JP2000040489A - Laser ionization mass spectrometer and measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To almost simultaneously measure at least two or more measuring object substances even by a compact measuring instrument, and to analyze the measuring object substances in high quantitatively determining accuracy by providing a laser beam wave length high speed converting mechanism in a pulse laser beam oscillator. SOLUTION: A resonator of a titanium sapphire laser of a laser beam wave length high speed converting mechanism used as a pulse laser beam oscillator, is composed of a rear mirror 5, a plane mirror 6, a titanium sapphire crystal 2, a telescope 12 and an acoustic optical element 3. A control signal is outputted from a personal computer 20, and when an ultrasonic wave of a frequency of 100 MHz to 70 MHz is applied to the acoustic optical element 3 by an ultrasonic driver 7, a laser beam having a wave length of 700 to 1000 nm can be outputted from an output coupler 6. When the laser beam is introduced to an ionization chamber, it is turned into a laser beam having a wave length for exciting/ionizing 1.4-dichlorobenzene, and can be changed in a period of maximum 1 KHz.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser ionization mass spectrometer for ionizing sample molecules as a substance to be measured by irradiating a laser beam and measuring the mass spectrum of the ions to perform mass analysis of the sample. is there.

[0002]

2. Description of the Related Art Combustion exhaust gas including coal and heavy oil,
Municipal waste and industrial waste incineration exhaust gas, metal smelting process exhaust gas, plastic pyrolysis gas, etc.
Although it is a trace amount, it contains compounds such as nitrogen oxides, sulfur oxides, aromatic compounds, chlorinated organic compounds, chlorinated aromatic compounds, and other halogenated compounds, and in many cases, a plurality of these are together, Present in a mixed state. As a rapid measurement technique for these compounds, a method based on laser multiphoton ionization mass spectrometry, which has selectivity for detection of a compound to be measured, is excellent in principle.

An example in which a mixed gas sample is measured by laser multiphoton ionization mass spectrometry is described in Analytical Che.
Mistry, Vol. 66, pp. 1062-1069 (1
994). That is, in the laser multiphoton ionization mass spectrometry technique using ordinary sample introduction, since the absorption line of light in the ultraviolet and visible regions of each compound is wide due to molecular motion, even if monochromatic light is irradiated, a plurality of compounds can be obtained. Ionize. Therefore, it is often impossible to separate and detect only a specific molecule (compound). That is, the accuracy is reduced due to the influence of other compounds coexisting in the quantification. Therefore, a gas sample is introduced into a vacuum ionization chamber through a sample introduction valve having a small hole diameter, irradiated with a laser, ionized, and measured by a mass spectrometer. This allows the gas sample to expand adiabatically and cool to near absolute zero,
Vibration and rotation of molecules of each compound are suppressed. In this state, the absorption line of ultraviolet / visible light of each compound becomes sharp, and a specific molecular species can be selectively ionized using monochromatic laser light. That is, even if various compounds coexist, that is, exist as a mixture, only a specific substance can be selectively measured by a laser ionization mass spectrometer. This method is sometimes called supersonic molecular beam spectroscopy or supersonic molecular jet spectroscopy because the velocity of the introduced molecules is about several tens of times the speed of sound. In this document, a standard laser beam irradiation time is described as 10 ns.

[0004]

In order to manufacture a measuring device having a fast response time by using this laser multiphoton ionization mass spectrometry technique, a laser oscillator and its power supply occupy about 60 to 80% of the apparatus. That is, when the compound in the exhaust gas is to be measured, for example, the width is about 1000 mm and the depth is 1000 to 2000 mm at the current technical level.
And the height is about 1200 to 1500 mm. Therefore, there is a problem that an attempt to measure at least two substances to be measured substantially simultaneously leads to a further increase in the size of the measuring instrument. That is, the oscillation of the variable laser light in the ordinary ultraviolet region is a dye laser, and the oscillation wavelength is changed by exchanging the dye or changing the angle of the dye cell. On the other hand, in the case of an optical parametric oscillation laser, the wavelength is changed by the angle rotation of the crystal. In any case, it takes several minutes to several hours. To measure two substances to be measured almost simultaneously, it is necessary to provide two ultraviolet tunable lasers.

