US3296434A - Method of operating an ion source for a time of flight mass spectrometer - Google Patents

Description

Jan. 3, i967 M. H. STUDIER E3,296,434

METHOD OF OPERATING AN ION SOURCE FOR A TI OF FLIGHT MASS SPECTROMETER Filed May 26, 1964 3 Sheets-Sheet 1 'F'- l ,Pulse DG. Pozaef :E @iff/@5501 ,y

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METHOD OF' OPERATING AN ION SOURCE FOR A TIME OF FLIGHT MASS SPECTROMETER Jan. 3, 1967 M. H. STUDIER 3,296,434

METHOD OF OPERATING AN ION SOURCE FOR A TIME OF FLIGHT MASS SPECTROMETER Filed May 25, 1954 3 Sheets-Sheet 5 x) M 5 am QB w u W Y \g\ x\\ wF/ w// LU C l l i o w n i? N s f. Ow A 'n w QM Q 1 L4M Ll I W www w n INVENTOR. ,lyafizjz Sida/er United States Patent O 3,296,434 METHOD OF OPERATING AN ION SOURCE FOR A TIME F FLIGHT MASS SPECTROMETER Martin H. Stndier, Downers Grove, Ill., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed May 26, 1964, Ser. No. 370,387 11 Claims. (Cl. 250-41.9)

The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

This invention relates to ion sources for use with time of flight mass spectrometers and more specifically to an impuroved method of operation therefor.

The conventional time of ight mass spectrometer comprises an ion source, a flight tube and a detector all housed in an evacuated structure. Operation of the ion source is critical for the proper function of the instrument. The ion source utilizes an electron beam to produce ions with the beam being generally operated in a pulsed mode. This mode, however, has the disadvantage of a low duty cycle with resulting low sensitivity. Attempts have been made to operate in a continuous mode but to date, for a slight increase in sensitivity, poorer resolution and greater noise exist therefor. Thus, the continuous mode is not generally recommended for use over the pulsed mode. (D. B. Harrington, Encyclopedia of Spectroscopy, Reinhold Corp., New York 1960, pp. 628-647.) Further, no method of operation is presently available wherein surface ionization sources may be used in a time of ight mass spectrometer.

It is therefore one object of the present invention to provide an improved method of operating in a continuous mode an ion source of a time of flight spectrometer.

It is another object of the present invention to provide a method of operating an ion source of a time of flight spectrometer in a continuous mode with improved sensitivity, resolution and no increase in noise to signal ratio over the conventional pulse mode operation.

It is still another object of the present invention -to provide a method of operating an ion source of a time of flight mass spectrometer wherein a surface ionization source may be used.

Other objects of the present invention will become more apparent as the detailed description proceeds.

In general, the present invention comprises the steps of injecting a gas to be analyzed between the backing plate and the first grid of an ion source in a time of flight mass spectrometer and passing a continuous electron beam between the backing plate and the first grid to ionize the gas. A negative potential pulse is periodically applied to the first grid to attract ions created therefrom to the first grid. The negative potential pulses are also integrated and applied to the backing plate to create a compensating ion attraction effect for the time duration of the tail end of the negative potential pulses. A negative potential is maintained on the second grid of the spectrometer Where by ions periodically attracted to said first grid are accelerated. A solid may be analyzed instead of a gas by making a filament thereof or depositing a sample on a filament and mounting the filament between the backing plate and the first grid of the spectrometer. No electron beam is required, ions -being produced by continuous electrical heating of the filament. A slight continuous bias is applied to the filament. The remaining steps therefor are the same as set forth for a gas.

Further understanding of the present invention will best be obtained from consideration of the accompanying drawings wherein:

FIG. l is a schematic diagram illustrating the practice 3,296,434 Patented Jan. 3, 1967 ICC Vof the present invention with a modified ion source for a time of flight mass spectrometer.

