US3226543A

US3226543A - Pulsed time of flight mass spectrometers

Info

Publication number: US3226543A
Application number: US259890A
Authority: US
Inventors: Melzner Friedhelm
Original assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Current assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Priority date: 1962-02-22
Filing date: 1963-02-20
Publication date: 1965-12-28
Anticipated expiration: 1982-12-28
Also published as: GB1005053A; DE1498870A1

Description

Dec. 28, 1965 F. MELZNER 3,226,543

PULSED TIME OF FLIGHT MASS SPECTROMETER Filed Feb. 20, 1963 2 Sheets-Sheet l Fig. 1

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PULSED TIME OF FLIGHT MASS SPECTROMETER Filed Feb. 20, 1965 2 Sheets-Sheet 2 Fig. 3

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WieQeLm MeLzner Fittorness United States Patent 3,226,543 PULSED Till IE 0F FLIGHT MASS SPECTROMETERS Friedhelm Melzner, Munich, Germany, assignor to Max- Planclr-Gesellschaft zur Foerderung der Wissenschaften e.V., Goettingen, Germany Filed Feb. 20, 1963, Ser. No. 259,890 Claims priority, application Germany, Feb. 22, 1962, M 51,908 9 Claims. (Cl. 250-413) The present invention relates to dynamic mass spectrometers employing electrical fields in which the ions oscillate in a potential trough and ions of different ratios of charge e to mass m are separated by the diiference between their times of flight or their oscillation frequency.

It is an object of the invention to provide a mass spectrometer which is structurally simple, and which in its preferred embodiment requires only very simple supplementary electronic devices for its operation.

A mass spectrometer in accordance with the invention employs only electrical fields; ions of a particular kind (i.e. of a given ratio of charge e to mass m) are arranged to oscillate axially in an evacuated vessel whereas ions of other kinds are removed from the oscillation path of the first mentioned ions, so that disturbing space charges cannot build up in the region of ion oscillation and impair the sensitivity and power of resolution of the instrument.

The sensitivity and the power of resolution of mass spectrometers according to the invention are relatively high, event at low partial pressures of the vapour or gas under test, despite their simplicity of construction and compactness. A preferred though not an exclusive application of the invention is that of leakage detection.

An object of the invention is to provide a mass spectrometer including a longitudinally extending vacuum tube, means for producing ions in said tube, means for producing differences of potential along a portion of the length of said tube such that there is a relatively positive potential at one end of said portion and a relatively negative potential at an intermediate point between the ends of said portion, and means for periodically producing a relatively positive potential at the other end of said portion.

Another object of the invention is to provide a mass spectrometer including a longitudinally extending vacuum tube, means for producing ions in said tube, and means for producing differences of potential along a portion of the length of said tube such that each end of said portion is periodically raised to a potential positive with respect to a point intermediate between said ends.

A further object of the invention is to provide a mass spectrometer comprising a vacuum-tight envelope containing an ion source and a set of apertured coaxial relatively spaced electrodes, means for biasing at least one electrode intermediate between the ends of the electrode set, means for biasing an electrode at one end of the set at least during certain times by the application of a potential which is more positive than the potential of the intermediate electrode, and a source of pulses for applying to an electrode at the other end of the set a positive pulse train which keeps this electrode at a first potential during a first part of each pulse cycle and at a second potential more positive than said first potential during a second part of each pulse cycle.

The vacuum-tight envelope of a mass spectrometer according to the invention is equipped with means for introducing into it a gas or a vapour which it is desired to analyse. Inside the evacuated envelope which may preferably be in the form of a tube, there are provided several relatively spaced electrodes with coaxial aper- 3,226,543 Patented Dec. 28, 1965 tures. The electrodes are connected to lead-in wires which enter the evacuated envelope from the outside through vacuum-tight seals. In association with a suit able source of potential the electrode arrangement is suit table for the generation of an axial electric potential distribution which is, at least at certain times, more positive at the ends of the electrode system than in its centre. An ion source is located away from the centre of the electrode system, preferably at one end thereof. The more negative potential in the centre of the system accelerates the ions produced by the ion source in the direction from the source to the centre of the system. Ions of a particular kind are reflected at the end of the system remote from the source by a positive potential which preferably exists at this end only when the ions of the said kind are likewise present at said end. The electrodes in the centre of the system are preferably biased in such a way that the potential throughout an extensive central region is at least substantially constant. One of the outer electrodes of the system is connectable to a source of potential pulses supplying a cyclic voltage which, during the first and preferably longer part of the cycle, remains at a lower positive level, and during a second preferably shorter part of the cycle, is at a higher positive level.

