US2373151A - Analytical system - Google Patents

Description

April 10, 1945. D. D. TAYLOR ANALYTICAL SYSTEM Filed July 29, 1942 2 Sheets-Sheet 1 /T0 VACUUMPUMP ANALYZER MAGNE T ION COLLECTOR A a o w 0 IQ 0 o M E v w m k A N 0 Wm. B EM 7 3 NM @6 J RECORDER INVEN TOR, DAN/EL DWIGHT TA YLOR.

rd 9 O .r 1 X J W H m w WM w M. c m m April 10, 1945. D D, A L 2,373,151

ANALYTICAL SYSTEM Filed July 29, 1942 2 Sheets-Sheet 2 'IIIIII:

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A I K TABLE 1.

GAS ION APPEARANCE cassava) POTENTIAL N2 28 N2+ 16.5 '14 N+ 24.0

l6 o+ 20.5 E 8; co 2a co+ |4.| gl2 'c-+ 23. R l6 0+ 24. i co 44 co |4.4 l6 0+ l9.6 9 2a co+ 20.4 I VOLTAGE l2 c+ 28.3 v 32- 0 20.0 J

- IN VEN TOR DAN/E L OW/GH T TA VLOR.

Patented Apr. 10, 1945 UNITED STATES "PATENT orrlcs ANALYTICAL SYSTEM Daniel Dwight Taylor, Alena, 6., asslgnor to Consoiidated Engin 1., Corporation, Pasadena, cent, a. corporatinoil car-ma Application any so, was, semi No. assess a @laims.

from a sample region into an ionization region in gaseous or vapor form. In the ionization region, molecules of. the mixture are ionized by subjecting them to the ionizing action of particles such as electrons. Such electrons are com-I monly'directed by the action of an electric field into the ionization region where they encounter and ionize molecules of the mixture. The ions formed are then withdrawninto an analysis region where ions of .diflerent mass-to-charge ratios are segregated into beams which may be detected by successive focusing thereof upon an ion collector. As the beams are successively detected at the collector, the beam intensities may eitherbe measured directly with a suitable indicator or else automatically and permanently recorded prior to indication. The peak intensities of the ionbeams are representative of either the amounts oi the withdrawn ions or the rates of formation of theresp'ective ions. Such a record or set of measured peak intensities forms a mass spectrum.

Either before or after a mass spectrum 01' a mixture has been obtained, reference samples containing diflerent proportions of the components that may be present in the mixture are similarly subjected .to analysis in a mass spectrometer under substantially the same ionizing conditions to which the mixture was subjected, and corresponding mass spectra obtained for the reference samples. Usually. though not necessadly, the reference samples are relatively pure samples or the respective components that may occur in the mixture.

The composition of the mixture is then determined by comparing the mass spectrum of the mixture with the mass spectra 01 the reference samples.- Such a comparison is most simply carried out by a mathematical procedure involving the comparison of the peak intensities of the beamsiormingthespecu'a.

' In practice, the analysis of such a mixture is greatly facilitated if the intensity of each peak occurring in the mixture spectrum represents the sum of the intensities of corresponding peaks that would be obtained in mass spectra of the separate components if present alone. A method for producing this efiect, which is known as linear superpositiomis disclosed and claimed in copending patent application, Serial No. 513,526, filed December 9, 1943, by Harold W. Washburn. Briefly, this method involves maintaining the sample in a sample chamber homogeneous at all times during analysis, flowing the components from the sample chamber into an ionization chamber through a gasinlet at mutually independent rates, ionizing each component in proportion to its partial pressure in the ionization chamber and independently of the amounts of the other components there, and providing such pressure conditions in the mass spectrometer that collisions between ions withdrawn from the ionization chamber with any molecules either in the ionization chamber or in the analyzing chamber are relatively infrequent. 25 Whenccnditions suitable for achieving linear superposition during-the analysis of a mixture are obtained, the intensity of any peak occurring in the mass spectrum of the mixture due to ions of'mass-to-charge ratio m may be represen by the following equation:

=the partial. pressure or component i in the mixture, and P';=a sensitivity coeiilcient representing the eillciency of he mass spectrometer in producing ions of mass-to-charge ratio 11:. obtained from component i by particles having an ionization energy or V electron volts.

. The sensitivity c'oemcient may be expressed as the ratio of the number of divisions of measured tensity per micron (u) of partial pressure of component 1 present in the chamber. Values of the sensitivity coefllcients may be most readily determined from the intensities of the beams occurring 60' in the mass spectra, obtained from pure components. In the event that linear superposition is not achieved, the analysis or the mixture maybe made by obtaining mass spectra of known mixll tures approximating tho-unknown in composition peak height or the number'ot units of beam inand comparing the spectrum of the unknown mixture with the spectra of the known mixture by the method which is more fully explained in my copending patent application, Serial No. 324,950, filed March'20, 1940.