FIG. 6 illustrates the practice of the present invention with a surface ionization source for the apparatus of FIG. 5.

In FIG. l the backing plate 10 and two grids 12 and 14 are shown schematically for a conventional ion source of a time of flight mass spectrometer. The gas to be ionized and analyzed is fed via an inlet 16 to the space between backing plate 10 and first grid 12. An electron source 18 generates an electron beam which is directed through the injected gas to cause ionization thereof. In FIG. l, the electron beam is shown so that its direction is into the paper.

For the practice of `the present invention the electron ,beam is operated continuously, thereby producing positive ions with a duty cycle approaching It has been found that the electron ybeam operates to confine the positive ions so formed within its boundaries. It is believed that the electron beam creates a potential well wherein the formed positive ions are confined.

A pulse generator 20 periodically generates a negative potential pulse which is applied to -the first grid 12. With lthe backing plate ungrounded, this negative potential pulse destroys the potential well by dellecting the electron beam. The negative potential of the applied pulse to grid 12 also acts to draw out the positive ions formed by the electron beam. To prevent unfocused ions from being drawn out into the flight tube of the spectrometer lthe pulse applied to grid 12 is integrated by resistor 22 and capacitor 23 applied to the backing plate 10. Since the effect on the posiltive ions of a voltage applied to the backing plate 10 is opposite to that of a voltage of the same polarity applied to the first grid 12, the effect of the pulse on the backing plate 10 is an algebraic subtraction from the pulse applied Ito `the first grid 12. T'he wave shapes of the negative potential pulse applied to grid 12 and the integrated pulse derived therefrom and applied to the backing plate 10 are illustrated in FIGS. 2 and 3, respectively. The algebraic resultant of these two pulses is shown in FIG. 4

As may be readily seen from FIG. 2, the negative potential pulse applied to first grid 12 has a long tail 26 thereon. The integration circuit 22 is adjusted so that the waveform of the pulse applied to the backing plate 10 is such that the tail of the pulse applied to the first grid is subtracted and `the voltage gradient on the positive ions is actually reversed. Thus, the integrated pulse applied to the backing plate 10 provides a compensating ion attraction effect for the time duration of the tail end of the negative potential pulse applied to the first grid 12. This compensating ion attraction effect vastly reduces the noise and hence improves the resolution and sensitivity of the machine by bunching the formed positive ions. A high negative bias potential is maintained on the second grid 14 of the ion source. This potential accelerates the positive ions to a constant energy before they enter the drift tube of the spectrometer.

The above described method illustrates the basic method of the present invention as applied to a conventional two grid ion source. FIG. y5 illustrates a modified ion source whereto the preferred method for the present invention V3 may be applied. In the conventional ion source for continuous operation a three grid ion source is used (D. B. Harrington, Encyclopedia of Spectroscopy, Reinhold Publishing Corp., New York 1960', pp. 628-647).

The embodiment of FIG. 5 adds a fourth grid to the conventional structure. Thus, the embodiment of FIG. 5 comprises a backing plate 28, and four grids 30, 32, 34 and 36. The backing plate 28 is grounded through a variable resistor 38. The first grid 30 is biased slightly positive via D.C. voltage supply 40, potentiometer 42 and resistor 43. The second grid 32 is similarly positively biased from a D.C. voltage supply 44 visa potentiometer 46 and resistor 47. A negative bias is applied to third grid 34 from D.C. supply 48 via potentiometer 50 and resistor 51.