In one embodiment of the invention electrodes at both ends of the electrode system may be connected to two output terminals of a single source of pulses or to two synchronised separate sources of pulses. The positive pulses applied to the electrodes at opposite ends of the electrode system may appear simultaneously. However, it is preferred that they should be in antiphase.

In a particularly simple embodiment of the invention a single source of pulses applies positive pulses only to the end of the electrode system remote from the ion source. The ion source functions continuously, providing an uninterrupted ion stream. The end of the electrode system near the ion source may either be kept permanently at a positive potential which is adjustable for the detection of a particular kind of ions, or it may be cyclically variable to sweep a given e/m range. Since the potential existing at the point where the ions are formed determines their energy, it also detremines the velocity at which the ions enter the middle part of the electrode array which forms a drift path. In this region, as has been mentioned, the potential is preferably constant.

The nature of the invention will now be explained in greater detail by describing embodiments thereof by reference to the drawings, but these embodiments are not intended to limit the scope of the invention in any way. In the drawings:

FIGURE 1 is a simplified representation of a mass spectrometer according to the invention,

FIGURE 2 is a graph showing the potentials along the axis 6 of the mass spectrometer tube illustrated in FIG- URE l, and

FIGURE 3 is a graph illustrating a particular method of operating a mass spectrometer according to FIG- URE 1.

The embodiment of the invention shown in FIGURE 1 comprises a mass spectrometer tube in the form of an elongated tubular envelope consisting for instance of glass and equipped with conventional means, not shown in the drawing, for the introduction into the tube of a vapour or gas. The interior of the envelope contains an electrode

system comprising electrodes

101, 102, 103, 104 and in coaxial, relatively spaced, alignment along an axis 106. The electrodes 101 to 105 are formed with coaxial apertures. The electrodes 101 and 105 are roughly cupshaped, with apertured bottoms, whereas

electrodes

102, 103 and 104 are cylindrical.

During operation the

electrodes

102, 103 and 104 in the middle of the system are maintained at least approximately at the same potential, say at ground potential. The electrodes 101-and/ or 105 may be biased, according to the method of operation of the tube, with DC. and/o1 pulsed potentials, e.g., by bias generator 122. Electrodes to which pulsed potential are applied are preferably at the same potential as the electrodes 102 to 104 during the intervals between the pulses.

In the illustrated embodiment, electrode 101 also serves as the anode of an electron gun 107 which forms the ion source. Apart from anode 101 the electron gun 107 also comprises a cathode S and a control electrode 109. The electron gun 107 may be of conventional kind, However, the electron gun 107 may, if desired, be replaced by a different kind of ion source of known kind, such as a plasma ion source. Moreover, the electron gun may be so disposed that the electron beam crosses the tube axis 106.

The manner in which the mass spectrometer according to the invention functions will be described by reference to a particular mode of operation and to this end reference .will be made to FIGURES 2 and 3. FIGURE 2 represents the potential distribution along axis 106 of the mass spectrometer tube in FIGURE 1, whereas FIGURE 3, on the left-hand side, represents an ion timetable. That is to say, the time t is plotted on the ordinate from t downwards, whereas the position s of the ions along the tube axis 106 (FIGURE 1) are plotted on the abscissa. Zero on the abscissa roughly corresponds with the position of electrode 101, whereas the broken line parallel to the ordinate on the right-hand side of the chart roughly corresponds with the position of electrode 105 at the other end of the table. The left-hand part of the chart in FIG- URE 3 therefore reveals the position of particular ions at particular times. However, the chart has been somewhat simplified by neglecting decelerations and accelerations occuring when ions are reflected at positively biased electrodes. The graph on the right-hand side in FIGURE 3 is a pulse diagram. This graph shares the same ordinate ie. time t with the chart on the left, but the pulse amplitudes are plotted ona separate abscissa. For the sake of greater clarity two pulse trains p and p are shown with relatively displaced zeros on the abscissa. In actual practice the potentials during the intervals between pulses and at the pulse peaks respectively are the same in both pulse trains.

