US 3270773 A
Description (OCR text may contain errors)
Sept. 6, 1966 c. BRUNNE 3,270,773
CLOSABLE INLET DEVICES FOR ADMITTING GAS INTO HIGH VACUUM CONTAINERS Filed Feb. 11, 1963 3y I, M} I United States Patent 2 Claims. 61. 137-561) This invention relates to a olosable inlet device for admitting gas into high vacuum containers, which comprises a capillary tube having throttle means, and which is intended particularly for alternately admitting, into the ion source of a mass spectrometer, a standard gas and a sample gas, each out of a separate storage tank and by way of a separate capillary tube having throttle means. This inlet has hitherto been so arranged that the gas was conducted, .after flowing through the throttle means, by way of additional change-over valves and pump systems, either into the ion source or into a pumping-off pipe. This conventional method, however, has considerable disadvantages. Additional valves as well as an additional high vacuum pump are required; moreover, the method entails regrettable gas losses in the intervals between measurements.
These disadvantages can be avoided according to the invention by making the inlet closable by a valve body, for example in the form of a diaphragm, adapted to be pressed against the end of the capillary tube in rear of the restrictor. In this way the effect is achieved that the volume between the restrictor and the closure point is so small as to be negligible, so that it is unnecessary to keep this space permanently under high vacuum.
In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example, one embodiment thereof, and in which:
FIG. 1 shows an inlet system such as is customarily used in determining the incidence of isotopes in mass spectrometers, and
FIG. 2 shows a corresponding inlet system in accordance with the present invention.
With a continuous inflow of a gas mixture to be analysed entering the ion source from the storage container of a mass spectrometer, usually decomposition of the gases occurs in the container, which is due to the fact that the quantity of gas flowing per unit of time through the throttle point located between the storage container and the ion source, of a determined component of said mixture, is dependent on the molecular weight of this component, so that in fact the smaller the molecular weight, the greater quantity of gas flows out. If there is a purely molecular gas influx (mean free path of molecules large compared with dimensions of container and those of the throttle point), the quantities of gas 0 :0 flowing out per unit of time behave as m zm where m, and m respectively represent the molecular weights of the respective component. In the course of time, therefore, an impoverishment of the light component occurs.
This undesirable decomposition of the gas can be countered by letting the gas flow through the throttle point not molecularly but viscously. This can be efiected by increasing the pressure in the storage container. The pressure must be so high that the mean free path of the gas molecules becomes small compared with the dimensions of the throttle point (condition for viscous flow). With the conditions obtaining in a mass spectrometer, this is the case with a pressure of 100 to 500 torr. The purely viscous flow exhibits no mass dependence. However, the decomposition cannot be entirely avoided in this way.
3,270,773 Patented Sept. 6, 1966 Since on the ion source side, a high vacuum is always present, that is to say, a purely molecular flow obtains, an area of transference between viscous and molecular flow is always present at the throttle point. The purely viscous flow, immune from decomposition, thus cannot be immediately obtained at the throttle point. Closer consideration reveals, however, that the overlap of the residual decomposition occurring at the throttle point over the quantity of gas in the storage container can be practically completely prevented by interposing between the throttle point and the storage container a capillary tube of a diameter of approximately 0.1 millimetre. This of course implies the necessity of keeping the volume between the end of the capillary tube and the throttle point as small as possible, so as to keep the time constant small in the flow of gas. For this reason, it is most advantageous to squeeze the capillary tube at one end, and use this squeezed point as the throttle point, in which case the disturbing volume is practically nil.
