WO1991015029A1

WO1991015029A1 - Plasma mass spectrometer

Info

Publication number: WO1991015029A1
Application number: PCT/GB1991/000437
Authority: WO
Inventors: Andrew John Dixon; David Gregson
Original assignee: Fisons Plc
Priority date: 1990-03-23
Filing date: 1991-03-22
Publication date: 1991-10-03
Also published as: GB9006532D0

Abstract

The invention comprises a plasma mass spectrometer wherein ions are sampled from an inductively-coupled or microwave-induced plasma through an orifice in a sampling member and subsequently pass through a second orifice (46) in the smaller end of a hollow tapered skimmer member (43) to a mass analyzer. The skimmer member (43), conveniently conical, is typically such that the external included angle (51) of a first portion immediately adjacent to the orifice (46) is between 90° and 180° and the external included angle (44) of a second portion adjacent to the first portion is typically less than 60°. A plasma mass spectrometer incorporating such a skimmer has a higher ion transmission efficiency than prior types.

Description

Plasma Mass Spectrometer

This invention relates to a mass spectrometer wherein a sample is ionized in a plasma, for example an inductively coupled plasma (ICP) or a microwave induced plasma (MIP).

ICP or MIP mass spectrometers are widely used for the determination of elemental composition. Samples may be introduced into the plasma either in an aerosol prepared by nebulizing a solution of the sample with an inert gas, or as a vapour generated by laser ablation or by electrothermal vaporization. Ions are sampled from the plasma through a first orifice in a cooled sampling member into a first evacuated region, and then pass through a second orifice in a skimmer member into a second evacuated region maintained at a lower pressure than the first region. Mass analysis may be accomplished by a guadrupole mass analyzer disposed in the second evacuated region or by a magnetic sector analyzer which receives ions passing through the second orifice. An electrostatic lens system is disposed in the second evacuated region to efficiently transmit ions from the second orifice to the mass analyzer.

Up to now, the sampling member - skimmer member interface has been designed as though it were a molecular beam source of the nozzle-skimmer type described by Campargue (J. Phys. Chem, 1984 vol. 88 pp. 4466-4474) and Beijerinck, Van Gerwin, et al. (Chem. Phys. 1985 vol. 96 pp. 153-173), in which the external included angle of the skimmer member must be less than about 60° if reasonable transfer efficiency is to be obtained. When the angle is less than 60° the Mach Disk is downstream of the tip of the skimmer member and sampling takes place from the "zone of silence" between the sampling member and the Mach Disk. If the angle is too large the Mach Disk becomes separated from the skimmer member and turbulence between it and the skimmer member causes a serious loss of source efficiency. Similarly, Campargue and others teach that the edge of the second orifice in the skimmer member should be as sharp as possible, and that the internal included angle of the skimmer member should be as large as possible. Thus a typical prior skimmer member may be a hollow cone with external and internal included angles of 55° and 45° respectively, and an orifice with a very sharp edge, as shown in figure 1A. Manufacture of such a skimmer presents considerable difficulty, however, and consequently in most practical skimmer members used in prior plasma mass spectrometers the second orifice is a small drilled hole as shown in figure IB.

We have proposed to use skimmer members having at least a portion whose interior included angle is considerably greater than 60° (see, for example, PCT publication number WO 90/09031). Use of a larger included angle permits a stronger extraction field to be generated inside the skimmer member which can improve the efficiency of ion transmission. Best results are obtained by the use of a "dual taper" skimmer member, such as that illustrated in figure 1C, wherein the smaller end of the member comprises a cone of external included angle 55° which extends for approximately 3mm from the orifice before broadening out into a cone of external included angle of 120° or greater. When used in a plasma mass spectrometer, such cones result in improved ion transmission and reduced matrix suppression effects in comparison with the type of skimmer member shown in figure IB. Nevertheless, the efficiency of ion transmission of even these improved interfaces is still low and is a major limitation on the sensitivity of prior plasma mass spectrometers.

