US3634683A - Time-of-flight mass spectrometer with step-function-controlled field - Google Patents

Time-of-flight mass spectrometer with step-function-controlled field Download PDF

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US3634683A
US3634683A US27803A US3634683DA US3634683A US 3634683 A US3634683 A US 3634683A US 27803 A US27803 A US 27803A US 3634683D A US3634683D A US 3634683DA US 3634683 A US3634683 A US 3634683A
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ions
time
deflection
voltage
ion
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Johannes M B Bakker
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Shell USA Inc
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Shell Oil Co
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J49/00—Particle spectrometers or separator tubes
    • H01J49/02—Details
    • H01J49/06—Electron- or ion-optical arrangements
    • H01J49/061—Ion deflecting means, e.g. ion gates
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J49/00—Particle spectrometers or separator tubes
    • H01J49/26—Mass spectrometers or separator tubes
    • H01J49/34—Dynamic spectrometers
    • H01J49/40—Time-of-flight spectrometers

Definitions

  • Time-of-flight mass spectrometers have the following basic components.
  • a time-of-flight mass spectrometer is provided with means whereby a step-functioncontrolled electrical field perpendicular to the ion beam is employed to provide bunching.
  • This may be achieved by applying to a pair-of deflection plates between which the ion beam passes, a voltage varying in accordance with a step-function.
  • These deflection plates may conveniently be a single pair of deflection plates such as constitute the X- or Y-plates of a conventional cathode-ray tube.
  • two sets of deflection plates are provided which may conveniently be arranged as the X and Y-plates of a conventional cathode-ray tube. It is however not essential for the electrical fields between the two sets of plates to be at right angles to each other when viewed in a direction parallel to that of the ion beam. Any angular relationship between the fields from to 90 may be used.
  • the time taken for all the types of ions of a given bunch to actuate the collector is known as the spectrum, and it is necessary for the electrical field to be maintained at a substantially constant value during this spectrum.
  • the ratio of transit time of the ions through this field to the rise time of the step-function is such that the rise time must be approximately equal to or less than half the transit time of the heaviest ions through the field if adequate separation of ion bunches is to be effected.
  • the spectrometer in accordance with the present invention produces a continuous beam of ions which has a periodic effect on the collector as a result of the change in the perpendicular electric field or fields produced by the controlling step-function and a resulting change in direction of the ion beam.
  • Another aspect of the present invention comprises a method for repeatedly obtaining a mass spectrum of ions according to the principle of accelerating a group of ions to a certain voltage so as to impart to the ions velocities corresponding to their mass/charge ratio, and measuring the current of ions arriving at a target as a function of the time it takes the various ions of the group to reach the target.
  • a group of accelerated ions is formed time and again by intermittently directing a beam of accelerated ions towards the target by means of an oscillating transverse electrical field, with the aid of a set of deflection electrodes between which an oscillating deflection voltage is applied.
  • the electrical field oscillates between two levels chosen such as to cause the beam to be deflected repeatedly towards, across and beyond the target, spectra as represented by the varying ion current that reaches the target being measured or registered only for groups of ions directed towards the target as a result of (field) changes in one direction of the deflection voltage.
  • the step-function-controlled voltage must remain substantially constant at its higher or altered value for the duration of the spectrum and means have to be provided to ensure that the deflection of the ion beam when this voltage falls or returns to its original value, does not produce a display on the oscilloscope which interferes with the display arising from the original increase vor change in the step-function-controlled voltage.
  • These means could comprise an additional pulse generator triggered by the fall or return of the step-controlled voltage, together with synchronizing or delay networks which would cause the two traces to be superimposed. This however involves complications in design which are not warranted by the results achieved. It is preferable to arrange the system so that the ion beam deflection occasioned by the fall or return of the voltage to its original value, is without effect on the oscilloscope.
  • This last-mentioned mode of operation can conveniently be carried out by arranging for the application to a second set of plates of the step-function-controlled voltage which by its return to its original value, will prevent the ion beam from reaching the collector.