[0005] In order to avoid this, ionization with a laser beam having a non-resonant wavelength results in a significant decrease in sensitivity because the ionization efficiency does not increase, and almost all compounds present because of the inability to selectively ionize. Is ionized. Therefore, for example, Rapid Comm
communications in Mass Spectr
Ometry, vol. 11, p. 1099 (1997)
As shown in FIG. 3, when a gas containing various compounds such as a combustion exhaust gas or a metal smelting process exhaust gas is used as a sample, a peak caused by another compound overlaps with a peak of a substance to be measured. In many cases, quantitative measurement values cannot be obtained.

[0006] Further, when trying to continuously measure various combustion exhaust gas or metal refining process exhaust gas for a long time,
The analyzer is contaminated with high boiling compounds contained in the exhaust gas and the initial performance cannot be maintained. In other words, if a high-boiling compound adheres to the window (inner wall) of the ionization chamber through which the laser beam passes, the intensity of the laser beam applied to the molecular jet changes, and even if a certain amount of molecules are introduced, the ions generated There was also the problem that the amount was different. That is, there has been a problem that the result is inferior in quantitative accuracy.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem. In a laser ionization mass spectrometry technique for introducing a sample by a supersonic molecular jet, at least two or more measurement methods can be performed while using a compact measuring instrument. Provided is a laser ionization mass spectrometer capable of measuring a target substance at substantially the same time and analyzing the target substance with high quantitative accuracy regardless of contamination inside the analyzer (attachment of a high-boiling compound) or deterioration with time of an excitation laser oscillator. The purpose is to:

[0008]

The above object is achieved by a laser multiphoton ionization mass spectrometer equipped with a high-speed tunable laser oscillator.

That is, the present invention provides a sample introduction unit having a pulse valve for forming a molecular jet, a pulsed laser light oscillator, and a vacuum ionization chamber or window having a window through which laser light emitted from the oscillator can pass. And a laser ionization mass spectrometer having a mass spectrometer for analyzing the mass of molecules ionized by the laser light, wherein the pulse laser light oscillator is provided with a laser light wavelength high-speed conversion mechanism. The present invention relates to an ionization mass spectrometer.

According to the above configuration, in a laser multiphoton ionization mass spectrometer for introducing a supersonic molecular jet sample,
An instruction is issued by a personal computer that controls the entire system to rapidly change the wavelength of the laser beam for ionization to a wavelength that can selectively excite and ionize two or more compounds to be measured. Therefore, at least two or more compounds can be ionized and detected substantially simultaneously without the need for two ultraviolet laser oscillators. Therefore, since at least one ultraviolet variable laser oscillator is not required, the laser system which occupies most of the laser multiphoton ionization mass spectrometer can be simplified, and the apparatus can be downsized.

Further, a predetermined amount (preferably a fixed ratio of the sample such as exhaust gas) of an internal standard substance is added to a sample such as an exhaust gas in advance, and sampling is performed. By performing the correction, an accurate quantitative result can be obtained. According to this technique, even if the exhaust gas to be sampled has a pressure fluctuation and the ionization efficiency changes due to a change in the shape / density of the molecular jet, it can be a means for solving it.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The laser light wavelength high-speed conversion mechanism of the present invention uses an optical resonator having mirrors disposed on both sides of a laser medium.

The laser medium is titanium sapphire crystal,
BBO crystals, dyes and the like can be used.

The mirror reflects the laser light and repeatedly passes the laser medium. One mirror (rear mirror) constantly reflects the laser light, for example, a reflectance of 99.9 in a wavelength range of 680 to 1000 nm. % Or more is appropriate.

As the other mirror (output coupler), a flat mirror having a reflectance of about 70 to 90% in the above-mentioned wavelength range can be used.

An acousto-optic element is provided in the path of the laser light reciprocated by the two mirrors.

An acousto-optic element has the property of diffracting light of a specific wavelength in a certain direction in accordance with the frequency of an applied ultrasonic wave, and is a birefringent optical crystal of Te.
It can be manufactured using O ₂ crystal, CaMoO ₄ crystal, or the like.

