US2970215A - Mass spectrometer - Google Patents

Description

Jan. 31, 1961 H. S. SPACIL MASS SPECTROMETER Filed Jan. 10, 1958 STEADY POTENTIAL SOURCE CONTROLLING ACCELERATION AND THRESHOLD GRIDS RADIO FREQUENCY PULSE CIRCUIT CONTROLLING GATE GRIDS PREAMPLIFIER AND AM- PLIFIER CONTROLLED ,70

BY COLLECTOR CATHODE RAY OSCILLI- SCOPE CALIBRATED IN MASS vs. MAGNITUDE RECORDER CALIBRATED I IN TIME VS. MAGNITUDE I S? TOR dfl/ ATTORNEYS MASS SPECTROMETER Henry S. Spacil, Boston, Mass., assignor to Alloyd Re- 'nited tates Patent search Corporation, Watertown, Mass, a corporation of Massachusetts Filed Jan. 10, 1958, Ser. No. 708,323

8 Claims. (Cl. 250-419) The present invention relates to mass spectrometry and, more particularly, to processes and devices for analyzing a chemical sample by electrically charging particles that have been emitted from the sample and by physically separating such particles in an electrostatic and/or magnetic field system that imparts to given particles of selected mass a velocity and/or deflection by which they may be isolated from remaining particles in order to indicate qualitatively and quantitatively the composition of the chemical sample.

A typical mass spectrometer comprises: a vaporizing region in which a rarefied gas is produced; an ionizing region in which particles of tfe rarefied gas are positively charged by electron bombardment and the resulting ions are accelerated to a kinetic energy of the order of 1,000 electron volts (e.v.); an analyzing region in which the ions are directed through a field system that discriminates between particles of different mass to charge (m/e) ratios; and a collecting region for determining the presence and intensity of beams discriminately-produced in the analyzing region. Important related factors in the design of a mass spectrometer are its sensitivity, i.e., its ability to indicate extremely small concentrations of components in a sample being analyzed, and its resolution, i.e., its ability to distinguish between ions of closely similar masses. One purpose of accelerating particles in the ionizing region is to render their kinetic energies substantially uniform. Thus, where the sample is either a gas or a liquid, partices emanate from the sample at energies of a few e.v., more or less, the differences between the kinetic energies of the partic es becoming relatively negligible when the particles are accelerated uniformly to kinetic energies of the order of 1,000 e.v. In the analysis of solid samples, however, vaporization by high voltage discharge, for example, produces such a wide energy spread, e.g. of the order of 1 to 100 e.v. among like ions that the sensitivity and resoltuiou of the mass spectrometer are adversely affected. Also, in the analysis of solid samples, vaporization by heating has not been feasible because of the difiicu'ty in thermally isolating the relatively high temperature vaporizing region from the relatively low temperature remaining regions while at the same time avoiding condensation of the ions in their path from the vaporizing region into the remaining regions. The present invention contemplates the production, from a solid chemical sample, of ions having closely similar kinetic energies, from which the mass spectrometer may produce monoenergetic ion beams of the type common in the analyses of gases and liquids.

The primary object of the present invention is to provide, for the production of mass spectra, processes and devices that involve dissolving a relatively small quantity of a chemical sample in a relatively large quantity of a known liquified solid in order to produce sample particles of uniform kinetic energies for ionization and analysis. The mass spectrum of particles emitted from the dissolved sample, which posses kinetic energies within the range of from .5 to 5 e.v. and usually of the order of from 1 to 2 e.v., is indicative of the partial vapor pressures of the components of the sample and, in consequence, their chemical identities and proportions.

Another object of the present invention is to provide, for the production of mass spectra, processes and devices, in which particles emitted from a sample in liquified solid solution move in a direct path from the vaporizing region, which is at relatively high temperature, into the remaining regions, which are at relatively low temperature, with a minimum of condensation.

Other objects of the present invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the process involving the several steps and the relation and order of one or more of such steps with each of the others and the device possessing the construction, combination of elements and arrangement of parts, which are exemplified in the following detailed disclosure, and the scope of which will be indicated in the appended claims.

