JPH07118296B2 - Ionization method for high pressure mass spectrometry. - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for ionizing a sample for mass spectrometry, and more particularly to an ionization method for high pressure mass spectrometry that ionizes at atmospheric pressure. It is about.

(Prior Art) Mass spectrometry is often used to detect trace components. In mass spectrometry, a gasified sample is ionized, and each ion is separated according to its mass by passing through an analysis field consisting of a magnetic field or an electric field, and this is detected and measured. Therefore, it is possible to perform analysis on a molecular or atomic basis, and it is particularly useful for detection of trace biometabolites, analysis of pollutant components or ultratrace impurities contained in semiconductor manufacturing process gas.

By the way, in order to carry out such mass spectrometry,
It is necessary to ionize the sample. Conventionally, the ion bombardment method in which a high-speed electron is applied to the sample has been generally used for the ionization of such a sample.

However, in order to ionize the sample by electron impact, the electrons must be accelerated to high speed,
It is necessary to maintain a high vacuum in the ionization chamber that ionizes the sample. Therefore, the whole mass spectrometer becomes large-scale. Further, when electrons are applied at a high speed, the sample may be destroyed and a low molecular weight substance may be generated. When such a substance is produced, it becomes difficult to interpret the analysis result.

Therefore, in the ionization chamber under atmospheric pressure, the radiation source ⁶³ Ni or corona discharge is used to generate chemically reactive ions, and the ions, that is, the primary ions, are reacted with the sample. An ionization method for ionizing cesium has been developed, and high-pressure mass spectrometry using the method has been performed.

(Problems to be Solved by the Invention) However, since such an ionization method indirectly ionizes a sample, it is difficult to increase the ionization efficiency. Especially when the ion is a certain kind of organic substance, the chemical reaction with the primary ion is difficult to occur,
This method makes it difficult to ionize the sample.

Also, in the case of generating primary ions by radiation or corona discharge, the same number of electrons as the primary ions are generated at the time of generation, and the electrons recombine with the primary ions, so the concentration of the primary ions in the ionization chamber is sufficiently increased. I can't. Therefore, the number of ionized sample molecules is limited.

As described above, the conventional ionization method under atmospheric pressure cannot sufficiently ionize the sample, which is a hindrance factor in improving the accuracy of the high-pressure mass spectrometry.

The present invention has been made in view of such a problem, and an object thereof is to provide an ionization method suitable for high-pressure mass spectrometry, which can efficiently ionize a sample under atmospheric pressure. is there.

(Means for Solving the Problem) In order to achieve this object, in the present invention, an emitter made of a metal is arranged in an ionization chamber of the high-pressure mass spectrometer, which is maintained under atmospheric pressure, and the emitter is appropriately adjusted. The sample is put on a carrier gas and introduced into the ionization chamber while being heated and maintained at the temperature so that the sample collides with the emitter. As a material for the emitter, a material having a high work function such as platinum or iridium is desirable.

(Operation) When the sample is made to collide with the metal emitter, the sample is ionized by surface ionization. In particular, when the emitter is made of a metal having a high work function, its ionization degree becomes high. Since the emitter is placed in a heating environment, the ionized sample is desorbed without being adsorbed by the emitter. In that case, the electrons of the sample are taken by the emitter. Therefore, the generated ions do not recombine with the electrons.

Ionization by such surface ionization is more efficiently performed for a sample having a smaller ionization energy. Also, many organic substances have low ionization energy. Therefore, even a sample containing an organic substance can be efficiently ionized.

(Examples) Examples of the present invention will be described below with reference to the drawings.

In the drawings, FIGS. 1 and 2 show an example of a high-pressure mass spectrometer to which the present invention is applied, and FIG. 1 is an overall configuration diagram thereof.
The figure is a detailed view of the ion source part.

As is clear from FIG. 1, the high-pressure mass spectrometer 1 includes an ion source section 2 for ionizing a sample and accelerating it, and an analysis field 3 for separately analyzing an ion beam introduced from the ion source section 2. And a detector 4 for detecting and measuring the separated ions. The analysis field 3 is configured as a lens having a uniform magnetic field, and the ions are separated by their masses and reach the detection unit 4. Ion current is detected in the detector 4.

As shown in FIG. 2, the ion source part 2 is provided with an ionization chamber 5. The ionization chamber 5 is open under atmospheric pressure. A sample introduction port 6 is provided in the ionization chamber 5, and the gasified sample is introduced into the ionization chamber 5 together with a carrier gas composed of an inert gas such as nitrogen gas.
Is introduced into the ionization chamber 5. Further, an ion source slit 7 for guiding the ionized sample to the analysis field 3 is provided on the front end surface of the ionization chamber 5.

