Mass spectrometer

Details Mass spectrometer Mass spectrometer

1999-08-31

2002-10-08

Berman, Jack (Department: 2881)

Radiant energy

Ionic separation or analysis

Cyclically varying ion selecting field means

C250S3960ML

Reexamination Certificate

active

06462338

ABSTRACT:

The present invention relates to a mass spectrometer having an ionization chamber in which a sample is ionized under a pressure as high as near atmospheric pressure. Mass spectrometers of this type include, for example, an Inductively Coupled Plasma Mass Spectrometer (ICP-MS), an ElectroSpray Ionization Mass Spectrometer (ESI-MS), an Atmospheric Pressure Chemical Ionization Mass Spectrometer (APCI-MS).
BACKGROUND OF THE INVENTION
FIG. 7
schematically shows the construction of a conventional electrospray ionization mass spectrometer. The mass spectrometer includes an ionization chamber
10
provided with a nozzle
11
connected to, for example, the outlet of a column of a liquid chromatograph, and an analyzing chamber
18
in which a quadrupole filter
19
and an ion detector
20
are accommodated. A wall separates the space between the two chambers
10
,
18
into two parts, which are referred to as the first and second interface chambers
12
,
15
. The ionization chamber
10
and the first interface chamber
12
communicate only through a heated capillary
13
, which is a pipe of a small inner diameter. The first interface chamber
12
and the second interface chamber
15
communicate only via skimmer
16
having an orifice
16
of a very small diameter.
The pressure in the ionization chamber
10
is maintained at about the atmospheric pressure by a continuous supply of a sample gas from the nozzle
11
. The first interface chamber
12
is evacuated with a rotary pump (RP) so that the inside is kept at a low vacuum of about 10
2
Pa. The second interface chamber
15
is evacuated with a turbo molecular pump (TMP) so that the inside is kept at a middle-vacuum of about 10
−1
to 10
−2
Pa, and the analyzing chamber
18
is evacuated with another turbo molecular pump (or the same TMP mentioned above) so that the inside is kept at a high-vacuum of about 10
−3
to 10
−4
Pa. Thus, the analyzing chamber
18
is maintained at the high vacuum by decreasing the pressure gradually from the ionization chamber
10
to the analyzing chamber
18
.
In an electrospray method, sample liquid is sprayed from the nozzle
11
into the ionization chamber
10
and the sample molecules are ionized when the solvent contained in the fine liquid particles vaporizes. The mixture of the liquid particles and the ions are drawn into the capillary
13
due to the pressure difference between the ionization chamber
10
and the first interface chamber
12
, where the ionization further proceeds when the mixture flows through the capillary
13
. The first interface chamber
12
is provided with a ring electrode
14
inside, which generates an electric field for assisting the drawing-in of ions to the capillary
13
for converging ions to the orifice of the skimmer
16
.
The ions introduced through the orifice of the skimmer
16
into the second interface chamber
15
are converged and accelerated by an ion lens
17
, and enters the analyzing chamber
18
. In the analyzing chamber
18
, only ions of a particular mass number (i.e. ratio of mass (m)
10
to charge (z), m/z) pass through the longitudinal space around the central axis of the quadrupole filter
19
. Ions passing through the quadrupole filter
19
are detected by the ion detector
20
.
The ion lens
17
in the second interface chamber
15
generates an electric field to accelerate and converge travelling ions as described above, and various types of ion lenses have been proposed conventionally.
FIG. 8
is a perspective view of one of such lenses, a so-called electrostatic lens. The ion lens
21
shown in
FIG. 8
is composed of plural lens electrodes made of ring metal plates. The lens electrodes are applied the same DC voltage. When the DC voltage is determined appropriately, ions travelling through the ion lens
21
on or near the ion beam axis C are accelerated. The ion lens, however, is deficient in that the converging efficiency is not very high, especially when the pressure is as high as 10
−1
Pa or higher. Accordingly, when, for example, ions travelling through the ion lens disperse, only a part of the ions pass through the ion lens and enter the section behind.
FIG. 9
shows another type of practically used ion lens, a so-called multi-pole type. The ion lens
22
shown in
FIG. 9
is composed of four rod electrodes, but the number of rod electrodes may be any number so long as it is even. The rod electrodes are applied the same DC voltage and a high frequency AC voltage superimposed on it, where the phases of the high frequency AC voltages of adjacent rod electrodes are reversed. Electric field generated by the rod electrodes influences the ions introduced along the ion beam axis C so that they oscillate while travelling through the ion lens
22
. By this type of ion lens, the converging effect of ions is very high, so that more ions pass through the ion lens and enter the section behind.
This type of ion lens, however, is also deficient in that ions are not accelerated while travelling in the space surrounded by the rod electrodes, since the potential gradient in the longitudinal direction of the space is zero. Therefore, when the ion lens is used under a condition where the pressure is as high as in the first interface chamber
12
, only a small number of ions can pass through the ion lens, because the ions lose their kinetic energy as they collide with molecules of gas in the chamber.
With regard to the above-described problem, one object of the present invention is to propose a mass spectrometer having an ion lens whereby the convergence and acceleration of ions are performed effectively even under a pressure as high as near atmospheric pressure.
SUMMARY OF THE INVENTION
Thus, the present invention proposes a mass spectrometer having an ion lens for converging ions, characterized in that the ion lens is composed of an even number of virtual rod electrodes positioned separately around the ion beam axis, where each of the virtual rod electrodes is composed of a plurality of separate metallic plate electrodes aligned in a row, and a voltage is applied to each of the plate electrodes.
In the above-described mass spectrometer, the voltage applied to each of the plate electrodes constituting a virtual rod electrode is determined with respect to the position of the plate electrode in the virtual rod electrode. For example, when a voltage composed of a DC voltage and a high frequency AC voltage superimposed thereon is applied to each of the plate electrodes, the DC voltage may be changed according to the position of the plate electrode while the high frequency AC voltage is set at the same irrespective of the position. The high frequency AC voltage applied to a virtual rod electrode should be reversed in phase against that applied to the adjacent virtual rod electrode.
When ions produced in an ionization chamber enter the ion lens, the ions travelling through the ion lens oscillate transversally due to the electric field generated by the high frequency AC voltage, and converge on a focal point of the ion lens. Meanwhile, the voltage gradient due to the change in the DC voltage applied to the plate electrodes accelerates the ions. Thus, the ions keep travelling without being displaced too much from due converging paths even when they collide with molecules of residing gas. Therefore, when, for example, a skimmer having is set behind the ion lens so that the orifice is positioned at the focal point of the ion lens, a large number of ions can pass through the orifice and enter the section behind it.
Thus, by the mass spectrometer according to the present invention, the convergence and acceleration of ions are effectively performed even when the pressure is as high as near atmospheric pressure. As a result, an adequate amount of ions can enter the mass filter set behind the ion lens, and the sensitiveness and accuracy of the mass spectrometry are improved. Also, according to the present invention, various forms of electric field that are hardly realized by conventional solid electrodes can be realized without diffic

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