GB2138201A

GB2138201A - Mass spectrometer

Info

Publication number: GB2138201A
Application number: GB08308445A
Authority: GB
Inventors: Thomas William Ottley
Original assignee: Prutec Ltd
Current assignee: Prutec Ltd
Priority date: 1983-03-28
Filing date: 1983-03-28
Publication date: 1984-10-17
Also published as: IT1173606B; GB2138201B; EP0138898A1; IT8420251A0; AU2659384A; WO1984003994A1; GB8308445D0

Abstract

A mass spectrometer has four electrodes (40) constituting a quadrupole structure which are supported at least at one end by being urged against the outer surface of a tube (42) of predetermined outer diameter, at least the radially inner surface of the tube (42) being electrically conductive. This construction improves the accuracy of mounting while reducing manufacturing costs.

Description

SPECIFICATION Mass spectrometer The present invention relates to a mass spectrometer and is particularly concerned with a mass spectrometer of the quadrupole type.

Several types of mass spectrometer are known which work on the principle of ionising atoms or molecules and then separating the ions based upon mass or mass to charge ratio. One method of separation uses the fact that if the ions pass through a transverse magnetic field they are deflected by different amounts and a collector arranged behind a slit can then serve to detect ions with different mass to charge ratios.

Other forms of mass spectrometer are also known but the type of spectrometer with which the present invention is concerned is a quadrupole mass spec trometer which for the purpose of background information will now first be described by reference to Figure 1, this being a schematic diagram of such a spectrometer.

An ion source 10 introduces accellerated ions into a quadrupole structure made up of four electrodes 12 which are arranged symmetrically at the corners of a square. At the end of the electrodes 12 remote from the ion source 10 there is arranged a collector 14 sensitive to any ions which have traversed the length of the electrodes 12.

The ion source 10 may be of different types and as an example may rely on pyrolysis or may employ a plasma. The collector may also be constructed in various ways and may for example include an electron multiplier for higher sensitivity. The invention is however primarily concerned with the quadrupole structure, which will now be considered in greater detail.

In use, the two electrodes arranged on a first diagonal are connected to have a mean D.C. voltage U/2 and a high frequency voltage (V/2).cos(w.t) is applied between the two electrodes. The two electrodes on the other diagonal, on the other hand, have a mean D.C. level -U/2 and a high frequency voltage (-V/2).cos(w.t) is applied between the two electrodes. In other words, alternating voltages are applied in anti-phase across the opposite diagonals and the electrodes have D.C. levels which are positive in one case and negative in the other.

In such an arrangement, if an ion travels down the quadrupolefield created by the electrodes 12 it will follow a complex path tending to spiral or oscillate under the effect of the high frequency field. Depend ing on the mass m, frequency f (f=2 7r w) and the values U and V some ions will succeed in reaching the collector 14 while in other cases their path will be perturbed causing them to collide with one of the electrodes 12 whereupon they will not be sensed by the collector 14. The quadrupole structure therefore constitutes a selective filter which only permits ions of which the mass falls within a particular range determined by the frequency and the various voltages to reach the collector.

Figure 2 is a graph of two parameters plotted against each other, the parameters being termed A and Band being given by the following equations A = 4.q.U /(m.w2.r,2) B = 2.q.V/(m.w2.rO2) where q is the electronic charge, m is the mass of the ion, w is the anguiar frequency, and is S the radius of the inscribed circle which just touches the four electrodes.

The lines drawn in Figure 2 represent the boundary of the stable conditions and all ions satisfying the conditions within the shaded area will succeed in reaching the collector. In the region of the base of this shaded area, for any given value of A there will be many values of B which satisfy the conditions for stability. In other words, the quadrupole filter will not be particularly selective and will allow a wide range of masses to reach the collector. As the object of the spectrometer is to discriminate between ions of different mass, the ratio of the voltages U and V is set to work along the operating line 20 in Figure 2 which intersects the stable shaded area very near to its peak so that for a given value of the parameter A ions satisfying only a very small range of the parameter B will succeed in traversing the filter.If the radius of the inscribed circle is constant and the angular frequency w is constant then since B is fixed the only remaining variable is the mass m of the ion.