An emitter 8 made of platinum, which is a metal having a high work function, is arranged in the ionization chamber 5. The emitter 8 is connected to an external power source through a pair of lead wires 9 and is electrically heated.

Next, the operation of the high pressure mass spectrometer 1 configured as above will be described.

When performing mass spectrometry, the emitter 8 is heated to an appropriate temperature, for example, 500 to 600 ° C. Then, the sample introduction port 6
The sample is introduced into the ionization chamber 5 together with the carrier gas. Then, the sample collides with the emitter 8.

As shown in FIG. 3, when the sample molecule M collides with the heated solid surface S, the molecule M is repelled without being adsorbed on the solid surface S. Then, when repelled, some of the molecules M are deprived of the electrons and ionized. That is, the surface is ionized.

The number N of ions M ⁺ generated by such surface ionization
_ion is N, the number of neutral sample molecules M reflected from the solid surface S, I _E is the ionization energy of the molecules M, Φ is the work function of the solid, and T is its temperature. Is expressed as Here, A and k are constants.

As is clear from this equation, the higher the work function of the solid and the lower the ionization energy of the sample molecule, the more the sample molecules are ionized. That is, the degree of ionization is high. Moreover, such surface ionization occurs even under atmospheric pressure.

In this case, if the temperature of the solid is high, the ratio of the number of molecules that are ionized will decrease, but if the temperature of the solid is decreased, the molecules that collide with the surface of the solid will be adsorbed on the surface and will not be reflected from the surface of the solid. Will end up. Therefore, the solid surface needs to be heated and maintained at a temperature at which the ionized sample molecules are desorbed. That temperature is generally
500 to 600 ° C.

Therefore, when the sample collides with the platinum emitter 8 heated to 500 to 600 ° C. as described above, the sample is efficiently ionized even under the atmospheric pressure. Moreover, when the ionization energy of the sample is small, it is more efficiently ionized. Then, the robbed electrons are trapped inside the emitter 8.

The ions thus generated are accelerated in the ion source unit 2 into an ion beam, guided from the ion source slit 7 to the detection unit 4 through the analysis field 3, and detected for each mass.

As described above, by utilizing the surface ionization of the chemical species on the heated surface of the emitter 8 having a high work function, the sample can be efficiently ionized even under the atmospheric pressure. Moreover, since a sample having a smaller ionization energy is more efficiently ionized, this method is particularly effective when, for example, detecting trimethylamine, which is a pollution component having a strong odor contained in the atmosphere.

In addition, in the above-mentioned embodiment, an example in which the present invention is applied to a high-pressure mass spectrometer has been described, but it will be apparent that the present invention can also be applied to a high-pressure mass analyzer.

Further, the example in which platinum is used as the emitter 8 has been described, but other metals can be used as the emitter. However, it is desirable that the metal has a work function as high as possible.

(Effects of the Invention) As is clear from the above description, according to the present invention, the sample is made to collide with the heated metal surface and ionized by surface ionization. It can be directly ionized. Therefore, the structure of the mass spectrometer can be simplified, and the sample is less likely to be destroyed, so that highly accurate mass spectrometry can be performed. Moreover, since the generated ions are not recombined with the electrons, the sample can be efficiently ionized, and the accuracy of mass spectrometry can be further improved.

Further, since a chemical species having a smaller ionization energy is more efficiently ionized, a high pressure mass spectrometric method can be provided which is particularly effective in analyzing a sample containing such a chemical species.

[Brief description of drawings]

FIG. 1 is a vertical sectional view showing an example of a high-pressure mass spectrometer to which the present invention is applied, FIG. 2 is an enlarged vertical sectional view showing an ion source portion of the high-pressure mass spectrometer, and FIG. It is explanatory drawing which shows the principle of. 1 ... High-pressure mass spectrometer (high-pressure mass spectrometer) 2 ... Ion source part, 5 ... Ionization chamber 6 ... Sample inlet, 8 ... Emitter

Claims (1)

[Claims] 1. An emitter made of metal is placed in an ionization chamber of an high-pressure mass spectrometer, which is maintained under atmospheric pressure, and the emitter is heated to an appropriate temperature, and a sample is placed in the ionization chamber together with a carrier gas. An ionization method for high pressure mass spectrometry, which comprises bombarding a sample with the emitter upon introduction.