  • the leading edge of the square pulse acts to overcome a deflecting bias voltage on both sets of plates to allow ions to travel substantially axially down the apparatus to the collector, the trailing edge of the pulse reestablishes the deflecting bias on the second set of plates and thus prevents ions which have already left the first set of plates from reaching the collector.
  • One of the principal advantages derived from the mass spectrometer of the present invention is an increase in sensitivity due to the continuous ionization process, a considerable increase in resolution due to the lack of space defocusing and the improved ion bunching, combined with the speed of operation normally associated with a time-of-flight mass spectrometer.
  • the mass spectrometer according to the present invention may be arranged to achieve currently acceptable sensitivity and resolution with a shorter path-offlight than has heretofor been achieved; this permits a more compact instrument to be constructed.
  • the spectrometer of the present invention also offers the possibility of employing a collector slit of variable width by means of which resolution can be improved at the cost of sensibility and vice versa.
  • FIG. 1 shows a diagram of the entire mass spectrometer in accordance with the present invention
  • FIG. 2 shows diagrammatically the effect on the ion beam of the step-function-controlled voltage
  • FIGS. 2a, 2b, 2c, 2d, and 2e are curves showing voltages at different times
  • FIGS. 3 to 10 show typically oscilloscope displays produced with the mass spectrometer of the present invention.
  • the spectrometer comprises an ion gun indicated generally by reference 8, an ionization chamber indicated by the reference 1, and a cathode-ray tube structure generally indicated by the reference 2 adrift tube 3, a collector slit 4, a collector indicated by the general reference 5, and an oscilloscope 6, a pulse generator 7 producing a stepfunction-controlled voltage, and supplying a trigger pulse to the oscilloscope.
  • the power supplies for the ionization chamber 1 and cathode-ray tube structure 2 are provided by a power pack indicated generally by the reference 9.
  • High-voltage power supplies l0 and 11 are provided respectively for the drift tube 3 and collector 5.
  • the ionization chamber 1 may conveniently be of the type known as the Nier ionization chamber, and comprises an inlet 13 for the gas or vapor under examination (generally from a gas/liquid chromatographic analyzer) which flows in the direction indicated by the arrows and is passed through an ionization space 14 where it is bombarded with a stream of electrons 15 passing between a filament 17 and an electron trap 18 operating in combination with an ion repeller 19.
  • a collimating magnet not shown, may conveniently be used to collimate the electron beam and thereby increase the ion production.
  • a beam of ions derived from the molecules of the original sample leaves the ionization space 14 through the orifice 20 and thereafter passed through the accelerator electrode a.1, the focusing electrode a.2 and the astigmatism electrode :13 of the cathode-ray tube structure. Thereafter it passes a pair of Y-plates 21, a geometry control electrode :1 and a pair of X- plates 22.
  • the ion beam After passing down the drift tube 3 the ion beam passes through the collector slit 4 and strikes the cathode 23 of the collector 5 releasing electrons which activate an electron multiplier of known type comprising a field strip 24 and a dynode strip 25 before reaching an anode 26 thereby supplying a current which passes through the resistance 27 to give a voltage impulse to the oscilloscope 6. Voltages and resistance values are indicated at various points in FIG. 1.
  • the Y-plates 21 are normally biased in such a way that the ion beam does not strike the cathode 23 of the collector 5.
  • the generator 7 produces the step-function-controlled voltage, which may conveniently be a square pulse of 40-volt amplitude and 50 1. duration. If this is applied to the Y-plates 21 only, and the deflecting bias on these plates is overcome, the ion beam swings across the drift tube and ions pass through the slit 4 on to the cathode 23; or a square pulse of 40-volt amplitude and 1 1 duration may be applied to both the Y-plates 21 and the X-plates 22 to achieve the same result.
  • FIG. 2 shows diagrammatically the ef fect of a step-function-controlled electrical field on the ion beam.
  • a homogeneous monoenergetic ion beam of width B and consisting ofions with mass m and energy eU travels from left to right through an electrical field of strength E produced between a set of deflection plates of effective length l and effective separation D. After which, the ions travel through a space L, assumed to be free of any fields, until the ions reach a slit 4.
  • ions that have passed through the electrical field before the electrical field change has taken place will have experienced, while travelling through the electrical field, a downwardly directing force.
  • the position of these ions is indicated at three different instants in time by n 11,, a a,,, a and a,.
  • Any ions travelling through the electrical field after an electrical field change has taken place experience an upwardly directing force while travelling through the deflection plates.
