JPH0512664B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method of ionizing a sample for mass spectrometry, and particularly relates to a method of ionizing a sample using the principle of surface ionization. It concerns ionization methods for analytical methods.

(Prior Art) Mass spectrometry is often used to detect trace components. This mass spectrometry method involves ionizing a gasified sample, passing it through a mass separation unit made of a magnetic field or an electric field, separating each ion according to its mass, and detecting and measuring the ions. This type of mass spectrometry enables analysis on a molecular or atomic basis, so it is particularly effective in detecting trace amounts of biological metabolites and analyzing trace amounts of impurities contained in pollutant components or semiconductor manufacturing process gases. Demonstrate.

By the way, in order to perform such mass spectrometry, the sample must be ionized. As the ionization method, various methods such as electron impact method have been proposed and put into practice.

One such ionization method uses the principle of surface ionization. This is a method in which a heated solid surface with a high work function is placed in the ionization chamber of a mass spectrometer, and a gasified sample is guided to this. When sample molecules with low ionization energy are brought into contact with a solid surface with a high work function in a heated environment, the sample molecules are efficiently ionized by surface ionization.

Such an ionization method using surface ionization is particularly capable of efficiently ionizing elements, and is therefore still an indispensable method for isotope dilution methods and the like. It has also recently been found to be useful for certain organic compounds. Moreover, their spectra are often simple and contain a lot of information regarding their structures. Therefore, this ionization method is
Its use as a unique method is becoming more and more widespread.

When a sample is ionized using surface ionization in this manner, conventionally the sample is simply gasified and then introduced into the ionization chamber.

(Problem to be Solved by the Invention) However, in such a conventional ionization method using surface ionization, the proportion of sample molecules ionized by surface ionization is determined by the ratio of the work function of the solid surface and the ionization energy of the sample molecules. It will be determined by the difference. Therefore, chemical species with relatively high ionization energy are difficult to ionize, and sufficient ionization efficiency cannot be obtained. In particular, it becomes completely powerless against compounds with ionization energy of 8 eV or more.

The present invention was made in view of these problems, and its purpose is to obtain an ionization method for mass spectrometry that can efficiently ionize a wider range of chemical species by surface ionization. That's true.

(Means for Solving the Problem) In order to achieve this object, in the present invention, a light gas such as hydrogen or helium is used as a carrier gas, and the sample is ejected together with the carrier gas from a nozzle into a vacuum ionization chamber. There is. Gas ejected from the nozzle is guided to a heated solid surface within the ionization chamber.

(Function) When gas is ejected from the nozzle into a vacuum in this manner, a supersonic free jet is formed. Then, the heavy sample molecules are accelerated to the speed of the light carrier gas molecules while repeating two-body collisions. Therefore, the sample molecules have extremely large kinetic energy.

Gas phase molecules with such large kinetic energy are surface ionized extremely efficiently in the field of interaction between the gas phase and the solid phase. Therefore, when the sample molecules ejected from the nozzle are guided to the heated solid surface, the molecules are ionized with high efficiency.

(Example) Hereinafter, an example of the present invention will be described using the drawings.

In the drawings, FIG. 1 is a schematic configuration diagram showing an example of a mass spectrometer to which the present invention is applied, and FIG. 2 is an explanatory diagram showing a state in which gas is ejected from a nozzle used in the mass spectrometer.

As is clear from FIG. 1, the mass spectrometer 1 includes an ion source section 2 that ionizes a sample and accelerates it;
It includes a mass separation section 3 that separates the ion beam introduced from the ion source section 2. In this embodiment, the mass separator 3 is of a quadrupole type in which two sets of electrodes 3a and 3b facing each other are arranged. The ions are transferred to their electrodes 3
By passing through the high frequency electric field formed by a and 3b, the light is separated into masses and detected in a detection section (not shown).

The ion source section 2 and the mass separation section 3 are arranged in a vacuum chamber 5 connected to a vacuum evacuation device via a trap 4, and are always kept under a vacuum atmosphere.

The ion source section 2 is provided with an ionization chamber 6. This ionization chamber 6 is open into the vacuum chamber 5. Therefore, the inside of the ionization chamber 6 is also kept in a vacuum atmosphere.

A sample introduction section 7 is provided outside the vacuum chamber 5 . A gasified sample is introduced into the sample introduction section 7 together with a light carrier gas such as hydrogen or helium. Further, a nozzle 9 is connected to the outlet side of the sample introduction section 7 via a conduit 8. The tip of the nozzle 9 is designed to protrude into the ionization chamber 6.

On the other hand, in the ionization chamber 6, an emitter 10, which is a solid surface that ionizes the sample by surface ionization, is arranged at a position facing the nozzle 9. The surface of the emitter 10 is heated to an appropriate temperature by an external temperature control device 11.

As shown in FIG. 2, a gas outlet 9a is provided at the tip of the nozzle 9. The gas outlet 9a has a minute diameter of about 50 to 100 μm. Further, a heater 12 whose temperature is controlled by an appropriate power source is attached to the outer periphery of the tip of the nozzle 9 so that the gas inside the nozzle 9 can be heated.

The emitter 10 is made of either a solid surface with a low work function or a solid surface with a high work function. In order to obtain a material with a low work function, alumina silicate doped with an alkali metal compound such as a rubidium salt is formed into beads and attached to a heating wire such as a platinum coil. Further, in the case of using a material with a high work function, for example, a metal such as platinum, rhenium, tungsten, or a metal oxide thereof may be formed into a coil shape and heated electrically.

