US2341551A - Mass spectrometer - Google Patents

Description

F. 15 1944. H. HOOVER, JR

MASS SPECTROMETER Filed May 4, 1940 INVENTOR, HERBERT HOOVER, JR.

ATTOiPNI-TVSv Patented F eb. 15, if JV MASS srnornomrmn Herbert Hoover, Jr.. Sierra Madre, Qalif... assignor to Consolidated Engineering (Corporation, Pasadena, Gall? a corporation oi'lilaliiornia Application May Q, 1940, Serial No. 333,391

(CL tit-=51) i Claims.

My invention relates to mass spectrometry and in particular provides apparatus for simultaneously and continuously identifying ions of different mass-to-charge ratios.

In the customary method of mass spectrometry, ions of various mass-to-charge ratios are successively collected by a single collector and converted to easily measured currents by means oi an electrometer tube. This technique has .many limitations. For example, I have found that the analysis of extremely small samples is very dificult when only a single collector is used.

One reason for this is that the speed at which successive readings of difi'erent ion intensities may be taken, is limited by the sensitivity of the apparatus. In general, the greater the sensitivity the greater the time required between successive observations when ions are successively focused on a single collector, as increasing the grid leak resistance increases the R.-C. time constant of the electrometer tube circuit connected to the collector. In other words, the period of time required for the electrometer tube circuit to produce a full indication of the collected ion current increases in accordance with the product of the grid leak resistance R and the capacitance between the grid and cathode. Thus, in analyzing small samples, two conflicting requirements are made upon the system, one for high speed and one for high sensitivity. If the sensitivity is high, the rate at which readings may be taken may be so slow that the entire sample is exhausted by the apparatus before all desired readings are obtained.

When analyzing intermediate or large size samples by the usual method of mass spectrometry, the time consumed in making successive readings of ionic currents introduces important economic limitations on the system.

Another difficulty with the usual single-collec-' tor type of mass spectrometry arises when attempts are made to introduce automatic recording of a plurality of ion currents.

In another method of mass spectrometry, ions of various mass-to-charge ratios are brought to a. focus at various points on a photographic plate. While a complete mass spectrum of even a small sample may be obtained by this system, the complexity of apparatus required to change plates introduces economic objections.

The principal object of my invention is to overcome the above-mentioned limitations of prior art mass spectrometry.

Another object is to rovide a system for continuously providing indications of the contents of gas flowing through a gas line.

Another object is to provide means for performing accurate mass spectrometry free from background errors. I accomplish these results by utilizing a plurality of ion collectors predisposed to collect ions of different predetermined mass-to-charge ratios and separately reproducing indications of the intensities of said different ion currents.

My invention possesses numerous other ob jects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus.

In the drawing:

Fig. 1 shows the general organization of my apparatus suitable for analyzing individual samples, partly schematic and partly in cross-section.

Fig. 2 is a wiring diagram of an amplifier suitable for use in a mass spectrometer.

Fig. 3 is a cross-sectional view of a connection between a gas line and a mass spectrometer.

In general, a mass spectrometer comprises a gas source connected to an ionization chamber, an analyzing chamber in which ions of different mass-to-charge ratios are separated and associated recording or indicating instruments.

In Fig. 1, I have shown an apparatus suitable for analyzing discrete samples of gas. The mass spectrometer proper comprises an ionization chamber 2 wherein gas introduced through the inlet system t is ionized by bombardment of electrons drawn into the space within a helical filament anode 8 by means of the positive potential maintained by battery l0 between a cathode 6 and said anode 8. The electrode assembly 6-43 is maintained at a high positive potential by means of battery l2, which is grounded on the negative side thereof. .Ions formed within said electrode assembly are drawn towards slit M in grounded collimator tube l6 by virtue of the high electric field maintained between said electrode assembly and said collimator slit. Some of the fast traveling ions which pass through collimator slit It also pass through collimator slit l8 in grounded disc 20. The ions passing through both collimator slits l4 and 20 form a collimated heterogeneous ion. beam. Said collimated beam is deflected downward by the electrostatic field maintained between plates 2222 by grounded battery 2t. The deflected beam enters the gap 26 between opposite. poles of electromagnet 28.

