US2769910A - Mass spectrometer - Google Patents

Description

Nov. 6, 1956 G. A. ELINGS 2,769,910

MASS SPECTROMETER Filed Sept. 1'7, 1953 INVEN TOR I GARMT ALLARD ELINGS AGENT Flee Patented Nov. 6, 1956 MASS SPECTROMETER- Garmt Allard Elings, Hartford National Conn., as trustee Application September 17, 1953, Serial No. 380,813

Claims priority, application Netherlands September 30, 1952 Claims. (Cl. 250-413) Eindhoven, Netherlands, assignor to Bank and Trust Company,Hartford,

The invention relates to a mass spectrometer, i. e. a device for analysing or, if desired, decomposing substances into their constituents, the substance to be examined being ionized in a gaseous state, the ions then being subjected to an electric and/or magneticfield in a manner such that a decomposition of the constituents according to the mass or at least the ratio between the mass and the charge is obtained.

Most known mass spectrometers require a comparatively strong magnetic field of large extension and adequate homogeneity. The apparatus are thus bulky, heavy and costly. These mass spectrometers may, it is true, provide a sharp decomposition of substances, but this is not always of importance, since for many kinds of examinations a comparatively general impression of the composition of a mixture may frequently be sufficient.

The invention has for its object to provide a mass spectrometer of simple construction, which may nevertheless yield satisfactory results.

A mass spectrometer according to the invention comprises an ion gun and a collecting electrode, between which two conductors shaped in the form of a double helix are arranged in a manner such that the axis thereof coincides with the junction line of the ion gun and the collecting electrode, an alternating voltage, taken from a generator, being applied between the conductors of the double helix, the frequency of this voltage being such that the ions of a given velocity traverse the length of one pitch in one period, the ions deflected from their paths under the action of the radial field in the double helix being collected, and measured or recorded.

The ions of which the velocity fulfils the given condition are deflected from their paths, whereas the others are deflected to a smaller degree within the double helix. After they have left the double helix, the ions maintain their velocity in the direction of deflection. By varying the frequency of the alternating voltage applied to the double helix, ions of difierent masses may be deflected and collected. The fact that only ions of a definite velocity and hence of a definite ratio between charge and mass are deflected, may be accounted for in a simple manner as follows:

If it is assumed that the diameter of the ion beam travelling along the axis of the double helix is sufliciently small relative to the diameter of the helix and to the pitch, it may be assumed, as a first approximation, that the strength of the radial field acting upon the ions is independent of the position in the beam and of any deflections already performed.

The radial force exerted on the ions varies as a cosine function with time owing to the alternating voltage applied to the double helix. The component of the force parallel to a given plane through the axis varies also as a cosine function, with the path covered in the helix, owing to the helical field. Consequently the total radial 2 velocity maybe expressed, with the exception of one factor by:

t f cos wt cos 21r dt 0 a s wherein x designates the path covered in the helix in the time t and s designates the pitch of the helix. However, for ions fulfilling the aforesaid condition may be expressed by wt; for all others this is nt, wherein 11. may be higher or lower than 1. In the first case :c t a 1 1 1 cos wt cos21r-dt= cos wtdt=(wt+ sin wt cos wt 0 s o w 2 2 This means that the lateral velocity obtained increases proportionally to the path covered in the helix, with the exception of the term sin wt cos wt, which becomes soon negligible. For the second case the result of the integration is. the sum of two sine functions; this means that. the lateral velocity does not increase with the path covered, but it varies with time as those functions which can attain only a low value.

The direction in which the lateral velocity is determined for one kind of ions, depends on the phase of the voltage applied to the double helix at the entrance of the ions. Since this differs for all ions, the directions of deflection arealso different. Sinceafter' thtraversal of the double helix the lateral velocity no longenih creases, the deflected ions move along a cone surface. It is not necessary to construct the double helix in the form of a cylindrical helix, but the influence of inhomogeneity of the field on ions already deflected may be better avoided by a double helix wound in an adequately chosen plane of revolution, of which the generatrix, viewed from the entry of the ions, has a positive first and second derivative.

