US3405263A - Dual mass spectrometer ion source comprising field ionization and electron bombardment sources and the method of use - Google Patents

Description

8, 1968 G. 5. WANLESS ETAL 3,405,263

DUAL MASS SPECTROMETER ION SOURCE COMPRISING FIELD IONIZATION AND ELECTRON BOMBARDMENT SQURCES AND THE METHOD OF USE Filed Jan. 14, 1966 2 Sheets-Sheet 1 Tic-+1- INVENWS emu/112 A/VLHJ' 0x06 awe/sad).

ATTORNEY EMA.

3,405,263 DUAL MASS SPECTROMETER ION SOURCE COMPRISING FIELD Oct. 8, 1968 G. G. WANLESS ETAL IONIZATION AND ELECTRON BOMBARDMENT SOURCES AND THE METHOD OF USE 2 Sheets-Sheet 2 Filed Jan. 14, 1966 INVENTORS ATTORNEY United States Patent "ice ABSTRACT OF THE DISCLOSURE The instant invention involves the novel structural combination of a field ionization source with a conventional electron bombardment type of ion source for use in a mass spectrometer. The combination of the instant invention facilitates the switching from one type of electron analysis of a sample under test to the other type of analysis of the sample with minimum changes being required. The specific construction is such that the separate ion sources share several common components. The field ionization source produces mainly parent ions and electron bombardment source produces more fragmented ions, thus allowing more versatility in the analysis of complex samples.

The present invention relates to a dual mass spectrometer and a method of analysis. It pertains particularly to a dual mass spectrometer instrument designed to produce both field ionization spectra and electron-bombardment spectra. It pertains also to a method by which dual and supplementary analytic data may be obtained.

The mass spectrometer is an important instrument in modern analytical laboratories. It is widely used not only for analysis but also for plant control; for example, in connection with petroleum refinery streams and in chemical plants. As is well known, the mass spectrometer operates to separate ionized molecules, etc. of different masses, thereby to obtain certain analyses or characteristics of materials by which the ingredients of mixtures may be identified.

The most common type of mass spectra are those produced by an electron-bombardment type of ion source. Spectra of this type are usually characterized by a predominant share of fragmentary ion peaks. In most cases there is produced some share of parent-ion peaks. Such spectra, however, are exceedingly useful in interpreting the molecular structure of simple mixtures of gases and vapors. On the other hand, in analysis of samples which contain a large number of components, including materials having widely dilferent molecular structures, the spectra obtained by electron bombardment become exceedingly complex. Analyses then become confused and difficult to interpret.

Obviously it is highly desirable to have a simple and practical means for acquiring mass spectra of a type that can be clearly interpreted, especially in analyzing sample materials having a number of components. Mass spectra which can be interpreted more clearly are badly needed when complex samples are being studied. Those spectra which are obtained from parent ions, i.e. ions produced by removal of only a single electron from the molecule, are generally much simpler and more easily studied than those obtained when molecules are broken up into fragments. Hence, it is also very desirable in many cases to have spectra which consist chiefly of parent-ion peaks. Usually, pure parent ions cannot be obtained to complete exclusion of fragments, but records containing a maximum of parent ions, together with a minimum amount of fragment-ion spectra are very use- Patented Oct. 8, 1968 ful when obtainable. One object of the present invention is to make it possible to obtain such.

For complete and accurate analytical studies it is also desirable, in many cases, to be able to determine relative proportions of the parent ions and the fragmentary ions. Some recent techniques have made it possible to obtain more of the parent type ions mentioned above than formerly, but it is often desirable to be able to treat a sample in such a way as to get a desired or controllable balance of the two. To accomplish this is a further object of the invention.

According to the present invention, a method and apparatus have been devised for obtaining selectively a combination of parent-ion peaks, together with a more controllable amount of fragment-ion spectra.

The invention involves adapting a field-ion source, such for example as that used in the Mueller field-ion microscope, an instrument of known type, and combining such source into the mass spectrometer to serve as a prime means for getting parent ions. The Mueller field-ion microscope source or equivalent instrument may be incorporated directly into a mass spectrometer of more or less conventional type for this purpose. Such an adaptation is capable of creating positive ions by abstracting electrons from previously uncharged molecules. According to the present invention, this may be done, for example, when a vaporized sample of material to be tested is brought into the neighborhood of a very intense electrostatic field. Single electrons are abstracted from molecules and the resulting ions in the field will then yield mass spectra which consist almost entirely of the desired parent-ions. At the same time, this process of ionization is more gentle and less destructive to the sample than that usually carried out by the more conventional electron-bombardment process.

There are, of course, some important differences between the two types of spectra obtained from field ionization and electron bombardment, respectively, as will be pointed out in further detail. In some cases kinds of spectra both may be desirable.

