US3478204A - Mass spectrometer ion source having a laser to cause autoionization of gas - Google Patents

Description

Nov. 11. 1969 w. M. BRUBAKER ETA!- 3,478,204

MASS SPECTROMETER ION SOURCE HAVING A LASER To CAUSE AUTOIONIZATION OF GAS Filed Aug. 24, 1964 2 Sheets-Sheet 1 Nov. 11, 1969 w. M. BRUBAKER ETAL 3,473,204

MASS SPECTROMETER ION SOURCE HAVING A LASER T0 CAUSE AUTOIONIZATION OF GAS Filed Aug. 24, 1964 2 Sheets-Sheet 2 .Vfi JM "P '9? 7 X l I 5i:

INVENTORE W/LMA/ M EPZ/EAKEA 000 020 5:709): armzria United States Patent US. Cl. 25041.9 6 Claims ABSTRACT OF THE DISCLOSURE A mass spectrometer having as a gas ion source a laser having a field strength intensity sufficient to cause autoionization of the gas.

This invention relates to ionizing gas atoms or molecules with a strong beam of light from a laser.

There are a number of ion sources used in mass spectrometers, each having features which are characteristic of a particular manner of acquiring ions. The ideal source should have a high efliciency of ionization to obtain a maximum number of ions from a small sample, and the source should be capable of delivering a large ion current when larger amounts of sample are available so as to make the detection in measurement of the beam as simple as possible.

The available ion sources include the elctron-bombardment source which is generally used for organic chemical analytical analysis and is typically a molecular beam of gaseous vapor directed into an ionization chamber where it is intercepted by a beam of electrons. Surface ionization, sometimes called thermoemission, depends for its operation on a substance that is evaporated from a heated surface; there is a probability that the substance will evaporate as positive ions and approximately 54 stable elements can be examined in this manner. Vacuum spark sources consist of mechanically breaking and making a circuit between metallic electrodes in a high vacuum. The field emission sources are based on the fact that substances which can be absorbed on the surface can be desorbed as positive ions under the action of sufficiently strong electric fields which are in the nature of Volts per centimeter on a sharply pointed electrode. In photo-ionization, a beam of ultraviolet ray light of sufliciently short wave length is absorbed by an atom molecule raising it to a state from which an electron may be ejected. Other ionization sources are available for individual materials and for special types of examination.

In the spectrometric study of some compounds which are non-conductive and also those compounds which are highly unstable, the electron discharge types of ionization sources are not acceptable and to date only the photoionization source could be used to advantage. With conventional electron-bombardment sources and the like, even if none of the ions recorded come from molecules which have reached the filament and decomposed there, the walls of the ion chamber are themselves above 200 C. above room temperature. Therefore, ion sources, which do not decompose or condense from gaseous samples, have been limited to date to the photo-ionization variety.

It is well known that if a molecule or atom is placed in a sufficiently strong electric field, a process of autoionization occurs. The applied field reduces the potential barrier seen by an electron in the molecule or atom, allowing tunneling of the electron to take place and thereby resulting in the production of an ion. Fields in the order of 10 volts per centimeter are required to produce autoionization.

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In accordance with the present invention, we have found that atoms and molecules are ionized by the extremely high electric field produced in a localized region by the use of a focused laser beam.

Lasers have been constructed which produce a peak power in the order of 109 watts in a beam with a cross section of the order of one square centimeter. A short focal length lens can focus this beam to a spot in the order of 0.001 centimeter diameter. Thus, in this spot the energy density is in the order of 10 watts per square centimeter giving an electric field strength of the order of 10 volts per centimeter, which is more than adequate to produce ionization in most atoms or molecules.

One feature of the present invention includes the autoionization of molecules or atoms in which energy density is the important factor as distinguished from photoionization in which the wave length of light used is important.

Accordingly, the invention comprises an ion source which includes a laser directed upon an optical system for focusing the laser light beam at an ionization zone. Such a source is ideally suited for use with a mass spectrometer.

