Mass spectrometer with electron source for reducing space...

Details Mass spectrometer with electron source for reducing space... Mass spectrometer with electron source for reducing space...

2001-01-12

2003-10-14

Wells, Nikita (Department: 2881)

Electric lamp and discharge devices

With positive or negative ion acceleration

C250S281000, C250S282000, C250S42300F, C250S424000, C250S251000

Reexamination Certificate

active

06633114

ABSTRACT:

The present invention is described in the following three papers published by the applicants.
The first paper is N. Praphairaksit et al., “Reduction of Space Charge Effects in Inductively Coupled Plasma Mass Spectrometry Using a Supplemental Electron Source inside the Skimmer: Ion Transmission and Mass Spectral Characteristics”,
Analytical Chemistry
, Vol. 72, No. 11, Jun. 1, 2000, pp. 2356-2361 (the Praphairaksit I paper). An earlier version of this paper is included in U.S. provisional application Serial No. 60/175,688 referred to above as Appendix I.
The second paper is N. Praphairaksit et al., “Attenuation of Matrix Effects in Inductively Coupled Plasma Mass Spectrometry with a Supplemental Electron Source inside the Skimmer”,
Analytical Chemistry
, Vol. 72, No. 11, Jun. 1, 2000, pp. 2351-2355 (the Praphairaksit II paper). An earlier version of this paper is included in U.S. provisional application Serial No. 60/175,688 referred to above as Appendix II.
The third paper is N. Praphairaksit et al., “Reduction of Mass Bias and Matrix Effects in Inductively Coupled Plasma Mass Spectrometry with a Supplemental Electron Source in a Negative Extraction Lens”,
Analytical Chemistry
, Vol. 72, No. 18, Sep. 15, 2000, pp. 4435-4440 (the Praphairaksit III paper). An earlier version of this paper is included in U.S. provisional application Serial No. 60/175,688 referred to above as Appendix III.
The Praphairaksit I, Praphairaksit II, and Praphairaksit III papers referred to above are incorporated herein by reference in their entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to mass spectrometers, and in particular to mass spectrometers wherein space charge effects occur in a sample beam due to an excess of positive ions in the sample beam. The present invention is particularly useful in inductively coupled plasma-mass spectrometers (ICP-MS).
2. Description of the Related Art
FIG. 1
shows a prior-art mass spectrometer which is basically the same as that described in K. Hu et al., “Inductively Coupled Plasma Mass Spectrometry with an Enlarged Sampling Orifice and Offset Ion Lens. I. Ion Trajectories and Detector Performance”,
Journal of the American Society for Mass Spectrometry
, Vol. 4, 1993, pp. 16-27 (the Hu I reference), and K. Hu et al., “Inductively Coupled Plasma Mass Spectrometry with an Enlarged Sampling Orifice and Offset Ion Lens. II. Polyatomic Ion Interferences and Matrix Effects”,
Journal of the American Society for Mass Spectrometry
, Vol. 4, 1993, pp. 28-37 (the Hu II reference). One of the authors of the Hu I and Hu II references, Robert S. Houk, is also one of the applicants of the present application. The Houk I and Houk II references are incorporated herein by reference in their entirety.
The prior-art mass spectrometer in
FIG. 1
includes an ion source
1
, a sampling interface
2
, and a mass analyzer
3
.
In the Hu I and Hu II references, ion source
1
is an inductively coupled plasma ion source, and mass analyzer
3
is a quadrupole mass analyzer.
Sampling interface
2
includes a sampler
4
, a skimmer
5
, an ion lens
6
, and a differential pumping plate
7
. Ion lens
6
includes seven electrodes
8
,
9
,
10
,
11
,
12
,
13
, and
14
, which are typically DC electrodes.
In the Hu I and Hu II references, a positive voltage +V
1
is applied to a first electrode
8
of ion lens
7
, meaning a voltage that is more positive than a potential of skimmer
5
.
First electrode
8
may be a cylindrical electrode having holes in it as indicated by the dashed lines in
FIG. 1
, and may be formed of a mesh.
Ion source
1
generates a quasineutral beam
15
of positive ions and electrons wherein the total positive charge of the positive ions is substantially equal to the total negative charge of the electrons. Sampling interface
