US3514594A - Mass spectrometer employing a structure similar to an electron multiplier tube to generate electron pulses indicative of mass concentration - Google Patents

Description

May 26, 1970 L. N. HEYNICK ,5

MASS SPECTROMETER EMPLOYING A STRUCTURE SIMILAR TO AN ELECTRON MULTIPLIER TUBE TO GENERATE ELECTRON PULSES INDICATIVE OF MASS CONCENTRATION Filed Feb. 13, 1968 IF j 2 Sheets-Sheet I.

' VARIABLE I l l g ggggg FIG. 1 I "LL I 17C J15 I f. 34 9 36 I DETECTOR] E G CIRCUIT 57 CONTROL I53 55 CIRCUIT I F I C} 2 III r 59 GAS 52 10 54 SOURCE 40 I L r U g M 3 Q DETECTOR M C T HQI I CIRCUIT 50\ R 38 DIVIDER w 104 f 1054 -CATE'I 11 s R III [NVIL'NTUR FIG 3 LOUIS N. HEYNICK BYEKIKZ 4 Q-Q ATTORNEYS May. 26, 1970 L. N. HEYNICK 3,514,594

MASS SPECTROMETER EMPLOYING A STRUCTURE SIMILAR TO AN ELECTRON MULTIPLIER TUBE TO GENERATE ELECTRON PULSES INDICATIVE 0F MASS CONCENTRATION Filed Feb. 13, 1968 2 Sheets-Sheet 3 1 l l 150 PERIODIC 9,154 156 158 VARYING l|i-DETECT0R VOLTAGE 162 1 SOURCE 1 i \I\ 180 1L I l 1 T 2% X X V 1 DETECTOR TO AC SOURCE FIG. 5

INVIJNTOR.

LOUIS N. HEYNICK BY iwaLAL x ATTORNEYS United States Patent O" US. Cl. 25041.9 12 Claims ABSTRACT OF THE DISCLOSURE A mass spectrometer comprising a channel apparatus with an axial electric field similar to the type of apparatus used in channel electron multipliers, but which also includes apparatus for creating a perpendicular field component which varies rapidly between zero and an appreciable value. Ions of a gas to be analyzed are created near the positive end of the channel, and they are impelled toward the negative end by the axial electric field. The perpendicular field component forces ions toward the channel walls, with an acceleration dependent upon the ion weight, and causes ions of only a particular weight to impact the channel walls at a time when the perpendicular field component is zero. Upon impact, ions of that particular weight eject electrons which are able to multiply in passing along the channel toward the positive end where they are detected. Other ions impact when the perpendicular field component is not zero and therefore the electrons they eject cannot multiply. As an additional means of discriminating, the channel length is relatively short, so ions which are heavier than those to be detected pass out of the channel without ever striking the channel walls.

BACKGROUND OF THE INVENTION This invention relates to mass spectrometers utilizing channel electron multiplier type devices.

Distributed dynode, or channel electron multipliers, are well known devices which operate by providing an electric field directed along the channel to accelerate electrons. Electrons entering the negative end are multiplied in passage through the channel and the multiplied stream of electrons is detected at the positive end. The multiplication of electrons occurs because each entering electron strikes the channel wall, causing the ejection of additional electrons. These additional electrons similarly multiply, until a large stream of electrons is received at the positive end of the channel.

Ion feedback is a phenomenon sometimes observed in electron multipliers, wherein electrons striking residual gas molecules in the channel ionize some of the molecules to create ions which travel toward the negative end of the channel. These ions may strike the channel walls near the negative end with sufiicient energy to cause the ejection of the electrons, referred to as secondary electrons. These secondary electrons move along the channel toward the positive end, and multiply in the same manner as the original electrons which entered the channel. Thus, the secondary electrons caused by ion feedback produce a large number of additional electrons at the positive end.

Where electron multiplication is intended to be accurately controlled, steps are taken to avoid additional electrons created by ion feedback. Various designs and devices are used to reduce the effect of ion feedback. These generally operate by preventing any ions from reaching the negative end where they can produce sec- 3,514,594 Patented May 26, 1970 ice ondary electron emissions. However, in accordance with the present invention, instead of avoiding the phenomenon of ion feedback, it is utilized to identify the types of residual gases present which produce the ions.