More particularly, this invention includes an optical means which collects light from the laser and focuses it at the ionization region. In a preferred form, the light passed through the ionization zone is reflected and focused back at the zone to improve the efliciency of the source. The optical means includes a light collecting lens disposed between the laser and the ionization zone, with a reflector disposed on the side of the zone opposite the lens. The lens may be a plano-convex lens having its flat portion partially silvered and abutting the laser crystal to perform the dual function of reflecting light back into the crystal and focusing it at the ionization zone. The planoconvex lens may also have a spherical reflector disposed opposite the zone to reflect light back through the zone to improve efiiciency.

The present invention is not necessarily limited to the use of any particular type of laser, or a mass spectrometer.

Other features and advantages of this invention will become apparent from the following detailed description and the accompanying drawings of which:

FIG. 1 is a perspective view of a laser used in the present invention;

FIG. 2 is a schematic view of a laser used in the present invention;

FIG. 3 is schematic illustration of a mass spectrometer having an ion source constructed according to the present invention;

FIG. 4 is a partial cross sectional schematic view of the mass spectrometer ion source illustrated in FIG. 3, the section taken along line 44 of FIG. 3;

FIG. 5 is another embodiment of a laser and an optical means arranged according to the present invention;

FIG. 6 is another embodiment of a laser and an optical means arranged according to the present invention;

FIG. 7 is another embodiment of a laser and an optical means arranged according to the present invention; and

FIG. 8 is yet another embodiment of a laser and an optical means arranged according to the present invention.

Referring now to FIGS. 1 and 2, a laser 10 comprises a ruby crystal 12, machined in the form of a cylindrical rod and having its partially silvered ends 14, 16, polished optically flat and parallel, is disposed within a powerful electronic helical flash tube 18 attached to a power source 20 to provide an intense source of pumping light. The trigger electrode 19 aids the cascade of photons. A glass tube 22, disposed around the ruby crystal 12, has a coolant passing therethrough such as liquid nitrogen to reduce thermal excitation in the ruby crystal 12 and thus narrow the spectral lines and increase the relaxation time. In the usual manner, the pumping light from flash tube 18 is turned on and photons are reflected back and forth parallel to the longitudinal axis of the laser being reflected by the partially silvered ends 14, 16 of the ruby crystal 12 stimulating a cascade of photons. The cascade culminates in a coherent beam of light which flashes through the partially silvered mirror as a pulse with an intensity of millions of watts.

The high output of the laser is accomplished in part by a fast acting conventional shutter (not shown) interposed between one end of the laser crystal 12 and the mirror normally found there. As soon as the shutter is open and light can reach the mirror, laser oscillation builds up almost instantly and most of the excitation energy stored in the rod is delivered in one huge burst of light. This giant pulse can deliver up to 50 megawatts lasting for about nanoseconds. For speed and convenience, an electro-optical means may be used as the shutter such as the conventional Kerr cell (not shown) which rotates a plane of polarization and exploits the natural polarization of the stimulated emission of ruby which is observed when the optic axis of the ruby does not coincide with the cylinder axis. The laser must produce a field strength of an intensity of at least 10 volts per centimeter which is suflicient to cause autoionization of gas atoms and molecules.

The mass spectrometer shown in FIG. 3 comprises an analyzer tube 23 immersed in a magnetic field produced by a conventional magnet pole 24 and an oppositely disposed magnet pole (not shown). An ion source 25 is housed within one end of the analyzer tube 23, and includes a sample inlet conduit 26, a repeller electrode 27, an ionizing light beam focused at P developed by a laser (see FIG. 4), and first and second accelerating electrodes 28, 29. A collector electrode 30 is disposed in the opposite end of the analyzer tube 23. Ions passing through the analyzer tube are focused through a resolving slit S in a barrier electrode 31 on the collector electrode. A metastable suppressor electrode 32 having an ion slit S is disposed intermediate the barrier electrode 31 and the collector electrode 30. An exhaust conduit connects the analyzer tube with an evacuating system (not shown).