  • the position of these ions is indicated at three different instants in time by e,,, e,, e e,, e, and 2' Any ions travelling through the electrical field at the instant an electrical field change takes place will experience both downwardly and upwardly directing forces.
  • the motion of these ions is dependent on their exact position within the electrical field at the time of field change.
  • the position of eight ions at time T is indicated by a 11' c c d ta and 2' If at T an electrical field change takes place then the a ions will have travelled through an electrical field, as shown in FIG. 2a.
  • the a-ions are therefore not affected by the changeover and these ions will move along the path indicated by a a,, a a',,, a, and a'
  • the 0,, and c -ions are exactly halfway through the electrical field at time T These ions will have travelled through an electrical field, as shown in FIG. 2c.
  • the e and e -ions are at the entrance of the deflection plates at time T These ions will travel through an electrical field as indicated by FIG. 2e. For these ions the changeover occurred at 1 0, and they will, therefore, experience only an upwardly directing force.
  • the position of the e-ions at three instants in time is given by e e,, e 2' e, and 0' lfa slit 4 is placed in a position as shown in FIG. 2, then of the a, c, and e-ions only the c-ions will pass through this slit.
  • the limiting case ofions which will pass through the slit 4 is indicated by the ions b and d.
  • the b -ion will have travelled through an electrical field as indicated by FIG. 2b.
  • the changeover occurred slightly after hi, or 2 k1,.
  • the ion will, therefore, have experienced a downwardly directing force over a time interval slightly greater than /2t,. It will also have experienced an upwardly directed forece over a time interval slightly less than V21
  • the net result is that ion b will leave the electrical field with a small velocity component in a downward direction.
  • the position of the b-ion at four different instants in time is shown by b b b and h
  • the changeover time 1 is directly related to the position of an ion at the time of the electrical field change. Ion 1), presents a limiting case in that any ion for which I; is larger than the 1 of ion 12 will have received too much downward acceleration and therefore, these ions will arrive below the slit 4.
  • the d -ion will have experienced an electrical field while travelling through the deflection plates as indicated in FIG. 2d.
  • the changeover occurred slightly before /2!,, or t /2t,.
  • the ion experiences a downwardly directing force over a time interval slightly less than /21,, followed by an upward directed force over a time interval slightly greater than /fil
  • the net result is that ion d will leave the deflection plates with a small velocity component in an upward direction.
  • the position of the d-ion at four different instants in time is indicated by d d d and d Any ions for which t is less than the t of ion d will arrive above the slit 4.
  • a homogeneous beam of ions passes through an electrical field which deflects the beam downwards.
  • a change in the electrical field will produce a deformation of kink in the beam.
  • This kink comprises the ions having positions between the e-ions and the a-ions at the instant of field change.
  • the beam as a whole, including the kink will continue to travel from left to right with the same velocity.
  • the kink will expand both in the upward and downward direction.
  • the ions at the upper end of the kink (e-ions) will have the largest upward velocity.
  • the ions at the bottom of the kink (a-ions) will have the largest downward velocity.
  • the time duration of the ion bunch emerging from the slit aperture can be directly related to the changeover time 1
  • ion b passes first through the slit
  • the ion bunch b -c d -c' represents a packet of very light ions
  • the ion bunch b c d c' represents a packet of medium weight ions
  • the ion bunch b c,-a ,c represents a packet of very heavy ions.
  • the ion-bunching method of the present invention has two outstanding featureslFirstly, because At is mass dependent the resolution will be independent of mass, and secondly because 1 is independent of mass the number of ions in each packet b-c-aJ-c is independent of mass and, therefore, the sensitivity of the mass spectrometer is independent of mass.
  • B Beam width (typically 1-5 mm.)
  • Predeflection accelerator voltage U Postdeflection accelerator voltage U Total accelerator voltage (typically 2000-5 ,000 v.)
  • eU Total ion energy (typically 2,000-5,000 ev.)
  • a.m.u. Atomic mass unit l.66 l0'kg).
  • V(tt) deflection plate voltage V0 Magnitude of deflection plate voltage change 10-100
  • E V(t)D electrical field strength between deflection plates v (2eU/m) ion beam velocity t, l(m/2eU,)" transit time in deflection plates of length t L(m/2eU) transit time of ions in drift space of length L.
  • t (d) same as t (b), except this is last ion of a particular bunch to pass through slit 4.
  • FIGS. 3, 4 and 5 show the effects of differing rise times in the step-function-controlled voltage applied to X- and Y-plates, and demonstrate that the rise time of the voltage change must be approximately equal to or less than half the transit time of the heaviest ions through the electrical field.