The identification of the types of gas present in a channel multiplier device can be made by measuring the time between ionization of the gas and receipt of the pulse of secondary electrons created when the ions strike a channel wall. This can be done because the ion transit time, or time between creation of the ion and the time at which it strikes a channel wall, varies directly with its mass-to-charge ratio, Q. Most ions created in such devices have lost a single electron, so they all have the same charge. Therefore Q varies in proportion to ion weight and transit time varies in accordance with ion weight. It should be noted that electrons travel so rapidly through the channel that the secondary electrons reach the positive end almost instantaneously after the ion impact which created them. The analysis of gas present in the channel could be made by noting the time of arrival of secondary electron pulses. The strength of the first pulse would indicate the quantity of the lightest gas, hydrogen, while successive pulses would indicate the quantity of. successively heavier materials.

While the separation of ions can be made solely upon the difference in times at which each secondary electron pulse in a train of pulses is received, such a method is likely to have low accuracy because of variations in the initial positions and velocities of the ions. If additional separating phenomena were used, besides the time of arrival of pulses, higher resolutions could be obtained.

OBJECTS AND SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide a simple and accurate mass spectrometer.

Another object of the invention is to provide a mass spectrometer of a channel multiplier type which provides an accurate identification of residual material present therein or of material introduced therein.

In accordance with the present invention, a channel is provided with a time varying electric field, to produce a different trajectory for each ion weight. The field is varied in a manner which allows only ions of a particular Q value to strike a channel wall at an area or at a time wherein the ions can produce appreciable secondary electron pulses. Those ions with a different Q do not produce appreciable secondary electron pulses.

The varying electric field enables the separation of ions by reason of two phenomena: the difference in the positions at which ions of different Q value may strike the channel wall, and the difference in the times at which they strike. Differentiation by the position of which an ion strikes is based upon the fact that only ions of a particular Q value strike a channel will near the negative end of the channel, where their impact creates many secondary electrons. Ions of a higher Q pass beyond the channel, while ions of a lower -Q pass only a short distance down the channel before impact, and their impact with the channel wall gives rise to few secondary electrons.

Differentiation based upon the dilference in the time at which an ion strikes is based upon the fact that only those ions of a particular Q value strike the channel wall at a time when the field is directed straight along the channel. Only if an ion strikes a channel wall when the electric field is substantially straight along the channel will the secondary electrons it produces multiply. Those ions with a different Q have a transit time which causes any impact to occur when the electric field is not straight along the channel. The electrons produced by their impact do not multiply appreciably in passing toward the positive end of the channel. Therefore, such ions impacts do not give rise to large numbers of secondary electrons. Thus,

the varying electric fields, by allowing only certain ions to impact near the negative end of the channel, and only at a time when the electric field is substantially straight along the channel, allow a high discrimination between difierent ions.

The present invention provides an apparatus for use as a mass spectrometer comprising a channel and voltage supplies for producing a varying electric field along the channel. The spectrometer utilizes means which create ions at the positive end of the channel, and the electric field moves the ions toward the negative end. When the ions strike a channel wall near the negative end, they create electrons. The electric field moves the electrons from the negative end of the channel toward the positive end. The electrons are amplified in movement toward the positive end by successive collisions with the channel walls. The amplified electron pulse is detected at the positive end.

For an ordinary multiplier with a constant electric field, all ions have the same trajectory, although lighter ions (smaller Q) move faster and therefore have a smaller transit time between creation and impact. Generally, there is a certain critical ratio of channel length to channel width, which is approximately 83, which causes most ions to strike the channel walls near the negative end. All of the ions thereby produce secondary electrons that undergo maximum amplification by multiplication as they move along the channel to the positive end. The ions could be differentiated by time of arrival of the secondary electron pulses which they give rise to. Pulses due to lighter ions arrive first, followed by pulses due to the presence of successively heavier ions.