In order that the invention may be readily carried into effect, it will now be described with reference to the accompanying drawing, which shows diagrammatically a mass spectrometer according to the invention.

Reference numeral 1 designates an elongated glass tube having widened ends, in which an ion source 2 and a collecting electrode system 3 are arranged. The double helix is designated by 4 and the collecting electrode system comprises a ring 5 and a plate 6. A high-frequency generator 7 having push-pull output is connected to the double helix and a current measuring apparatus 8, for example, a cathode-ray oscillograph, is connected to the ring 5. The ion gun has an acceleration voltage of l kv., so that the two electrodes 5 and 6 and an acceleration electrode 10 also have a voltage of 1 kv. (negative) with respect to the cathode 9 of the ion source. The double helix has 2X54 turns and a pitch of 6 mms. and a diameter of 12.5 mms. The high-frequency generator 7 supplies an alternating voltage having frequencies variable from 3 to 40 mc./s. and a peak voltage of a few times 10 volts. Thus the mass spectrometer is suitable for use with masses of about to 10 (with single charge). Thecharge of the deflected ions, collected by the ring electrode 5, is passed through the measuring apparatus 8. If the latter is a cathode ray oscillograph, the vertical deflection of the electron beam therein may, for example, indicate the value of the ion flow and the horizontal varying the frequency of the high-frequency generator 7 in synchronism with the time base of the oscillograph. Although with the same number of turns, the same pitch and the same high-frequency voltage, the deflections obtained are less great, the double helix may be, as an alternative, wound on the outer side of the tube. Then the apparatus may be manufactured in a simpler manner.

deflection may indicate the mass thereof by i What is claimed is:

1. A mass spectrometer comprising an ion gun, a colto a particular frequency of said alternating voltage are deflected away from the axis of the helix.

2. A mass spectrometer comprising an ion gun, a collecting electrode including a plate and an annular member, a pair of conductors shaped in the form of a double helix between said gun and said collecting electrode and coaxially arranged therewith, means for applying an alternating voltage of variable frequency between said conductors whereby ions of a given mass corresponding to a particular frequency of said alternating voltage are deflected toward said annular member, and indicating means coupled to said annular member for indicating the extent of ion collection thereof. 3. A mass spectrometer comprising a glass envelope having widened ends, an ion gun disposed at one widened end of said envelope, a collecting electrode including a plate 'and an annular member disposed at the other widened end of said envelope, a pair of conductors shaped in the form of a double helix disposed within said envelope between said gun and said collecting electrode and coaxially arranged therewith, means for applying an alternating voltage of variable frequency between said conductors whereby ions of a given mass corresponding to a particular frequency of said alternating voltage are deflected toward said annular member, and indicating means coupled to said annular member for indicating the extent of ion collection thereof.

4. A mass spectrometer comprising a glass envelope, an ion gun disposed at one end of said envelope, a collecting electrode including aplate and an annular member disposed at the other end of said envelope, a pair of conductors shaped in the form of a double helix surrounding said envelope between said gun and said collecting electrode and coaxially arranged therewith, means for applying an alternating voltage of variable frequency between said conductors whereby ions of a given mass corresponding to a particular frequency of said alternating voltage are deflected toward said annular member, and indicating means coupled to said annular member for indicating the extent ofion collection thereof.

5. A mass spectrometer comprising an ion gun, a collecting electrode, a pair of conductors shaped in the form of a double helix wound in a plane of revolution having a generatrix having a positive first and second derivative and disposed between said gun and said collecting electrode and coaxially arranged therewith, means for applying an alternating voltage of variable frequency between said conductors, whereby ions of a given mass corresponding to a particular frequency of said alternating voltage are deflected away from the axis of the helix, and means coupled to said collecting electrode for measuring the current of the deflected ions as a function of the frequency of said applied voltage.

References Cited in the file of this patent UNITED STATES PATENTS

Images