Mass spectrometry of solution and apparatus therefor

Details Mass spectrometry of solution and apparatus therefor Mass spectrometry of solution and apparatus therefor

2001-05-18

2002-08-20

Nguyen, Kiet T. (Department: 2881)

Radiant energy

Ionic separation or analysis

With sample supply means

Reexamination Certificate

active

06437327

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to mass spectrometry of a solution, and particularly to a mass spectrometer for analyzing substances in a solution and an apparatus for combining a liquid chromatograph used for separation and analysis of a mixed sample and the mass spectrometer.
At present, development of mass spectrometry of biological substances is regarded as important in the field of analysis. Biological substances are generally dissolved in a solution as a mixture, so that an apparatus for combining a means for separating a mixture and a mass spectrometer is under development. As a typical apparatus of this method, there is a liquid chromatograph-mass spectrometer (hereinafter abbreviated to LC/MS) available. A liquid chromatograph (hereinafter abbreviated to LC) is superior in separation of a mixture but cannot identify each substance, whereas a mass spectrometer hereinafter abbreviated to MS) is highly sensitive and superior in identification ability but not suitable to analysis of a mixture. Therefore, an LC/MS using an MS as an LC detector is very useful in analysis of a mixture.
The LC/MS using the conventional atmospheric pressure chemical ionization method which is disclosed in Japanese Patent Application Laid-Open 5-325882 will be explained hereunder by referring to FIG.
9
.
A sample solution eluted from a liquid chromatograph
14
is introduced into a metallic tube
3
via a pipe
1
and a connector
2
. The sample solution comprises a sample and a mobile phase (a buffer solution which flows into the separation column when the sample is separated in the LC). However, for the purpose of improving the ionization efficiency in the ion source (according to the present invention, the term “ion source” means a portion for converting a substance to be analyzed existing in the liquid phase to ions in the gas phase and includes a spray portion for spraying a solution, a vaporization portion for vaporizing droplets generated by the spray portion, and an ionization portion for ionizing a substance), a solvent different from the mobile phase may be added. The metallic tube
3
is embedded in a metallic block
4
a
. When the metallic block
4
a
is heated by a heating means such as a heater, a sample solution introduced in the metallic tube
3
is sprayed. Fine droplets generated by spraying are introduced into and vaporized in a vaporization portion
5
comprising a heated metallic block
4
b.
Sample molecules vaporized in the vaporization portion
5
are introduced into the ionization portion
6
. A needle electrode
7
is installed in the ionization portion
6
. When a high voltage is applied to this needle electrode
7
from a high voltage source
8
a
, a corona discharge is generated in the ionization portion
6
.
Assuming A to be sample molecules to be analyzed and B to be molecules of a reaction gas, the atmospheric pressure chemical ionization method is a method for ionizing A by a chemical reaction of A and B. As a typical ion chemical reaction, there are a protonation reaction and a deprotonation reaction available as shown below.
A+BH
30
→AH
+
B
(Protonation reaction)
A+B
−
→(
A−H
)
−
+BH
(Deprotonation reaction)
According to the prior art shown in
FIG. 9
, hydronium ions (H
3
O
+
) are generated when a corona discharge is generated in the atmosphere and ions Ali+ of the sample A are generated by using the following reaction between the hydronium ions and the sample molecules A.
A+H
3
O
+
→AH
+
+H
2
O
Ions of the sample generated by chemical ionization in the ionization portion
6
have their trajectory deflected by a voltage applied to a deflection electrode
31
by a power source
30
and drift toward an ion introduction aperture
9
a
. The ions pass through the ion introduction aperture
9
a
and are introduced into a high vacuum portion
12
which is exhausted to a high vacuum by an exhaust system
10
b
via a differential pumping portion
11
which is evacuated by an exhaust system
10
a
and an ion introduction aperture
9
b
. When ions and solvent molecules pass through the ion introduction apertures
9
a
and
9
b
, they are cooled by adiabatic expansion, so that so-called clustering for condensing the ions and solvent molecules again occurs. To prevent the clustering, electrodes in which the ion introduction apertures
9
a
and
9
b
are formed are heated. The mass of ions introduced into the high vacuum portion
12
is analyzed by a mass spectrometric portion
13
. Nonvolatile compounds that are not ionized in the ionization portion
6
are diffused in the atmosphere and captured by a capture plate
32
.
An LC/MS using the conventional electrospray method which is disclosed in Japanese Patent Application Laid-Open 6-102246 will be explained by referring to
FIG. 10. A
sample solution eluted from the LC is introduced into the metallic tube
3
via the pipe
1
and the connector
2
. A high voltage is applied between the metallic tube
3
and an electrode
21
c
in which the ion introduction aperture
9
a
is formed by using a high voltage source
8
b
and the sample solution is electrostatically sprayed. To assist electrospray, gas such as nitrogen gas is let flow from a spray gas outlet
40
. When fine charged droplets generated by electrospray are vaporized, gaseous ions are generated. However, the diameter of droplets at the center of the jet is large and it is difficult to vaporize droplets with a large diameter, and furthermore when droplets with a large diameter adhere to the electrode
21
c
, the temperature of the electrode
21
c
drops and the ion intensity obtained in the mass spectrometric portion may vary. Therefore, a shielding plate
41
for shielding the center of the jet is installed between the metallic tube
3
and the electrode
21
c
and the outer periphery of the jet is sprayed toward the ion introduction aperture
9
a
. Generated ions are introduced into the high vacuum portion
12
via the ion introduction aperture
9
a
, the differential pumping portion
11
, and the ion introduction aperture
9
b
and analyzed by the mass spectrometric portion installed in the high vacuum portion
12
.
For analysis of biological substances and environmental contaminants, a method for analyzing a sample solution containing nonvolatile compounds of high concentration is required.
For example, in the LC, the mobile phase including a nonvolatile salt is often used experientially so as to enhance the separation ability and the reproducibility of the retention time. As a result, a method for analyzing a sample solution containing a nonvolatile salt is desirable for a detector of the LC.
Nonvolatile compounds are contained not only in samples obtained from a living organism such as urine, perspiration, and blood, but also is samples relating to the environment such as effluents from a factory and water of a lake or marsh. To remove nonvolatile compounds from these samples, a complicated pretreatment such as desalting is required. Therefore, in order to analyze biological substances and environmental contaminants quickly, a method for analyzing a sample solution containing nonvolatile compounds is required.
However, in a mass spectrometric apparatus using the conventional atmospheric pressure chemical ionization method shown in
FIG. 9
, when a sample solution containing nonvolatile compounds of high concentration is introduced into the ion source of the mass spectrometric apparatus comprising an ion source, a differential pumping portion, and a mass spectrometric portion, a problem arises that ions of a substance to be analyzed cannot be analyzed stably for many hours. The reason is that since the sample solution is sprayed by using the heated metallic tube, solvent molecules are vaporized in the tube and nonvolatile compounds are salted out on the inner wall of the tube. The inner diameter of the metallic tube becomes smaller because nonvolatile compounds are salted out on the tube wall, and the metallic tube finally clogs up. Therefore, the

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