GB2315362A

GB2315362A - Mass spectrometers

Info

Publication number: GB2315362A
Application number: GB9614676A
Authority: GB
Inventors: Dennis Leigh
Original assignee: ANALYTICAL PRECISION Ltd
Current assignee: ANALYTICAL PRECISION Ltd
Priority date: 1996-07-12
Filing date: 1996-07-12
Publication date: 1998-01-28
Also published as: GB9614676D0

Abstract

A mass spectrometer, e.g. of magnetic sector type, is provided with a retardation grid (14) between the ion source and detector (13). The retardation grid (14) is connected to a variable voltage source whereby a selected voltage can be applied to the grid (14) in order that ions below a selected energy value are unable to pass through the grid (14). Ions which have had their expected energy varied by scattering do not therefore give rise to tails which interfere with the detected spectrum.

Description

IMPROVEMENTS IN OR RELATING TO MAGNETIC SECTOR MASS SPECTROMETERS This invention relates to improvements in or relating to magnetic sector mass spectrometers and more particularly to means for increasing the accuracy of measurements made of low mass ion-beam intensities.

In magnetic sector mass spectrometers an image of the source slit is formed on a collector plane for each mass. These images are often spread out due to variation in ion energy caused by, inter alia, scattering of ion beams between the source and collector plane. When measurements are made, the scattering of ion beams results in the formation of "tails" on the low energy side of detected large peaks.

A problem arises as a result of the formation of tails insofar as the tails can modify smaller adjacent peaks which can accordingly lead to erroneous measurements. For example, the equation which relates the radius of curvature of an ion with mass m and energy ev in a magnetic field B is: RZ = 2Vm B2e Therefore if a variation in energy of 25% occurs in a 4He ion then such an ion can appear in the same position as a 3HD ion thereby causing interference in the 3HD peak.

It is an object of the present invention to provide means for increasing the accuracy of measurements made of low mass ion beam intensities in a mass spectrometer.

According to a first aspect of the present invention therefore there is provided in or for a mass spectrometer, retardation means disposed between an ion source and a detector, said retardation means being adapted to prevent passage from the ion source to the detector of ions having an energy which is below a selected fraction of the energy with which said ions leave said ion source.

With this arrangement it is possible to minimise tailing in the detected peaks by effectively "screening out" ions which have had their expected energy varied by scattering.

Preferably said retardation means comprises a grid which is connected to a variable voltage source related to the mean ion energy whereby the energy of ions which are prevented from passing to the detector can be set by appropriate setting of the voltage.

According to a second aspect of the present invention therefore there is provided the use of a retardation means in a mass spectrometer between an ion source and a detector to prevent passage of ions below a selected fraction of the energy with which said ions leave said ion source from reaching the detector.

The invention will now be described further by way of example only and with reference to the accompanying drawings of which: Fig. 1 shows a schematic representation of one form of prior art mass spectrometer; Fig. 2 shows a graphical representation of Intensity v Energy of detected masses for a theoretical ideal and for an experimental situation; Fig. 3 shows a schematic representation of a part of one form of mass spectrometer in accordance with the present invention; and Fig. 4 shows a graphical representation of Intensity v Energy of detected masses using the mass spectrometer of Fig. 3.

Referring now to Fig. 1, there is shown a part of a prior art form of mass spectrometer. Three incoming ion beams A, B and C are shown which pass between a series of slits 11 in a collector plate 1 2 positioned in a common collector plane X. The ions are detected by a series of Faraday collectors 13 which are disposed behind each slit 11. In the Figure each of the three beams are formed by ions of three different masses and each of these is detected by a respective Faraday detector 1 3. Such a prior art arrangement of mass spectrometer will produce a detected mass spectrum of the type shown in Figure 2b in which there is considerable tailing at the low mass edge of each mass peak which can cause the problems mentioned above. The ideal theoretical detected spectrum would be as shown in Figure 2a in which the mass peaks are sharp with no tailing, which it will of course be realised, will allow much more accurate measurement to be taken.

Figure 3 shows an improved arrangement over the prior art of Figure 1 in which a retardation grid 14 is disposed interposed between the incoming ion beams A, B and C and the collector slits 11. The retardation grid 1 4 is connected to a voltage source (not shown), the output voltage of which is variable in any suitable manner. In one embodiment the voltage is varied using a voltage divider arrangement although it will be appreciated that the voltage can be varied in any suitable manner as desired or as appropriate.

The collector plate 12 is connected to electrical ground.

It will be appreciated that with a voltage applied to the retardation grid 14, it can be arranged, with a suitable level of voltage, for ions of energy below a selected value to be unable to pass through the grid 14. Thus, in this way, it is possible to selectively control the ions which are able to reach the detectors 13 and therefore low mass ions with lower than the expected energy which would normally cause the tailing of Figure 2b, can be prevented from reaching the detector 13. Thus in this way, the measurement accuracy of the mass spectrometer can be increased and an expected mass spectrum obtained using this form of mass spectrometer is shown in Figure 4.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiment which are described by way of example only.

Claims

1. In or for a mass spectrometer, retardation means disposed between an ion source and a detector, said retardation means being adapted to prevent passage from the ion source to the detector of ions having an energy which is below a selected fraction of the energy with which said ions leave said ion source.

2. In or for a mass spectrometer, a retardation means according to claim 1 wherein said retardation means comprises a grid which is connected to a variable voltage source related to the mean ion energy whereby the energy of ions which are prevented from passing to the detector can be set by appropriate setting of the voltage.

3. In or for a mass spectrometer, a retardation means according to claim 2 wherein the variable voltage source includes a voltage divider arrangement.

4. Use of a retardation means in a mass spectrometer between an ion source and a detector to prevent passage of ions below a selected fraction of the energy with which said ions leave said ion source from reaching the detector.

5. In or for a mass spectrometer, retardation means substantially as hereinbefore described with reference to Figs. 2, 3 and 4 of the accompanying drawings.

6. Use of a retardation means substantially as hereinbefore described with reference to Figs. 2, 3 and 4 of the drawings in a mass spectrometer.