It is obvious to those skilled in the art that the two types of mass spectra will complement each other in a very practical way. By their use, one may greatly facilitate the analysis of spectra of unknown mixtures of compounds. In the past it has usually been necessary to have available two entirely different types of mass spectrometers in order to achieve comparable results. Thus, another object of the present invention is to make possible the production of either type, or even both types, of mass spectra from a single sample in a single analytical instrument. As far as applicants are aware, this has not been possible in systems of the prior art.

While two different and separate types of ion sources may be combined in various ways, and some of these are described hereinbelow, it is possible to couple such sources either loosely or closely together so as to obtain controlled ionization with a selective balance between the two main types of ions mentioned above. Close coupling in an eflicient manner, to obtain a compact instrument, is a further object of the invention.

The essential components of certain preferred embodiments of the invention will be shown and described in connection with specific examples of the new instrument. For this purpose, reference will next be made to the drawings which accompany this specification and form a part thereof.

Referring to the drawings, FIGURE 1 shows a combination comprising a more or less conventional mass spectrometer which has been modified to include a fieldionization source as well as an electron-bombardment source.

FIGURE 2 represents a modification generally similar to FIGURE 1, wherein the two separate ion sources are more closely coupled to make a more versatile. and more convenient instrument.

FIGURE 3 represents a further improvement in a combination of multiple and different ion sources wherein the ion sources are brought together still more closely than in FIGURE 2.

Referring first to FIGURE 1, there is shown at the left an instrument structure in which is incorporated a field ionization source 20 of the electrostatic type. This ionization source comprises a probe or electrode 11 to which a high electrostatic voltage may be applied, the electrode having a tip 13 of relatively very small radius. This small radius greatly intensifies the potential gradient when an activating voltage is applied to the electrode. Such a device, as previously suggested, is well known per se, being a basic element in the ion microscope of the type developed by Mueller. In a typical case, the probe or electrode tip may have a radius of the order of about 500 to 1200 Angstroms. When a voltage potential of 10,000 to 15,000 volts is applied to the electrode 11, from a source not shown but well known in the art, a much higher voltage gradient than normal is imposed around the tip 13. This tip is spaced from a perforate plate member 15 by a distance of the order of only one or two millimeters, the voltage gradient being a maximum between the tip 13 and plate member 15. Successively lower positive voltage potentials are applied to the succeeding accelerator plate or gate elements 17 and 19 of the gun. As the electrode tip 13 extracts an electron from a molecule, the latter becomes a positively charged ion which is then strongly repelled towards plate 15. As the ions are formed from molecules of the sample coming in through the outlet line 21 of a sample tube 23, they are passed through openings 25, 27, 29 of conventional accelerator and guide elements arranged in succession, causing the ions to move to the right at progressively accelerated velocity. Ordinarily, the instrument will probably be arranged for the ions to flow vertically downward.

In lieu of the small radius-tipped electrode 11 a finewire type electrode, of form for example as suggested by Professor Robertson of the University of London, may be substituted. This may be a fine platinum wire of the order of about 2 /2 microns or so in diameter, which replaces the tip 13. It will be understood that other suitable electrodes may be used, as long as they are suitable to abstract single electrons from previously uncharged molecules in the sample.

The probe 11, in any case, bears a high charge potential. When a molecule is converted to an ion, the ion is repelled from the probe through the openings 25, 27, etc., coming under the influence of the previously mentioned accelerating field produced around the accelerator plate elements 17 and 19. It may also require some guidance in direction. Electrostatically charged members 29 of conventional type operate to guide the movement of the ions along the axis of the instrument. These ions then pass through a narrow gap or slit 31 in a plate 33 and into a tubular walled chamber 35. They may go on to the detector, not shown, in the conventional manner. The detector is of conventional type and need not be described. It is assumed for the moment that apparatus elements farther to the right, FIGURE 1, have no effect on the ion stream.

Sample gas, which may include vapors, is supplied from a source, not shown, to the inlet 41 of the tube 23. The glass tube 23 is attached to the metal flange at 24 by means of a Kovar seal. The inner half of the flange 26 is similarly attached at point 23A by a ceramic-to-metal seal to the ceramic insulator 10.

The sample emerges from the tube outlet 21 into the highly evacuated chamber 45. A vacuum pump applied to an outlet 47, for example, keeps the pressure inside the instrument at the necessary low value. The well known tunnelling reaction at the tip 13 of the electrode 11 strips 4', 1 1 a off single electrons from moleculespresent to convert them to ions. The various accelerator elements of the gun serve in conventional manner to impel the ions at the desired velocity into the main mass analyzer tube. The latter is indicated at at the right of FIGURE 1.