In making measurements by means of a spectrometer it is often required, particularly in the case of determining the frequency of isotopes by the double catcher process, to permit the inflow of a standard gas and .a sample gas alternately into the ion source h, each from separate storage container a and b, and each by way of a separate capillary tube 0 and d and separate throttle point f and g, respectively. This has hitherto been done in the manner indicated in FIGURE 1, by conducting the gas, after flowing through the throttle point 1 or g, through change-over valves i, k and n, m and pump means, either by way of a duct 0 into the ion source h, or else by way of a pumpingoff line p with a high vacuum pump 1' (of. C. R. McKinnay, J. M. McCrea, S. Epstein, H. A. Ellen, H. C. Urey in Rev. Sci. Instr. 21,724 (1950)). This method, however, entails two substantial disadvantages, namely:
(1) Auxiliary valves k, n are necessary, as well as an auxiliary high vacuum pump r, while in respect of the final vacuum extremely heavy demands have to be imposed upon the pumping system, since this pump branch is situated on the ion source side of the throttle point;
(2) The gas flows continually out of the storage containers a, b even when no measurements are made. As a storage container usually contains the standard gas which very often is difficult to prepare and which goes to waste in this way in the intervals between measurements, such a gas loss is highly undesirable. Moreover, it is frequently the case that only a small amount of the sample gas is present, so that there is an unprofitable loss of the sample.
These difficulties are overcome by using an inlet system in accordance with the present invention, as shown in FIGURE 2. The throttling is effected in the capillary tubes c, d by squeezing between two short clamp jaws s, t at a geometrically accurately defined point 1 and g, respectively. Immediately behind the throttle point, the capillary tube is cut OE and its edges sharpened to a blade-like edge. The short capillary end u on the ion source side is seated in a bored valve stem v, which is sealed against the housing x of the valve by a diaphragm w and may be shifted in a usual manner by a valve spindle y with handle z. By turning the spindle inwards valve stem v can be pressed against the capillary end u, whereby the gas influx into the ion source it can be interrupted. The essential feature of this arrangement is that the closure is effected at the capillary itself, and thus directly at the throttle point. The capillary end it may be made considenably shorter than is shown in the diagrammatic representation in the drawing. If, in the embodiment of FIG. 1, the closure were to be effected only in the inlet duct 0 by means of a valve, the gas influx could indeed be interrupted; but, after the closing of such a shut-off valve the gas would continue to flow into the volume between the throttle point and the valve until the pressure prevailing in this volume was equal to that in the storage container (i.e. approximately atmospheric pressure). Reopening of the valve Would then no longer be possible, since then the entire accumulated quantity of gas would be expanded all at once into the high vacuum chamber of the ion source. With the arrangement as described with reference to FIG. 2, no such pressure thr-ust occurs, for the reason that the volume between the throttle point and the closure point is so small as to be negligible.
Within the framework of the present invention numerous modifications and other forms of construction are feasible; in particular, the invention is applicable to all cases where it is required to have a closable inlet consisting of a capillary tube throttle means for admitting gas into high vacuum vessles. Instead of a capillary tube with a throttle point, it would be possible in such case to use a suitable fine capillary tube of appropriate length not having a throttle point.
What I claim is:
1. In a mass spectrometer having a gas storage tank and a high vacuum ion source, the improvement comprising a closable inlet device interconnecting said gas storage tank and said ion source,
said closab le inlet device comprising a capillary tube having a first end operatively connected with said gas storage tank,
a second end of said tube comprising an outlet means to said high vacuum ion source,
a throttle means disposed immediately adjacent and interconnected only with said second tube end of said tube without interconnection of a high vacuum pump between said throttle means and said second tube end, and
valve means operatively associated with said outlet means for movement into positions seating against and closing said outlet means.
2. The improvement in accordance with claim 1 wherein said capillary tube has an inner diameter of approximately 0.1 mm. and said valve means comprises a bore valve stem encompassing said second end of said tube and mounted by means of a diaphragm for reciprocation into positions opening or closing said outlet means.
References Cited by the Examiner UNITED STATES PATENTS 219,178 9/1879 Schmidt 1376l4 930,635 8/1909 Warter 251331 X 2,244,311 6/1941 Nee et al 138-44 2,334,166 11/1943 Allen 251118 X OTHER REFERENCES 153,392 11/1920 Great Britain.
M. CARY NELSON, Primary Examiner.
I. FENNELL, L. KAMPSCHROR, A. ROSENTHAL, Assistant Examiners.