Radiant energy – Ionic separation or analysis – Static field-type ion path-bending selecting means
Reexamination Certificate
2002-03-29
2004-07-13
Wells, Nikita (Department: 2881)
Radiant energy
Ionic separation or analysis
Static field-type ion path-bending selecting means
C250S281000, C250S296000
Reexamination Certificate
active
06762407
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a mass spectrometer that includes an improved quadrupole mass analyser arrangement. The invention will be described mainly with reference to an inductively coupled plasma-mass spectrometer (ICP-MS) having an inductively coupled plasma ion source, however it is to be understood that the invention encompasses other types of mass spectrometers employing other types of ion sources, examples of which are disclosed hereinbelow.
BACKGROUND
The subject disclosure refers to a mass spectrometer having an ion reflecting or an ion transmissive optics system. The spectrometer includes an ion source for providing a supply of particles including ions representative of chemical elements present in an analytical sample and an ion optics system between the ion source and a mass analyzer for producing a beam of ions from the source and establishing a reflecting electrostatic field for reflecting ions form the beam through an angle, for example 90°, and for focussing them into the mass analyzer entrance.
It has been found that the present invention as embodied in an ICP-MS instrument gives excellent sensitivity for detection of elemental isotopes having relatively high atomic masses (for example, the sensitivity for thorium, atomic mass 232, was over 650,000 counts per second per microgram per liter). However the sensitivity for elemental isotopes having low atomic masses is relatively poor (for example the sensitivity for beryllium, atomic mass 9, was less than 10,000 counts per second per microgram per liter). Furthermore, the background count rate (the count rate detected at a selected mass-to-charge ratio when no ions having that selected mass-to-charge ratio where expected to be present) was higher than desired, and when the voltages applied to the ion optics electrodes were increased to improve the focussing to increase sensitivity for detection of low atomic mass isotopes, the background count rate unfavourably increased.
The best possible Limit of Detection (LOD) for an elemental isotope in an ICP-MS is given by
LOD=
3×(background count rate/measurement time)
1/2
/sensitivity
Thus the relatively high background count rates and relatively low sensitivities for elemental isotopes having low atomic masses means that detection limits for such low atomic mass isotopes are undesirably high.
Although this problem has been highlighted by use of a mass spectrometer which employs a reflecting ion optics system, it is considered (in view of what is thought to be the mechanism for causing the high background count rates, as explained hereinbelow) that the same problem would exist in mass spectrometers that do not use a reflecting ion optics system.
It is known to arrange a separate set of four short straight sections of rod at the entrance of a quadrupole mass analyser and operate them with only radio-frequency (rf) voltage applied thereto or with the ratio of the DC to AC voltage substantially zero. Such a set of rods is often known as “fringe rods” because their function is to alleviate the effect of the fringing fields at the entrance of a quadrupole mass analyser and so improve the efficiency of transmission of ions into the mass analyser (see Peter H Dawon's book “Quadrupole Mass Spectrometry and its Applications”, Elsevier Scientific Publishing Co., 1976, at p. 105 and FIG. 1(
b
); and the earlier disclosure of U.S. Pat. No. 3,371,204 (Wilson M Brubaker)). While these straight fringe rods are not directly related to the problem of excessive background in quadrupole mass spectrometry, similar structures have been involved in efforts to solve that problem.
Thus U.S. Pat. No. 3,473,020 (Wilson M Brubaker) discloses a quadrupole mass filter having a curvilinear entrance section and a rectilinear section. A charged particle source directs particles (normally ions) into the analyser where they are resolved and the sorted beam is then directed into a detector section. The curvilinear quadrupole section can be operated in a strong focussing mode with low resolving power such that ions in a small mass range are transmitted from this section into the quadrupole rectilinear section of high resolving power. The curvilinear entrance section also reduces the number of photons from the charged particle source reaching the analyser detector and thus provides a substantial improvement in the signal to noise ratio in the output of the analyser. This arrangement would also remove neutral particles emanating from the source as well as photons because these particles would not be affected by the electrostatic field in the curved quadrupole section and so would continue straight ahead and strike the curved electrode rods. In a subsequent U.S. Pat. No. 3,410,997, Brubaker discloses the use of a similar curved quadrupole section at the exit of a linear quadrupole mass analyser to separate ions from photons from the source. It is disclosed that this curved quadrupole section may be operated with AC voltages only.
Peter H Dawson in his above mentioned book “Quadrupole Mass Spectrometry and its Applications” at pp 34-35 describes that background signal limits the ability to measure trace concentrations and originates from excited neutrals which easily pass through the “line-of-sight” analyser. He goes on to describe that “curved quadrupoles . . . or curved sections . . . have also been used to avoid the problem”.
European Patent Application 0 237 259 A2 (J. E. P. Syka) discloses tandem quadrupole mass spectrometer arrangements that include a bent quadrupole placed in front of a mass analysing quadrupole for reducing output noise. This bent quadrupole removes fast neutral particles generated in the ion source or from a collision cell (for producing daughter ions) in front of the bent quadrupole. In Syka's invention the bent quadrupole is separated from the mass analysing quadrupole by aperture plates and electrostatic lenses. The bent quadrupole does not act as a set of ‘fringe rods’.
D. J. Douglas in his article “Some Current Perspectives on ICP-MS” (Canadian Journal of Spectroscopy, Vol. 34, No. 2, 1989, pp 38-49) reported, in relation to seeking to reduce the high level of background noise in inductively coupled plasma mass spectrometry, the use of a curved (90°) RF only quadrupole (which he terms a “bent quad”) at the exit of the analysing quadrupole, which is essentially the same arrangement as that disclosed by Brubaker in U.S. Pat. No. 3,410,997. Douglas states, however, that the background noise (i.e. count rate) was a strong function of mass, that is, for high mass ions the background was reduced dramatically, but for low masses the background remained high (which is similar to the problem described hereinbefore in relation to the invention of WO 00/17909). Douglas describes, “Apparently at the exit of the analysing quadrupole, photons or metastable atoms from the source were somehow producing low mass ions which were efficiently transmitted to the detector to produce a high background level. When the voltage on the RF quad was high (corresponding to high mass analytes) these low mass ions had unstable trajectories and were not transmitted. Thus the “bent quad” almost but did not quite solve the background problem” (ibid p.41).
U.S. Pat. No. 5,939,718 (N. Yamada et al) discloses an ICP-MS having an ion lens section, including a multipole (at least four electrode rods) ion beam guide located in front of mass filtering and ion detection sections. In some embodiments (FIGS. 9-12) the rods of the ion beam guide are tilted or bent with respect to the moving direction of an ion beam “so as to prevent an (sic) direct entrance of photons of light from an inductively coupled plasma into (the) mass filter . . . Consequently the noise from direct light can be reduced . . . and it can highly enhance the S/N ratio and the measurement accuracy.” Thus this patent addresses a problem that is essentially the same as that addressed in U.S. Pat. No. 3,473,020 (Brubaker) and claims a solution that is generally similar, but specifically applied to an inductivel
Fishman Bella
Kalivoda Christopher M.
Varian Australia PTY LTD
Wells Nikita
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