Ion Beam Intensity Control With Pulsed Beam Deflection And Synchronized Ion Source Blanking

Abstract

A method and apparatus for selectively introducing an intermittent ion beam to an analyzer by passing the intermittent ion beam through deflection electrodes to which generated pulses are applied to direct the ion beams in a manner that the intermittent ion beam does not pass the electrodes at a rising and falling time of the pulses. The pulses selectively project into the analyzer the intermittent ion beam in accordance with the amount of exposure.

Description

This invention relates to mass spectrometers and, more particularly, to an apparatus for controlling the amount of exposure of an ion beam at a detector.

In the general analysis of elementary composition by a mass spectrometer, the mass spectra of the composition represented by varying amounts of exposure is directed onto a sensitive layer. Analysis of the elementary composition is made by measuring the total amount of ions in one spectrum and the blackness degree of each spectrum line in the respective spectrum. The ratio of blackness degree between the lightest and darkest spectrum line on the sensitive layer can vary to a maximum of about 100 times. Therefore, in order to compare different spectrum lines of the same blackness degree, the mass spectrum must be detected by changing the entire exposure of the ion beam over a range of 10.sup..sup.-13 Q to 10.sup..sup.-6 Q. To achieve this, the exposure time must be varied in order to obtain such a wide exposure ratio.

However, the above method of controlling the exposure is impractical sine the exposure time must be controlled over a wide range such as 1.8.times.10.sup..sup.+4 sec to 30 min. In view of this, in the conventional method, the amounts of ions generated in the ion source are controlled. For instance, in a spark type ion source, the amounts of ions generated per unit time are controlled by changing the spark voltage or spark pulse width or repetition frequency to be applied to the spark electrodes. However, the condition of ionization changes in accordance with the change of either the spark voltage, spark pulse width or repetition frequency, resulting in a variation in the efficiency of ionization between the various elements constituting the sample. Therefore, even though spectra with the same ion exposure are obtained from the same sample, the ratio of the blackness degree between each spectrum line is different due to the variation of the ion source condition. Hence, the measurement is reduced 15 percent to 30 percent and further, it is not uniform.

In order to overcome this defect various methods have been used, the most typical being to control the exposure of the ions by chopping the ion beam. In this method, the chopper electrodes are arranged in front of the electrostatic field to which the chopper voltage is applied. The ion beam which flows from the ion source to the analyzer is chopped by the chopper voltage so as to control the amounts of ions arriving at the detector. In other words, the ion beam is deflected from its path when the chopper voltage is not applied, but continues in its path when the chopper voltage is applied so by reaching the sensitive layer.

However, since the chopper voltage has finite rising and falling times, the ion beam passing between the chopper electrodes during these times is focused on the shifted position of the sensitive layer and, as a result, the spectrum lines become dim and the resolution is consequently decreased. Moreover, even though the ion beam is chopped in the lengthwise direction of each spectrum line, the spectrum becomes dim due to second order aberration when a spherical electric field is used.

My invention permits an ion beam to be passed between the deflection electrodes without chopping, yet a clear image can be focused on the sensitive layer. Furthermore, the intensity of the ion beam which reaches the magnetic field is controlled without affecting the focusing of the ion beam.

My invention provides a method and apparatus for selectively introducing the ion beam to the analyzer so as to obtain the desired amount of exposure of the intermittent ion bear emitted from the ion source on the sensitive layer by arranging the deflection electrodes at a selected point along the ion path. The pulses are applied to the deflection electrodes so that the rising time and falling time of the pulses coincide with the period when the ion beam does not pass through the electrodes.

These and other features and advantages of the present invention will become apparent after perusing the following specification in conjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of a typical double-focusing mass spectrometer showing one embodiment of the present invention which is useful for controlling the amount of exposure of an ion beam; and

FIG. 2 is a waveform diagram showing the wave forms necessary for explaining the present invention.

Referring now to FIGS. 1 and 2: the specimen to be analyzed is arranged, on the spark electrodes 1 and is ionized by a spark generated between said electrodes. The ions thus formed are accelerated between an accelerating slit 2 and are propelled into the analyzer tube through an earth slit 3 and a main slit 4.

The spark electrodes 1 are connected to the output terminals of a high frequency generator 10 through a transformer 9. A clock pulse generator 11 generates periodic pulses with constant intervals as shown in FIG. 2(a) which are fed into a delay circuit 12 (such as a delay cable) and a gate circuit 14. The clock pulse with constant intervals fed into the delay circuit 12 are delayed by a fixed time .DELTA.t.sub.1 (for example 5.mu.sec) and are then fed into a first pulse generator 13 which forms spark pulses with a pulse width .DELTA.t.sub.1 (for example 20.mu.sec) as shown in FIG. 2 (b). The spark pulses are then applied to the high frequency generator 10 which generates an intermittent high frequency and, after being boosted by transformer 9, they are applied to the spark electrodes 1 which generate a spark resulting in the specimen fixed to the spark electrodes being ionized.

