JPH1074479A - Laser ionization mass spectrometric device and mass spectrometry - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a measuring device and method for high sensitivity detection in a laser multiple photon ionization mass spectrometric technology in which a sample is introduced by a supersonic molecule jet. SOLUTION: A mass spectrometric device is equipped with a sample introduction portion 1 which is provided with a pulse bulb 11 forming a molecule jet, a pulse laser radiation oscillator 2, a vacuum ionization chamber 3 or an equivalent part having a window 31 through which a laser radiation emitted from the oscillator 2 can pass, and a mass spectrometer 4 which analyzes the mass of an ionized molecule by the laser radiation 22. The pulse laser oscillator 2 can oscillate a ultrashort pulse laser radiation whose peak output is 1MW or more.

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 spectrometry technique for ionizing a sample molecule as an object 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,
Nitrogen oxides, sulfur oxides, aromatic compounds, chlorinated organic compounds, chlorinated aromatic compounds, and other halogenated compounds are present in trace amounts in incineration exhaust gas from municipal solid waste and industrial waste, and gas generated by pyrolysis of plastics. Such a compound is contained in many cases,
That is, they exist in a mixed state. One of the rapid techniques for measuring these compounds is a method using laser multiphoton ionization mass spectrometry, which has a selective detection of the compound to be measured.

[0003] When a mixed gas sample is measured by laser multiphoton ionization mass spectrometry, an Analytical Che is used.
Mistry, Vol. 66, pp. 1062-1069 (1
An example of the technology is introduced in (994). That is,
In the laser multiphoton ionization mass spectrometry technique using the normal sample introduction, the peaks corresponding to the respective compounds are broad, and therefore, the peaks overlap and it is difficult to quantify. Therefore, a gas sample is introduced into a vacuum ionization chamber through a sample introduction valve having a small hole diameter, ionized by laser irradiation, 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. Therefore, the peak corresponding to each compound is sharpened and separated, which facilitates quantification. 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 the introduction of a supersonic molecular jet into a sample, in order to maintain a high vacuum in the ionization chamber, the amount of gas sample introduced per unit time is continuously or pulsed. There is a restriction that it must be reduced. For this reason, since the amount of the sample to be measured is extremely small, there is a problem that the sensitivity is reduced as a whole. As a countermeasure, increasing the energy of laser beam irradiation can be easily considered. However, when the energy of the laser beam is increased, there arises a problem that molecules to be measured are decomposed, that is, fragmented, and accurate quantification cannot be performed.

SUMMARY OF THE INVENTION The present invention has been made to solve such problems, and a measuring apparatus and a method for high-sensitivity detection in a laser multiphoton ionization mass spectrometry technique for introducing a sample by a supersonic molecular jet. The purpose is to provide.

[0006]

Means for Solving the Problems As a result of intensive studies, the present inventors have found that the fragmentation of molecules depends on the energy of the irradiated laser beam, but the ionization of molecules is related to the peak power of the laser beam. I found something.
When irradiating an ultrashort pulse laser beam with a large peak output,
It has been found that ionization efficiency can be improved without increasing laser light energy beyond the limit energy at which molecules undergo fragmentation. That is, when a laser beam having a large peak power is used, the amount of molecular ions generated can be increased while suppressing fragmentation of molecules.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a sample introduction section having a pulse valve for forming a molecular jet, a pulsed laser light oscillator, and a vacuum ionization chamber having a window through which laser light emitted from the oscillator can pass. And a mass spectrometer for analyzing the mass of the molecules ionized by the laser light, wherein the pulsed laser light oscillator can emit an ultrashort pulsed laser light having a peak output of 1 MW or more. Solved by the characteristic laser ionization mass spectrometer.

In addition, a sample gas is jetted into a vacuum ionization chamber or a corresponding portion by a pulse valve capable of forming a molecular jet to form a pulse-like molecular jet, and the molecular jet has an ultrashort pulse having a peak output of 1 MW or more. The problem is solved by a mass spectrometric method characterized by irradiating with a laser beam to ionize and analyzing the mass of a molecule ionized by the laser beam.