In the mode of operation to be described first, positive pulses are applied alternately to electrodes 101 and 105. At the time t the potential of electrode 101 is that of a positive pulse 120 of a first pulse train p which in the neighbourhood of electrode 101 generates a potential hill 113 (FIGURE 2) with a positive peak at 114. Ions are produced substantially only during the presence of pulse 120, the electron beam being emitted only during this period. In the region of the peak at 114 ions are thus formed and these are accelerated down the potential slope 113 towards the centre portion of the electrode system. They enter the region 117, Within the

electrodes

102, 103, 104, Where the potential is at least substantially constant, at a velocity which depends upon the ratio of their charge e to their mass m. The ion drift path extends through this region 117. As the ions drift through region 117, ions having different e/m ratios therefore begin to separate because their velocities and hence their transit times are different.

The ions of a particular kind (such as the ions of a gas whose presence is to be detected in a leakage detector) arrive at electrode 105 at the right-hand end of the electrode system at a time t At this time t electrode 105 has a positive potential due to the arrival of a pulse 118 of a second pulse train p This pulse gives rise at the right-hand end of the electrode system to a positive potential hill 119 (FIGURE 2) the top of which must be at least as high as the peak at 114 where the ions originated. The ions of a particular e/m ratio which correspond to the time lag between the pulses of pulse trains p and p are reflected by this potential bill 119 and return ,through region 117 to electrode 101 wherethey arrive at a time t At this time t electrode 101 is again positively biased by the next positive pulse 120 of the pulse train 17 The ions therefore ascend the potential hill 113 to their original starting point in the region of the peak at 114 where they change their direction to begin a fresh oscillation as above described. Whilst the required ions are back in the region of the peak at 114 fresh ions are pref erably formed, for instance, by activating the electron gun 107 by the application of a positive pulse to the control electrode 109.

In other words, the required ions perform a periodic oscillation between the electrodes 101 and 105 in the manner indicated by the full zig-zag line in FIGURE 3. Apart from the ratio e/m of the ions the frequency of these oscillations depends upon the length of the drift path 117 and upon the velocity of the ions traversing this path. The velocity of the ions in turn is a function of the potential energy at their point of origin, that is to say it is a function of the potential difference between the potential at the point of origin 114 of the ions and the potential in region 117.

An output signal can be derived from the centre electrode 103. The frequency of this signal will then be twice the repetition frequency of the reflecting pulses in trains p and p The indicating instrument may be an oscillograph 123, as schematically indicated by a block in the drawing. The voltage tapped from electrode 103 (after amplification if required) may be used for the vertical deflection of the electron beam of the CRO, whereas its horizontal deflection is synchronised with the repetition frequency of pulse trains p and p The

pulses

118 and 120 may be supplied by two separate synchronised pulse generators or by a single pulse generator of suitable constructions. Since such arrangements are well known to a person skilled in the art, it will be unnecessary to describe them in detail.

The conditions required for the periodical reflection of the required ions will not cause oscillations of ions of different mass, subject to a few exceptions which will now be considered. Assuming the required ions have a mass m=4 and a charge e==l (the possibility of multiply charged ions may here be neglected), then ions of lower mass, say m=2, will drift through region 117 at a greater velocity and therefore arrive at electrode 105 before the latter has been positively biased by pulse 118. This is indicated by dot-dash line 121. In other Words, upon arrival at the right-hand end of the electrode system ions of mass 2 will not be faced by a potential barrier and they will therefore exit through electrode 105 to be discharged by impinging upon the wall of the tube.

The same also applies to heavier masses, say m=6. Ions having this mass do not arrive at electrode 105 until pulse 118 has already died down. They cannot therefore be reflected. This is indicated by dot-dash line 122 in FIGURE 3.