  • the time basis of the oscilloscope was 0.1 tsecJcm. and the transit time through the field of the Y- plates was approximately 450 nanoseconds.
  • the rise time of the applied voltage was 17 nanoseconds and the sharpness of the oscilloscope trace and the high resolving power (800) are clear.
  • FIG. 3 the rise time of the applied voltage was 17 nanoseconds and the sharpness of the oscilloscope trace and the high resolving power (800) are clear.
  • the rise time was 300 nanoseconds and the indistinct nature of the pulses and the consequent loss of resolving power is beginning to be appreciable.
  • the rise time was 500 nanoseconds and from this figure it is clear that the resolving power has decreased to a degree which is unacceptable.
  • FIGS. 6, 7 and 8 show the effect of pulse width, i.e., the time between step-functiomcontrolled voltage increase and decrease, when using only Y-plates and demonstrate the desirability of returning the step-function-controlled voltage to its original value only after the spectrum has taken place.
  • the oscilloscope display was obtained with a time base of l usec/cm. and a square pulse having a width of 0.5 usec; the oscilloscope trace due to the trailing edge of the pulse overlaps with that resulting from the leading edge of the pulse.
  • the time base was the same as in FIG. 6 and the pulse width was 6 #sec. and it will be seen that the overlap between the two traces has been materially reduced in comparison with FIG. 6.
  • the trace shown in FIG. 8 was produced with a time base of 5 used/cm. and a pulse width of 18 14sec.
  • leading edge and the trailing edge traces are entirely separate.
  • FIGS. 9 and 10 show the effect on the oscilloscope trace (in both cases of l used/cm.) of pulse width when using X- and Y-plates, and demonstrate the desirability, when the stepfunction-controlled voltage has the form of a square pulse, of a pulse duration greater than the time of flight of the ions between the two sets of plates.
  • the pulse width was 1 sec. and this has given a satisfactory trace showing heavier ions on the right and lighter ions on the left.
  • this pulse width was reduced as shown in FIG. 10 to approximately 0.5 sec. the heavier and slower-moving ions have failed to reach the collector and are absent from the trace.
  • a time-of-flight mass spectrometer wherein a mass spectrum of ions is obtained from a material under investigation by accelerating a group of the ions to velocities corresponding to their mass-to-charge ratios, making the group cover the distance to a target to allow the ions with different mass-tocharge ratios to separate and measuring the current of ions that reach the target, and displaying the course in time of the measured ion current;
  • a source of voltage and means for varying the voltage as a step-function said source being coupled to said deflection plates;
  • a target slit and current detector disposed in the target area traversed by the ion beam so that for each such step function a group of the ions passing through said electrical field pass through said target slit and current detector;
  • Time-of-flight mass spectrometer characterized by a source of trigger pulses which are generated at instants corresponding to the step-function changes in the deflection voltage, said source of trigger pulses being connected to an input of the said means for recording the course in time of the ion current detector output signal.
  • Time-of-flight mass spectrometer characterized in that the source of trigger pulses is capable of generating only trigger pulses at instants corresponding to step-function changes proceeding in one direction.
  • Time-of-flight mass spectrometer characterized by another pair of deflection electrodes connected to a second source ofa step-function deflection voltage which, apart from possible phase shifts, can vary synchronously with the first source of voltage.
  • Time-of-flight mass spectrometer characterized in that the two pairs of deflection electrodes are arranged to produce substantially perpendicular electrical fields.
  • Time-of-flight mass spectrometer characterized in that the target is a slit in a diaphragm.
  • a method according to claim 8 characterized in that the said step-function as described by the deflection voltage has a rise time not longer than approximately half the transit time of the ions with the highest mass-to-charge ratio through the space between the deflection electrodes.
  • a method according to claim 8 characterized in that the changes in deflection voltage between the said levels are separated by time intervals chosen sufficiently long to prevent the periods of arrival at the ion current detector of successive groups of ions from overlapping at least during a part of interest of these periods.
  • a method according to claim 8 characterized in that the ion beam is passed between another pair of deflection electrodes to which a second deflection voltage is applied which varies synchronously with the first-mentioned deflection voltage, thereby preventing the first-mentioned deflection voltage in one direction of alternation between levels from giving rise to groups of ions that would reach the target.
US27803A 1969-04-18 1970-04-13 Time-of-flight mass spectrometer with step-function-controlled field Expired - Lifetime US3634683A (en)