The collector electrode 30 is connected to a conventional recorder 34 which includes a suitable amplification system.

A power supply 34A is connected across a voltage divider 35 to which the repeller electrode 27 and accelerating electrodes 28, 29 of the ion source are connected at 36, 37, 38, respectively. The metastable suppressor electrode 32 is connected to the voltage divider 35 at 39, and in the particular embodiment illustrated is maintained at a potential intermediate that of the ion source repeller electrode 27 and the first accelerating electrode 28. To this extent the illustrated mass spectrometer is conventional.

As shown in FIG. 4, a laser 50 has an optical axis X which is substantially parallel to the electrodes 27, 28, and 29. Light rays 50A emitted from the laser 50 pass through a light collecting lens 52 which may have a focal length of approximately one centimeter to focus the beam at point P. A second lens 54 also having a focal length of approximately one centimeter is positioned on the opposite side of point P collecting all light passing from point P collimating it and directing it toward the flat reflector 56. It should be noted that point P is preferably positioned opposite the ion slits in the acceleration electrodes 28, 29 with the laser being focused at a minute spot, such that the gas is ionized into predominately molecular ions with few fragmentation products. In this respect, the ionization caused by the laser gives mass spectra resembling those obtained with field emission sources.

The apparatus of FIGS. 3 and 4 operates as follows: A sample entering the ion chamber 25 through the inlet conduit 26 is ionized under the influence of the light beam focused at P, and the resultant ions are accelerated into the analyzer chamber 23 under the influence of the potentials established between electrodes 27, 28 and 29 of the ion source. As is well known, ions of a particular mass may be focused on the resolving slit S of the barrier electrode 31 by adjustment of the potentials applied to the accelerating electrodes to strike and discharge at the collector electrode 30. The potential on the metastable electrode is such as to prevent metastable ions from gaining access to the collector electrode in a manner familiar in the art.

While the mass spectrometer as a whole is not a part of the present invention, its details are described in full in the US. Patent to Brubaker et al. 2,975,278.

The manner of introducing the gases and volatile liquid samples into the ion source are discussed in detail in the book Mass Spectrometry, by I. H. Beynon, Elsevier Publishing Co., New York (1960), in the chapter starting on page 124.

Referring now to FIG. 5, there is illustrated another embodiment of the present invention wherein a laser 60 directs a light beam 60A through a collecting lens 62 having a very short focal length similar to the lens 52 described in FIG. 4. The light beam passes through the lens 52 and focuses on point P. A spherical reflector 64, having an optical axis collinear with the optical axis X of lens 62, reflects and focuses all light at point P. The advantage of this embodiment is the obvious removal of a second lens from the system and reduction in the loss in the optical system. The spacial relation of the laser and point P to the accelerator electrodes is quite similar to the arrangement illustrated in FIG. 4.

Referring now to FIG. 6, another embodiment of the present invention is disclosed in which the number of optical elements in the system 'is reduced to a minimum. In the previous embodiments described, the laser has one end which has a 100% reflector at one end and a partially silvered face on its opposite end. In this embodiment, a laser 70 has a 100% reflector 71 formed on one end of the crystal. A lano-convex lens 72 abuts the other end of the laser 70 and positioned therebetween is a partially silvered reflector which may be placed for convenience sake, either upon the crystal or the lens 72. This in essence places a spherical end on the crystal such that the light beam 70A is immediately focused upon point P, and reduces the amount of loss in the system to a minimum. Arrangement of the laser 70 and point P are quite similar to that disclosed in the embodiment illustrated in FIG. 4. The system is not ideal in that no provision is made for reflecting any light which may be lost or not directed at point P such that only pulses of light will be directed at point P to ionize gas molecules at that point.