However, the selectively condition is satisfied for ions which have a mass equalling (2n+l) times the mass of the required ions, n being a positive integer. Therefore, if the mass spectrometer were used as a leakage detector for helium as the test gas, then the presence of ions having the masses 36 and might cause interference. Ions of mass m=36, as shown in FIGURE 3, arrive for the first time at electrode at the time i and they are therefore likewise reflected. However, ions of mass 36 could be present only in the form of hydrocarbon ions and such substances can be readily decomposed inside the tube, for instance, by a hot tungsten wire and an electron beam. Indeed, small concentrations of hydrocarbon can be lowered below the detectability limit by the electron emitting system of the ion source. Alternatively, the effect of interference of this kind can be eliminated by electronic means as Will be hereafter explained.

It will be readily understood that it is possible to sweep the mass range by varying the pulse frequencies p and p and that a mass spectrogram can thus be obtained.

The above described working principle may be modified in diverse ways. The following method of operation is particularly simple. Only electrode 105 is connected to a pulse generator and the pulse frequency is constant. Electrode 101 and the preferably continuously operating ion source have a positive bias which determines the particular type of ion which will be selected. The output signal can be derived from the central electrode 103, as was done in the previously described method of operation. However, in the present instance, the output signal may also be derived at detecting means 124 from electrode 101 which then operates as an influence collector. Alternatively, pulses may be applied to electrode 101 and the signal derived from electrode 105.

Without taking further steps, the ion source can thus be made to work intermittently even though the operational voltage of the electron emitting system remains constant.

Since the potential energy of the ions at their point of origin determines their transit time and hence their frequency of oscillation the magnitude of the DC. voltage and/ or of the pulse voltage applied to electrode 101 adjacent the ion source 107 is preferably arranged to be adjustable.

For simplifying the electronics of the arrangement, both reflecting electrodes 101, 105 may be jointly pulsed. However, the reflection conditions will then be satisfied for ions having only four times the selected mass.

Unwanted ions which as such comply with the existing reflection conditions (e.g., ions of mass 36 in FIG- URE 3), may, as has been mentioned, be eliminated by electronic means. One possibility would be to reduce the width of the reflection pulses 118 and/ or 120 to such an extent that, although the light ions to be detected (e.g. ions of mass 4) reach the constant potential level along the drift path 117 before the pulse (and the potential hill 113 and/ or 119) has died down, nevertheless, the heavier and therefore slower ions (e.g. of mass 36) will still be in the course of descent when the potential slope collapses. The heavier ions will then fail to attain their full energies and velocities, and will reach the other end of the tube too late for reflection.

Another possibility lies in applying to the ions at the end of the electrode system remote from the ion source a very short voltage pulse such as to apply to them a momentum which is twice as large as, and of opposite sign to, the momentum mv of the selected ions of mass m which arrive at velocity v. Mathematically speaking: mv (momentum of the arriving selected ion)+(2 mv) (momentum imported by the electrical pulse)=mv (momentum of the reflected ion). This ensures that only ions of the required mass will return to the drift path 117 at the same velocity as before. Ions of heavier mass re turn at a slower speed and are therefore unable to ascend to the potential level of their original starting point. Ions of lighter weight are propelled beyond the peak at 114 and are thus likewise eliminated.

The magnitude of the reflecting pulse may be suitably chosen to drive the required ions just over the peak 114 to enable them then to enter a secondary electron multiplier not shown in the drawing. In such a case the electron stream is directed perpendicularly across the tube axis. Pulse-reflected lighter weight ions which are also able to ride over the peak 114 can be recognised by their greater energy. They can be detected in an anticoincidence device or eliminated from the desired signal by subtracting a corresponding direct current. The reflection of the ions by the described sign-reversal of their momenta is particularly advantageous when combined with the above-described simple method of operation. When the ion source is continuously in operation undesirable ions are always reflected, which then produce an interfering signal of the same frequency as the desired signal. Pulse-reflected heavier ions cannot now reach electrode 101 which functions as an influence collector because they accept too little energy in reflection.