In the crystal of this acousto-optic element, the laser light is partially deviated from the traveling direction and diffracted in a certain direction due to the interaction with the ultrasonic wave traveling in the crystal. Therefore, a laser beam having a specific wavelength can be oscillated by configuring the optical resonator so as to match the wavelength to oscillate, that is, diffracting the laser beam having the wavelength to oscillate so as to resonate. That is, the wavelength of the pumping laser can be converted in accordance with the frequency of the ultrasonic wave to be added, and the frequency of the ultrasonic wave can be changed at a high speed.
Also, by applying ultrasonic waves of two different frequencies,
In some cases, it is possible to create laser light of two wavelengths.

The sample introduction part in the mass spectrometer of the present invention may be either continuous introduction or pulse introduction as long as it has a nozzle (including an orifice) capable of producing a supersonic molecular jet. However, in consideration of the burden on the exhaust system such as a pump, pulse-like introduction is preferred.

The pulse valve is used for fuel injection of an engine or the like. As described in the Chemical Society of Japan, 4th edition, Experimental Chemistry Course, Vol. 8, pages 127 to 129 (1993), Usually, a plunger pressed against a seal surface by a spring is electromagnetically attracted rearward by instantaneous energization of a rear solenoid (electromagnetic coil) and is opened only during that time. In addition, a Gentry-Giese type pulse valve, a pulse valve using a piezo element, or the like can be used.

The pulsed laser light oscillator is not particularly limited as long as it can oscillate a high-output pulsed laser light. For example, the following can be used as long as it oscillates a pulsed laser light on the order of nanoseconds. be able to. That is, a titanium sapphire laser or a dye laser is most commonly used. For example, in the case of a titanium sapphire laser, an excimer laser or a second harmonic of a yag laser is used as a pumping light source.

This laser beam is guided to a titanium sapphire laser as it is, and a variable laser beam in the ultraviolet region can be oscillated. Therefore, the acousto-optic device is installed in the resonator of the titanium sapphire laser as described above so that the wavelength can be selected by mechanically free complete electronic control. The adoption of the completely electronic control method makes it possible to directly control the laser oscillation wavelength and its output for each pulse by a signal from the laser system control personal computer, thereby achieving a high-speed wavelength change.

The focusing of the laser beam is not limited at all, and it has various shapes, such as a normal beam cross section or an elliptical shape close to the cross section of a molecular jet formed using a special lens (cylindrical lens). Can be used. The irradiation time is usually several nanoseconds to several tens of picoseconds, but may be shorter if the laser energy is almost the same. That is, it may be in a femtosecond region.

The ionization chamber has a structure capable of forming a high vacuum, and may be provided with a window made of a material that transmits laser light. In some cases, the vacuum ionization chamber is connected to the vacuum chamber of the mass spectrometer so that there is no partition. In that case, the site where the ionization is performed is the site in charge of the vacuum ionization chamber.

As the mass spectrometer, a time-of-flight type,
Any type such as a quadrupole type and a double focusing type can be used.

Ionization chamber and adjacent mass spectrometer
Oil rotary pump, mechanical booster pump, oil
Connect a diffusion pump, turbo molecular pump, etc.^-6~
10 ^-8It can be maintained at about torr.

As for the introduction of the sample, since the ionization chamber (or the corresponding part) is normally kept at 10 ^-6 torr or less, a pressure near normal pressure is sufficient as long as it is in a gaseous state, and the driving force is sufficient. Therefore, pressure adjustment such as pressurization does not need to be performed.

The determination and detection of the mass number of the molecular ion can be performed by operating the mass spectrometer in a normal operation state, and the recording can be performed by a general digital oscilloscope or recorder.