For a fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein:

Figure 1 illustrates, partly in cross-section and partly in diagram, a mass spectrometer embodying the present invention; and a Figs. 2 and 3 illustrate graphs explaining features of processes of the present invention effected in the mass spectrometer of Fig. 1.

p The processes of the present invention illustrated specifically herein generally involve determining the composition of an inorganic chemical sample by dissolving a relative y small quantity of the sample in a relatively large known quantity of a liquified known inorganic solid, which contains a material selected from the class consisting of the metals and the metalloids. Particles emanating from the liquified solution, then are transmitted in a mechanically direct or free path, through an ionizing region in which the particles are charged, an accelerating region in which the kinetic energies of the resulting ions are increased to values within the range of from 50 to 1000 e.v., an analyzing region in which an adjustable applied radio frequency potential selectively passes particles moving at a selected velocity to a collecting region. In the collecting region a current is generated, from which the presence and quantity of ions of a selected mass may be determined. By total weight of the liquified solvent and sample solution, the concentration of the overall sample, should be no greater than 10% and the concentration of the major component of the sample preferably should be no greater than 5%. When the sample is in such a minor concentration, as will be more fully discussed below, a selected component of the sample may be considered to be in binary solution with the liquified solid solvent. In consequence, the concentration of the selected component may be calculated from its vapor pressure without consideration of the concentrations of other components. Thus, the current in the collecting region resulting from a beam of ions of any selected mass is a direct indication of the concentration of material of that selected mass in the sample. The liquified solid solvent may be composed of any of a wide variety of low vapor pressure metallic and metalloidal elements and compounds, for example: the metallic and metalloidal elements of groups .I, II, III, IV and VIII of the periodic table of chemical elements, e.g. aluminum, gallium, titanium, germanium, tin, and, particularly, the noble metals, including ruthenium, rhodium, paladium, osmium, iridium, gold and silver; and metallic and intermetallic compounds embodying at least in part the aforementioned elements,

tag. antimony antiminide and aluminum phosphide. Preferably the liquified solid has a low oxide stability that precludes the formation of an oxide film on its free surface. In the foregoing system, particles are emitted from the free surface of the solution at uniformly low energies within the range of from 0.5 to 5.0 e.v. in a steady flux that is substantially devoid of short term variations. In general the minimum temperature of the solution is that required to give a high evaporation rate. The upper temperature limit, approximately 4000 C. as a practical matter is limited only by the nature of refractory material of the crucible containing the solution. Although the solution thus is maintained at a temperature considerably higher than that of the other components of the mass spectrometer, condensation upon or interaction with these other components by particles emanating from the solution is avoided by operating at such a low pressure, of the order of 10" to 10- millimeters of mercury (mm. Hg), that the mean free particle path is many times greater than the dimensions of the various regions of the mass spectrometer.

Figure 1 discloses, in accordance With the present invention, a mass spectrometer providing a vaporizing region 10, an ionizing region 12, an analyzing region 14 and a collecting region 16. The components of these regions are confined by and supported within a hermetically sealed jacket having a vertical tubular portion 18, a base portion 20 integral with the tubular portion and a removable ciover portion 22. Tubular, base and cover portions 18, 20 and 22, for example, are composed of a glass or metal that is sufficiently strong and impermeable to permit the production of a high vacuum within the jacket by rapid and continuous exhaustion, for example, through an outlet 24 in cover portion 22. This exhaustion may be effected by a multistage mercury diifusion pump.

The components of vaporizing region 10 include a crucible 26 containing a predetermined quantity of an inorganic solid to serve as a liquified solvent, a radio frequency current in induction coil 30 ranges from 28 to the liquid state and agitating it to ensure that it dissolves the sample rapidly and completely in a manner to be described below. Preferably the radio frequency curernt in induction coil 30 ranges from 10 l0 to l 10 cycles per second, lower frequencies being conducive to increased agitation. Crucible 26 is composed of a material that melts at a higher temperature than does liquifiable solid 28, for example, is composed of a refractory metal such as platinum, tungsten or tantalum or a ceramic, for example, a refractory oxide such as zinc oxide or beryllium oxide. In order toisolate crucible 26 thermally from adjacent remaining components of the system, which are at relatively low temperature, crucible 26 is supported by a rod 32, the upper end of which projects into a hub 34 depending from the base of the crucible and the lower end of which projects into a well 36 depending from base portion 20 of the jacket. Rod 32, which is either similar to or different from crucible 26 in composition, serves as an insulating support to prevent the conduction of heat from crucible 26 to base 20. A radiation screen 38 that blocks radiation directed outwardly from crucible 26 to the jacket, has a dished conformation that generally surrounds crucible 26. The upper free edges of the conformation are outwardly flanged for the purpose of centering the conformation in the container and the base of the conformation has a circularly flared opening through which rod 32 may be seated in well 36 and hub 34 may be seated on rod 32. Shield 38 is characterized by an interrupted metallic portion, for example, has a spiral strip construction, of which the successive substantially isolated convolutions provide only short electrically conducting paths. Thus, shield 38, although substantially unheated by eddy currents generated by induction coil 30, is capable of reflecting the bulk of thermal radiation emanating from liquifiable solvent 28.