[0009]

DETAILED DESCRIPTION OF THE INVENTION The sample introduction unit uses a pulse valve having a nozzle or orifice capable of producing a supersonic molecular jet. The pulse valve is used for fuel injection of an engine and the like, and is edited by The Chemical Society of Japan, 4th edition, Experimental Chemistry, Vol. 8, pp. 127-129 (19)
(1993), a plunger normally pressed against a sealing surface by a spring is electromagnetically attracted rearward by instantaneous energization of a rear solenoid (electromagnetic coil), during which time the plunger is attracted rearward. Only open. A valve that opens and closes using a piezo element has also been developed and can be used.

In the present invention, the time during which the laser light interacts with the molecule of the compound to be measured for ionization depends on the oscillation time of the pulsed laser light, and is substantially equal to the oscillation time (irradiation time) of the pulsed laser light used. A pulse valve that operates in a very short time is preferred. As a specific operation time, about 10 μs to 5 ms, particularly 5 μs
It is preferably about 0 μs to 2 ms. Therefore, when the operating time of a commercially available pulse valve is long, the length of the spring is increased, the strength is increased at the same time, the electric resistance of the coil is reduced so that a large current can flow, and the operating voltage is reduced. When the size is increased, the valve can be operated in a shorter time. The opening of the nozzle may be a slit in addition to a normal circular hole. The size of the opening is determined so as to create a supersonic molecular jet, which depends on the evacuation capacity of the vacuum ionization chamber.
mm ² approximately, in particular about 0.2 to 0.5 mm ² is usually appropriate.

The present invention is characterized in that an ultrashort pulse laser light having a peak output of 1 MW or more oscillated by a pulse laser light oscillator is used. The preferred peak power is 10 MW to 100 GW, particularly preferably 10 MW.
0 MW to 10 GW. Here, the peak output indicates the intensity of the laser beam, and is expressed as laser beam energy (J) / oscillation time (s).

As a method of increasing the peak output, there are a method of shortening the laser light oscillation time of one pulse and a method of increasing the laser output. As for the irradiation time, it is preferable to increase the peak output as much as possible with laser light energy at which the molecules do not fragment, since the ionization efficiency increases. On the other hand, theoretically, laser multiphoton ionization means that the molecule of the compound to be measured transitions from the ground state to an excited state by a photon having an energy corresponding to the energy difference between the ground state and the excited state, and is further changed by the photon energy. This is the process of ionization. Therefore, irradiation with an intense pulsed laser beam to the extent that molecules are not extremely decomposed, based on the excitation lifetime, which is the time spent in the excited state, improves ionization efficiency and significantly increases the amount of generated ions. The preferred irradiation time is 3 times or less, more preferably 2 times or less, particularly preferably about the same as the excitation lifetime. On the other hand, the preferable lower limit of the irradiation time is 1/10000 or more, more preferably 1/4000 or more, and particularly preferably 1/2.
000 or more. Generally, a preferred irradiation time is 1
About 00 to 500 fs, more preferably 200 to 300 fs.
fs.

[0013] Regarding the energy of the laser beam, it is theoretically preferable to increase the energy so that the peak output can be increased. Therefore, it is preferable that the energy is large as long as the molecule is not decomposed. However, there is a slight difference in the density between the center of the laser beam and the outside thereof. In some cases, fragmentation that occurs when irradiated with a molecular jet may also be spatially distributed,
The effect may be obtained even if the laser light energy is slightly reduced. The preferred pulse laser light energy is 5 mJ or less, more preferably 4 mJ or less, and particularly preferably 3 mJ or less. On the other hand, a preferable lower limit is 1 mJ or more, and more preferably 2 mJ or more.

The wavelength of the laser beam to be applied is in principle a wavelength corresponding to the energy difference between the ground state and the excited state specific to each molecule to be measured, that is, a resonance wavelength is preferable. The effect can be obtained.

