Mass spectrograph

Details Mass spectrograph Mass spectrograph

2001-11-14

2003-12-09

Nguyen, Kiet T. (Department: 2881)

Radiant energy

Ionic separation or analysis

With sample supply means

C250S292000

Reexamination Certificate

active

06661002

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a mass spectrograph of the type for ionizing a sample under a relatively near atmospheric condition of pressure such as an inductively coupled plasma mass spectrograph (ICP-MS), an electro spray mass spectrograph (ES-IMS) or an atmospheric pressure chemical ionization mass spectrograph (APCI-MS).
A prior art ESI-MS is shown schematically in FIG.
4
and an portion thereof around its skimmer is shown enlarged in FIG.
5
. This mass spectrograph is provided with a first intermediate chamber
12
and a second intermediate chamber
15
between an ionization chamber
10
having a nozzle
11
connected to the outlet of the column of a liquid chromatographic apparatus and an analyzing chamber
18
with a quadrupole filter
19
and an ion detector
20
, each being mutually separated by a partition wall. The ionization chamber
10
and the first intermediate chamber
12
are connected only through a heated capillary of a small inner diameter serving as a solvent-removing pipe
13
. The first intermediate chamber
12
and the second intermediate chamber
15
are connected only through a conically shaped skimmer
16
having an orifice
16
a
of a small diameter at its tip.
The interior of the ionization chamber
10
is nearly in the atmospheric condition due to the gasified molecules of the sample liquid continuously supplied thereinto through the nozzle
11
. The interior of the first intermediate chamber
12
is at a low vacuum condition of about 10
2
Pa by means of a rotary pump (RP). The interior of the second intermediate chamber
15
is at a medium vacuum condition of about 10
−1
-10
−2
Pa by means of a turbo-molecular pump (TMP). The interior of the analyzing chamber
18
is at a high vacuum condition of about 10
−3
-10
−4
Pa by means of another turbo-molecular pump (TMP). In other words, the degree of vacuum increases as one moves from one chamber to the next, starting at the ionization chamber
10
towards the analyzing chamber
18
such that the interior of the analyzing chamber
18
is maintained at a high vacuum condition.
A sample liquid is sprayed (or electro-sprayed) through the nozzle
11
into the ionization chamber
10
, and the sample molecules are ionized while the solvent contained in the liquid drops is evaporated. Small liquid droplets with ions mixed in are pulled into the solvent-removing pipe
13
due to the pressure difference between the ionization chamber
10
and the first intermediate chamber
12
. As they pass through the solvent-removing pipe
13
, the solvent is evaporated and the process of ionization proceeds further. A pair of mutually facing planar electrodes or a ring-shaped electrode
14
is provided inside the first intermediate chamber
12
. The electric field generated by this electrode
14
serves not only to pull in the ions through the solvent-removing pipe
13
but also to converge the ions to a point (“backward focal point”) F near the orifice
16
a
of the skimmer
16
.
The converged ions are caused to pass through the orifice
16
a
of the skimmer
16
by the pressure difference between the first intermediate chamber
12
and the second intermediate chamber
15
and is directed into the analyzing chamber
18
after being converged and accelerated by means of an ion guide
17
(also referred to as the ion lens or the ion-transporting lens). Inside the analyzing chamber
18
, only those of the ions having a specified mass number (the ratio of mass m to charge z) are passed through the longitudinal space at the center of the quadrupole filter
19
and reach the ion detector
20
to be detected thereby.
The function of the ion guide
17
is to accelerate flying ions while causing them to be converged. Ion guides with many different shapes have been proposed. The so-called multi-pole type is one of known types, having a plurality of approximately cylindrically shaped rod electrodes arranged so as to circumscribe a circle of diameter d
1
and mutually separated and having a voltage difference superposing high-frequency voltages with phases mutually inverted by a same direct-current voltage applied between each mutually adjacent pair of these rod electrodes. Such a high-frequency electric field causes the ions introduced in the direction of the optical axis C to move forward while vibrating at a specified frequency. As a result, the ions can be converged more effectively and more ions can be sent into the analyzing chamber
18
on the downstream side.
For the purpose of passing ions as efficiently as possible through the first intermediate chamber
12
and the second intermediate chamber
15
, it is desirable to reduce the distance as much as possible between the orifice
16
a
and the space surrounded by the rod electrodes of the ion guide
17
. For this reason, the end surface of the ion guide
17
facing the skimmer
16
is formed with a slope so as to match the sloped surface of the skimmer
16
and the ion guide
17
is disposed such that its sloped end surface protrudes into the conically shaped portion of the skimmer
16
. This makes it time-consuming to fabricate the rod electrodes, affecting the production cost adversely.
Another problem is that the orifice
16
a
of the skimmer
16
and its neighboring parts become contaminated with sample ions that stick to them, and the skimmer
16
must therefore be designed to be detachable. With the skimmer
16
and the ion guide
17
as formed above, either of them should be made slidable in the direction of the aforementioned optical axis C or the skimmer
16
must be attached to be rotatable by means of a hinge. This causes the attachment mechanism of the skimmer
16
and the ion guide
17
to be complicated.
SUMMARY OF THE INVENTION
It is therefore an object of this invention in view of the problems described above to provide a mass spectrograph having an ion guide with a simplified structure and a simplified attachment mechanism for the skimmer while maintaining a high level of efficiency in passing ions.
A mass spectrograph embodying this invention, with which the above and other objects can be accomplished, may be characterized not only as being of the kind having an ionization chamber for ionizing a sample, a skimmer in a conical shape with an orifice, an analyzing chamber at a lower pressure than inside the ionization chamber such that the generated ions are pulled through the orifice into the analyzing chamber, and a multi-pole ion guide which is disposed immediately behind the skimmer and comprised of an even number of cylindrically shaped electrodes all elongated in an axial direction but also wherein these electrodes are disposed so as to circumscribe an inscribed circle and the bottom surface of the conically shaped skimmer has a smaller diameter than the inscribed circle of the ion guide such that the ions can reach the analyzing chamber more efficiently.


REFERENCES:
patent: 4358302 (1982-11-01), Dahneke
patent: 4863491 (1989-09-01), Brandt et al.
patent: 5432343 (1995-07-01), Gulcicek et al.
patent: 5447553 (1995-09-01), Apffel, Jr. et al.
patent: 5793039 (1998-08-01), Oishi et al.
patent: 5847386 (1998-12-01), Thomson et al.

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