Quite related to the embodiment illustrated in FIG. 6 is the embodiment illustrated in FIG. 7. Here again a laser has a plano-convex lens 82 abutting the laser crystal with a partially silvered reflector formed therebetween. Light rays 80A are focused at point P by the lens and the spacial arrangement of the crystal and point P are similar to that disclosed in the previous embodiment. The reflector 84 has a focal length of one centimeter and focuses light rays 80A from the laser 80 back to point P thus making the embodiment illustrated in FIG. 6 more eflicient. Reflector 84 has an optical axis coaxial with the axis X of laser 80 and lens 82. It is recognized that the laser may be operated either on the pulsed mode or on a continuously pulsed mode, thus the embodiments illustrated in FIGS. 6 and 7 may have special applications in the particular type of laser being used in the ionizing source.

Referring now to FIG. 8, a laser 90 has an optical axis X which is collinear with the axis of a spherical reflector 92 spaced from it. Thus all light rays emitted by the laser are reflected and focused at point P to perform the ionizing function. It is recognized that this systern is not as efiicient since the light rays are not first focused through point P, and then collected and rereflected externally of the laser. However, this embodiment is useful when single pulses of light are used in the ionizing source.

What is claimed is:

1. A mass spectrometer comprising:

a housing,

an analyzer channel in the housing,

a gas ion source disposed at one end of the analyzer channel for injecting ions into the channel,

means for mass separation of injected ions while such ions are within the analyzer channel,

an ion detector disposed at a second end of the analyzer channel, and

means for focusing separated ions on a detector,

the gas ion source including:

(a) an ionization region,

(b) electrodes forming an acceleration field upon application thereto of suitable electrical potentials, at least one of the electrodes having an ion aperture therein communicating with the ionization region,

(c) a laser having a field strength intensity sufficient to cause autoionization of gas focused toward the ionization region of the ion source, and

((1) optical means for causing a beam from the laser to focus in the ionization region, including a pair of lenses having coaxial optical axes and positioned on opposite sides of the ionization region, and means aligned with the lenses optical axes for reflecting light, the lenses being positioned between the light reflecting means and the laser.

2. A mass spectrometer comprising:

a housing,

an analyzer channel in the housing,

a gas ion source disposed at one end of the analyzer channel for injecting ions into the channel,

means for mass separation of injected ions while such ions are within the analyzer channel,

an ion detector disposed at a second end of the analyzer channel, and

means for focusing separated ions on a detector,

the gas ion source including:

(a) an ionization region,

(b) electrodes forming an acceleration field upon application thereto of suitable electrical potentials, at least one of the electrodes having an ion aperture therein communicating with the ionization region,

(c) a laser having a field strength intensity suflicient to cause autoionization of gas focused toward the ionization region of the ion source, and

(d) optical means for causing a beam from the laser to focus in the ionization region, including a pair of light collecting lenses positioned on opposite sides of the ionization region with the focal point of each lens at the same point in the ionization region, and a flat reflector in spaced relation with one of the lenses on the side of the ionization region opposite the laser.

3. A mass spectrometer comprising:

a housing,

an analyzer channel in the housing,

a gas ion source disposed at one end of the analyzer channel for injecting ions into the channel,

means for mass separation of injected ions while such ions are within the analyzer channel,

an ion detector disposed at a second end of the analyzer channel, and

means for focusing separated ions on a detector,

the gas ion source including:

(a) an ionization region,

(b) electrodes forming an acceleration field upon application thereto of suitable electrical potentials, at least one of the electrodes having an ion aperture therein communicating with the ionization region,

(0) a laser having a field strength intensity sufiicient to cause autoionization of gas focused toward the ionization region of the ion source, including a crystal having a reflector at one end thereof, and

(d) optical means for causing a beam from the laser to focus in the ionization region, including a plano-convex lens focused at the ionization region and abutting the laser crystal at an end opposite said one end with the'plano portion of the lens having a partial reflector (as a component of said laser) disposed thereon, and a reflector for focusing light at the ionization region having an optical axis passing through the ionization region and disposed on the side of the ionization region opposite the laser.