It is an object of the present invention to provide a plasma mass spectrometer which has higher ion transmission efficiency than prior types. It is another object to provide an improved skimmer member for use with a plasma mass spectrometer, and yet another object to provide a method of manufacture of such a member. The invention provides a mass spectrometer comprising a mass analyzer, means for generating a plasma in a flow of gas, means for introducing a sample into said plasma, a sampling member adjacent to said plasma comprising a first orifice through which ions characteristic of said sample may pass into a first evacuated region, and a hollow tapered skimmer member disposed with its smaller end closest to said sampling member, wherein said skimmer member has an axis and, formed in said smaller end and on said axis, a second orifice through which ions may pass from said first evacuated region to a second evacuated region maintained at a lower pressure than said first evacuated region and subsequently to said mass analyzer, said skimmer member being such that said smaller end is downstream of the intersection of an axial projection of said second orifice with a notional extension of the external surface of said skimmer member from a point where its external included angle is a minimum.

Viewed from another aspect the invention provides a mass spectrometer comprising a mass analyzer, means for generating a plasma in a flow of gas, means for introducing a sample into said plasma, a sampling member adjacent to said plasma comprising a first orifice through which ions characteristic of said sample may pass from said plasma to a first evacuated region, and a hollow tapered skimmer member comprising a second orifice in its smaller end through which ions may pass from said first evacuated region to a second evacuated region maintained at a lower pressure than said first evacuated region and subsequently to said mass analyzer, wherein the external included angle of a first portion of said skimmer member immediately adjacent to said second orifice is substantially greater than the external included angle of a second portion of said skimmer member downstream of said first portion. Preferably the included angle of the first portion is greater than 60° (typically, between 90° and 180°) and the included angle of the second portion is 60° or smaller.

In a preferred embodiment the skimmer member is such that its smaller end is downstream of the intersection of an axial projection of the second orifice and a notional extension of the external surface of the skimmer member from a point where its external included angle is both a minimum and less than approximately 60°. In such a case the external included angle of a portion of the skimmer member immediately adjacent to the second orifice will be substantially greater than 60°, for example in the range 90°-180°.

Further preferably, both the sampling member and the skimmer member are conical (strictly, frustro-conical) . The skimmer member may have an internal included angle in the immediate vicinity of the second orifice substantially greater than 60°, and preferably greater than 90°, and the internal included angle of the portion of the skimmer member where the external included angle is a minimum should be as great as possible within the limits imposed by its external angle.

Another skimmer member suitable for use in the invention may comprise three portions, a short first portion immediately adjacent to the second orifice where the external included angle is substantially greater than 60°, a second portion adjacent to and downstream of the first portion where the external included angle is about 60° or smaller, and a third portion adjacent to and downstream of the second portion where the external included angle is substantially greater than 60°, and preferably greater than 90°. The internal included angles of each portion should also be as great as possible, particularly in the first portion adjacent to the second orifice where advantageously it may be in the range 90°-180°.

Skimmer members suitable for use in the invention need not comprise portions with linear surfaces, such as are present on a cone. The external surface may comprise curved surfaces which blend into each other. In such a case, references to the included angles of the surfaces are taken to mean the angles between tangents to the curved surfaces at an appropriate point. For example, the portion of the skimmer member immediately adjacent to the orifice advantageously may be rounded or even part-spherical, so that the external included angles referred to above are defined by tangents drawn at the orifice.

A convenient method of manufacture of a skimmer member suitable for use in the invention is to roll the tip of a conical straight-sided skimmer so that the external surface in the vicinity of the orifice becomes rounded. As explained below, as well as causing the external included angle to be increased, the internal angle may also be increased. The increased angles result from a displacement by the rolling process of the orifice in the skimmer member to a point downstream of the intersection of a tangent drawn from the point of minimum included angle of the skimmer and an axial projection of the orifice, as required in a previous definition of the invention.

The axial depth of the orifice in the skimmer member should be as short as possible for maximum advantage to be realized from the invention. This is conveniently achieved by rolling a conventional sharp-edged conical skimmer as described.

The applicants teach that in a plasma mass spectrometer advantage is to be had by making the external angle of the skimmer member in the vicinity of the orifice considerably larger than has been conventional. This is contrary to the teachings of Campargue and others that the edges of the skimmer orifice should be as sharp as possible and that the angle should be less than 60°. The applicants believe that the improved efficiency of the skimmer members described may be due to the reduced loss of ions on their interior surfaces, particularly in the vicinity of the orifice. It appears that the lack of a sharp edge at the orifice is not important in the case of a plasma mass spectrometer, possibly because the stagnation pressure of the plasma is lower than the nozzle stagnation pressures which are encountered in Campargue type nozzle-skimmer molecular beam sources.