The amount of ions of mass-to-charge ratio m originating iromya given component i is found to vary over a wide range with changes in the energies of the ionizing particles. For simplicity, hereinafter, the ionization of a sample by particles having a given amount of energy corresponding to the kinetic energy of electrons which have been accelerated by some predetermined voltage will be referred to simply as ionization of the sample or component at such a voltage.

The ionization curves of ions of a given massto-charge ratio formed from diflerent components which may be present in a mixture are not Benerally of the same shape. Generally, such ions have different appearance potentials .when formed from the different components. I utilize this fact in the analysis of a mixture of two such components by ionizing the mixture at a voltage intermediate the appearance potentials of such ions for the two components, measure'the amount of ions of such mass-to-charge ratio formed from onlyone of the components,-and determine the amount of that component present in the mixture from the amount of such ionsformed and the sensitivity coemcient of that component at the intermediate voltage.

In one form of my method all the ions measured are formed at a sufllciently low ionization voltage -so as to produce ions of a different mass-tocharge ratio for each component present.

In another form of my invention, the ionizing potential is changed during analysis, a low potential being used to produce ions of one mass-tocharge ratio from one component, and a high ionization'p'otential being used to produce ions of a different mass-to-charge ratio from another component.

vide a method for the analysis of a mixture containing homologous components.

My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description embodying and utilizing my novel method. It is thereforeto be understood that my invention is applicable to analyses of a variety ofmixtures and may be employed with several types of mass spectrometers which may utilize other kinds of ionizing particles, and that I-dot not limit myself, in any way, to the analyses, to the apparatus, or to the ionizing particles, of the present application, as I may adopt various other modifications of my invention utilizing the method, within the scope of the,

appended claims.

g by direct reference to the drawings in which:

7 connected to an ionization chamber 3 through a My method may also be applied to the analysis of a multi-component mixture when the production of ions of a given mass-to-charge ratio from a large number of components may occur at a high voltage, but from a fewer number of components at a lowionization voltage. In this case the use of an ionization voltage intermediate the highest and the lowest appearance potentials of such ions from those components, greatly simplifies the computation of the mixture computation by making the values of some of the sensitivity coeillcients "p? equal to zero.

My method is particularly applicable to the analysis of a mixture containing a plurality of homologous components, that iscomponentso! diiferent molecular weights, which contain some atoms or radicals common to their molecules.

Accordingly, the principal obiect'of my invention isto provide an improved method for analyzing chemical mixtures with a mass spectromete'r. Q

Another object is to provide a method of mass spectrometry in which the-contribution of one component to' the production of certain ions may be made practically negligible compared to the contribution of some other component.

Another obiect or. my invention is to provide a method for obtaining-independent indications ofindividual components of a mixture, even though the components may under some conditions, produoe ions of the some nass-to-charge ratio.

Still another object of my invention is to pro- Fig. l'shows a general organization of a mass spectrometer to which my method may be applied.

Fig. 2 is a schematic drawing partly showing a section taken one. line 22 of Fig. l of part of the mass spectrometer including the ionization chamber and the ionization energy con-' trols. .Fig. 3 is a graph including two curves showing typical variation of the intensity of an ion beam with accelerating voltage of electrons used as particles for ionizing twodiflerent components. Fig. 4 is a table showing the appearance potentials of various ions formed from nitrogen, oxygen, carbon monoxide. and carbon dioxide.

Referring to the drawings:

In Fig. 1, I have shown a sample chamber i tube 5 containing a restricted orifice 1..

A sample to be analyzed is introduced through the sample inletiinei into the sample.., ch'am- .11., her I, which has been previouslyreyacuated through the vacuum pumping line II. 'when the pressure of the sample is at a suitable value as determined by a pressure gauge 13 and the mass spectrometer is otherwise in condition for operation, a valve H5 is opened to admit the sample into the ionization chamber. 4

As illustrated in Figs. l and 2, electrons emitted from a heated filaments II are directed in a beam ll through aperture IS in the electron beam intensity control electrode 3|, through aperture 23' in electron accelerating electrode 25 which is formed by part of the wall of said ionization chamber 3, and through aperture 21, in the opposite part of the wall of said ionization chamber, said beam being directed along the line perpendicular to theface of a magnetic pole 3| by the combined action of the magnetic field-incatcher 82 electrically connected to electrode 28.

dicate by arrow II in Fig. 2, which field is directed downward perpendicular to the plane of 'thedrawing in Fig. 1, and electric fields parallel to the magnetic field. The electrons passing through said apertures impinge on electron The electric fields are provided by potentials applied from voltage supply circuit 33 to filament I Land electrodes II and. Said voltage supply circuit includes means for varying the relative potentials between said filament l1 and said electrodes.