The pulse laser light oscillator used in the present invention is not particularly limited as long as it can oscillate a high-output pulse laser light. The following can be used. That is, dye lasers are most commonly used. In this method, an excimer laser or a yag laser is used as a pumping light source, and the wavelength can be continuously changed from 330 to 1000 nm by exchanging a laser dye. By utilizing the overtone generation and mixing of the dye, the oscillation region can be extended to the short wavelength side of 220 nm. Recently, an optical parametric oscillation laser is commercially available, and it is possible to oscillate using the laser instead of the dye laser. The laser light of the order of femtoseconds can be roughly oscillated by a system composed of a femtosecond pulse dye laser excited by a XeCl excimer laser and a KrF excimer laser for amplification. This quench the nanosecond dye laser to further excite the short cavity laser and pass it through a saturable absorber to generate a 9ps pulse. This light pulse is amplified by a dye amplifier and used as pump light of a distributed feedback dye laser. Eventually, a pulsed laser beam having a wavelength in the ultraviolet region and a maximum output of about 20 mJ on the order of femtoseconds can be obtained. When the oscillation of the femtosecond laser section is interrupted, laser light on the order of nanoseconds can also be emitted.

The focusing of the laser beam is not limited at all, and it is possible to use a laser beam of various shapes such as a normal beam cross section or a flat shape formed by using a special lens (cylindrical lens). .

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 ionization is performed is a site corresponding to a vacuum ionization chamber.

As a 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 site) 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. Thus, the pressure does not need to be particularly increased, but there is no problem even if the high-pressure sample is directly introduced. Further, as is well known, when the pressure is reduced, the density of the molecular jet increases, and the sensitivity may be slightly improved, which is sometimes preferable.

The determination and detection of the mass number of molecular ions 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.

[0022]

【Example】

Example 1 A laser ionization mass spectrometer shown in FIG. 1 was produced.
Many of the components used in this apparatus are commercially available, and a pulse valve (PN91-47-900 (85 kg / cm ² ) manufactured by General Valve) is installed in the sample introduction unit 1.
L using a dye laser for the pulse laser light oscillator 2
ambda Physik LPD500fs type laser system, mass spectrometer 4 is a time-of-flight type having a 450 mm long flight tube, and detector 45 is Hamamatsu Photonics F1094 type microchannel plate. A Lecroy Model 9360 digital oscilloscope was used as a recorder (not shown). The opening of the nozzle 12 of the pulse valve 11 was a circular hole having an inner diameter of 0.8 mm.

The vacuum chamber 40 of the mass spectrometer was evacuated with a UTM150 type turbo molecular pump manufactured by Japan Vacuum Engineering Co., Ltd. having a pumping speed of 190 l / s. In addition, the ionization chamber 3 by laser light irradiation was evacuated with a ULK-06A type oil diffusion pump manufactured by Japan Vacuum Engineering Co., Ltd. having an evacuation speed of 1200 l / s.

The pulse laser beam 22 emitted from the oscillator 2 is condensed by the lens 21 and enters the vacuum ionization chamber 3 through the window 31. On the other hand, the sample gas is intermittently introduced by the pulse valve 11 of the sample introduction unit 1 and is ejected from the nozzle 12 to form a molecular jet 13. This molecular jet 13 enters the vacuum ionization chamber 3. Then, the molecular jet 13 is irradiated with a laser beam 22 to be ionized, and enters the mass spectrometer 4. In the mass spectrometer 4, the direction of the molecular jet flow is first changed by 90 degrees by the repeller electrode 42 and then accelerated by the high-voltage accelerating electrode 43. Further, the ion passage holes 4 provided in the partition 41
4, each ion is detected by the ion detector 45.
This detection signal is measured by a digital oscilloscope.

The mass spectrometry was performed using chlorobenzene as the sample gas.

Laser light of 4 ns to 1 ps and 1 to 1000 MW was oscillated by changing the pulse width and oscillation wavelength of the laser light. The wavelength was 248 nm.

Chlorobenzene was flowed at a constant concentration together with argon gas, and introduced into a vacuum ionization chamber in a molecular jet state by a pulse valve. Energy is 1mJ and 4m
1 MW, 10 MW, 100 MW and 1
Irradiation was performed by irradiating a laser beam having a different power of 000 MW. The laser light irradiation time at this time is 1 ps to 4 ns
Was in the range. The ions generated by irradiating a pulsed laser beam in synchronization with the sample introduction were detected by a microchannel plate of a time-of-flight mass spectrometer, and integrated by a digital oscilloscope 200 times to obtain a spectrum. The results are shown in FIG.