That part of the instrument to the right of plate 33 is maintained at high vacuum by application of a suitable evacuating pump, not shown, connected to another suction line52."

A mounting plate of stainless steel material is attached tothe metallic frame 56 of the instrument. In the apparatus ,or. structure shown in the right hand part of FIGURE 1, a-standard electron bombardment ionizer is provided, as indicated generally at 60. This device operates in the usual and well known manner. Sample gas or vapor which may be brought in through an inlet 61, if desired; is ionized by electronic collision. Electrons are produced by a hot filament and pass across the chamber 70, striking molecules in their path. In some cases they produce parent ions by taking off a single electron. In other cases they break up some of the molecules into smaller or fragmented ions.

In using the device to produce mainly parent ions, the bombardment unit 60 atthe right of FIGURE 1 will not be operated. The field ionization source 20 is operated and ions produced therein flow straight through the inactive structure 60. Both devices, however, can be used, if desired, to first ionize and then break up molecules from the same sample. As a rule, they will be operated independently and alternatively.

If unit 20 is operated, and unit 60 is inactivated, the instrument operates precisely as though unit 60 were not present. Likewise, if unit 60 is operated and unit 20 is inactivated, the instrument behaves as a conventional bombardment type mass spectrometer.

When it is desired to obtain mass spectra of both types, the field ionization source 20 will be operated first, e.g. to obtain a visual pattern, based mainly on parent or nonfragmented ions, on the detector output. Then source 20 will be shut off momentarily and the bombardment source 60 will be operated to obtain a separate visual indication or pattern from the same sample gas. The latter pattern will indicate fragmented as well as parent ions. The procedure may be repeated or varied as many times and in any desired sequence to obtain the various characteristics needed for complete analysis of a complex mixture.

The embodiment just described in connection with FIGURE 1 is essentially, therefore, an effective combination of a Mueller type source with a more or less conventional electron-bombardment type mass spectrometer. The detector or analyzer elements per se are not shown, being of conventional type, as will be well understood by those skilled in the art. It will be understood that it is not always necessary to have separate sample inlets 41 and 61.

Suitable power sources are connected, for example, to electrodes 75, contained in suitable insulators 77, at the left of FIGURE 1, and to other electrodes 79 associated, for example, with the field ionization source 20 and the bombardment source 60.

The arrangement described above is quite satisfactory for many purposes. It has the advantage that it can be readily built by a relatively simple addition, with minor modifications, of the standard to a conventional mass spectrometer of suitable type.

FIGURE 2 shows an arrangement which is essentially the same in principle as that of FIGURE 1 but which constitutes in some respects a more compact and more convenient instrument for many purposes. In this case the electrostatic or ion microscope type ion source and the bombardment source are 'built together into the single mass spectrometer. This arrangement involves reconstruction or redesign but it has advantages of simplicity and convenience. I

The parts not shown to the left of FIGURE 2 are essenti-ally the same as in FIGURE 1. The electrode 111 and the sample inlet 121 are similar, as is the perforate gate member 115, and the openings 125, 127. The aci celerator plates 117 and 119 and the electrostatic guide elements 129 correspond respectively to elements 17, 19 and 29 of FIGURE 1, as previously described.

All of the parts just mentioned are mounted by insulator rods 133, 135, of glass, ceramic, or equivalent, which are secured at their right end to a plate 155. The latter is attached on its right side to other ceramic rod insulator members 150. The latter hold together the conventional bombardment, accelerator and guide elements 180, 181, 183, 185, and 187.

As in the previous example, a sample may be introduced through inlet -121 around the highly positively charged electrode 111. As the latter extracts electrons, the positively charged ions thus produced are repelled into the slit 125 in plate 115. They are further accelerated by the elements 117, 119 and are guided to the next slit by the members 129' which are appropriately charged.

The instrument is maintained under high vacuum by a suitable vacuum pump attached to the outlet 147. The member 155 has slit 161 through which ions from the left can pass into and on through the second zone of the mass spectrometer.

Alternatively, sample material can be bombarded to ionize it in the second zone. A heated filament is mounted within the space 180 so that molecules coming either from the left or from sample material introduced through tube 170 may be bombarded with electrons. Following this, the ions so produced are accelerated through the slits in plates 181, 183, 185, 187 and through an adjustable pair of slits 190 mounted in plates 192 and on into the main outlet tube 200. From here the ions are passed in a curved path and through the conventional magnetic field so that their pattern can be recorded in a conventional manner.

Means 201 are provided for adjusting the slits 192, etc., laterally, the adjusting means 202 being connected through a vacuum seal of bellows type 203. If desired, that part of the instrument to the right of the plate 155 may be kept under vacuum by means not shown, the arrangement preferably being such that the pressure in both compartments is essentially the same.