In operation, .a continuous electron beam is collimated and `directed into a gas to be analyzed which is injected between the backing plate 28 and the first grid 30. The electron beam, as hereinbefore described, acts to ionize the gas and confine positive ions resulting therefrom in a potential Well formed by the electron beam. The ions are drawn out from the well by the application of a negative pulse to the first grid. This negative pulse is shown in detail in FIG. 2. A variable resistor 52 and capacitor 54 connected between the first grid and ground reduce the tail of the negative pulse by providing a low impedance return to ground. Variable resistor 56 and capacitor 58 integrate the negative pulse and apply it to the backing plate 30 as hereinbefore described for FIG. 1. The backing plate pulse so derived subtracts the tail of the pulse applied to the first grid 30 and reverses the potential gradient on the positive ions. This prevents the drift of positive ions past the first grid 30 after the main bunch thereof has passed. The variable resistor 38 connected between backing plate 28 and ground provides a D.C. return thereby preventing the buildup of a charge thereon. Resistor 38, to some degree, also shapes the pulse applied to the backing plate 28. The positive bias on rst grid 30 provides a focusing action for the spectrometer through its effect on the location of the electron beam and the ions produced therefrom.

The negative pulse applied to first grid 30 is also applied via a coupling capacitor 59 to second grid 32. This pulse combined with the positive bias applied to grid 32 operates to make the grid 32 an effective filter for stray ions that drift past first filter 30.

The third grid 34 as previously stated is continuously negatively biased. This negative bias on the third grid 34 acts to minimize current leakage between grids and also to impr-ove sensitivity. Similarly, the fourth grid is more negatively biased than the third grid, thereby providing the final acceleration of the positive ions into the flight tube of the spectrometer.

Using a model 12 spectrometer havin-g a 100 cm. flight tube manufactured by Bendix Aviation Corporation with a modified ion source as shown in FIG. 5, improved sensitivity and resolution resulted with no increase in noise using the following valued components.

Variable resistor 38 100K As described above, the potentiometers and resistors are adjusted so that the biasing and pulse shaping effects thereof achieve maximum sensitivity and resolution with minimum noise. It is to be noted that the conventional spectrometer such as the model 12, identified above, has adjustable permanent magnets mounted on the ion source for collimating the electron beam. It has been found for the purposes of the present invention that the adjustment of these magnets is also critical to the sensitivity and resolution of the machine. They should be adjusted to collimate the beam from which maximum ionization is achieved within a minimum area. The model 12 spectrometer was operated satisfactorily at a pulse repetition rate of 10 kc. with 100 ,usec. between pulses applied to the first grid 30. The amplitude and duration of the applied pulses are shown to scale in FIGS. 2 and 3.

It is to -be understood that though the foregoing method is described as applicable to the analysis of a gas injected into the area between the backing plate and the first grid it is not to be so limited thereto. The method is equally applicable to the analysis of a solid.

Where the test sample is in the form of a solid, the present invention may be applied thereto by the use of surface ionization. FIG. 6 is a schematic of the backing plate 28 and first grid 30 of the apparatus of FIG. 5 showing the mounting of a solid test sample 62 therebetween. When using a solid as a test sample the solid should be deposited on or made into a filament 64 as shown in FIG. 6. This filament is spatially mounted between grid 30 and backing plate 28. A second filament 66 is spatially mounted adjacent filament and directly above it as shown. A.C. supplies 68 and 70 are connected across filaments 64 and 66 via filament transformers 72 and 74 as shown. A positive D.C. bias is applied to each of the filaments 64 and 66 from D.C. supplies 76 and 78 which are connected to the center taps and 82 of the output windings 84 and` 86 of transformers 72 and 74.

In operation, the A.-C. supply 68 is adjusted to provide a continuous heating current through filament 64 such that molecules of the solid 62 under test are boiled therefrom. A.C. supply 70 is adjusted so that it provides a continuous heating current to filament 66 to cause the filament to be heated to a high temperature. Molecules boiled from solid 62 strike the hot (up to 2500 C.) filament 66 which causes ionization of such striking molecules.