In this invention, higher resolution, or ion species separation, is achieved by utilizing a channel having a lengthto-width ratio less than the critical value. This channel would eliminate ion feedback if operated in an ordinary fashion, with the field constantly directed along the channel. However, it is operated with a varying perpendicular electric field component which is directed toward the channel walls. The perpendicular component causes ion impact with the walls at the negative end, so impact can occur before an ion passes out of the channel.

In one embodiment of this invention, the channel is formed by two electrically separated half-channel portions and each portion is connected to a different voltage source. The voltages vary with time to produce a curved field whose curvature varies with time, or to produce a straight electric field whose angle of tilt varies with time. Both modes of operation produce a field component directed toward the channel walls, which causes the trajectories of ions to vary in accordance with their Q values.

In another embodiment of this invention, an unsplit channel having a length-to-width ratio less than the critical value is used, and a force component toward the channel walls is introduced by a time-varying magnetic field.

The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified perspective view of a mass spectrometer constructed in accordance with the invention;

FIG. 2 is a simplified side elevation view of the spectrometer of FIG. 1;

FIG. 3 is a simplified side-elevation view of a spectrometer constructed in accordance with another embodiment of the invention;

FIG. 4 is a simplified side elevation view of a spectrometer constructed in accordance with still another embodiment of the invention; and

FIG. 5 is a simplified perspective view of a spectrometer constructed in accordance with yet another embodiment of the invention.

4 DESCRIPTION OF THE PREFERRED EMBODIMENTS The mass spectrometer of FIG. 1 comprises a first plate 10 and second plate 12, each constructed of electrically resistive material. The plates have a length L and are separated by a distance W, representing the width of the channel. A first voltage source 14 is connected across the ends 16 and 18 of the first plate, while a second voltage source 20 is connected across the ends 22 and 24 of the second plate. The two plates 10 and 12 serve as wall segments of a channel within which electrons and ions move, impelled by the voltage gradient, or electric field therealong.

The voltage sources 14 and 20 provide a voltage gradient which is directed to a large extent along the length of the channel. This causes the apparatus to function generally like an electron multiplier, and such operation will first be described in detail. If an electron, shown by the negatively marked particle at 26, is impelled through the inner end 28 of the channel, it accelerates along the length of the channel under the force of the electric field. The field is negative at the inner end 28 and positive at the outer end 30, so the electron moves toward the outer end 30. The electron 26 generally will also have a component of motion perpendicular to the length of the channel, so that it strikes one of the plates, such as at the point 32. When an electron of sufficient energy strikes a plate, it causes the emission of several electrons, such as the two electrons shown at 34 and 36. These electrons also pass along the length of the channel toward the outer end 30, and wherever they strike one of the plates, additional secondary electrons are emitted. At the outer end 30, the large stream of electrons is captured by an anode 38 which is positively charged. The current passing through the anode 38 is detected by a detector circuit 40 connected through a battery 41 to the channel. Thus, the channel multiplies electrons which begin moving near the inner end 28.

The area within the channel may contain a low pressure gas. When an electron strikes a gas molecule or atom, it may ionize it and create a positive ion. The positive ion, such as the one indicated at 42 is ac celerated by the electric field toward the inner end 28. The ion trajectory is curved, and the ion may strike a wall of the channel, such as the plate 12, where it may be neutralized. However, when it strikes the wall, the ion may cause the emission of secondary electrons such as those indicated at 44 and 46, if the ion has received sufficient energy through acceleration by the electric field. If the secondary electrons 44 and 46 are created near the inner end 28 of the channel, they will undergo many multiplications, on the order of twenty, before reaching the outer end 30 where they are captured. In passing along the channel, the secondary electrons multiply in the same way as an electron 26 initially impelled into the channel, and they create a large current at the anode 38.

A straight electric field constantly directed along the length of the channel would exist if the potentials at points 16 and 22 were equal and the potentials at points 18 and 24 were equal. Then all of the ions which were created near the outer end 30 would have a common impact location, that location being an area a predetermined distance from the outer end 30. If the ratio of the length L of the channel to the spacing W between the plates slightly exceed a certain critical value, which is approximately 83, the common impact location would lie within the length of the channel, and all of the ions would strike the channel wall and create secondary emissions.