Projecting through tubular portion 18 of the jacket is a tube 40 composed of the same material as is the container. The outer end of tube 40 normally is sealed. The inner end of the tube, which is open, projects sub stantially to a point in superposition above the edge of crucible 26. At the open inner end of tube 40 may be placed a specimen 42, the composition of which is to be determined. In the remainder of tube 40 is a magnetic slug 44, which when actuated by a solenoid 46, propels specimen 42 from'the open end of tube 40 into liquifiable solid 28 for dissolution. Slug 44 is encased in glass for the purpose of preventing any chemical activity between the slug and the remainder of the system. Particles thermally escaping from the free surface of the solution in crucible 26 are directed generally upwardly into ionizing region 12, analyzing region 14 and collecting region 16, now to be described.

Resting upon the upper flanged edges of shield 38 are the lower edges of an annulus 48 suitably cut away in the vicinity of tube 40. Annulus 48 is inwardly flanged at 50 to support an annulus 52 that mounts an electron emitter 54 and an electron collector 56. Emitter 54 and collector 56, respectively, present opposed horizontally elongated emitting and collecting surfaces that generate a planar electron beam of a sufficiently broad horizontal extent to substantially cover the circular opening defined by flange 50 of annulus 48. Details of the filament and accelerating grids of emitter 54 and of the anode and secondary electron shield of collector 56, which are designed in accordance with known techniques, need not be discussed in detail. A proportion of the particles emitted from liquifiable solid 28, become ionized upon passing through the planar electron beam generated between emitter 54 and collector 56. Particles so ionized are selectively transmitted, in accordance with their mass to charge ratio, through analyzing region 14, now to be described.

The components of analyzing region 14 include ten accelerating, control and threshold grids to be described in detail below. Accelerating grids are designated by 60, control grids by 62 and threshold grids by 64. These grids are positioned in horizontal planes by spacers 58, composed for example of aluminum oxide, that are supported one above the other upon annulus 52 and present central circular openings each equal in extent to the opening presented by annulus 52. Spacers 58 are centered with the aid of projections that are integral therewith and that extend to the inner surface of tubular portion 18. Steady negative potentials are applied to accelerating grids 60 for the purpose of imparting to the positively ionized particles a selected kinetic energy usually within the range of from 50 to 1,000 e.v. Radio frequency signals are applied to control grids 62 and positive steady potentials are applied to threshold grids 64. Grids 62 and 64 coact to transmit or reject ions in accordance with their m/e ratio in the following manner. Ions of mass greater than a predetermined magnitude will not acquire a sufficient velocity from an accelerating grid 60 to reach a control grid 62 until after the brief accelerating portion of the radio frequency cycle has been terminated. On the other hand, ions of mass smaller than this predetermined magnitude will acquire so great a velocity that they will reach the control grid 62 before the brief accelerating portion of the radio frequency cycle is initiated. The arrangement is such that ions which are either too heavy or too light will not have suificient energy to overcome the positive repelling voltage on a threshold grid 64. p p

Ionsthat succeed in passing through analyzing region 14 impinge upon a collector plate 66, which is mounted upon a spacer 68, .of the type described above. The resulting current through collector plate 66 is applied the atomic fraction.

to an amplifying circuit 70, which applies a control signal either to a cathode ray oscilloscope 72 or a continuous recorder 74. Cathode ray oscilloscope 72, for example, is calibrated in units representing mass vs. current. This arrangement is particularly useful when an overall mass spectrum indication is desired. Recorder 74, for example, is calibrated in units representing time vs. current. This arrangement is particularly useful when a precise indication of the concentration of a particular mass is desired. In either case, the calibration will depend upon functional relationships between the vapor pressure of particles emitted by the solution (the number of ions collected per unit time in collecting region 16 is determined by this vapor pressure) and the concentration of the component of that mass in the solution, as will become apparent in the following discussion.