4. A mass spectrometer comprising:

a housing,

an analyzer channel in the housing,

a gas ion source disposed at one end of the analyzer channel for injecting ions into the channel,

means for mass separation of injected ions while such ions are within the analyzer channel,

an ion detector disposed at a second end of the analyzer channel, and

means for focusing separated ions on a detector,

the gas ion source including:

(a) an ionization region,

(b) electrodes forming an acceleration field upon application thereto of suitable electrical potentials, at least one of the electrodes having an ion aperture therein communicating with the ionization region,

(c) a laser having a field strength intensity sufficient to cause autoionization of gas focused toward the ionization region of the ion source, including a crystal with a reflector formed at one end at right angles to its optical axis, the crystal .being positioned to have light emitted therefrom directed toward the ionization region, and

(d) optical means for causing a beam from thev laser to focus in the ionization region, including means abutting the laser crystal at an end opposite said one end for partially reflecting said light (as a component of said laser) and focusing lased light toward the ionization region, and means for reflecting light to a point in the ionization region disposed on the side of the ionization region opposite the laser. 5. A mass spectrometer comprising: a housing, an analyzer channel in the housing, a gas ion source disposed at one end of the analyzer channel for injecting ions into the channel, means for mass separation of injected ions while such ions are within the analyzer channel, an ion detector disposed at a second end of the analyzer channel, and means for focusing separated ions on a detector, the gas ion source including:

(a) an ionization region, (b) electrodes forming an acceleration field upon application thereto of suitable electrical potentials, at least one of the electrodes having an ion aperture therein communicating with the ionization region,

(c) a laser having a field strength intensity suflicient to cause autoionization of gas focused toward the ionization region of the ion source, the laser having its optical axis directed toward the ionization region, and

((1) optical means for causing a beam from the laser of focus in the ionization region, including (i) light focusing means positioned intermediate the laser and the ionization region and aligned with the laser so that light rays emitted by the laser are received thereby, and

(i) means disposed opposite said light focus ing means for reflecting and focusing light at the ionization region.

6. A mass spectrometer comprising:

a housing,

an analyzer channel in the housing,

a gas ion source disposed at one end of the analyzer channel for injecting ions into the channel,

means for mass separation of injected ions while such ions are within the analyzer channel,

an ion detector disposed at a second end of the analyzer channel, and

means for focusing separated ions on a detector,

the gas ion source including:

(a) an ionization region,

(b) electrodes forming an acceleration field upon application thereto of suitable electrical potentials, at least one of the electrodes having an ion aperture therein communicating with the ionization region, a laser having a field strength intensity sulficient to cause autoionization of gas focused toward the ionization region of the ion source, the laser having its optical axis directed toward the ionization region, and (d) optical means for causing a beam from the laser to focus in the ionization region, including a spherical reflector having (i) an optical axis aligned with the laser optical axis, and 1 (ii) a focal point disposed at the ionization region and positioned with relation to the ionization region on the side opposite the laser.

References Cited UNITED STATES PATENTS OTHER REFERENCES Observation of Ionization of Gases by a Ruby Laser, by E. K. Damon et al. from Applied Optics, vol. 2, No. 5, May 1963, pp. 546 and 547.

WILLIAM F. LINDQUIST, Primary Examiner US. 01. X.R.

mg? UNITED s'm'ncs lA'll-lN']. OFFHIE CERTIFICA'IE ()b CORRECTION Pat 3. 478. 204 JmemherlLJQbi..-

lnvent fl Wilson M. Brubaker et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 2, line 6, "109" should read --10 line 49, after "is" insert -a-.

SIGNED AND SEALED 'FEB 2 4 1970 mm: 1. mm. .m. Atteating Officer dominionof Patents

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