In FIGURE 2 the accelerator plates 181, 183, 185, and 187 are supported by the rods 150 which are supported at the left in plate 155, as previously mentioned, and are supported at the right in another heavy plate 210 fitted and sealed between facing plates 211 and 212. The adjustment device 201 is slidably mounted in the plates 201A and 201 B, the latter being relatively fixed.

Referring now to FIGURE 3, the general arrangement is quite similar to that of FIGURE 2 and will not be repeated in detail, except to state that the parts may be more closely coupled than in FIGURE 2. This is accomplished by attaching the two ion sources to either side of the heavy base plate 255. The latter corresponds with plate 155 of FIGURE 2. Adjustment mechanism 301 for the slit controls 292 extends slidably through the plates 250A and 250B, manipulating means 302 being mounted outside the outer wall 300 of the apparatus, similar to the manner in which parts 201 and 200 of FIGURE 2 are mounted. A bellows means to retain the vacuum seal is indicated at 303, being equivalent to the element 203 of FIGURE 2. Electrodes 310 and 311 are introduced through glass or ceramic seals, as is the case in the previously described modifications.

It will be understood that the various electrodes, filaments, accelerator plates, electrostatic members, etc. are all appropriately wired to suitable sources of voltage and power. Since such elements are conventional and well known they are omitted in the interest of simplicity of detail.

Also, while dual sample inlets are shown and dual vacuum pump connections in the case of FIGURE 1, it

will be understood that single inlets and/or single vacuum connections will usually suflice. Furthermore, although the apparatus has, for convenience, been described as if the ions moved from left to right along a horizontal path (as shown in the drawings) it will usually be more convenient to .arrange the instrument in a more or less vertical position so that the ions flow downwardly. For some installations, however, the instrument may be oriented as desired so that its axis may be at any desired angle.

Obviously, the particular arrangement and number of parts is not critical. A greater or smaller number of accelerator and/or beam guiding elements may be used, as is well known to those skilled in the art. Moreover, other materials may be used for the insulators, packings, glands, etc., as will be obvious to those skilled in the art.

It is intended by the claims which follow, to claim the invention as broadly as the prior art properly permits.

What is claimed is:

1. In a mass spectrometer of the type having an evacuated housing, an ion detector mounted in said housing, and a single analyzer for resolving ions according to their mass the improvement which comprises a dual ionization source, said source having both a field ionization source for producing an ionization field of at least 10,000 volts and an electron bombardment source, said dual source producing ions in an elastic fluid confined at very low pressure.

2. A mass spectrometer according to claim 1 wherein the field ionization source comprises an electrode of very small radius, a plate member and means for imposing a high electrostatic potential between said radius and said plate member.

3. A mass spectrometer according to claim 1 wherein the field source .and the bombardment source are axially aligned and closely coupled in the entrance portion of the analyzer.

4. A mass spectrometer according to claim 1 wherein the field source produces predominantly parent ions.

5. A mass spectrometer according to claim 1 wherein both sources are mounted on a single transverse mounting plate perforated at its center.

6. The method of analysis of an elastic fluid such as gas or vapor at very low pressure, which comprises passing a sample of said fluid into an ionization zone, momentarily applying a field ionization force at high electrostatic voltage to extract single electrons from molecules of said fluid to produce predominantly parent ions, then momentarily bombarding said fluid with electrons to produce a mixture of ions containing fragments of the fluid molecules, and analyzing said mixture.

7. Method according to claim 6 wherein the procedure is repeated several times to obtain alternative analyses of said parent ions and said mixture of ions.

8. Method according to claim 6 wherein the application of said field ionization force and the bombarding of said fluid with electrons is done simultaneously.

9. In .a mass spectrometer of the type having an evacuated housing, an ion detector mounted in said housing, and a single analyzer for analyzing a vaporized and ionized sample, the improvement which comprises a dual ionization source having in combination:

(a) a plate transversely mounted within said housing,

said plate being perforated at its center;

(b) a field ionization source having an electrode of very small radius, a plate member and means for imposing a high electrostatic potential between said radius and said plate member, said field ionization source producing predominantly parent ions and be ing mounted on one side of said perforated plate;

(0) an electron bombardment source, axially aligned 3,405,263 7 8 with said field ionization source and being mounted References Cited on the other side of said perforated plate, said UNITED STATES PATENTS bombardment source primarily producing fragmerited ions; and 2,809,314 10/1957 Herb 31363 (d) means for introducing vaporized test samples into 5 3,115,591 12/1963 Bnfnnee 313-231 the field produced by said field ionization source 5,274,436 9/1966 Relch 313 63X and in the region of sald bombardment source. WILLIAM F. LINDQUIST, Pfimw'y Examiner.

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