As a result of the combined eifects of the electric and magnetic forces acting on the ions and the geometry of the mass spectrometer, positive ions of diiferent mass-to-charge ratios are brought to focus at different points in the exit plane 38-38.

According to my invention, I simultaneously collect ions of different mass-to-charge ratios which appear in the ion image field at plane 38-38 and separately indicate their intensities. One way of accomplishing this result is to utilize two grounded Faraday cages 32-34, having narrow slits 36 and 38 on one side thereof adjacent plane 38-38. Ionsof predetermined mass-tocharge ratio pass through the narrow slits 38, 38. Other ionsfalling within the area of the Faraday cages are discharged to ground on the outer surface of said grounded cages. Within said Faraday cages corresponding ion collectors 48, 42 are mounted on the grid caps of electrometer tubes. 44 and 46 in such a manner that each of the respective ion collectors may collect all of the ions of predetermined mass-to-charge ratio entering the Faraday cages through the corresponding slit 36 or 38. The ions falling upon the respective collectors 48 and 42 discharge to ground through the corresponding large resistors 48 and 58 in a conventional manner.

Any gasesthat would otherwise accumulate 6 within Faraday cages 32 and 34 are permitted to exhaust themselves from said cages through apertures 52 in the side walls of said cages.

Networks 58 and 68 connected to the corresponding electrometer tubes 44 and 46 and the corresponding resistors 48-58 through the cables 49 and in convenitional manner, provide means for transforming collected ionic currents into readily reproducible voltages. Voltages appearing in the outputs 54-56 of networks 58 and 68 in accordance with the collected ion currents, are simultaneously recorded in any conventional manner desired by automatic recorder 62.

The wiring diagram of a single reproducing circuit, including an electrometer tube 44, resistor 48, network 58 and galvanometer unit 88, is shown in Fig. 2. A collector 48 is connected to the control grid 64 of said electrometer tube 44. Positive ions discharge on ion collector 48 to ground 66 through the large resistor 48. Suitable voltages are supplied to the elements of said electrometer tub 44 by low voltage battery 68 through rheostat I8, control grid resistor I2, filament I4, and compensating resistors I6 and I8. The voltage of anode 88 is supplied through variable resistor 82 and the voltage of space charge grid 84 is supplied through resistor 86.

Voltages impressed upon control grid 64 by discharge of ionic currents through resistor 48 are reproduced by galvanometer 88 connected between anode 88 and screen grid 84. The voltages appearing across galvanometer 88 may be read directly on an indicating meter or may be reproduced if desired in recorder 62' as illustrated in Fig. 1.

The output voltage appearing between electrodes 88 and 84 of electrometer tube 44 may be further amplified if desired prior to application to recorder 62.

The function of the various compensating resistors 16, I8 and other regulating resistors 18, I2, 86, and 82 shown in Fig. 2, is to maintain the amplification of electrometer tube 44 substantially invariable regardless of small fluctuations in temperature and battery supply voltage and other variable conditions as is well known in the art. K

In the schematic diagram shown in Fig. 1, the electric networks 58 and 68 contain the various elements required to complete the amplifier connections between the electrometer tubes 44 and 46 and grid resistors 48 and 68 and the associated galvanometers contained in recorder 62.

5 While I have shown in Fig. 1 only two ion current collectors and reproducing elements, it is to be understood that a larger number may be used in accordance with my invention. The particular arrangement of collector slits, collectors,

l0 electrometer tubes,. etc., adopted may be varied within wide limits according to convenience, number desired, and geometry of the mass spectrometer to which they are applied. The number of independent ionic currents which should be reproduced should preferably be at least equal to the number of constituents to be detected in the gas sample under examination.