The internal standard substance used in the present invention is different from the substance to be measured and has an ionization wavelength at least about 0.1 nm, preferably 1 nm, from the substance to be measured.
Those different from the above are appropriate. Preferred examples of the internal standard substance include a halogenated aromatic compound, a deuterated aromatic compound, an aromatic compound substituted with carbon having a mass number of 13, and the like, which may not be present in the measurement target substance (sample). is important. In the case of halogenated aromatic compounds, chlorine compounds, especially chlorobenzene, are present in the sample and often become a substance to be measured, so it is preferable to exclude them. For example, bromobenzene which can be ionized by laser light having a wavelength of 266 nm is used. And iodobenzene are preferred. In addition, deuterated benzene and derivatives, chlorobenzene composed of carbon having a mass number of 13, and the like can also be used.

The addition mechanism of the internal standard substance is, for example, a pump capable of sucking the exhaust gas from a flue in which the exhaust gas flows, a tower, a tank, or the like, and an internal standard substance diluted with a solvent or the like in its piping. A device consisting of a pump capable of quantitatively feeding can be used. If necessary, it may have a heating / heating function. Also,
As long as the internal standard substance becomes gaseous at a temperature near normal temperature, it may be sent in gaseous form, or may be diluted with another inert gas and introduced into the exhaust gas sampling pipe. The point is that a predetermined amount can be added to the exhaust gas. The concentration of the internal standard substance is 1pp
b to about 100 ppb, preferably 1 ppb to 10 pp
It is appropriate to inject so as to be about b.

[0031]

Embodiment 1 An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a drawing of a titanium sapphire laser as an example and an optical system, in particular, a mechanism for converting a laser beam to high-speed wavelength light. A laser for generating a laser in an ultraviolet / visible region is a pulse yag laser (for example, Nd: YAG laser system of New Wave Research Inc.).
It oscillates with a Model Mini Laser-III 10GS) and utilizes the second harmonic. Light is collected on the titanium sapphire crystal 2 by the convex lens 11. The resonator of the titanium sapphire laser includes a rear mirror 5, a plane mirror (output coupler) 6, a titanium sapphire crystal 2, a telescope 12, and an acousto-optic device 3 (Brimrose).
(Made by the company). Personal computer 20
Output a control signal from the
When an ultrasonic wave having a frequency of 00 MHz to 70 MHz is applied to the acousto-optic element 3, a laser beam having a wavelength of 700 to 1000 nm can be output from the output coupler 6. This is guided to a third harmonic converter (not shown), for example, a laser beam having a wavelength of 269.8 nm for exciting and ionizing monochlorobenzene in exhaust gas is guided to an ionization chamber of a laser ionization mass spectrometer. At the next moment, the laser light (274.1 nm) having a wavelength for exciting and ionizing 1,4-dichlorobenzene can be obtained. It can change at a cycle of up to 1 kHz.

The ionization chamber and the mass spectrometer are constituted as follows. An ionization chamber having a window through which laser light is transmitted, a turbo-molecular pump with a pumping speed of about 200 l / s, and a mass spectrometer are a 1200 mm long flight tube reflectron type time-of-flight type, and are detected. A microchannel plate is used for the vessel. The mass spectrometer is equipped with a similar turbo-molecular pump. Also,
Since a pulse valve for introducing an exhaust gas sample is installed in the ionization chamber, when this pulse valve is operated in synchronization with the yag laser, the exhaust gas enters the ionization chamber, and the monochlorobenzene contained therein has a 269.8 nm laser. Light is irradiated. Then, monochlorobenzene ions are generated and measured by a mass spectrometer at a later stage.

At the next moment, the wavelength of the laser light is changed to 27.
When switching to 4.1 nm, 1,4-dichlorobenzene is ionized and measured.

[Embodiment 2] FIG. 2 is a block diagram of a laser multiphoton ionization mass spectrometer 32 provided with the internal standard substance addition mechanism 31 of the present invention and the laser light wavelength high-speed conversion mechanism shown in FIG. .

The internal standard substance addition mechanism 31 sucks exhaust gas from a flue 33, for example, downstream of a bag filter of a municipal solid waste incinerator by a pump using a heated and heated stainless steel pipe (sampling pipe) having a diameter of 5 to 10 mm. I do.
Concentration in exhaust gas is 1ppb-100ppb in the middle of piping
Bromobenzene is injected using a metering pump or the like so as to be constant within the range. A laser multiphoton ionization mass spectrometer 32 is connected to a sampling pipe downstream of the position where bromobenzene was injected.

