Mass spectrometer

Details Mass spectrometer Mass spectrometer

1999-03-12

2002-05-21

Anderson, Bruce (Department: 2881)

Radiant energy

Ionic separation or analysis

Cyclically varying ion selecting field means

Reexamination Certificate

active

06392226

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a mass spectrometer, and particularly to a liquid chromatograph/mass spectrometer in which a liquid chromatograph is coupled with an ion trap type mass spectrometer.
BACKGROUND ART
Recently, in the field of analysis, it is required to establish a technique of analyzing a mixture. For example, in the case of analyzing harmful substances in environments, a sample taken for analysis (for example, water in lakes and marshes) contains a variety of substances. The same is true for analysis of substances associated with organisms. A sample derived from an organism, such as blood or urine, contains a variety of substances. In this way, the technique of analyzing a mixture is essential to analysis of substances associated with environments and substances associated with organisms.
In general, it is difficult to directly analyze a mixture. Accordingly, a mixture is separated into components, each of which is in turn detected and identified. In such circumstances, a liquid chromatograph/mass spectrometer (hereinafter, referred to as “LC/MS”) and a capillary electrophoresis/mass spectrometer (hereinafter, referred to as “CE/MS”) in which a liquid chromatograph and a capillary electrophoresis good in separation are respectively coupled with a mass spectrometer good in identification of a substance are very useful for analysis of the above-described substances associated with environments and organisms.
A prior art LC/MS using a mass spectrometer having an ion trap type mass spectrometric unit will be described with reference to FIG.
14
.
A liquid chromatograph
1
includes a liquid supply pump
2
, a mobile phase solvent bath
3
, a sample injector
4
, a separation column
5
, and a pipe
6
. The mobile phase solvent is supplied at a specific flow rate from the liquid supply pump
2
to the separation column
5
. A mixture sample is introduced from the sample injector
4
disposed between the liquid supply pump
2
and the separation column
5
. The sample, which has reached the separation column
5
, is separated into components by interaction with a filler charged in the separation column
5
. The sample, whose components have been separated by the liquid chromatograph
1
, is introduced together with the mobile phase solvent into an ion source
7
.
Of known various type of ion sources, a typical electrostatic spraying type will be described below. The sample, which has reached the ion source
7
, is introduced in a metal tube
9
a
via a connector
8
. When a high voltage of several kilovolts is applied from a high voltage power supply
11
between the metal tube
9
a
and an electrode
10
disposed opposite to the metal tube
9
a
, electrostatic spray is generated in the direction of the counter electrode
10
from the end of the metal tube
9
a
. The flow rate of a solution allowing to sustain stable electrostatic spraying is about several microliters per minute; however, the flow rate of the solution supplied from the liquid chromatograph
1
to the ion source
7
is about one milliliter per minute, and accordingly, a spraying gas
13
supplied from a gas supply pipe
12
is allowed to flow around the metal tube
9
a
to assist electrostatic spraying with the gas
13
. Droplets created by electrostatic spraying, which contain ions associated with sample molecules, are dried into gaseous ions. The ions thus created are introduced in a vacuum unit
17
pumped by a pumping system
15
b
via an ion introducing pore
14
a
opened in the counter electrode
10
, a differential pumping portion
16
pumped by a pumping system
15
a
, and an ion introducing pore
14
b
. An electrostatic lens
19
a
composed of electrodes
18
a
and
18
b
is disposed in the differential pumping portion
16
, which lens acts to converge ions for improving the permeability of the ions through the pore
14
b
. The ions introduced in the vacuum unit
17
are converged through a converging lens
19
b
composed of electrodes
18
c
,
18
d
and
18
e
, and then introduced in an ion trap mass spectrometric unit
20
.
Next, the operational principle of the ion trap mass spectrometric unit will be described. The ion trap mass spectrometric unit
20
includes a ring electrode
21
and end cap electrodes
22
a
and
22
b
.
FIG. 15
is a diagram showing a control of the amplitude of a high-frequency voltage applied to the ring electrode with an elapsed time in a period of time required for obtaining a first mass spectrum (the change in voltage applied to an electrode with an elapsed time, as shown in the figure, is hereinafter referred to as “scan function”). First, in an ion storage period.
201
, a high-frequency voltage is applied to the ring electrode
21
to form a potential for confinement of ions in a space surrounded by the ring electrode
21
and end caps electrodes
22
a
and
22
b
. The ions trapped in the vacuum unit
17
are converged through the converging lens
19
b
to enter into the space surrounded by the ring electrode
21
and the end cap electrodes
22
a
and
22
b
from an opening
23
a
formed in the end cap electrode
22
a
. An impingement gas such as helium is introduced in the space surrounded by the ring electrode
21
and the end cap electrodes
22
a
and
22
b
, and is kept at a pressure of about 1 milli-Torr. The ions impinge on molecules of the impingement gas to lose the energies thereof, and are confined in the confinement potential formed in the space surrounded by the ring electrode
21
and the end cap electrodes
22
a
and
22
b
. Next, in a scan period
202
, a voltage applied to either of the electrodes
18
c
,
18
d
and
18
e
constituting the converging lens
19
b
is changed, to prohibit the ions from passing through the converging lens
19
b
, thereby preventing entrance of the ions into the ion trap mass spectrometric unit
20
. The mass analysis is performed by gradually increasing the amplitude of the high-frequency voltage applied to the ring electrode
21
. It is known from a document “Practical Aspects of Ion Trap Mass Spectrometry, vol. 2, p. 10(CRC Press, 1995)” that in the ion trap mass spectrometric unit, the ion trajectory becomes unstable in the direction of the end cap electrode (the direction of the Zo axis shown in
FIG. 14
) if a q value defined in the following equation is more than 0.908.
q=
8
z
V/
m
(
r
o
2
+2
Z
o
2
)&OHgr;
2
  (Equation 1)
In this equation, z designates the electric charge of an ion; V is the amplitude of a high-frequency voltage applied to the ring electrode; m is the mass of the ion; r
0
and Z
0
are a radius of the circle inscribed with the ring electrode
21
and the distance from the center of the circle to each of the end cap electrodes
22
a
and
22
b
respectively; and &OHgr; is an angular frequency of the high-frequency voltage applied to the ring electrode
21
. Accordingly, in the scan period
202
, as the amplitude V of the high-frequency voltage applied to the ring electrode
21
is gradually increased, the trajectories of the ions become unstable sequentially in the order from an ion having a smaller value obtained by dividing the mass of the ion by the electric charge of the ion (hereinafter, referred to as “m/z”) to an ion having a larger value of m/z, and the ions are sequentially discharged from openings
23
a
and
23
b
formed in the end cap electrodes
22
a
and
22
b
to the outside of the mass spectrometric unit
20
. The discharged ions are detected by an ion detector
24
, and detection signals are supplied to a data processor
26
via a signal line
25
, to be thus processed. After termination of the scan period
202
, the voltage applied to the ring electrode
21
is cut off, to destroy the ion confinement potential, thereby removing the ions remaining in the mass spectrometric unit
20
(ion removing period
203
). These sequences of operations (ion storage period
201
, scan period
202
, and remaining ion removing period
203
) are repeated, to perform mass analysis of the samples sequentially supplied from the liquid chromatograph
1
.
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