If several solutes are present in a solvent, the interaction of these with each other can be neglected if the solution is dilute. Thus each of the solutes can be considered, to the exclusion of the other solutes, as being in binary solution in the solvent. The activity of the ith solute in the solvent is defined by the expression where p; is the vapor pressure of the ith solute in the solvent, and p may be defined in any convenient way. One preferred way will be considered below. The activity of the ith solute is related to the composition of the ith solute by the expression where f, is the activity coeflicient of the ith solute, and x is the composition of the solution in terms of the ith solute. For example, x, may be the weight percent or In the case of dilute solutions, it is convenient to specify that the activity coefiicient becomes equal to unity as the solution becomes infinitely dilute, or

lim i,=1.0

The value of p may be determined by introducing the activity coefiicient, as above defined, into the expression for activity, thus:

Now, by specifying, as above, that the activity coefiicient 73, becomes equal to unity as the solution becomes infinitely dilute, 12, can be specified mathematically as In practice, p is obtained by measuring p at known values of x; and plotting a curve of p /x as a function of Jr This curve then is extrapolated to x =0 in order to obtain p as shown in Fig. 2. Once p has been determined as above, measurements of p, at known values of x, are used to produce a chart of a, as a function of Such a chart is plotted for each The value of activity thus obtained, is used to read the concentration of the solute directly from the curve of activity vs. composition. When the solute is introduced into the solvent as a small solid or liquid sample, if the weights of the solvent and the sample are known, the composition of the sample may be calculated.

In an exemplary mass spectrometer of the type herein specifically described: thermal evaporation takes place from a liquified metal solvent of 10 cm. free surface area; the included angle from the normal to the solvent surface, within which evaporating atoms or molecules are collected is 60 degrees; 10% of the evaporating atoms or molecules are singly ionized; and of the ten linearly superposed grids, each produces a 5% transmission loss. Under these conditions, the number of ions per second passing to collector plate 66 (with suitable settings of grid potentials) is given by:

where M, is the molecular weight of particles being analyzed; T is the solvent temperature expressed in K/ p, is the vapor pressure of the particles at the surface of the solvent in mm. Hg; and E is the overall current efiiciency of the mass spectrometer. As stated above, the concentration of a particular component of the sample is derived from the vapor pressure of that component as indicated by the intensity of its associated ion beam impinging upon collector plate 66.

The current efiiciency, E depends inversely upon the resolving power of the mass spectrometer, that is, upon the ability of the spectrometer to reject ions which are not of a selected m/e ratio. Resolving power may be defined as Example I Thus the required current efficiency is The ion current is the number of ions per second arriving at the collector plate multiplied by the charge per ion, or

At 350 C., the vapor pressure of pure magnesium is 3 x 1O- mm. Hg. Thus p =3.0 10* mm. Hg, so that the net ion current is This current is detectable with reasonably sensitive electrical instrumentation.

. Example II In another process performed in the above described mass spectrometer, lead is dissolved in nickel to the extent of one p.p.m. at 1500 C. The molecular weight of lead is about 207. The mass spectrometer will have to exclude bismuth, molecular weight 209, and thallium, molecular weight 204. Accordingly, the required resolving power is Now the required current efiiciency is The ion current is At 1500" C., the vapor pressure of pure lead is 200 mm. Hg. Thus so that the ion current is '58, are placed in registered relation in superposed planes within the container and their leads extended through an opening in cover 22. This opening is sealed by heating.

Then, induction coil 30 is energized to liquify liquifiable solid 28 and solenoid 44 is actuated to propel specimen 42 into solvent 28. Finally, information about the mass spectrum of the various components of the sample 42 is presented on cathode ray oscilloscope 72 or recorder 74 by varying the radio frequency applied to control grids 6%.

Since certain changes may be made in the above processes and devices without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative and not in a limiting sense.

What is claimed is:

1. A mass spectrometer for determining the composition of an unknown specimen of relatively small mass, said mass spectrometer comprising a jacket and components within said jacket providing, in sequence, a vaporizing region, an ionizing region, an analyzing region and a collecting region, said ionizing region and said analyzing region providing a path between said vaporizing region and said collecting region, the components of said vaporizing region including a crucible containing a known solid liquifiable at relative high temperature to provide a bath of relatively large mass, said specimen being soluble in said bath, said crucible being thermally isolated from adjacent portions of said jacket and preventing said bath from flowing to said adjacent portions of said jacket, said adjacent portions of said jacket being at relatively low temperature.