One way of accomplishing this variation is to connect the filament to the negative end of a pair ofparallel connected rheostats 35 and '31 through which a current is flowing from a battery ll. and to connect said electrodes 2| and II to rhesotats Stand 31 respectively through sliding contacts 36 and 38 for picking on suitable positive potentials.

When electrons in said electron beam l8-encounter molecules of the sample in the ionization chamber, positively charged ions are formed inamounts characteristic of the structure of the molecules and in amounts proportional to the electron beam intensity. The amounts of ions produced are dependent on the energy ofthe electrons in beam I8. The positive ions formed are accelerated toward first slit electrode 4| by action of a small electric potential which maintains said-first collimating slit electrode negative with respect to a pusher electrode 43 on the opposite side of said electron beam. Some of the accelerated ions pass through a narrow slit 45 in said first collimating electrode 4i and are thereupon accelerated by a large negative potential between said first collimating slit electrode 4| and second collimating slit electrode 41.

Some of the accelerated ions then pass through a second slit 49' in electrode.

Owing to the combined action of said ion ac-.

' celerating fields and the-magnetic field, positive ions passing through said collimating slits follow circular. paths, and ions of a predetermined mass-to-charge ratio are focused at exit slit i positioned in front ofv an ion collector 53. when an ion beam impinges ion collector 53 a corre-' sponding ion current is produced, which current is amplified by amplifier 55 and indicated by galvanometer 51.

In order to provide for automatically recording a mass spectrum of a sample, an automatic recorder 59 is connected to the amplifier so as to record the intensities of ion beams of diflerent mass-to-charge ratios which are successively moved past said collector slit ii by automatically changing voltages supplied to said pusher electrode 43', and said collimating slit electrodes 4i and '41, by electrical connection to suitable points of a potential divider 60 in ion beam deflection control circuit 61, which circuit may be prepared for operation by closing key 62 and set into operation by opening said key.

As beams comprising ions or different corresponding mass-to-charge ratios are swept past said exit slit 5|, ions in the different beams are successively discharged at the ion collector 53, and corresponding ion currents actuate recorder 59 thereby producing a mass. spectrogram i! of the sample, said mass spectrogram being in the form of a trace 6! in which successive trace displacements represent correspondingintensities.

In order to determine how the intensity ot'any particular ion beam varies with the energy of the electrons in the electron beam, I adopt one of two procedures. In the first, the total accelerating voltage to which ions are subjected may be set at a predetermined value by adjustment of the total potential between pusher electrode 48 and collimating slit electrodes and 41 by suitable positioning of the slide contact 61 on potentiometer S9 in the deflection control circuit Bl. thereby focusing ions or any predetermined mass-to-charge ratio desired at the exit slit 5|. With the mass spectrometer adjusted to detect ions of predetermined mass-to-charge ratio, the energy of the electrons in the ion beam may be said second collimating slit peak of interest in each component of interest. In the second procedure I set contact 38 successively at diiierent points of rheostat 31 to establish different values of electron accelerating voltages as indicated by a voltmeter 66, and make a mass spectrogram o! the sample underinvestication at each of these voltages for each component in which I am interested, by the recording method hereinabove described. If desired, the beam deflection control circuit 6i may be operated to sweep over only a single peak of interest at each of the voltages by suitable adjustment of contact 8'! and proper manipulation of key 62.

In both of these methods the voltage between the filament l1 and the electron beam intensity control electrode 2| is maintained constant and preferably at a value such that the total electrical current borne by electron beam I8 is constant. In Fig. 3 I have plotted a typical ionization curve a representing a plot of the intensity of the beam of ions comprising ions of predetermined 'mass-to-charge ratio obtained from a pure component as a function of electron accelerating voltage. As indicated by this curve, practically no ions are formed when-this voltage is at a value less than that indicated by the, point A, which represents the appearance potential of the ions in question. Curve b represents the ionization curve of another component having a lower appearance potential B. By reference to these, curves, it isseen that, if the ions to which the curves refer are of the same mass-to-char'ge ratio, the component corresponding to curve b may be determined by measuring the amount of ions being formed when the ionization voltage is between the values corresponding to the appearance potentials of the two components. 7

In the table shown in Fig. 4, I have shown the appearance potentials of various ions formed from nitrogen (N2) oxygen (0;) carbon monoxide (CO), and carbon dioxide (CO2). Examples of the use of my invention in the analysis of various mixtures of these components are discussed hereinbelow.

Example I-Mixture of CO and C02 7 A mixture of carbon dioxide and carbon monoxide may be analyzed by obtaining a mass spectrum or the mixture at an ionization voltage between 14.4; and 20.4 volts, thus producing ions (30* and CD2+ respectively from C0 and C02 without introducing anyC0+ ions from C02. The

; amounts or said components present may then adjusted to successivelyjdiflerentvalues indicated by voltmeter 88 and the corresponding intensities of the ionbeam measured by means of salvanolneter 51. This procedure is repeated tor each be determined readily from the observed beam intensities by dividing the respective beam intensities by the corresponding sensitivity coeflicients "of the respective components at the ionization voltage used on the mixture.