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Cited By (12)

* Cited by examiner, † Cited by third party
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US4295046A (en) * 1975-09-11 1981-10-13 Leybold Heraeus Gmbh Mass spectrometer
EP0304525A1 (de) * 1987-08-28 1989-03-01 FISONS plc Gepulste mikrofokussierte Ionenstrahlen
EP0613171A1 (de) * 1993-02-23 1994-08-31 Hans Bernhard Dr. Linden Massenspektrometer zur flugzeitabhängigen Massentrennung
US5543624A (en) * 1993-07-02 1996-08-06 Thorald Bergmann Gasphase ion source for time-of-flight mass-spectrometers with high mass resolution and large mass range
US5614711A (en) * 1995-05-04 1997-03-25 Indiana University Foundation Time-of-flight mass spectrometer
AU685113B2 (en) * 1993-07-02 1998-01-15 Bergmann, Eva Martina Time-of-flight mass-spectrometer with gasphase ion source, with high sensitivity and large dynamic range
US5753909A (en) * 1995-11-17 1998-05-19 Bruker Analytical Systems, Inc. High resolution postselector for time-of-flight mass spectrometery
US5981946A (en) * 1995-11-16 1999-11-09 Leco Corporation Time-of-flight mass spectrometer data acquisition system
US6770874B2 (en) * 2000-07-14 2004-08-03 Epion Corporation Gas cluster ion beam size diagnostics and workpiece processing
US6831272B2 (en) * 2000-07-14 2004-12-14 Epion Corporation Gas cluster ion beam size diagnostics and workpiece processing
US20050087684A1 (en) * 2003-10-23 2005-04-28 Farnsworth Vincent R. Time of flight mass analyzer having improved mass resolution and method of operating same
US20080029697A1 (en) * 2006-07-12 2008-02-07 Willis Peter M Data Acquisition System and Method for a Spectrometer

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WO1989006044A1 (en) * 1987-12-24 1989-06-29 Unisearch Limited Mass spectrometer
DE3920566A1 (de) * 1989-06-23 1991-01-10 Bruker Franzen Analytik Gmbh Ms-ms-flugzeit-massenspektrometer
GB9010619D0 (en) * 1990-05-11 1990-07-04 Kratos Analytical Ltd Ion storage device
US5180914A (en) * 1990-05-11 1993-01-19 Kratos Analytical Limited Mass spectrometry systems

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US2612607A (en) * 1947-04-05 1952-09-30 William E Stephens Mass spectrometer
GB780999A (en) * 1953-12-12 1957-08-14 Tno Improvements in or relating to mass spectrometers
US3307033A (en) * 1963-07-19 1967-02-28 William H Johnston Lab Inc Coincidence mass spectrometer with electrostatic means to separate positive and negative ions and detectors and method of use

Patent Citations (3)

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Publication number Priority date Publication date Assignee Title
US2612607A (en) * 1947-04-05 1952-09-30 William E Stephens Mass spectrometer
GB780999A (en) * 1953-12-12 1957-08-14 Tno Improvements in or relating to mass spectrometers
US3307033A (en) * 1963-07-19 1967-02-28 William H Johnston Lab Inc Coincidence mass spectrometer with electrostatic means to separate positive and negative ions and detectors and method of use