2. A mass spectrometer apparatus for determining the composition of an unknown specimen of relatively small mass, said mass spectrometer apparatus comprising a jacket and components within said jacket providing, in vertical sequence, a vaporizing region, an ionizing region and a collecting region, said ionizing region and said analyzing region providing a straight line path between said vaporizing region and said collecting region, the components of said vaporizing region including a crucible containing a known solid liquifiable at relatively high temperature, said crucible being thermally isolated from adjacent portions of said jacket and preventing said bath from flowing to said adjacent portions, said adjacent portions being at relatively low temperature.

3. The mass spectrometric apparatus of claim 2 wherein the components of said analyzing region include a series of superposed grids.

4. The mass spectrometric apparatus of claim 2 wherein the path of particles from the free surface of said liquifiable solid through said vaporizing region, said ionizing region, said analyzing region and said collection region, is of substantially uniform cross-sectional extent, said cross-sectional extent being substantially equal to the extent of said free surface of said liquifiable solid.

5. A mass spectrometric apparatus for determining the composition of an unknown specimen of relatively small mass, said mass spectrometer apparatus comprising a jacket and components within said jacket providing, in vertical sequence, a vaporizing region, an ionizing region, an analyzing region and a collecting region, said ionizing region and said analyzing region providing a substantially straight path between said vaporizing region and said collecting region, the components of said vaporizing region including a crucible containing a known solid liquifiable at relatively high temperature, said crucible being thermally isolated from adjacent portions of said jacket and preventing said bath from flowing to said adjacent portions, said adjacent portions being at relatively low temperature, the path of particles from the free surface or" said liquifiable solid through said vaporizing region, said ionizing region, said analyzing region and said collecting region being of substantially uniform crosssectional extent, said cross-sectional extent being substantially equal to the extent of said free surface of said liquifiable solid.

6. The mass spectrometric apparatus of claim 5 wherein the components of said analyzing region include a series of superposed accelerating, radio frequency control and threshold grids.

7. A mass spectrometer for determining the composition of an unknown specimen of relatively small mass, said mass spectrometer comprising a jacket and components within said jacket providing, in sequence, a vaporizing region, an ionizing region, an analyzing region and a collecting region providing a path between said vaporizing region and said collecting region, the components of said vaporizing region including a container, a known solid liquifiable at relatively high temperature within said container, means for heating said known solid to said relatively high temperature to form a bath of relatively large mass for receiving said specimen of unknown composition, said liquifiable solid comprising a materail selected from the class consisting of elements and compounds containing metal and metalloids, said specimen constituting less than ten percent of the total weight of said bath and said specimen.

8. A mass spectrometric apparatus comprising a jacket and components within said jacket providing, in verti cal sequence, a vaporizing region, an ionizing region, an analyzing region and a collecting region, said ionizing region and said analyzing region providing a straight line path between said vaporizing region and said collecting region, the components of said vaporizing region including a crucible containing a solid liquifiable at relatively high temperature, said crucible being thermally isolated from adjacent portions of said container, said adjacent portions being at relatively low temperature, the

path of particles from the free surface of said liquifiablesolid through said vaporizing region, said ionizing region, said analyzing region and said collecting region being of substantally uniform cross-sectional extent, said cross sectional extent being substantially equal to the extent of said free surface of said liquifiable solid, means adiacent to said crucible for projecting said specimen into said liquifiable solid, said liquifiable solid being of known composition, said specimen being of unknown composition, said liquifiable solid being of relatively large proportion, said specimen being of relatively small proportion, said liquifiable solid being composed of a material selected from the class consisting of elements and compounds containing metals and metalloids, said specimen 10 constituting less than 10% of the total weiglit of said liquified solid and said specimen. 5 References Cited in the file of this patent UNITED STATES PATENTS 2,703,843 Cameron Marl 8, 1955 2,715,682] Lawrence Augi 16, 1955 2,721,271 Bennett Oct! 18, 1955 10 2,768,301 Bennett OCtJ23, 1956 Britten Docs 31, 1957

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