Prior to introducing a gas sample into the mass spectrometer, the ionization chamber and the analyzing chamber are brought to suitably low pressures by evacuation of the gas contained therein through vacuum lines I88, I82, and I84. The amplifiers are then turned on by closin key I86 shown in Fig. 2 and the elements of electrical networks 58 and 68 adjusted to produce zero readings in the respective reproducing elements of recorder 62 and are balanced so as to provide against variations in amplification due to temperature or supply voltage fluctuations.

The current supplied to cathode 6 by battery I88 is then turned on by closing key 8. Due to the residual gas remaining in the walls of ionization chamber 2, ions will thereupon be formed within electrode assembly 68 and will be focused at 86 their respective positions in the ion image field in plane 38-38. After a few minutes equilibrium is reached and the collected ion currents reach steady background values.

a tact I8I on low voltage potentiometer I83 in such a manner as to cause the galvanometer indicauum line II8. Stop-cock I28 is then closed and stop-cock I22 opened to admit a sample of gas from sample bottle I24. The approximate quantity of gas sample introduced into sample chamber II2 may be measured by pressure gauge I26. The quantity of a sample in sample chamber I I2 may be limited to any suitable value, if desired, by manipulation of stop-cocks I28 and I22.

After the gas sample is introduced into sample chamber I I2 and is found to be at a suitable pressure, the inlet stop-cock H4 is opened to admit the'sample into the ionization chamber 2. Simultaneously, the recorder 62 is set into operation and records simultaneously obtained of different ion currents derived from said gas sample.

The amplitude of the recorded galvanometer current produced at any instant is substantially proportional to the collected ion current which in turn is substantialy proportional to the quantity of gas from which the respective ions are derived. As the sample gradually becomes exhausted, the recorded indications gradually diminish. The rates of ion current diminution 78 which are characteristic of the molecular weights I provide for com-' pensating for this background by adjusting con- 7 the tubulation II6 are evacuated through vacof the respective gases from which the ions are derived serve as a check in the identification of the various gases, the corresponding ions of which are recorded. To illustrate how the rate of ion current diminution may serve as a check in identifying a gas from which the ions are derived, consider two examples, one involving the measurement of or 0+ ion currents derived from C0: and the other in which the same ions are derived from C0. In both cases the gas is assumed to be flowing from the sample chamber H2 at low pressure. In either case, when the sample chamber is first connected to the ionization chamber the pressure of the sample in the ionization chamber will gradually rise. After a short while stable flow conditions will be achieved and the pressure will then gradually, diminish due to the gradual exhaustion of the sample. Inasmuch as CO molecules have a. greater mean velocity than CO2 molecules at the same temperature, the percentage rate of diminution of pressure for C0 in the ionization chamber would be greater than for CO2 once the stable flow conditions have been reached. It is clear that the ion current corresponding'to anyoneoi the ions 0+, CO+, and CD2+ derived :from a single component gas COaWill then each be. substantially proportional to the amount of CO2 present in the ionization chamber at any instant, and that the rate of diminution of each of these ion currents will be proportional to the rate or diminution of the C02 pressure in the. ionization chamber. Similarly, the rate of diminution of ioncurrents uous recording of the contents of a gas line is made. An inlet system useful for this purpose is illustrated in Fig. 3. Here gases are flowing downward in gas line I. As sample gas flows into the tube E22 pointing upstream, said gas flows through chamber I24 and is exhausted into gas line I20 through the tube I26 pointing downstream. In this manner the composition of the gas within chamber I24 continuously reflects the composition of the gas flowing in gas line I20.

A portion of the gas within the sample chamber I24 is admitted to the ionization chamber 2 through the pressure reducing inlet line 828'. The inlet line shown has therein a pair of capillary tubes H and lit mounted in corresponding plates Wt and I36, respectively, ring-sealed in the inlet line i 28. The space between the two plates lit and itt is exhausted at a controlled rate corresponding to 00+, and 0+, derived from a single component 00, will be proportional to the rate of diminution of CO pressure in the ionization chamber. Accordingly. as only one of these gasses is assumed to be present, it becomes a relatively simple matter to determine from the rate of diminution of either the 0+ or C0+ ion current which of these gasses is present. Broadly speaking, if several gases having difierent molecular weights are capable of producing ions of the same mass-to-charge ratio in amass spectrometer, the percentage rate of diminution of the corresponding ion current will vary .as an inverse function of the molecular weight of the component from which it is derived and this fact may be utilized to assist in identifying the component.