For a constant electric field, all ions would impact at a common location (which is at the inner end 28 for a ratio L/W of approximately 83). The time required for ions to move from the outer end 30 to such a common impact location varies according to the ratio Q between the mass and charge of the ion. Most of the ions have a charge equal to that of a single electron, so their Q varies according to the atomic weight of the gas. As compared to the transit or flight time of the ions, the flight time of the electrons (which have a Q which is of the lightest ion, hydrogen) created in the secondary emission is very short, so a secondary emission pulse occurs at the anode 38 almost immediately after an ion strikes a channel wall. Ions created at the outer end 30 could be separated by noting the time at which secondary emission pulses were detected at the anode 38. Light ions such as those of hydrogen would move very rapidly from the outer end 30 to the impact point, and pulses due to their creation would be received before the pulses resulting from ions of any other type.

Accordingly, a gas sample with many constituent materials, which was injected into the channel at the outer end 30 would produce a train of pulses, in the order of increasing Q of constituent material (and therefore of increasing weight). The time of arrival of each pulse in the train would indicate the type of ion represented by that pulse, and the strength of that pulse would indicate the proportion of the gas sample containing that type of material.

While a mass spectrometer could be constructed which depended solely upon the difference in flight times of ions as described above, it would not provide a highly sensitive discrimination between different types of material. The difierence in flight times of different ions may be small, and pulses due to ions of one material present in small amounts may be masked by large pulses of ions of a material with almost the same Q. Discrimination between the different pulses detected by the detector circuit 40 is very difficult when such pulses are of very different magnitudes and occur at almost the same time. Furthermore, the flight times between the points of creation of the ions and their points of impact may vary due to creation at slightly different times. Therefore, sharp distinctions could not be made between different ion types if such distinctions were based solely upon time of arrival in a constant electric field.

In accordance with one embodiment of the present invention, a channel with a ratio between its length L and width W less than the critical value is used. In addition, the electric field is varied between a straight field directed along the length of the channel, and a field with a component directed along the width of the channel. This variation is achieved in the apparatus of FIG. 1 by including a variable voltage source 15 in series with a direct current source 17. The voltage produced by direct current sources '17 and 20 are equal and are typically several thousand volts. The varying voltage produced by source 15 may have a maximum amplitude of only several volts, but it is sufficient to produce an appreciably curved field at its maximum amplitude.

In the operation of the circuit of FIG. 1, ions of a material to be analyzed are created or injected at the outer end 30 of the channel at a predetermined instant of time. Various means for establishing the ions at an instant, such as a thermionic cathode, which is turned on at an instant, can be used. The time at which ions begin to travel may be the instant of maximum curvature of the field, which occurs when the source 15 has a maximum voltage and begins to decrease toward zero. For a predetermined choice of amplitude, frequency, and phase of the time varying field, ions of one Q value, such as helium, which start moving at the outer end 30 will impact the channel walls just inside the inner end 28, at a time when the electric field is straight along the channel length. This is when the source 15 becomes zero. The electrons which the helium ion creates upon impact pass down the channel toward the outer end 30 where they strike anode 38 and are detected by a detector 40.

Continuing with the above example, heavier ions, such as those of oxygen, pass through the inner end 28 without striking a channel wall, and therefore do not give rise to a pulse at detector 40. Lighter ions, such as those of hydrogen, strike the channel Walls near the outer end 30 and strike at an earlier time 'while the field is still curved and therefore has a perpendicular field component. As a result of the perpendicular field component, the electrons created upon channel impact of the lighter ions do not travel far before they impact the channel. Therefore, the electrons do not attain suflicient velocity to produce a large secondary electron yield. The net result is that a decrease rather than a multiplication in such electrons occurs, and the number of electrons reaching the anode 38 is almost zero. Thus, only ions of a particular Q value, such as that of helium, cause appreciable pulses at the detector 40. The rate of change and amplitude of the source 15 can be varied to cause ions of any particular Q value to be detected. Also, the time at which ions are established at the outer end 30 can be varied in relation to the variation of voltage at 15 to select which particular ion weight will be detected.