The invention extends to a hollow tapered skimmer member suitable for use in a spectrometer as described above wherein said skimmer member has a large end and a small end spaced apart along an axis, and, formed in said small end and on said axis, an orifice through which ions may pass, said skimmer member being such that the distance between said small and large ends is less than the distance between said large end and the intersection of an axial projection of said orifice with a notional extension of the external surface of said skimmer member from a point where its external included angle is a minimum.

The invention further extends to a hollow tapered skimmer member suitable for use in a spectrometer as described above wherein said skimmer member has a large end and a small end spaced apart along an axis, and, formed in said small end and on said axis, an orifice through which ions may pass, said skimmer member being such that the external included angle of a first portion of said skimmer member immediately adjacent to said orifice is substantially greater than the external included angle of a second portion of said skimmer member closer to said large end than said first portion. Preferably the included angle of the first portion is substantially greater than 60° (typically, between 90° and 180°) and the included angle of the second portion is 60° or smaller.

The invention will now be described in greater detail by way of example and with reference to the figures, in which:-

figures 1A-1C are drawings of three types of skimmer members used prior to the present invention;

figure 2 shows a plasma mass spectrometer according to the invention; and

figures 3,4,and 5 show details of three types of skimmer members suitable for use with the spectrometer of figure 2.

Referring first to figure 2, a solution 1 of the sample to be analyzed is admitted to a pneumatic nebulizer 2 which is fed by a flow of argon gas in pipe 3 from a gas supply unit 4. The sample, entrained in argon gas, is introduced through a conventional ICP torch 6, and excess solution is drained from the nebulizer 2 through a drain 7. Gas supply unit 4 provides two other controlled flows of argon to the torch 6 through pipes 8 and 9. A radio-frequency electrical generator 10 supplies energy to coil 11 via leads 12 and 13 so that a plasma 14 is formed by the torch 6.

The torch 6 and its associated equipment including the gas supply unit 4, coil 11, generator 10 and the nebulizer 2 are conventional. Suitable equipment is described by Houk, Fassel, Flesch et. al. in Analytical Chemistry, 1980, vol 52, pp.2283-89, but other equipment is also suitable. Although figure 2 illustrates the use of a pneumatic nebulizer for introducing a sample into the plasma 14 it is within the scope of the invention to use other methods, for example, electrothermal vaporization or laser ablation.

The plasma 14 is directed against a sampling member 15 which is mounted on a cooled flange 33. A first orifice 16 is formed in the apex of the sampling member 15 and communicates with a first evacuated region 17, the pressure in which is maintained between about 0.01 and 10 torr by a vacuum pump 18. A hollow tapered skimmer member 19 separates the first evacuated region 17 from a second evacuated region 20 which is evacuated by a diffusion pump (not shown) and maintained at a lower pressure than the first region 17. The skimmer member 19 is discussed in greater detail below. An electrostatic lens assembly 21 is disposed in the second evacuated region 20 and transmits ions emerging from the skimmer member 19 to a quadrupole mass analyzer 22. In the particular embodiment shown in figure 1 the analyzer 22 is disposed in a third evacuated region 23, maintained at a lower pressure than the second region 20 and separated from it by an apertured diaphragm 30. In lower performance instruments this stage of differential pumping can be omitted and the mass analyzer 22 may be operated at the pressure of the second evacuated region 20.

Ions which pass through the mass analyzer 22 enter an ion detector 24 and strike a converter electrode 26, releasing secondary electrons which are detected by an electron multiplier 25. The signal from the multiplier 25 is amplified by the display unit 27 and is subsequently fed to a digital computer 28 and terminal 29 to allow further processing of the data. Computer 28 may also control the operation of the mass analyzer 22 and the RF generator 10, for example.

A magnetic sector mass analyzer may be employed in place of the quadrupole analyzer 22 if the sampling member 15 and the skimmer member 19 are electrically isolated from the grounded vacuum housings of the spectrometer. Sampling member 15 may then be maintained at approximately the accelerating potential required by the analyzer, and the electrostatic lens assembly 21 arranged to transmit ions to the entrance slit of the analyzer, which typically may be the apertured diaphragm 30. PCT application publication number WO 89/12313 gives full details of such an ICP spectrometer, and it will be appreciated that any of the skimmer member geometries discussed below can be incorporated in such a spectrometer.