Consequently, if U and V are changed simultaneously such that the ratio remains constant following the operating line 20 in Figure 2 then at different values of U and V only ions with a small range of masses will succeed in reaching the collector 14.

In practice the ratio of U to V can be maintaind constant if U is derived by rectifying and dividing the high frequency signal. By varying the voltages applied to the quadrupole in the manner described above one can thus scan the mass spectrum and determine the ions which are present.

It is naturally desirable for each mass to produce a sharp spike when scanning is effected by varying the voltages U and V in this manner but in practice in place of spikes one detects peaks which have a finite width tending to reduce the resolution of the spectrometer. One of the reasons for this is that it has been assumed that in the equations given for the parameters A and B that both the frequency remains constant during the traverse and the radius r0 does not vary along the length of the electrodes.The frequency can in practice be maintained substantially contant because of the stability of crystal oscillators but maintaining the separation of the electrodes constant over the length of the quadrupole presents problems and it can be shown that when measuring atomic mass units in the regions of 250 an inaccuracy of one part in 5,000 in r0 suffices to reduce the height of the peaks by some 10% with a corresponding spread in the width of the peaks. The invention is concerned with the manner of manufacturing the quadrupole structure so as to maintain r0 constant over the length of the electrodes.

Figure 3 of the accompanying drawings shows one known method of mounting the electrodes of the quadrupole structure. In Figure 3, the electrodes 12 are surrounded at each end by a ceramic collar 30 which has accurately ground surfaces against which there rest rods 32 constituting the electrodes. The electrodes 32 are retained in place by fastening means such as screws which pass into the rods through the collar 30.

This arrangement has several disadvantages.

First, it is desired to determine r0 with accuracy and this is the radius of the inscribed circle not that of the circumscribed circle. The two are, of course, related but any variation in the rod diameter will cause a variation in r0 even if the ceramic collar 30 has been accurately formed Also, from a constructional point of view, hollow grinding of the ceramic collar accurately is expensive to implement.

Another problem which affects the performance of the spectrometer is that the parts of the collar lying between the rods are of insulating material and these face the ion beam. This is undesirable because any ions which fall on the collar will tend to charge the collar and will disturb the electric field. The collar must however of necessity be of an insulating material given that the electrodes are at different potentials.

With a view to mitigating the foregoing disadvantages, the present invention provides a mass spectrometer having four electrodes constituting a quadrupole structure which are supported at least at one end by being urged against the outer surface of a tube of predetermined outer diameter, at least the radially inner surface of the tube being electrically conductive.

In the preferred embodiment of the invention, the tube is formed entirely of a conductive material and at least the ends of the electrodes in contact with the tube are formed of a ceramic or other insulating material. If desired the entire electrodes may be formed of an insulating material provided with an electrically conductive coating such as gold.

In order to secure the electrodes in position, there may be provided a collar surrounding the tube defining stop surfaces at 90 intervals and means for urging each rod against one of the stop surfaces and the outer surface of the said tube.

Advantageously, the tube has an extension spaced inwardly from the electrodes and axially overlapping the parts of the insulating end portions of the electrodes so as to shield the insulating portions of the electrodes from the ion beam.

The tube of predetermined outer diameter may at one end of the quadrupole structure be constituted by an electrode forming part of a gun for accellerating the ions in the ion beam.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a mass spectrometer of quadrupole type; Figure 2 is a graph for explaining the operation of the mass spectrometer in Figure 1; Figure 3 is a section through the end of a quadrupole structure in which the electrodes are mounted in a conventional manner; Figure 4 shows a side view of quadrupole structure of a mass spectrometer of the invention; and Figure 5 is a section along the line V-V in Figure 4.