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

When performing mass spectrometry, the inside of the vacuum chamber 5 is maintained in a vacuum atmosphere, and the surface of the emitter 10 is heated to an appropriate temperature. Further, the tip of the nozzle 9 is also heated by the heater 12.

In this state, a carrier gas is introduced into the sample introduction section 7, and a gasified sample is injected. The sample is then carried by the carrier gas through conduit 8 to the tip of nozzle 9.
The gas ejection port 9a of the nozzle 9 has a minute diameter, and gas is successively fed from the upstream side thereof, so that the inside of the nozzle 9 has a positive pressure. On the other hand, the outside of the nozzle 9 is maintained in a vacuum atmosphere. Therefore, an extremely large pressure difference occurs between the upstream side and the downstream side of the gas jet port 9a. As a result, the light carrier gas is ejected from the gas outlet 9a,
Along with this, heavy sample molecules are also blown out into the vacuum.

At this time, since the mean free path of the ejected gas is sufficiently smaller than the diameter of the gas ejection port 9a, the sample molecules are accelerated to the speed of light carrier gas molecules while repeating two-body collisions immediately after ejecting.
In this way, the gas ejected from the gas ejection port 9a is
This results in a supersonic free jet having a supersonic region as shown in FIG. Therefore, the kinetic energy of heavy sample molecules in the supersonic jet becomes extremely large.

An emitter 10 is provided in this jet stream.
Therefore, the sample molecules come into contact with the solid surface of the emitter 10 and are ionized by surface ionization. In this case, since the sample molecules have extremely large kinetic energy as described above, their ionization is significantly promoted. In other words, when the sample molecules become negative ions, the kinetic energy of the sample molecules has the same effect as amplifying their electron affinity. When a sample is introduced into a sample, its molecules or their constituent chemical species are extremely efficiently ionized into negative ions. In addition, if the sample molecules become positive ions, the kinetic energy of the sample molecules has the same effect as increasing the work function of the solid surface. When guided, the molecule or its constituent chemical species are ionized into positive ions with extremely high efficiency.

In this way, by setting the work function of the emitter 10 to be high or low depending on the characteristics of the sample to be detected, it becomes possible to efficiently ionize even compounds that could not be ionized with conventional devices. become.

The ions thus generated are accelerated in the ion source section 2 and guided to the mass separation section 3 as an ion beam. In the mass separation section 3, a high frequency electric field is formed, so that the trajectory of the ion beam is bent according to the mass of each ion, and the ion beam is separated by mass.

In such an ionization method, the kinetic energy given to the sample molecules is controlled by the gas pressure upstream of the gas outlet 9a, the degree of vacuum downstream, the temperature of the gas, the diameter of the gas outlet 9a, etc. . Among these, the degree of vacuum on the downstream side of the gas ejection port 9a is generally determined by the degree of vacuum in the vacuum chamber 5 of the mass spectrometer 1. Further, the minimum diameter of the gas ejection port 9a is approximately determined from the viewpoint of its workability. However, the gas pressure on the upstream side of the gas jet port 9a can be adjusted by the supply flow rate of the carrier gas.
The temperature of the gas can be adjusted by a heater 12 attached to the tip of the nozzle 9. Therefore, by adjusting them, the kinetic energy of the sample molecules can be optimized.

In the above embodiment, the mass spectrometer 1 in which the mass separator 3 is of a quadrupole type is taken as an example, but a mass spectrometer equipped with a mass separator that forms a uniform magnetic field, It will be obvious that the present invention can also be applied to mass spectrometers.

Furthermore, the present invention can also be employed when performing mass spectrometry on a sample separated by a gas chromatograph column.

(Effects of the Invention) As is clear from the above explanation, according to the present invention, the sample is ejected from the nozzle into vacuum together with a light carrier gas, and the sample molecules have large kinetic energy, so that heating is possible. surface ionization on the solid surface is significantly promoted,
Samples can be ionized with high efficiency. Therefore, compounds that cannot be ionized and detected using conventional ionization methods using surface ionization, or compounds that can be ionized but are present in such small amounts that sufficient detection sensitivity cannot be obtained. It also becomes possible to detect and expand the range of application.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing an example of a mass spectrometer to which the present invention is applied, and FIG. 2 is an explanatory diagram showing a state in which gas is ejected from a nozzle provided in an ionization chamber. 1... Mass spectrometer (mass spectrometer), 5... Vacuum chamber, 6... Ionization chamber, 7... Sample introduction section, 9
... Nozzle, 9a ... Gas outlet, 10 ... Emitter (solid surface), 11 ... Temperature control device, 12
……heater.

Claims (1)

[Claims] 1. A solid surface heated to an appropriate temperature is placed in an ionization chamber maintained in a vacuum atmosphere of a mass spectrometer, and a sample is ejected from a nozzle together with a light carrier gas toward the solid surface. An ionization method for mass spectrometry, characterized in that a sample is ionized by surface ionization on the solid surface. 2. The ionization method according to claim 1, wherein when molecules in the sample or their constituent chemical species have a large electron affinity, the solid surface is made to have a low work function. 3. The ionization method according to claim 1, wherein when the ionization energy of molecules in the sample or their constituent chemical species is small, the solid surface is made to have a high work function. 4. The ionization method according to claim 1, further comprising heating the gas ejected from the nozzle.