FIG. 2. shows the mass spectrometer of FIG. 1 with means for creating ions at the outer end 30. The device includes a gas source 52 'with an inlet to admit the gas to be analyzed. A cathode structure 50 provides pulses of electrons which enter the inner end 28 of the channel. If the outer ends of the two plates 10 and 12 are connected together, they produce a pattern of equipotential lines of a form shown at 54 when the electric field is curved.

The gas is analyzed by admitting some of it through the inlet of gas source 52 into the outer end of the channel. Electrons from the ionizing source 50 are injected in pulses synchronized with the instants when the field curvature is zero (if they are injected at other times, they do not multiply). The electrons pass down the channel to multiply themselves by secondary emissions. When an electron strikes a gas molecule at the outer end 30 of the channel, the gas molecule may become ionized and begin traveling toward the inner end 28 under the influence of the electric field. The field is then curved in order to make different ions follow different paths. An ion does not travel in a straight line but instead follows a curved or sinuous path down the channel toward the inner end 28. The amount of path curvature is in inverse relationship with the Q value of the ion, with the ion of highest Q having the least curved path.

As described above in connection with FIG. 1, the field curvature is established so that ions which are heavier than those to be detected will pass out of the inner end 28 of the channel before striking either plate 10 or '12 while ions lighter than those to be detected strike a plate near the outer end 30. The apparatus of FIG. 2 is able to distinguish between ions of different Q based upon the fact that ions of a Q larger than a predetermined amount do not generate any current pulse, while ions of a Q less than a predetermined amount generate very weak pulses, if any. For a given apparatus, appreciable pulses for each value of Q can be obtained for known field varia tions. The detection of such pulses and their strength indicates the presence and density of gases which readily produce ions of each Q.

The device of FIGS. 1 and 2 can eliminate pulses of lighter ions than those to be detected by two phenomena. First, such ions impact close to the outer end so their secondary electrons undergo fewer multiplication steps. Second, the electric field is not straight when the lighter ions impact, so the electrons are additionally prevented from multiplying because they impact at insufficient energy to produce an adequate secondary yield. In some modes of operation, only the first means of separation may be used. The effect of pulses due to lighter ions also can be eliminated by turning off the detector 40 until the time of arrival of pulses from those heavier ions to be detected.

FIG. 3 is an illustration of another embodiment of the invention, in which the field is continuously varied. A

constant voltage is applied between the ends of the Plates 104 and 108 by a single source 112. The output of a sinusoidal or other periodically varying voltage source 110 is added to the voltage at the inner end 105 of the plate 104 to provide a rapidly varying electric field. The varying source provides a curved field that is strong enough that it prevents electron multiplication except when the sinusoidal voltage is passing through zero and therefore the field is straight.

In the apparatus of FIG. 3, ions created at the outer end 116 of the channel travel toward the inner end 118 and strike one of the channel Walls 104 or 108 a predetermined time after their creation. The time of impact is not appreciably affected by variations in the field because curvature of the field aifects the ion trajectory but not its transit time. For a proper channel size, ion will have attained considerable energy before impact with a channel wall. Only if an ion strikes a channel wall while the voltage source 110 is near zero does a large multiplication of the emitted secondary electrons occur. Such multiplication causes a large current pulse to strike the anode 114 and be registred by the detector 120, which is connected to the anode through a voltage source 121. In order to detect the presence of a particular constituent in a gas injected at the outer end 116, the amplitude, frequency, and phase of the sinusoidal voltage source 110 must be at particular values. These values must be such that only ions of that molecule to be detected will strike a channel wall at the inner end at the time that the source 110 is passing through a node. It may be noted that the electron multiplication process occurs very rapidly as compared to the transit time of an ion from the outer end 116 to a channel wall, so that once an ion strikes a wall the electron multiplication process occurs before the voltage 110 can change appreciably.