Referring now to Figure 4, there are shown two electrodes 40 which consist of titanium rods 40a vacuum braised onto ceramic end portions 40b. The ceramic end portions 40b rest against two metal tubes 42 the outer diameter of which is accurately machined. Each tube 42 is formed as part of an axial end plate 44,46 of which the end plate 44 forms part of a gun for accellerating the ion beam and the end plate 46 may serve to mount the collector for detecting ions which have traversed the quadrupole structure.

In the section shown in Figure 5, it is seen that the ceramic end portions 40b of the electrodes 40 are arranged within recesses in the end plate defines both the tube 42 and a collar surrounding the end portions 40b, the collar in turn defining stop surfaces 50 which are equi-angularly spaced.

Each rod 40 is held in place by means of a plunger 52 which is slidable in a bore 53. A screw 56 is received in the end of the bore 53 and a spring 54 acts between the screw 56 and the plunger 52 to urge the latter in the general direction of one of the stop surfaces 50. The electrode end portions 40b therefore make three point contact with the tue 42, a stop surface 50 and the plunger 52. The radius of the inscribed circle of the quadrupole structure is therefore accurately predetermined and the position of the electrodes circumferentially is also accurately maintained without resorting to expensive machining operations in a ceramic.

The axial end plates 44 and 46 may conveniently be formed of a metal if the electrodes have insulating ends as in the embodiment described. As an alternative, however, it is possible for the ends to be of a ceramic or other insulating material and only the inner surface of the innertubular portion need be coated with a conductive material. Even if the end plates are made of an insulating material, the invention still offers the advantage of ease of construction over the prior art since it is less difficult to grind accuratelythe outer surface of the tubular portion 42 which accurately predetermines the radial spacing of the electrodes 40.

It will be noted that the tubular portion 42 has an extension 42a of reduced outer diameter which extends into the region of the electrically conductive portions 40a of the electrodes 40. Because of the reduction in the outside diameter, there is no short circuit but this extension 42a still shields the insulating parts of the mounting from the ion beam in order to prevent a build-up of charge on an insulator.

The entire electrodes structure is surrounded by a housing 58 onto which the axial end plates are clamped and terminals 60 mounted in an insulating manner on the outer surface of the housing 58 are connected to the electrodes 40 by leads 62 which pass through the housing 58. The potentials to establish the quadrupole field are applied to the terminals 60.

Instead of electrodes which are formed of titanium rods braised onto insulating end portions, it is alternative possible for the electrodes to be formed entirely of an insulator which is rendered conductive in its central region by being coated with a conductive material such as gold.

Though the embodiment described above employs rods of circular section, it is alternatively possible for the electrodes to be of hyperbolic section and they need not be constructed as rods.

Claims

1. A mass spectrometer having four electrodes constituting a quadrupole structure which are supported at least at one end by being urged against the outer surface of a tube of predetermined outer diameter, at least the radially inner surface of the tube being electrically conductive.

2. A mass spectrometer as claimed in claim 1, wherein the tube is formed entirely of a conductive material and at least the ends of the electrodes in contact with the tube are formed of a ceramic or other insulating material.

3. A mass spectrometer as claimed in claim 2, wherein the entire electrodes are formed of an insulating material provided with an electrically conductive coating, such as gold.

4. A mass spectrometer as claimed in any preceding claim, wherein in order to secure the electrodes in position, there is provided a collarsur- rounding the tube, the collar defining stop surfaces at 90 intervals, and means for urging each rod against one of the stop surfaces and the outer surface of the said tube.

5. A mass spectrometer as claimed in any preceding claim, wherein the tube has an extension spaced inwardly from the electrodes and axially overlapping the parts of the insulating end portions of the electrodes so as to shield the insulating portions of the electrodes from the ion beam.

6. A mass spectrometer as claimed in any preceding claim, wherein the tube of predetermined outer diameter at one end of the quadrupole structure is constituted by an electrode forming part of a gun for accellerating the ions in the ion beam.

7. A mass spectrometer having a quadrupole structure substantially as herein described with reference to and as illustrated in the accompanying drawings.