Ion lens for a mass spectrometer

Details Ion lens for a mass spectrometer Ion lens for a mass spectrometer

2003-05-21

2004-04-06

Nguyen, Kiet T. (Department: 2881)

Radiant energy

Ionic separation or analysis

Cyclically varying ion selecting field means

C250S3960ML

Reexamination Certificate

active

06717139

ABSTRACT:

The present invention relates to a mass spectrometer, especially to the ion optical system for transporting ions generated in an ion source to a mass analyzer such as a quadrupole mass filter.
Among various mass spectrometers, the Electrospray Ionization Mass Spectrometer (ESI-MS), the Atmospheric Pressure Chemical Ionizing Mass Spectrometer (ACPI-MS) and Radio-frequency Induction Plasma Mass Spectrometer (ICP-MS) are called atmospheric pressure type mass spectrometers (API-MS) because the sample is ionized under almost atmospheric pressure.
FIG. 9
is a schematic sectional view of a conventional ESI-MS, which includes the ionizing chamber
1
and the analyzing chamber
9
. In the ionizing chamber
1
, a nozzle
2
is provided which is connected to the exit of, for example, a liquid chromatographic column. In the analyzing chamber
9
, a quadrupole filter
10
and an ion detector
11
are provided. Between the ionizing chamber
1
and the analyzing chamber
9
, the first vacuum chamber
4
and the second vacuum chamber
7
are placed, where air-tight walls separate those chambers
1
,
4
,
7
,
9
. The ionizing chamber
1
and the first vacuum chamber
4
communicate with each other only with a desolvation tube
3
provided in the wall between them, where the desolvation tube
3
has a narrow conduit at its center. The first vacuum chamber
4
and the second vacuum chamber
7
communicate with each other only with a skimmer
6
provided in the wall between them, where the skimmer
6
has a very narrow orifice.
The pressure in the ionizing chamber
1
, which is the ion source, is almost atmospheric due to the vaporized molecules of the liquid sample continuously supplied from the nozzle
2
. The pressure of the first vacuum chamber
4
is lowered by a rotary pump to about 10
2
Pa, that of the second vacuum chamber
7
is lowered by a turbo molecular pump to about 10
−1
to 10
−2
Pa, and that of the analyzing chamber
9
is made as low as 10
−3
to 10
−4
Pa by a turbo molecular pump. Thus the pressures of those chambers are gradually decreased from the almost atmospheric pressure of the ionizing chamber
1
to the very high vacuum of the analyzing chamber
9
. This multi-stage differentiated evacuation system assures the high vacuum of the analyzing chamber
9
.
The liquid sample is sprayed from the tip of the nozzle
2
into the ionizing chamber
1
, wherein the sample is electrically charged (electrosprayed). When the solvent in the sprayed droplets evaporates, the sample molecules are ionized. The droplets containing such ions are drawn into the desolvation tube
3
due to the pressure difference between the ionizing chamber
1
and the first vacuum chamber
4
. Since the desolvation tube
3
is heated, the solvent in the droplets further evaporates and the sample molecules are further ionized. A first ion lens
5
, which may be constructed by a cylindrical electrode, is provided in the first vacuum chamber
4
. The first ion lens
5
, with the electric field created in it, assists the drawing-in of the ions coming through the desolvation tube
3
, and converges the ions to the orifice of the skimmer
6
.
The ions introduced into the second vacuum chamber
7
through the orifice of the skimmer
6
are converged and accelerated by the second ion lens
8
, which may be constructed by concentrically arrayed ring electrodes, and sent to the analyzing chamber
9
. In the analyzing chamber
9
, only such ions that have a certain mass to charge ratio can pass through the central space of the quadrupole mass filter
10
, and other ions dissipate while traveling through the space. The ions that have passed through the quadrupole mass filter
10
enter the ion detector
11
, which outputs an electrical signal corresponding to the number of ions detected.
In the above construction, the first ion lens
5
and the second ion lens
8
are generally called ion optical systems, whose primary functions are to converge flying ions with their electric fields, and, in some cases, accelerate them toward the next stage. Conventionally, various types of ion optical systems have been used or proposed.
FIG. 10
shows a multi-rod type ion lens
20
, which has four rods. The number of rods can be six or eight, for example, and generally it can be any even number no less than four. To any neighboring two rods among the rods of an even number, the same DC voltage plus the same RF (radio-frequency) voltages having opposite polarities are applied. Ions introduced along the central axis (“ion optical axis”) C of the space surrounded by the rods travel through the space vibrating at the frequency the same as that of the RF voltage. This structure has a better ion converging efficiency, so that larger number of ions can be passed onto the next stage.
In the multi-rod type ion lens
20
, however, the inscribing circle P
1
(which contacts the inner surfaces) of the rods
201
-
204
at the entrance and the inscribing circle P
2
at the exit have the same diameter, and thus the ion traveling space surrounded by the rods
201
-
204
is shaped almost cylindrical. As shown in
FIG. 9
, especially in the first vacuum chamber
4
, ions ejected from the desolvation tube
3
spread conically, so that the capturing efficiency of the first ion lens
5
having a rather small entrance is rather low. If the entrance of the multi-rod type ion lens
20
is broadened, however, the converging efficiency of ions toward the orifice becomes low, on the other hand, so that the overall ion passing efficiency cannot be improved. Since, further, the value of the DC voltage is constant on the ion optical axis C, ions are not accelerated in the space. Thus, in the first vacuum chamber
4
where the pressure is rather high, compared to the low pressure or high vacuum in the following chambers, ions are deprived of their kinetic energy due to collisions with the remaining gas molecules, and fewer ions can pass through the firs ion lens
5
.
Addressing the problem, the present applicant proposed a new ion lens in the Publication No. 2000-149865 of unexamined Japanese patent application.
FIG. 11
shows an example of the new ion lens
21
, where virtual rod electrodes
211
-
214
are used. A virtual rod electrode is composed of a plurality of metal plate electrodes aligned in a row along the ion optical axis C, where every metal plate is positioned substantially vertical to the ion optical axis C. Owing to such a construction of the virtual rod electrodes
211
-
214
, the plate electrodes can be arranged as shown in
FIG. 11
, where they are arranged closer to the ion optical axis C toward the exit of the virtual rod electrodes. Since, in this case, the ion passing space is conical with a broader entrance, more ions can be collected at the entrance and are gradually converged to the narrower exit by the electric field produced by the virtual rod electrodes. Thus the transporting or passing efficiency of ions is improved.
Further, since different voltages can be applied to the respectively independent plate electrodes constituting a virtual rod, a static electric field having a gradient can be produced, and the ions can be accelerated.
Though the virtual rod electrodes as described above have such advantages, it is necessary to set and arrange respective plate electrodes to the proper positions, and the holding or fixing structure is rather complicated and rather cost-inefficient.
SUMMARY OF THE INVENTION
The present invention addresses the problem. An object of the present invention is therefore to provide an ion optical system having a simpler structure and high ion passing efficiency.
According to the present invention, an ion optical system for converging ions includes:
an ion lens composed of platelet electrodes of an even number no less than four arranged radially and symmetrically around an ion optical axis connecting the ion source and the mass analyzer; and
a voltage generator for applying a voltage composed of a DC voltage and an RF voltage to a group of alternately arranged platelet electrodes and for

Affiliated with

Also associated with

Rating

Ion lens for a mass spectrometer does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Ion lens for a mass spectrometer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Ion lens for a mass spectrometer will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3221811