Plasma mass spectrometer

Details Plasma mass spectrometer Plasma mass spectrometer

1997-05-30

2001-04-24

Anderson, Bruce C. (Department: 2881)

Radiant energy

Ionic separation or analysis

With sample supply means

C250S292000

Reexamination Certificate

active

06222185

ABSTRACT:

This invention relates to a plasma (inductively-coupled or microwave induced) mass spectrometer, and in particular to such a spectrometer intended for the determination of isotopic ratios.
Two of the most significant problems which limit the performance of prior plasma mass spectrometers are firstly, the very low efficiency of transfer of the ions generated in the plasma through the interface into the vacuum system containing the mass analyzer, and secondly, the presence of interfering ion signals, sometimes very intense, due to species generated in the plasma other than the atomic ions characteristic of the elements present in a sample. These interfering ion species comprise atomic or molecular ions such as Ar
+
, Ar
++
, ArH
+
, ArN
+
etc. which are generated by the plasma in the absence of any introduced sample, and also molecular ions such as oxides, argides and hydride ions formed by reaction of the elements present in a sample with other species present in the sample. Not only do some of these interfering ions mask the signals from atomic ions for which a measurement is required because they have the same mass-to-charge ratio as that of an atomic ion to be measured, but they also result in a very high total ion current, much greater than that typically available from a sample. The maximum ion current that can be transmitted through any ion-optical system is generally limited by space-charge effects, and in practice the high ion current due to these unwanted species can saturate the spectrometer optics, reducing the number of sample ions transmitted and causing other undesirable effects such as mass discrimination and matrix effects.
Considerable research effort has been expended in trying to reduce both the quantity and the deleterious effect of these interfering ions, and the following is a review of that work relevant to the present invention. Rowan and Houk (Appld. Spectroscopy, 1989 vol 43(6) pp 976-980) and Rowan (Thesis, Iowa State University, submitted 1989) describe a failed attempt to reduce the number of polyatomic ions entering the mass analyzer of a plasma mass spectrometer by collision-induced dissociation. An RF-only quadrupole was disposed between the nozzle-skimmer interface and the mass-analyzing quadrupole of an otherwise conventional ICP mass spectrometer, and a collision gas, (typically xenon) was introduced into it at a pressure between 10
−5
and 10
−4
torr. It was hoped that this would induce dissociation of unwanted polyatomic species before they entered the mass analyzer by a mechanism similar to the collisional dissociation of molecular ions used in the triple quadrupole mass spectrometers intended for use in organic mass spectrometry. Although Rowan and Houk were able to demonstrate an improvement in the ratio of wanted to unwanted ions by this technique, the ion transmission efficiency of the instrument was greatly reduced and the intensity of the background signals increased, so that they concluded that any beneficial effect was in general outweighed by the disadvantages.
A similar approach was reported by Douglas (Can. J. Spectroscopy, 1989 vol 34(2) pp 38-49, in particular the passage bridging pp 47-48). In this work a triple quadrupole spectrometer was fitted with an ICP source with the aim of dissociating unwanted polyatomic ions in the centre quadrupole. This approach also failed, and Douglas predicted that it would not be possible to achieve large gains in the atomic ion to polyatomic ion ratio by collision-induced dissociation because the loss cross-sections for the atomic ions were found to be much higher than expected; so much higher, in fact, that they were comparable to those of the polyatomic ions. Thus the net effect of the collision process would be to cause roughly equal losses of both atomic and polyatomic ions. Douglas concludes that a more profitable approach might be to use ion-molecule chemistry in the centre quadrupole (that is, to chemically convert both wanted and unwanted ions, for example by reaction with oxygen) to species such as oxides. Certain polyatomic species generated in the plasma, for example oxides, would then be less likely to undergo further reaction, so that the ratio of reacted atomic ions to reacted polyatomic ions would in some cases be reduced. However, this approach is obviously highly specific and while reducing the effect of one interfering ion may introduce another that was not previously present.
Also in 1989, King and Harrison (Int. J. Mass Spectrom. And Ion Proc, 1989 vol. 89 pp 171-185) described the use of collision-induced dissociation to remove polyatomic ion interferences in glow-discharge mass spectrometry. Like Douglas, they employed a triple quadrupole mass spectrometer and used the centre quadrupole as a collision cell. Their results were similar to those of Rowan and Houk with an ICP spectrometer, namely, that although it was possible to demonstrate a reduction in the ratio of certain polyatomic ions to wanted atomic ions, the ion transmission was severely reduced, causing an overall reduction in detection limits.
Presumably because of the failure of the work in 1989 to demonstrate a worthwhile reduction in polyatomic ion interferences in ICPMS, and Douglas's comments that this was to be expected on theoretical grounds, research effort related to reducing interferences switched to development of other aspects of ICPMS, and it was not until 1996 that Eiden, Barinaga and Koppenaal (J. Anal. Atomic. Spectrom., 1996 vol 11 pp 317-322) described a method for the selective removal of plasma matrix ions such as Ar
+
from either an ion-trap ICP spectrometer or from the ion beam in a quadrupole ICP mass spectrometer by the reaction of added gaseous hydrogen with the ions sampled from the plasma. In practice, hydrogen was introduced into the vacuum system of the spectrometer downstream of the conventional nozzle-skimmer system (which is used to interface the plasma to the mass analyzer) at a pressure of about 10 mtorr, and it was found that Ar
+
ions were removed 45 times faster than typical atomic ions, leading to a large reduction in the intensity of the Ar
+
peak in a typical mass spectrum. The results were more spectacular in the case of an ion-trap spectrometer, leading to almost complete elimination of the Ar
+
peak. Eiden et.al. also suggest that the efficiency of the removal of Ar
+
in a quadrupole mass spectrometer might be increased by using a radio-frequency quadrupole ion guide (or other multipole device), into which hydrogen is introduced, between the skimmer and the mass analyzer. They suggest that operating the quadrupole guide with a low-mass cut-off of between 5 and 15 daltons might reject charged hydrogen ions generated by chemical reaction between the added hydrogen and the unwanted Ar
+
ions, thereby minimising the number of charged species passing into the mass analyzer and consequently reducing space-charge related problems. However, the method is dependent on chemical reaction between the added hydrogen and the unwanted ions, and similar reactions may take place between the hydrogen and the atomic ions to be determined, albeit at a much slower rate, generating unwanted mass discrimination effects and additional molecular ions. Because the removal of ions is a chemical process, Eiden, et al, do not teach that any gas other than hydrogen could be used.
Further art relevant to this invention is typified by U.S. Pat. 4,963,736 which teaches an atmospheric pressure ionization (API) quadrupole mass spectrometer in which an AC-only multipole (i.e., quadrupole or hexapole, etc) rod set is disposed between the API source and the quadrupole mass filter. Gas is introduced into the vacuum system in the vicinity of the additional rod set. The inventors claim that this results in improved mass resolution of the quadrupole mass analyzer and a narrow range of energies of the ions emerging from the additional rod set. More details of this technique were later published by the inventors (Douglas and French) in J.Am.

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