While one sample is being introduced into the ionization chamber 2 from sample chamber H2, stop-cock ltd may be kept closed and sample bottle I24 replaced by another containing a different sample. When a sufiicient recording of the partial mass spectrum of the first sample has been obtained. stop-cock ll i may be closed and stop-cocks lit and E28 opened to exhaust the sample chamber lit through vacuum line H8 and the entire process then repeated on the second sample. In this manner a number of samples may be analyzed in rapid succession and permanent records of partial mass spectra of each sample obtained.

The above-described method of analyzing a sample is especially applicable to small samples as very high amplification may be utilized in the respective amplifiers 58 and B0 and independent recordings simultaneously obtained for therespective concentrations of various ions derived from said sample. The indications of the respective ion currents are obtained with less danger of exhausting the sample before a complete set of readings is made.

In another application of my invention continthrough valve it? and vacuum line H38 in order to reduce the rate of flow of gas from sample chamber Mt into ionization chamber 2.

From the foregoing description it will be readily seen that I have provided a system of mass spectrometery which offers wide opportunities for adaptation to many different systems in which it i is desired-to ascertain the constitution of chemical mixtures. Those skilled in the art will readily perceive many modifications oi the above-described apparatus falling within the scope and spirit of my invention.

I claim:

1. In a mass spectrometer, the "combination which comprises an ionization chamber, means connected to said chamber and adapted to suc cessively sample portions of a mixture that varies in composition with time, means for focussing at diiierent points ions of difierent mass-to-charge ratios formed in said chamber from each of said portions, a plurality of ion collectors each positioned to simultaneously detect ions of a different mass-to-charge ratio, the relative rates of formation of the detected ions varying in time with the mixture composition, and individual ion current indicators electrically connected to the respective collectors for determining the relative rates of formation of the ions from the successive sampled portions, thereby indicating changes in composition between successive portion.

2. In a mass spectrometer for indicating changes in composition of a mixture to be analyzed, the combination which comprises an ionization g hamber, means adapted to successively introdue portions of the mixture of changing composition into the ionization chamber in gaseous form, means for focusslng at diiierent points ions of different mass-to-charge ratios formed in said chamber from each said portion of the mixture, a plurality of ion collectors positioned to simultaneously detect such ions of different mass-to-eharge ratios, a separate resistance connected to each collector, a separate amplifier connected to each resistance, and a separate electric meter connected to the output of each amplifier.

3. In a mass spectrometer for indicating changes in composition of a mixture to be analyzed, the combination which comprises an ionization chamber, means for introducing successive portions of the mixture into the ionization chamber in gaseous form, means for separating ions of different mass-to-charge ratios formed in said region from each said portion of the mixture, a plurality of ion collectors arranged to simultaneously detect selected ions of difierent mass-to-charge ratios, and individual recording galvanometers each electrically connected with the respective collectors and adapted to simultaneously record the respective rates of formation of the ions detected, whereby changes in the mixture composition from one portion to another may be indicated.

4. In apparatus for the analysis of a material of changing composition flowing in a conduit, the combination which comprises a mass spectrometer, sampling means connecting said conduit with the ionization chamber of the mass spectrometer and adapted to continuously admit a current of said material into the ionization chamber in gaseous form, means within said mass spectrometer for separating ions formed in the ionization chamber in accordance with their massto-charge ratios, means for separately and simultaneously detecting the charges from ions of different mass-to-charge ratios the relative proportions of which vary with the composition of the material, and means to simultaneously and separately record the rates of formation of detected 10 ions of diiferent mass-to-charge ratios.

HERBERT HOOVER, JR.

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