In reflection by reversal of momentum the resultant sensitivity to partial pressures is particularly high and this mode of operation is therefore especially useful for leakage detection. If helium is the test gas, only hydrogen ions in addition to helium ions can in practice influence a signal in electrode 101 or, when a multiplier is used, overcome the peak at 114. However, the hydrogen partial pressure is generally so small that interference need not be feared even if the above-mentioned arrangements for dealing with lighter ions have not been made.

If the ion source is pulsed, it is desirable that all newly formed ions should start at the same time. This can be achieved by providing an additional potential barrier 116 (FIGURE 2) with the aid of an auxiliary electrode 115 and thereby preventing the freshly formed ions from falling into the drift path until they are all released simultaneously at the end of the ion forming period. The auxiliary electrode 115 may be annular and located inside electrode 101.

The potential pulses applied to the pulsed electrode may sometimes give rise to an interference signal at the electrode from which the output signal is derived. Such interference can be eliminated as follows:

A compensating electrode may be interposed between the pulsed electrode and the output signal electrode and a pulse in antiphase of suitable amplitude may be ap plied thereto.

The interfering potentials at the output signal electrode can be neutralised in a manner known to the art.

Interference potentials will cancel each other out at the centre electrode 103 if electrodes 101 and 105 are simultaneously pulsed in antiphase, that is to say if a positive reflecting pulse is applied to one of the electrodes at the same time as a negative pulse of like amplitude is applied to the other electrode. This is indicated in FIGURE 2 by the dot-dash curves in FIGURE 2.

Finally, the ion source may be modulated with an arbitrary low frequency in such manner that the desired signal can be recognised by its modulation and isolated.

For focusing the ions radially in the tube axis an axially directed, substantially constant magnetic field may be provided in all the embodiments which have been described.

'In the last described mode of operation region 117 may also include arrangements additional to those already described for improving ion selectivity. For instance, a magnetic ion optical system or a Paul four-pole field may be provided. However, these are known steps and require no further description.

What we claim is:

1. .A mass spectrometer operative in a pulse cycle and comprising, in combination: a vacuum-tight envelope; at a set of apertured electrodes spaced along a common longitudinal axis within said envelope, at least one of said electrodes intermediate the ends of said electrode set forming an intermediate ion drift region; an ion source for providing ions at one end of said drift region; means for biass-ing said ion source positively with respect to said drift region so that ions produced by said source are accelerated along the negative potential gradient thus formed toward said drift region and continue to drift therethrough to the other end of said drift region; pulse generator means connected to an electrode at a second end of said drift region, opposite said one end, for delivering periodic pulses which are positive with respect to the potential of said drift region and which have such repetition rate and duration that of the accelerated ions which start at a given point in time, only those ions which have a selected charge/mass ratio are repelled back into said drift region; and means for detecting said ions of selected charge/ mass ratio which are periodically reflected by said positive biasses at the ends of said drift region.

2. A mass spectrometer as defined in claim 1 wherein said means for biassing said ion source produces a positive DC. bias.

3. A mass spectrometer as defined in claim 1 wherein said means for biassing said ion source produces a periodic pulse bias having the same repetition rate as the pulses produced by said generator means.

4. A mass spectrometer as defined in claim 3 wherein both said biassing means and said pulse generator produce alternatively pulses of positive and negative polarity.

5. A mass spectrometer as defined in claim 3 including means for providing a positive potential barrier directly in front of said ion source at all times except when the ions produced by said source are to be released, so that all ions so produced leave said source simultaneously.

6. A mass spectrometer according to claim 3, wherein the pulses applied to the electrode at the end of the electrode set remote from the ion source are very short in relation to the repetition rate and their amplitude is such as to apply a momentum to the arriving ions which in magnitude is equal to twice the momentum of the arriving desired ions but of opposite sign.