Cited By (27)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4295046A (en) * 1975-09-11 1981-10-13 Leybold Heraeus Gmbh Mass spectrometer
EP0304525A1 (de) * 1987-08-28 1989-03-01 FISONS plc Gepulste mikrofokussierte Ionenstrahlen
US4912327A (en) * 1987-08-28 1990-03-27 Vg Instruments Group Limited Pulsed microfocused ion beams
EP0613171A1 (de) * 1993-02-23 1994-08-31 Hans Bernhard Dr. Linden Massenspektrometer zur flugzeitabhängigen Massentrennung
US5543624A (en) * 1993-07-02 1996-08-06 Thorald Bergmann Gasphase ion source for time-of-flight mass-spectrometers with high mass resolution and large mass range
AU685113B2 (en) * 1993-07-02 1998-01-15 Bergmann, Eva Martina Time-of-flight mass-spectrometer with gasphase ion source, with high sensitivity and large dynamic range
US5614711A (en) * 1995-05-04 1997-03-25 Indiana University Foundation Time-of-flight mass spectrometer
US5981946A (en) * 1995-11-16 1999-11-09 Leco Corporation Time-of-flight mass spectrometer data acquisition system
US5753909A (en) * 1995-11-17 1998-05-19 Bruker Analytical Systems, Inc. High resolution postselector for time-of-flight mass spectrometery
US6770874B2 (en) * 2000-07-14 2004-08-03 Epion Corporation Gas cluster ion beam size diagnostics and workpiece processing
US6831272B2 (en) * 2000-07-14 2004-12-14 Epion Corporation Gas cluster ion beam size diagnostics and workpiece processing
WO2005040785A2 (en) * 2003-10-23 2005-05-06 Beckman Coulter, Inc. Time of flight mass analyzer having improved mass resolution and method of operating same
US20050087684A1 (en) * 2003-10-23 2005-04-28 Farnsworth Vincent R. Time of flight mass analyzer having improved mass resolution and method of operating same
US20050285030A1 (en) * 2003-10-23 2005-12-29 Farnsworth Vincent R Time of flight mass analyzer having improved detector arrangement and method of operating same
WO2005040785A3 (en) * 2003-10-23 2006-06-08 Beckman Coulter Inc Time of flight mass analyzer having improved mass resolution and method of operating same
US7186972B2 (en) * 2003-10-23 2007-03-06 Beckman Coulter, Inc. Time of flight mass analyzer having improved mass resolution and method of operating same
US20090014642A1 (en) * 2006-07-12 2009-01-15 Leco Corporation Data acquisition system for a spectrometer using horizontal accumulation
US20090014643A1 (en) * 2006-07-12 2009-01-15 Willis Peter M Data Acquisition System for a Spectrometer that Generates Stick Spectra
US20080029697A1 (en) * 2006-07-12 2008-02-07 Willis Peter M Data Acquisition System and Method for a Spectrometer
US7501621B2 (en) 2006-07-12 2009-03-10 Leco Corporation Data acquisition system for a spectrometer using an adaptive threshold
US20090072134A1 (en) * 2006-07-12 2009-03-19 Willis Peter M Data Acquisition System for a Spectrometer Using Various Filters
US20090090861A1 (en) * 2006-07-12 2009-04-09 Leco Corporation Data acquisition system for a spectrometer
US7825373B2 (en) 2006-07-12 2010-11-02 Leco Corporation Data acquisition system for a spectrometer using horizontal accumulation
US7884319B2 (en) 2006-07-12 2011-02-08 Leco Corporation Data acquisition system for a spectrometer
US8017907B2 (en) 2006-07-12 2011-09-13 Leco Corporation Data acquisition system for a spectrometer that generates stick spectra
US8063360B2 (en) 2006-07-12 2011-11-22 Leco Corporation Data acquisition system for a spectrometer using various filters
US9082597B2 (en) 2006-07-12 2015-07-14 Leco Corporation Data acquisition system for a spectrometer using an ion statistics filter and/or a peak histogram filtering circuit

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