A slight (up to 10 volts) continuous positive D.C. bias is applied to the filament 66 and a slight (up to 10 volts) continuous negative D.C. bias is applied to the filament 64. It has been found that these biases coupled with the bias on first grid 30 act to f-ocus ions formed by the surface ionization. It is to be noted that the polarity and val-ue of the biases applied to filaments 64 and 66 is dependent upon the operating temperatures of the filaments 64 and 66. Thus, if the filament 64 becomes overheated, thereby emitting electrons which ionize gases in the machine, the negative potential thereof will have to be reduced and possibly be reversed to a positive bias. Further, as filament 66 increases in temperature, the increase in temperature gives added velocity to ions formed therefrom which in turn causes defocusing. To correct for this, the positive potential on the filament 66 is therefore reduced. The remainder of the steps are as set forth for the method previously described for the apparatus in FIG. 5. It is to be noted that the utilization 0f the electron beam is negated, surface ionization forming a replacement therefor. The electron beam, however, may be utilized in conjunction with the surface ionization whereby molecules unionized by the heated filament 66 may be analyzed since the presence of the filaments 64 and 66 does not interfere with the electron gun mode of operation.

Filament arrangements may be used other than the one shown in FIG. 6. Successful operation has been obtained by the filaments being mounted at right Iangles instead of parallel as shown. Further, a single filament has been used instead of a double filament as shown. The position of the filaments between the backing plate 28 and first grid 30 is not critical. The filaments may be .positioned anywhere between backing plate 28 and first grid '30, but it is preferred that they be excluded lfrom the area therebetween defined by the aperture of the first grid 30.

The spacing -of the grids for any of the above methods is the same as in the conventional machine (MW apart). Thus, in the modified embodiment of FIGS. 5 and 6, the extra fourth grid 36 is similar to third grid 34 with the usual spacing of 1A therebetween.

When surface ionization is used (a solid test sample) the ionization efficiency may be higher than with the electron Ibeam continuous mode. However, the noise level thereof is higher than in the conventional pulsed or present continuous electron beam mode. The resolution for the surface ionization method is at least as good as for the conventional pulsed electron beam mode but lower than the presently described continuous electron beam mode. The preferred method for best sensitivity and good resolution for the surface ionization uses the filaments in the manner shown in FIG. 6 wherein they are parallel. This arrangement gives a better geometry for molecules hitting the hot filament 66 than where the filaments are at right angles 'with respect to each other.

Persons skilled in the art will, of course, readily adapt the teachings of the invention to methods far different than the methods described. Accordingly, the scope of the protection afforded the invention should not be limited t-o the methods illustrated and described above but shall be determined only in accordance with the appended claims.

What is claimed is:

1. A method of operating a time of flight mass spectrometer ion source having a backing plate and first and second grids comprising the steps generating positive ions between said backing plate and said first grid, periodically generating a negative potential pulse, applying each of said negative potential pulses to said first grid, integrating each of said negative potential pulses while applying said pulses to said first grid, applying each of said integrated pulses to said backing plate, and continuously `applying -a negative potential to said second grid.

2. The method according to claim 1 wherein said positive ion generation comprises injecting a gas between said hacking plate and said first grid, and passing a continuous electron beam between said backing lplate and said first grid to ionize said gas.

3. The method according -to claim 1 wherein said positive ion generation comprises forming the test sample into a filament, spatially mounting said test filament between said backing plate and said first grid, spatially mounting a second filament adjacent said test filament, passing a current through said test filament to cause heating thereof and the removal of molecules of said test sample therefrom, passing a current through said second filament to cause heating thereof and the ionization of molecules of said test sample Vremoved from said test filament, and applying bias potentials to said test and second filaments such that said ionized molecules of said test sample are repelled from said second filament and attracted toward said test filament.

4. A method of operating a time of flight mass spectrometer ion source having a backing plate and first and second grids comprising the steps continuously generating positive ions between said backing plate and said first grid, periodically generating a negative potential pulse having a short rise time and long decay time, applying each of said negative pulses -to said first grid to attract generated ions thereto, integrating each of said negative potential pulses while applying said pulses to said first grid, applying each of said integrated pulses to said backing plate to create a compensating ion attraction effect for the decay time duration of each of said applied negative potential pulses, and applying a continuous negative potential to said second grid to accelerate ions periodically attracted to said first grid.