The periodic pulses fed from the clock pulse generator 11 into the gate circuit 14 are in turn fed into a frequency divider 15 which is arranged so as to generate one pulse for N number of clock pulses. N can be widely varied to maximum of 10.sup.4. The pulses generated by the frequency divider 15 are transferred to a second pulse generator 16 which generates pulses as shown in FIG. 2(c). These pulses have a pulse width .DELTA.t.sub.2 (for example 50.mu.sec), that is to say, wider than the spark pulse width .DELTA.t.sub.1 . These pulses are applied to deflection electrodes 5 so that the intermittent ion beam ionized between spark electrodes 1 passes between the deflection electrodes 5 only when the said pulses are applied to the deflection electrodes 5. As a result, the intermittent ion beam is introduced into the electrostatic field 6. Each ion reaches the deflection field formed by the deflection electrodes 5 with a time difference due to the mass difference of each ion. The time difference persists through the deflection field also. However, since pulse width .DELTA.t.sub.2 of the pulses from pulse generator 16 is much larger than pulse width .DELTA.t.sub.1 of the pulses from pulse generator 13 and is advanced by .DELTA.t from the spark pulse passed through the delay circuit, ions to be analyzed pass between the deflection electrodes without any deflection. As a consequence the focusing of the ion beam is not affected. The ion beam of course is then passed through a standard electrostatic field 6 and magnetic field 7 and onto the sensitive layer photographic plate 8.

I have explained my embodiment in the case where one spark pulse is included in one pulse width .DELTA.t.sub.2 of the pulses to be applied to the deflection electrodes, but my invention is not so limited. It is possible to apply some of the spark pulses so that they are included in one pulse width .DELTA.t.sub.2 of the pulses to be applied to the deflection electrodes. Furthermore, this invention can be also applied to the single focusing mass spectrometer.

As mentioned above, my invention provides a method and apparatus for controlling the exposure amounts of the ion beam in such a manner that spark pulses with a fixed delay time delayed from the clock pulses generated by the clock pulse generator are applied to the spark electrodes, and the pulses with a width which is much greater than that of the spark pulses are synchronized with the clock pulses and applied for the pulse generator 16 to the deflection electrodes. By doing this, a desired ion beam can be passed between the deflection electrodes without chopping the ion beam and a clear image can be focused on a reasonable position of the sensitive layer. Moreover, by controlling the ratio between the input pulses to the frequency divider and the output pulses from the frequency divider, the intensity of the ion beam which reaches the magnetic field is controlled without adversely affecting focusing of the ion beam.

Claims

I claim:

1. A mass spectrometer comprising an ion beam source, a beam deflecting device and an analyzer for spatially separating said beam according to the mass to charge ratio of the ions comprising the beam and a control circuit, said ion source having means for releasing and directing intermittent ion beam pulses through said deflecting device to said analyzer, said deflecting device comprising spaced electrodes through which said intermittent ion beam passes, said control circuit providing first set of electrical pulses to direct the ion source to release the ion beam pulses of substantially uniform pulse width and frequency, said control circuit providing a second set of electrical pulses to the electrodes of said deflecting device such that the ion beam pulses may be selectively directed to said analyzer, the duration of the pulses in said first and second sets an the synchronization of said sets such that the ion pulses do not pass the electrodes at the rising and falling of said second set of pulses.

2. A mass spectrometer according to claim 1 wherein said control circuit may be adjusted to control the ratio of the frequency of the first set of pulses to the second set of pulses thereby controlling the intensity of the beam which is directed to the analyzer.

3. A mass spectrometer comprising an ion beam source, a deflection device having spaced electrodes, an analyzer for spatially separating said beam according to the mass to charge ratio of ions comprising the beam and a control circuit, said control circuit comprising a clock pulse generator for generating clock pulse signals, a first generator for generating a first set of intermittent signals with a fixed delay time from said clock pulse signals, said signals controlling the ion source to intermittently produce ion pulses and for intermittently emitting said pulses, a second generator for generating a second set of signals having a pulse width greater than the width of the signals generated by the first generator and selectively synchronized with said clock pulse signals, said second set of signals directed to the deflection electrodes to selectively direct the ion beam pulses to said analyzer.

4. A mass spectrometer comprising an ion beam source, a deflection device having spaced electrodes and an analyzer for spatially separating said ion beam according to the mass to charge ratio and a control circuit, said control circuit comprising a clock pulse generator for generating clock pulse signals, a first generator for generating intermittent high voltage signals with a fixed delay time from said clock pulse signals, said signals being directed to the ion source having two spark electrodes, a second generator generating a second set of signals having a pulse width greater than that of the signals generated by the first generator and selectively synchronized with said clock pulse signals, said second set of signals directed to the deflection electrodes to selectively direct the ion beam pulses to the analyzer.