Radiant energy – Ionic separation or analysis – Cyclically varying ion selecting field means
Patent
1993-06-02
1994-10-11
Anderson, Bruce C.
Radiant energy
Ionic separation or analysis
Cyclically varying ion selecting field means
250282, B01D 5944, H01J 4900
Patent
active
053549880
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a power supply for providing stable RF and DC supplies to the electrodes of a multipolar (eg a quadrupolar) mass filter, and to a method of stabilizing those supplies.
Quadrupole mass filters comprise four elongated electrodes to which time-varying potentials are applied to create a time-varying electrical field of particular characteristics. Ions entering the filter along its central axis undergo motion in the field in such a way that only ions of certain selected mass-to-charge ratios will have stable trajectories and will emerge from the filter to reach an ion detector.
Such a filter requires a radio-frequency supply of controlled amplitude and frequency and positive and negative stabilized DC supplies whose outputs are related to the amplitude and frequency of the RF supply. In simple terms, the center of the band of mass-to-charge ratios that are transmitted is determined by the amplitude and the frequency of the RF supply and the width of the band transmitted (ie, the mass resolution) is determined by the ratio of the DC to the RF amplitudes. Most, but not all, quadrupole mass filters operate at constant radio frequency (typically about 2 MHz), and variable amplitude (to select the mass transmitted).
There are many published designs for stabilized RF/DC generators suitable for quadrupole mass filters. The main problems to be solved are the stabilization of the RF amplitude over a range from almost zero to typically 2000 volts (the range necessary to scan a complete mass spectrum) and the control of the DC potentials in some defined relation to the RF amplitude. One advantage of operation at constant frequency is that the RF generator can be coupled to the filter electrodes by means of a resonant high Q transformer which steps up the RF amplitude to the necessary high value, obviating the need for a solid-state generator capable of generating a waveform of several thousand volts peak-to-peak amplitude.
There are two approaches to the design of suitable RF generators. The most common, especially suitable for high-performance filters, is to generate the RF at low amplitude, typically using a crystal-controlled oscillator operating at the desired frequency, and to use its output signal to drive a tuned power amplifier which is coupled to the filter electrodes through a transformer as described. Supplies of this type are disclosed, for example, by Pacak (Slabop. Obzor, 1983 vol 44 (10) pp 475-483), Toderean and Ristoiu (St. Cerc. Fiz. 1976 vol 28 (7) pp 665-672), V'yukhin and Kovalev (Instrum. & Exp. Tech. 1980 vol 23 (5) pt. 1 pp 1184-6), Tamura and Kitajima in U.S. Pat. No. 4,703,190, and by Bryndza in U.S. Pat. No. 3,621,464. All these publications further describe means by which the RF amplitude is stabilized and set accurately according to a control signal by means of which the mass transmitted by the filter is selected. Typically the generator is operated in a control loop with negative feedback derived from a detector circuit which generates a DC signal indicative of the actual RF amplitude. This is compared with the control signal and the gain of the RF amplifier set accordingly. The DC potentials are either derived by rectification of the RF applied to the electrodes or from separate stabilized DC power supplies controlled by the control signal.
More up-to-date methods of control of the RF and DC amplitudes using digital electronics are exemplified by Slomp, Chiasera, et.al. (Rev. Sci. Instrum. 1986 vol 57 (11) pp 2786-2790), but the principle of operation is the same as the older analogue methods. This, or similar techniques are used in most of the commercial quadrupole spectrometers presently available.
A fundamental difficulty with fixed-frequency RF generators is the need to maintain the resonant frequency of the coupling transformer at exactly the frequency of the oscillator over a long period of time. This is particularly difficult because the capacitance of the quadrupole filter itself forms part of the tuned circuit, and varies with time, temperature and contamin
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Anderson Bruce C.
Fisons plc
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