5. A method of operating a time of flight mass spectrometer ion source having a backing plate and first, second, third and fourth grids comprising -the steps generating positive ions between said backing plate and said first grid, periodically generating a negative potential pulse, applying each of said negative pulses to said first grid, integrating each of said negative potential pulses while applying said pulses to said first grid, applying each of said integrated pulses to said backing plate, continuously applying positive potentials to said first and second grids, and continuously applying negative potentials to said third and fourth grids.

6. The method according to claim 5 Iwherein said negative pulse has an amplitude of approximately minus 260 volts.

7. The method according lto claim `6 wherein the negative potentials applied to said third and fourth grids have approximate values of minus 1000 volts and minus 2.8 kilovolts respectively.

8. The method according to claim 7 wherein said positive ion generation comprises injecting a gas between said backing plate and said first grid, generating an electron beam, and collimating said electron Ibeam to pass between said backing plate and said first grid to ionize said gas.

9. The method according to claim 7 wherein said positive ion generation comprises forming the test sample into a filament, spatially mounting said test filament between said backing plate and said first grid, spatially mounting a second filament adjacent said test filament, passing a current through said test filament to cause heating thereof and the removal of molecules of said test sample therefrom, passing a current through said second filament to cause heating thereof and the ionization of molecules of said test sample removed from said test filament, and applying bias potentials to said test and second filaments such that said ionized molecules of -said test sample are repelled from said second filament and attracted toward said test filament.

10. The method according to claim 9 wherein the values of said bias potentials applied to said test and second filaments do not exceed approximately minus ten volts and approximately plus ten v-olts respectively.

11. A method of operating a Itime of flight mass spectrometer ion source having a backing plate and first, second, third and fourth grids comprising the steps continuously generating positive ions between said backing plate and said first grid, periodically generating a nega-tive potential pulse having a short n'se time and long decay time, applying each of said negative pulses to said first grid to attract generated ions thereto, integrating each of said negative potential pulses while applying said pulses to said first grid, applying each of said integrated pulses to said backing plate to create a compensating ion attraction effect for the decay time duration of each of said applied negative potential pulses, continuously applying positive potentials to said first and second grids, continuously applying a negative potential to said third grid, and continuously applying a potential to said fourth grid having a value more negative than that applied to said third grid.

References Cited by the Examiner UNITED STATES PATENTS RALPH G. NILSON, Primary Examiner.

W. F. LINDQUIST, Assistant Examiner.

Claims (1)

1. 4. A METHOD OF OPERATING A TIME OF FLIGHT MASS SPECTROMETER ION SOURCE HAVING A BACKING PLATE AND FIRST AND SECOND GRIDS COMPRISING THE STEPS CONTINUOUSLY GENERATING POSITIVE IONS BETWEEN SAID BACKING PLATE AND SAID FIRST GRID, PERIODICALLY GENERATING A NEGATIVE POTENTIAL PULSE HAVING A SHORT RISE TIME AND LONG DECAY TIME, APPLYING EACH OF SAID NEGATIVE PULSES TO SAID FIRST GRID TO ATTRACT GENERATED IONS THERETO, INTEGRATING EACH OF SAID NEGATIVE POTENTIAL PULSES WHILE APPLYING SAID PULSES TO SAID FIRST GRID, APPLYING EACH OF SAID INTEGRATED PULSES TO SAID BACKING PLATE TO CREATE A COMPENSATING ION ATTRACTION EFFECT FOR THE DECAY TIME DURATION OF EACH OF SAID APPLIED NEGATIVE POTENTIAL PULSE, AND APPLYING A CONTINUOUS NEGATIVE POTENTIAL TO SAID SECOND GRID TO ACCELERATE IONS PERIODICALLY ATTRACTED TO SAID FIRST GRID.

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