The electrons which ionize gas at the outer end 116 are created by a source of electrons 122 at the inner end of the channel. The ionizing electrons arrive during a node of the voltage source 110. During any other time, when the field is curved, electrons emitted at 122 will not generate a large number of secondary electrons at the inner end 116 to ionize the gas introduced there. If the Q of an ion created at a node of the source 110 is such that it strikes the channel wall at the same time as the source 110 achieves another node, then that ion creates a detectable pulse. A detectable pulse occurs because the secondary electrons resulting from impact of the ion with the wall are highly multiplied and are detected by the detector 120.

During the time of detection by detector 120, the electron source 122 must be turned off. If a sinusoidal source is used for the source 110, it is turned off by tying the source 110 to a gate 124 through a divider circuit 126. At every other node of the sinusoidal source 110, the divider 126 delivers a pulse to gate 124 to turn it off and prevent the emission of electrons by source 122. Various types of ions can be detected by changing the frequency and amplitude of the sinusoidal source 110. At any given frequency, the only ion type which gives rise to a pulse at detector is an ion whose of flight from the outer end 116 to a channel wall 104 or 108 is equal to the time of a half cycle (or an odd number of half cycles) of the sinusoidal wave at the source 110. The flight time of ions is on the order of microseconds for the usual channel electron multiplier constructions, and a radio frequency voltage is therefore generally required.

FIG. 4 illustrates an embodiment of the invention in which the electric field in maintained straight, but is varied by tilting it instead of introducing curvature. A voltage source 164 is applied across the channel plate and a separate source of equal magnitude 166 is applied across channel plate 152. In addition, a periodically varying voltage source 168 is applied between the inner ends 160, 162 of the plates. At instants when the periodic voltage source 168 is at a node, the inner ends 160, 162 are at the same potential and the outer ends 172, 174 are at the same potential, so the electric field is parallel to the channel. At other instants, one inner end, such as end 160, is made more positive in potential than the other inner end 162, and the equipotential lines are inclined or tilted. Consequently, the e ectric field is inclined to the direction of the channel and thus contains a component directed toward the channel walls. When the electric field inclination is varied over suitable magnitudes and at appropriate rates, this embodiment of FIG. 4 operates in the manner described above for the other embodiments.

FIG. 5 illustrates an embodiment of the invention wherein a component of force (which acts on charged particles moving along the channel) which is directed toward the channel walls is provided by a periodically varying magnetic field. In this embodiment, the same voltage source is applied to both channel plates 182 and 184, and a periodically varying magnetic field 186 is applied in a direction generally parallel to the plates/The electromagnet is formed by a pair of coils 190 and 192 which are supplied with alternating currents. A charged particle (ion or electron) experiences a component of force directed toward the channel walls. This component of force is proportional to the velocity of the particle in a direction perpendicular to the magnetic field and also is proportional to the strength of the magnetic field.

Similar magnetic operation can be obtained from an unsplit channel, such as a tubular channel, with the magnetic field applied in a direction generally perpendicular to the central axis of the channel.

While channel multiplier mass spectrometers have been illustrated having walls formed by two separate plates, it should be understood that many. different shapes may be employed, with many different manners of voltage application. For example, a tube with round cross-section, constructed of two or more parts may be used. As another example, voltages along the length of the channel may be produced by providing many defined voltage sources connected to points along the length of the channel.

While a particular embodiment of the invention has been illustrated and described, it should be understood that many modifications and variations may be resorted to by those skilled in the art, and the scope of the invention is limited only by a just interpretation of the following claims.

What is claimed is:

1. A mass spectrometer comprising;

elongated walls defining a channel therebetween having inner and outer ends; electron detection means disposed at said outer end of said channel;

field means for establishing an electric field component within said channel directed along its length and for establishing a time-varying field component within said channel which accelerates charged particles with a component of acceleration perpendicular to the length of said channel; and

means for establishing ions of a gas to be analyzed in said channel;

said time-varying field component varying at a rate which is slow enough to produce a substantially constant field during the flight time of an electron along the entire length of said channel, and which is fast enough to change appreciably during a period equal to thedifference in flight times of helium and hydrogen ions along said channel, so that an appreciably different level of electron multiplication occurs at times separated by said period.