The substance to be measured, which is generated by irradiation with a laser beam having a wavelength that excites and ionizes the substance to be measured, and the ion of bromobenzene, which is a non-resonant wavelength but is generated by irradiation with a laser beam having a wavelength of 266 nm, is a time-of-flight mass spectrometer. And a spectrum can be obtained with a digital oscilloscope. The substances to be measured are, for example, chlorobenzenes, chlorophenols, coplanar PCBs, chlorinated aromatic compounds of endocrine disrupting substances (environmental hormones), and the like.
It is about 1 ppb to 100 ppb.

Therefore, if the sensitivity ratio between the substance to be measured and bromobenzene is determined in advance, accurate analysis values can be automatically converted by a computer or the like.

Specifically, the measurement was performed using the reagent of the substance to be measured (high-purity product) and the reagent of bromobenzene (high-purity product), and the concentration conversion coefficient C = I (bromobenzene) / I (substance to be measured) is determined in advance. For this measurement, it is preferable to use nitrogen or the like as a carrier gas if necessary and to adjust the temperature to the actual average temperature of the exhaust gas. The measurement may be performed at one point, but it is preferable to measure about two or three points while changing the concentration and obtain an average value. Therefore, the analysis value can be calculated by the following equation. In addition,
The concentration of bromobenzene added is the same as when calculating the concentration conversion coefficient and when measuring the actual exhaust gas, and the concentration of the substance to be measured when calculating the concentration conversion coefficient is the average concentration of the actual exhaust gas or close to it. Shall be.

Analysis value = (Bromobenzene introduction concentration) ×
(I (actually measured value of target substance) / I (actually measured value of bromobenzene)) × C

[0041]

As described above, according to the present invention, the wavelength of the laser beam for ionization can be changed quickly and easily with a compact measuring instrument, and at least two or more substances to be measured can be measured almost simultaneously. Is obtained.

[Brief description of the drawings]

FIG. 1 is a view showing a laser light high-speed wavelength conversion mechanism according to an embodiment of the present invention.

FIG. 2 is a system diagram showing a state in which a laser ionization mass spectrometer equipped with this laser light high-speed wavelength conversion mechanism and an internal standard substance addition mechanism are mounted on a flue.

[Explanation of symbols]

 1: Yag laser (second harmonic) 2: Titanium sapphire crystal 3: Acousto-optic element 5: Rear mirror 6: Planar mirror 7: Ultrasonic driver 11: Lens 12: Telescope 13: Reflection mirror 20: Personal computer 31: Internal Standard substance addition mechanism 32: Laser multiphoton ionization mass spectrometer 33: Flue

Claims (3)

[Claims] 1. A sample introduction unit having a pulse valve for forming a molecular jet, a pulsed laser light oscillator, and a vacuum ionization chamber or a corresponding part having a window through which laser light emitted from the oscillator can pass. A laser ionization mass spectrometer having a mass spectrometer for analyzing the mass of a molecule ionized by the laser light, wherein the pulse laser light oscillator is provided with a laser light wavelength high-speed conversion mechanism. 2. The laser light wavelength high-speed conversion mechanism comprises an optical resonator having mirrors disposed on both sides of a laser medium, and an acousto-optical element is provided in a reciprocating path of the laser light.
2. The laser ionization mass spectrometer according to claim 1, wherein the acousto-optic device is controlled by changing the wavelength of the ultrasonic wave. 3. Using a laser ionization mass spectrometer capable of measuring two or more substances at substantially the same time, measuring a target substance and an internal standard substance and obtaining an ionic strength ratio between the target substance and the internal standard substance. Also, a known amount of this internal standard substance is added to the measurement sample, the ionic strength of the measurement target substance and the internal standard substance contained in the measurement sample is measured, and the concentration obtained from the ionic strength of the measurement target substance is calculated as the ionic strength. Method for measuring the concentration of a substance to be measured, which is corrected by a ratio