Radiant energy – Ionic separation or analysis
Reexamination Certificate
2003-02-25
2004-11-09
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
C250S492300
Reexamination Certificate
active
06815667
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an apparatus for and a method of detecting ions of interest by mass spectrometry, while the ions of interest or unwanted interference ions are being modified by collisions or reactions during their transport from an ion source to a detector. More specifically, the invention relates to the use of ion molecule reactions that modify either analyte ions or interfering species, in order to effect an m/z shift, to separate isobaric analyte and interference ions from one another, to give better resolution for the analyte ions.
BACKGROUND OF THE INVENTION
In inductively coupled mass spectrometry (ICP-MS), a sample is fed into a plasma that is maintained in an excited or energized state by inductive coupling. Typically, the plasma gas is argon. The plasma typically comprises the analyte, usually a metal and usually ionized, and various other constituents, such as argon, oxygen, hydrogen and also water vapor, all of which will commonly be neutral but some (about 0.1%) may be ionized. For wet plasma, which is typically used, the content of the reactive neutrals such as H, O, and their various polyatomic combinations, is as high as 17%. The plasma, including these ions and neutrals, passes into a chamber maintained at approximately 4 Torr. From this chamber, the plasma passes through a skimmer into a chamber maintained at a low pressure off approximately 10
−3
Torr. From this chamber, the ions are intended to pass into a reaction/collision cell. The reaction/collision cell commonly has a multipole rod set, and can be maintained at different pressures; for example when no reaction is required, it may be maintained at 10
−5
Torr, while a pressure of 5×10
−3
Torr to 10
−2
Torr is provided by a reaction/collision gas when reaction or collision induced dissociation (CAD) is required. The higher pressure is maintained in the reaction cell when it is desired to promote ion-molecule reactions or CAD. In such a case, a simple analysis would suggest that the higher pressure within the reaction cell would prevent neutral species from passing into the reaction cell, and only ions, driven by the potential gradient through the whole instrument, would overcome the pressure difference and pass into the reaction cell. However, this overlooks the significant velocity created by the expansion of the plasma from the atmospheric pressure to a region at 4 Torr, which creates a supersonic expansion jet. Consequently, individual ions and neutrals within the supersonic expansion jet, after passing through a skimmer into the region at 10
−3
Torr, may have sufficient kinetic energy to overcome the pressure differential between the higher pressure in the reaction/collision cell and lower pressure of the region at 10
−3
Torr, and pass into the reaction/collision cell. More specifically, and as detailed below, the present inventors have now realized that it is possible for neutral species to pass into the reaction/collision cell.
Ion-molecule reaction cells are widely used in ICP MS. Their successful operation depends on how pure the reaction gas is. Inductively coupled plasma is the source of neutral particles, because 99.9% of the gases that constitute the plasma are not ionized. Usually, about 4×10
18
-2×10
19
molecules/s
−1
flow of neutral plasma particles enters the mass spectrometer, which is equivalent of 0.1-0.4 scc/s. If these neutral gas particles are entrained into the flow into the reaction cell, the reactions are not controlled anymore. Instead of the high purity reaction gas introduced on purpose to the cell, it now has a mixture of the reaction gas with entrained plasma gases, and these plasma gases constitute up to 17% of the reactive neutrals H, O and various polyatomic combinations of these. Despite the fact that the pressure in the pressurized cell (with typical flow of 0.03-0.3 scc/s) may be higher than the background pressure of the vacuum compartment where the cell is positioned, the gases from the plasma can still enter the cell, because, as noted, the plasma gas undergoes supersonic expansion in the plasma-vacuum interface, after which particles travel with the terminal speed of about 2300 m/s, typically. The impact pressure of such high velocity gas particles can be sufficiently higher than the pressure of the reaction gas in the cell, so the neutral gas particles from plasma will be entrained into the reaction cell.
Similar processes are taking place in any other mass spectrometers, in which the ion source pressure is sufficiently higher than the pressure in a collision/reaction cell. A variety of the instruments now comprise collision devices for collisional cooling, collisional focusing or collision-induced dissociation. For example, in Electrospray Ionization Mass Spectrometry, the ion source is usually operated at atmospheric pressure, from which ionized and neutral particles are delivered into the lower pressure collision cell by a supersonic expansion. As noted above, the impact pressure of the expanding ion source gas may be greater than the collision cell pressure, so that the neutral gas particles from the ion source will be entrained into the collision cell, altering the composition of the collision gas. As a result, un-predicted and un-controlled dissociative and reactive collisions with the collision gas of altered composition may bring undesirable modifications to the ions that are to be detected by mass analysis.
A variety of ion-molecule reactions in pressurized mass-analyzing and ion transmitting devices have been successfully used in ICP Mass Spectrometry for chemical resolution of analyte ions from isobaric interfering species by use of a reaction cell. Douglas [Douglas, DJ. Canad. J. Spectrosc. 1989, 34, 38] was first to report on discrimination between the rare earth elements and their oxides through the specificity of oxidation by the reactive gas. Tb+was shown to oxidize more readily with O
2
than CeO
+
. The analyte ion (
159
Tb
+
) was moved to a higher m/z and could thus be measured as TbO
+
. The interfering ion (
142
Ce
17
O
+
) was not shifted to the same extent, thus providing a possible analytical advantage of achieving better signal-to-noise ratio for Th signal measured as TbO in the presence of Ce in the sample. Shortly after, Rowan and Houk [Rowan, J. T.; Houk, R. S. Applied Spectrosc. 1989, 46, 976] reported on the removal of the interfering argide ions from the m/z of analyte ions of interest due to lower reactivity of the latter towards reaction gas such as CH
4
.
The specificity of the analyte-interference chemical resolution in general and in both of the above-described cases is dependent on the reaction gas properties. When the interfering species are to be moved away from the m/z of the analyte ion, the reaction gas reactivity towards the analyte is desirably low, while being high towards the interfering species. On the other hand, when the analyte ion is to be moved from its m/z by conversion to a polyatomic ion, the reactivity of the gas towards the analyte ion should preferably be high and simultaneously should be low towards the interfering species. In the latter case, the reaction that converts the analyte ions should preferably have one or only few channels, so that the analyte ion current or signal is not distributed amongst many product ion currents and the detection capabilities are not compromised. The reactivity of the gas towards the interference should in this case be low, at least for any reaction channels that can produce from the interference product ions at the same m/z as that of the analyte product ions, i.e. one does not want any interference products to be isobaric with analyte product ions.
The inventors have recently shown that the highest effectiveness of reactive isobaric interference removal in ICP MS can be achieved only if the average number of ion-molecule collisions in the pressurized device is sufficiently high. Efficiency of 10
9
of suppression of Ar
+
si
Bandura Dmitry R.
Baranov Vladimir I.
Tanner Scott D.
Bereskin & Parr
Hashmi Zia R.
Lee John R.
MDS Inc.
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