Prior types of skimmer members are illustrated in figures 1A-1C and have been discussed previously. In each of the skimmers illustrated the smaller end is located at the intersection of the orifice and the surface of the skimmer at a point where the included angle is a minimum. Figure 1A illustrates an ideal Campargue type skimmer member 35 with a sharp orifice edge 38 and an external included angle of 55°. The internal included angle is as large as possible given the wall thickness required for mechanical strength. In view of the difficulty of manufacturing member 35, the sharp orifice edge 38 is frequently replaced by a drilled hole 36 in the apex of a thicker walled cone 37, figure IB. In that cone, the external included angle is still typically 55°, as taught by Campargue, but the very sharp edge is abandoned in the interests of mechanical strength and ease of manufacture. Skimmer members similar to that shown in figure IB are used in certain prior types of .mass spectrometers. An improved prior type of skimmer member 39 suitable for ICP mass spectrometers is illustrated in figure 1C. Member 39 comprises a front portion 40 adjacent to the orifice where the external included angle is a minimum (typically 55°) and a rear (downstream) portion 42 where the external and internal included angles are much greater. Such a member is described in PCT application publication number WO 90/09031.

In contrast, figure 3 illustrates the front portion of a skimmer member 19 according to the invention. The external included angle 44 of the major part 43 of the member 19 is approximately 55° and its internal included angle 45 is approximately 45°, similar to the values for a Campargue type cone. However, in case of the figure 3 member, the tip is rounded so that the orifice 46 is located downstream of the intersection 47 of a projection 48 of the external surface of the member 19 from a point where the included angle 44 is a minimum and a projection 49 of the orifice 46 in the direction of the axis 50 of the skimmer. The preferred value of the external included angle 51 in the immediate vicinity of the orifice 46 is substantially greater than 60°, and is typically in the range 90°-180° (160° is illustrated). In a preferred embodiment the internal included angle 52 in the immediate vicinity of the orifice 46 is also substantially greater than 60° (140° is illustrated) .

Another preferred type of skimmer member is shown in figure 4. Skimmer member 53 with an orifice 54 on its axis 55 comprises first, second and third portions (56, 57 and 58 respectively) disposed as shown. The first portion 56, immediately adjacent to the orifice 54, has an external included angle 59 substantially greater than the external included angle 60 of the second portion 57 (angle 60 is typically about 55°). The external included angle 61 of the third portion 58 is also much greater than angle 60. Member 53 is similar to the skimmer disclosed in W) 90/09031 but additionally incorporates the rounded first portion 56 which is a preferred characteristic of the present invention.

Another preferred type of skimmer member 62 is shown in figure 5. It comprises a first portion 64, immediately adjacent to a drilled orifice 65, which has an external included angle 66, and a second portion 63 with a smaller external included angle 67. Angle 66 is typically greater than 60°, and preferably in the range 90°-180°. Angle 67 is typically 60° or smaller. Skimmer member 62 is preferably conical and may be manufactured by turning. Its internal included angle 68 may be the same in both portions 63 and 64 and is of course slightly smaller than the external included angle 67. It is thought that the improved performance of a skimmer member of this type is due to the decrease in the depth 69 of the orifice 65 which is made possible by the larger included angle 66 of the first portion 64 without loss of mechanical rigidity of the member (compare the prior skimmers shown in figures 1A - 1C). As in the case of other skimmers suitable for use in the invention, it appears that the blunt tip does not reduce the transmission through the skimmer in the plasma mass spectrometer. It will be appreciated that the first and second portions of the skimmer of figure 5 may be used in the three-portion skimmer of figure 4, wherein they may replace portions 56 and 57 of the skimmer 53.

Claims

l. A mass spectrometer comprising a mass analyzer, means for generating a plasma in a flow of gas, means for introducing a sample into said plasma, a sampling member adjacent to said plasma comprising a first orifice through which ions characteristic of said sample may pass into a first evacuated region, and a hollow tapered skimmer member disposed with its smaller end closest to said sampling member, wherein said skimmer member has an axis and, formed in said smaller end and on said axis, a second orifice through which ions may pass from said first evacuated region to a second evacuated region maintained at a lower pressure than said first evacuated region and subsequently to said mass analyzer, said skimmer member being such that said smaller end is downstream of the intersection of an axial projection of said second orifice with a^'notional extension of the external surface of said skimmer member from a point where its external included angle is a minimum.

2. A mass spectrometer as claimed in claim 1 wherein the external included angle of the portion of the skimmer member immediately adjacent to the second orifice is greater than 60°.

3. A mass spectrometer as claimed in claim 2 wherein the external included angle of the portion of the skimmer member immediately adjacent to the second orifice is in the range 90° to 180°.