7. A mass spectrometer operative in a pulse cycle and comprising, in combination: a vacuum-tight envelope; at set of apertured electrodes spaced along a common longitudinal axis within said envelope, at least one of said electrodes intermediate the ends of said electrode set forming an intermediate ion drift region; an ion source for providing ions at one end of said drift region; a first source of periodic pulses for biassing said ion source positively with respect to said drift region, so that ions produced by said source are accelerated periodically along the negative potential gradient thus formed toward said drift region and continue to drift therethrough to the other end of said drift region; means for biassing positively with respect to said drift region an electrode at said other end of said drift region, opposite said one end, so that the ions incident at said electrode are repelled back into said drift region; and means for detecting ions :of a selected charge/mass ratio which are periodically reflected by said positive biasses at the ends of said drift region.

8. A mass spectrometer as defined in claim 7 wherein said means for biassing said electrode at the other end of said drift region periodically delivers pulses which are positive with respect to said drift region and which have a repetition rate and duration such that of the accelerated ions which started during the period in which said ion source was positively biassed by a pulse of said first pulse source, only those ions which have a selected charge/ mass ratio are repelled to said drift region, said first source of periodic pulses and said means for biassing being arranged to provide biassing .at the same time.

9. A mass-spectrometer operative in a pulse cycle and comprising, in combination: a vacuum-tight envelope; a set of ape'rtured electrodes spaced along a common longitudinal axis within said envelope; an ion source for providing ions at one end of said axis; means for biassing at least one electrode intermediate between the ends of the electrode set to a given potential for providing an ion drift region; pulse generator means connected to a first electrode at said one end of the electrode set for biassing said first electrode, at least during certain portions of said cycle, to a potential which is more positive than that of said intermediate electrode for preventing ions from passing said first electrode; pulse generator means connected to a second electrode at the other end of said electrode set from said first electrode for periodically raising the potential of said second electrode to a value above said given potential to deflect selected ions back along the longitudinal axis of said spectrometer; said pulse generator means being arranged to alternately bias said first electrode and raise the potential of said second electrode and means for detecting those selected ions so deflected.

References Cited by the Examiner UNITED STATES PATENTS 2,642,535 6/1953 Schroeder 25041.9 2,772,364 11/1956 Washburn 250-419 2,798,162 7/1957 Hendee 25041.9 2,810,075 10/1957 Hall et al 25041.9 2,908,816 40/195 9 Le Poole 25041.9 2,938,116 5/1960 Benson et a1 250-41.9

RALPH G. NILSON, Primary Examiner.

Claims

9. A MASS-SPECTROMETER OPERATIVE IN A PULSE CYCLE AND COMPRISING, IN COMBINATION: A VACUUM-TIGHT ENVELOPE; A SET OF APERTURED ELECTRODES SPACED ALONG A COMMON LONGITUDINAL AXIS WITHIN SAID ENVELOPE; AN ION SOURCE FOR PROVIDING IONS AT ONE END OF SAID AXIS; MEANS FOR BIASING AT LEAST ONE ELECTRODE INTERMEDIATE BETWEEN THE ENDS OF THE ELECTRODE SET TO A GIVEN POTENTIAL FOR PROVIDING AN ION DRIFT REGION; PULSE GENERATOR MEANS CONNECTED TO A FIRST ELECTRODE AT SAID ONE END OF THE ELECTRODE SET FOR BIASING SAID FIRST ELECTRODE, AT LEAST DURING CERTAIN PORTIONS OF SAID CYCLE, TO A POTENTIAL WHICH IS MORE POSITIVE THAN THAT OF SAID INTERMEDIATE ELECTRODE FOR PREVENTING IONS FROM PASSING SAID FIRST ELECTRODE; PULSE GENERATOR MEANS CONNECTED TO A SECOND ELECTRODE AT THE OTHER END OF SAID ELECTRODE SET FROM SAID FIRST ELECTRODE FOR PERIODICALLY RAISING THE POTENTIAL OF SAID SECOND ELECTRODE TO A VALUE ABOVE SAID GIVEN POTENTIAL TO DEFLECT SELECTED IONS BACK ALONG THE LONGITUDINAL AXIS OF SAID SPECTROMETER; SAID PULSE GENERATOR MEANS BEING ARRANGED TO ALTERNATELY BIAS SAID FIRST ELECTRODE AND RAISE THE POTENTIAL OF SAID SECOND ELECTRODE AND MEANS FOR DETECTING THOSE SELECTED IONS SO DEFLECTED.