.66kf~) Tektronics type 132 power supply
C~
f tt
Cs
~-it-
-T °
+ (U +Vcos wt)
terminates the amplifier and helps maintain a reasonably flat frequency response. Tapping points for the rectifying circuits are also taken from this resistor chain. Provision of the appropriate dc voltages (0-+6V), which were added to the output via the 470 kD resistors, was derived from the two diodes and associated smoothing condensers Cs, C4. These levels could be set identical to one another at the required value by the two 1 Mf~ potentiometers. As the resolution of the quadrupole is critically dependent on the rf/dc ratio it is essential that this remains constant over the operating frequency range. For this reason it is imperative that germanium diodes are used in the rectifying circuits. Measurement of the very small signal characteristics (1=10-7--+10 -5 A) have indicated that the forward voltage drop is a factor of 10 less than that for silicon diodes. A typical value for an OA5 was 0.02 V at 10-6 A. This low forward voltage drop ensures that the ratio rf/dc at the output is effectively constant for wide variations of the rf level. A change of only 0.3 per cent was calculated for a variation of the rf voltage from 36-->18 Vp.p. This circuit has been used with a quadrupole having field radius r0 = 0.41 cm and rod length 25 cm to observe ions from glow discharge source, operating in various helium admixtures. Adequate resolution in the range 4-40 amu was achieved if the rf/dc ratio was set for maximum resolution (5.96:1). In these studies the potential of point A, which is in fact the potential of the quadrupole axis, was at ground but if necessary this can be operated up to 500 V. Some difficulty was experienced for lighter masses ( < 4 ainu) due to high background currents ( ~ 10-11 A) brought about by the fall of the rf voltage at high frequencies. This fall off was due mainly to the high frequency limitation of the Type 132 Power supply and at present investigations are in progress to correct this by the inclusion of small inductors in parallel with the output to provide a wideband resonance at frequencies around 2 MHz. In spite of this limitation the existing arrangement has allowed qualitative observations to be made on hydrogen ions. D J Mellor
Figure L R~, R4 1.5 k.Q; R=, R8 330 ~; Rs, R6 470 k all 2 per cent ¼W. VR1, VR= 1 M ~ lin; (71, C~, C5, C~, 0.1 /~F 500 V, C3, C4 470 pF. D1, D~ matched pair OA5.
Department of Electronic and Electrical Engineering University of Sheffield Mappin Street Sheffield S1 3JD, England
Vacuum/volume 21/number 12. Pergamon Press LtdlPrinted in Great Britain
607