4. A mass spectrometer as claimed in any preceding claim wherein the internal included angle of the portion of the skimmer member immediately adjacent to the second orifice is greater than 60 .

5. A mass spectrometer as claimed in claim 4 wherein the internal included angle of the portion of the skimmer member immediately adjacent to the second orifice is greater than 90°.

6. A mass spectrometer as claimed in any preceding claim wherein the external included angle of the skimmer member where its external included angle is a minimum is less than 60° .

7. A mass spectrometer as claimed in any preceding claim wherein the internal included angle of the portion of the skimmer member where the external included angle is a minimum is as great as possible within the limits imposed by its external angle.

8. A mass spectrometer as claimed in any preceding claim wherein the skimmer member comprises three portions, a short first portion immediately adjacent to the second orifice where the external included angle is greater than 60 ^* , a second portion adjacent to and downstream of the first portion where the external included angle is about 60 or smaller, and a third portion adjacent to and downstream of the second portion where the external included angle is greater than 60°.

9. A mass spectrometer as claimed in claim 8 wherein the external included angle of the third portion is greater than 90".

10. A mass spectrometer as claimed in claim 8 or 9 wherein the internal included angles of each portion are as great as possible within the limits imposed by each external angle.

11. A mass spectrometer comprising a mass analyzer, means for generating a plasma in a flow of gas, means for introducing a sample into said plasma, a sampling member adjacent to said plasma comprising a first orifice through which ions characteristic of said sample may pass from said plasma to a first evacuated region, and a hollow tapered skimmer member comprising a second orifice in its smaller end through which ions may pass from said first evacuated region to a second evacuated region maintained at a lower pressure than said first evacuated region and subsequently to said mass analyzer, wherein the external included angle of a first portion of said skimmer member immediately adjacent to said second orifice is greater than the external included angle of a second portion of said skimmer member downstream of said first portion.

12. A mass spectrometer as claimed in claim 11 wherein the external included angle of the first portion is greater than 60°.

13. A mass spectrometer as claimed in claim 12 wherein the external included angle of the first portion is in the range 90° to 180°.

14. A mass spectrometer as claimed in any one of claims 11 to 13 wherein the internal included angle of the first portion is greater than 60°.

15. A mass spectrometer as claimed in claim 14 wherein the internal included angle of the first portion is greater than 90°.

16. A mass spectrometer as claimed in of claims 11 to 15 wherein the external included angle of the second portion is less than 60°.

17. A mass spectrometer as claimed in any of claims 11 to 16 wherein the internal included angle of the second portion is as great as possible within the limits imposed by its external angle.

18. A mass spectrometer as claimed in any of claims 11 to 17 wherein the second portion is adjacent to and downstream of the first portion and wherein the skimmer member further comprises a third portion adjacent to and downstream of the second portion where the external included angle is greater than 60°.

19. A mass spectrometer as claimed in claim 18 wherein the external included angle of the third portion is greater than 90°.

20. A mass spectrometer as claimed in claim 18 or 19 wherein the internal included angles of each portion are as great as possible within the limits imposed by each external angle.

21. A mass spectrometer as claimed in any preceding claim in which the portion of the skimmer member immediately adjacent to the orifice is rounded or part- spherical.

22. A mass spectrometer as claimed in claim 21 in which the rounded portion of the skimmer member immediately adjacent to the orifice is formed by rolling a conical straight-sided skimmer member.

23. A hollow tapered skimmer member suitable for use in a mass spectrometer between a plasma ion source and a mass analyser wherein said skimmer member has a large end and a small end spaced apart along an axis, and, formed in said small end and on said axis, an orifice through which ions may pass, said skimmer member being such that the distance between said small and large ends is less than the distance between said large end and the intersection of an axial projection of said orifice with a notional extension of the external surface of said skimmer member from a point where its external included angle is a minimum.

24. A hollow tapered skimmer member suitable for use in a mass spectrometer between a plasma ion source and a mass analyser wherein said skimmer member has a large end and a small end spaced apart along an axis, and, formed in said small end and on said axis, an orifice through which ions may pass, said skimmer member being such that the external included angle of a first portion of said skimmer member immediately adjacent to said orifice is substantially greater than the external included angle of a second portion of said skimmer member closer to said large end than said first portion.

25. A hollow tapered skimmer member is claimed in claim 24 wherein the included external angle of the first portion is greater than 60° and the included external angle of the second portion is 60° or less.