2
extracts a portion of quasineutral beam
15
to form a sample beam
16
which is analyzed by mass analyzer
3
.
Quasineutral beam
15
and sample beam
16
are shown schematically in
FIG. 1
as a single line representing the center lines of these two beams. The shapes of quasineutral beam
15
and sample beam
16
and the trajectories of the positive ions and the electrons in these two beams are described in the Hu I and Hu II references, and elsewhere in the prior art, for example, in H. Niu et al., “Fundamental aspects of ion extraction in inductively coupled plasma mass spectrometry”,
Spectrochimica Acta Part B
, Vol. 51, 1996, pp. 779-815 (the Niu reference). One of the authors of the Niu reference, Robert S. Houk, is also one of the applicants of the present application. The Niu reference is incorporated herein by reference in its entirety.
As sample beam
16
passes through sampler
4
and skimmer
5
, it is initially a quasineutral beam of positive ions and electrons wherein the total positive charge of the positive ions is substantially equal to the total negative charge of the electrons. However, as sample beam
16
travels downstream from skimmer
5
towards ion lens
6
, it changes to a positively charged beam with an excess of positive ions, causing space charge effects to develop in sample beam
16
. The reasons for this are described in detail in the Praphairaksit I and Praphairaksit II papers and the Hu I, Hu II, and Niu references.
However, a simplified explanation of the reasons for this is that electrons diffuse away from sample beam
16
as it travels downstream from skimmer
5
towards ion lens
6
, creating an excess of positive ions in sample beam
16
and causing sample beam to become positively charged. This causes a space charge field to develop in sample beam
16
, thereby causing space charge effects to develop in sample beam
16
as it travels downstream from skimmer
5
towards ion lens
6
. Other fundamental reasons for preferential loss of electrons, in place of or in addition to diffusion, are also possible.
The space charge effects which develop in sample beam
16
have numerous disadvantages, and adversely affect the performance of the prior-art mass spectrometer in
FIG. 1
as described in detail in the Praphairaksit I, Praphairaksit II, and Praphairaksit III papers and the Niu reference.
FIG. 2
is a diagram showing some consequences of these space charge effects, wherein (−) denotes electrons, (+) denotes light analyte ions, such as Li, and (+) denotes heavy analyte ions, such as U. As shown in
FIG. 2
, the electrons (−) diffuse away from sample beam
16
as it travels downstream from skimmer
5
towards ion lens
6
, creating an excess of positive ions in sample beam
16
and causing sample beam
16
to become positively charged.
For the reasons discussed in detail in the Praphairaksit I, Praphairaksit II, and Praphairaksit III papers and the Niu reference, this causes sample beam
16
to defocus as it travels downstream from skimmer
5
towards ion lens
6
, decreasing the number of the light analyte ions (+) and the heavy analyte ions (+) which are available to enter first electrode
8
of ion lens
6
. Also, this causes the light analyte ions (+) to defocus to a greater extent than the heavy analyte ions (+), such that the ratio of the abundance (or number density) of the light analyte ions (+) to the abundance of the heavy analyte ions (+) in the portion of sample beam
16
which actually enters first electrode
8
of ion lens
6
is less than the actual ratio of the abundance of the light analyte ions (+) to the abundance of the heavy analyte ions (+) in sample beam
16
where it enters skimmer
5
. These effects are particularly troublesome when attempting to measure a small amount of a light element such as Li, in a matrix of a heavy element, such as U, especially when ion source
1
has a high temperature which induces a large variation of ion kinetic energy with ion mass and/or ion mass-to-charge ratio, as described in the Niu reference.
One method of reducing space charge effects in a mass spectrometer is disclosed in S. Tanner et al., “Reduction of Space Charge Effects

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