If desired, the sensitivity of the analysis with respect to CO: may be increased by using a higher voltage when measuring the intensity or the 002+ ion beam.

Example Il -Mixture of O2 and CO2 The amounts of carbon monoxide, oxygen, and

II carbon dioxide present in amixture or these comintensities of 'corresponding'molecular ion beams comprising ions 01+, and CD2+ respectively formed at a voltage between 14.1 and 20.0 volts, which are respectively the appearance potentials of (30* from C0 and 601+ from CO2.

Example Iii-Mixture of CO and N:

The amount of carbon monoxide may be de- Example V-Mizture of N2 and co: By ionizing a mixture of nitrogen and carbon dioxide at a voltage between 16.5 volts, the appearance potential of Kr and 20.4 volts, the ap-- pearance potential oi'CO+, individual beams cor. responding to the two components maybe produced. I

Example VI-Mirture of N2, 0:, CO, and C0;

Individual indications of the amounts of the three components, oxygen,'carbon monoxide, and

carbon dioxide present in a mixture containingthese gases and nitrogen, may be obtained by measuring intensitiesot the corresponding beams comprising ions 0 00+, and CD2+, respectively,

produced at a voltage between 14 and 16.5 volts.

The amount of nitrogen present in such a'mixture can then be determined by measuring the intensity of a beam of atomic nitrogen ions N+ roduced at a voltage greater than 24 volts.

From the foregoing discussion it is apparmt that I have provided a new method of mass spectrometry which under many conditions simplifies computations, and in many cases makes it possibleto analyze a mixture with respect to some components without the necessity of determining the quantities of. other componentsalso-present.

While my invention has been illustratedin connection with the analysis of mixtures from which it is possible to obtain beams which correspond individually to the mixture components, it is clear that mymethod may also be utilized when the ionization voltage is such that other components affect the intensities of the beams as well as those hereinbe'fore considered. When more than one component ailects the intensity of abeam, simul- 2,878,151 ponents may be determined by measuring the the method which comprises ionizing a sample of said mixture at such a predetermined ionizataneous equations or the typ given in Equation 1 may be used to compute the composition oi the mixture under invenstigation, the advant ge of my improvement in this case being that there are fewer terms in such equationsrthereby increasing the speed of amount.

I claim: a 1 1. In analyzing a mixture containing a plurality oi components with respect to onecomponent,

the computation a corresponding oi said components.

tion voltage that ions of a selected mass-.tocharge ratio which may be formed from each of said components at some ionization voltage are actually formed from only. said one component,

ionizing at the same voltage a reference sample v containing a known amount of said one compom nent, measuring the amounts of said ions formed from both said samples, and determining from said measurements the quantity of said one comrponent present in the mixture.

2. In the analysis of a mixture containing two components both of which are capable of forming ions of the same mass-to-charge ratio at different ionizing potentials involving the determination of the amounts of said components in the mixture by comparison 01 measurements of the quantities of ions of different mass-to-chrage ratiosformed in a mass spectrometer from said mixture and from reference samples including difierent proportions of the components, the improvement which comprises ionizing said mixture at one ionization voltage to form ions of a given mass-to-charge ratio from only one of said components, ionizing said mixtureat another ionization voltage to form ions of the same mass-tocharge ratio from only the other component,

measuring the quantities or such ions formed at so the respective voltages, and utilizing measure- .ments of the quantities of ions formed at the respective voltages to determine'the quantities of the corresponding components present in the mixture.

3.1a the analysis of a mixture son of measurements of the quantities of ions of predetermined mass-to-charge ratios formed in a mass spectrometer from said mixture'and from 40 reference samples including diile'rent proportions of the components, the improvement which comprises ionizing said mixture at such an ionization voltage that ions of a given mass-to-chargeratio which may be formed from each of said components at some ionization voltage are formed at said ionization voltage from less than all of said components to provide a basis for determiningthe quantities of said latter components present in the mixture.

4. In the analysis of a mixture containlng'a plurality of components such that ions of the same mass-to-charge ratio can be formed from i more than one of the'oomponents, involving the determination of the amount of said components in the mixture by comparison of measurements of the quantitiesof such ion of diilerent predetermin'ed mass-to-charge ratios formed in a mass .spectrometer from said mixture and from reier-' ence'samples including diiterent proportions of the components, the improvement which comprises ionizing said mixture during the measurement of -the respective amounts of such ions at such an ionization voltage that such ions 01 each mass-to-charge ratio are Danna. nwmn'r TAYLOR containing a plurality of components involving the compariformed from only one

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