2. A mass spectrometer as defined in claim 1 wherein;

said field means comprises means for establishing a time-varying magnetic field component directed perpendicular to the length of said channel.

3. A mass spectrometer as defined in claim 1 wherein:

said means for establishing ions comprises gas source means for injecting a gas into the area within said channel adjacent to said outer end and electron source means for impelling electrons into said inner end of said channel, whereby to provide a multiplied stream of electrons at said outer end to ionize said gas. I

4. A mass spectrometer as defined in claim 1 wherein:

said field includes means for periodically varying said time-varying field; and

said means for establishing ions comprises means for creating ions of said gas at a predetermined phase of said time-varying field.

5. A mass spectrometer comprising:

means defining a substantially straight channel with inner and outer ends, said channel having a predetermined width and having a length less than 83 times as great as said width;

electron detection means disposed at said outer end;

means for establishing an electric field directed along the length of said channel;

time-varying means for establishing a field which accelerates charged particles perpendicular to the length of said channel, said field which accelerates particles perpendicular to the length of said channel rapidly varying between zero and a plurality of other values along an appreciable length of said channel; and

means for establishing ions of a material to be analyzed substantially at said outer end of said channel.

6. A mass spectrometer as defined in claim 6 wherein:

said time-varying means comprises means for establishing a time-varying magnetic field directed substantially perpendicular to the length of said channel.

7. A mass spectrometer as defined in claim wherein:

said means defining a channel comprises a plurality of elongated wall segments substantially electrically insulated from each other and extending along the length of the channel;

said means for establishing an electric field comprises first means for establishing a predetermined potential between the opposite ends of a first of said channel segments and means for establishing the same potential difference between the inner and outer ends of a second of said channel segments; and

said time-varying means comprises means coupling adjacent ends of said channel segments for applying a time-varying voltage between them.

8. A mass spectrometer comprising:

at least two electrically separated channel means forming a channel with inner and outer ends between them;

means for introducing a gas to be analyzed to said channel;

means for ionizing said gas;

electron detector means disposed at said outer end of said channel for detecting electrons; and

means for establishing voltage gradients along first and second of said channel means, including means coupled to points on said first and second channel means which are approximately the same distance from said outer end for varying the voltage between said points between a zero value and an appreciable value.

9. A mass spectrometer as defined in claim 8 wherein:

said means for establishing voltage gradients comprises means for establishing the same voltage gradient along the length of said first and second channel means, whereby to provide a tilting electric field.

10. A mass spectrometer as defined in claim 8 wherein:

said means for establishing voltage gradients comprises means for establishing the same voltage at one end of said channel on substantially all of said channel means and for establishing voltages at the opposite end sof said channel means which are time-varying with respect to each other between a zero difference and an appreciable difference.

11. A mass spectrometer comprising:

elongated walls defining a channel therebetween having inner and outer ends;

electron detection means disposed at said outer end of said channel;

field means for establishing an electric field which varies between an orientation directed along said channel along a majority of the length of said channel, and an orientation having an appreciable component perpendicular to the length of said channel; and

means for establishing ions of a gas to be analyzed in said channel.

12. The mass spectrometer defined in claim 11 wherein:

said field means varies at a rate which is slow enough to produce a substantially constant field during the flight time of an electron along the entire length of said channel, and which is fast enough to change appreciably during a period equal to the dilference in flight times of helium and hydrogen ions along said channel, so that if two ions which dilfer in weight by the dilference between helium and hydrogen ions start accelerating through said channel at the same time, only one of them can strike a channel wall at a time when the electric field is directed along said channel, whereby only one of said ions gives rise to an appreciable electron flow.

References Cited UNITED STATES PATENTS 2,932,768 4/1960 Wiley 25041.93

RALPH G. NILSON, Primary Examiner C. E. CHURCH, Assistant Examiner

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