COMPARATIVE EXAMPLE 1 Using the same apparatus as in Example 1, the laser of the femtosecond laser section was interrupted to emit pulsed laser beams having energies of 1 mJ and 4 mJ and peak outputs of 100 kW and 400 kW, respectively (irradiation time 10 ns). The same experiment as in the example was performed except for ()). FIG. 2 shows the results.

In the measurement shown in FIG. 2, only molecular ions were observed without fragment ions. Pulse laser beam energy is 4mJ and peak output is 400kW
Has an ionic strength (relative value) of about 0.4 and an energy of 1
Since the ion intensity at a peak output of 1 MW at mJ is about 0.5, the ion intensity is increased by 20% or more, and it is clear that the ion output depends on the peak output, not the energy of the laser beam. Also, with the same pulsed laser light energy, it can be seen that the ion intensity measured is higher when the peak output is higher than in the comparative example, and increases with the peak output.

Example 2 The same apparatus as used in Example 1 was used. The pulsed dye laser light on the order of femtoseconds is Lambda
It was oscillated by a Physik LPD 500 fs laser system. The wavelength was 248 nm as in the first embodiment.

Chlorobenzene, bromobenzene and iodobenzene are introduced into a high vacuum ionization chamber with a supersonic molecular jet of a certain concentration together with argon gas, and the peak power and irradiation energy are reduced to 0.1 in consideration of the excitation lifetime of each molecule. 500f laser light changed at 2-1.5mJ
s, 150 fs irradiation for ionization. Incidentally, the peak output at this time was 0.4 to 10 GW. The generated ions were detected by a microchannel plate of a time-of-flight mass spectrometer, and integrated by a Lecroy Model 9360 digital oscilloscope 200 times to obtain a spectrum. The results are shown in FIGS. 3, 4 and 5.

Comparative Example 2 Using the same apparatus as in Example 2, the same experiment as in Example 2 was performed except that the oscillation of the femtosecond laser section was blocked and a laser beam of 15 ns was applied. 3 to 5 show the results.

Here, the excitation lifetime of chlorobenzene is 60
0 ps, 500 fs of laser light irradiation time, 15
0 fs and 15 ns are 1/1200 and 1/400, respectively.
0, 25 times, and the excitation lifetime of bromobenzene is 30.
In ps, they correspond to 1/60, 1/200, and 500 times, respectively. Similarly, the excitation lifetime of iodobenzene is 400 fs.
It is reported that the degree is about 1 / 1.3, about 1 / 2.7, and about 37500 times, respectively. As is clear from FIG. 5, the effect is recognized with the irradiation time as short as about the excitation life, and from FIGS. 3 and 4, 1/10000 of the excitation life is obtained.
It can be seen that the effect is obtained up to the irradiation time. Further, from these figures, it is understood that when laser light is irradiated with the excitation life as a standard, a remarkable effect of improving ionization efficiency can be obtained.

Further, Embodiments 1 and 2 were carried out under the same apparatus and under the same conditions except that only the oscillation conditions of the pulsed laser beam were changed, that is, the peak power, irradiation time and energy of the laser beam were changed. Therefore, the ionic strengths can be compared although they are relative values. Therefore, it can be seen from FIGS. 2 and 3 that the ionization efficiency is improved as the peak output is increased with an energy in a range where no fragmentation of molecules occurs during irradiation with the pulsed laser beam. That is, in FIG. 3, the laser light energy is 1 mJ, the irradiation time is 500 fs, and the peak output is 2 GW (2000 MW) and 6.7 GW (6700
MW). The ionic strength at this time can be read as 1.5 and 1.7, respectively. Here, looking at FIG. 2, the ion intensity at a laser light energy of 1 mJ increases with an increase in the peak output, and becomes 1.4 at 1000 MW. It is clear that the ionization efficiency is improved by the increase in the peak power.

[0035]

As described above, according to the present invention, the ionization efficiency is improved by irradiating an ultrashort pulse laser beam having a large peak output, but the irradiation energy is not increased due to the short irradiation time, so that extreme fragmentation occurs. Since it does not cause, it is possible to perform high-sensitivity detection and lower the quantification (detection) lower limit.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an apparatus according to an embodiment of the present invention.

FIG. 2 is a graph showing a change in molecular ion peak intensity of chlorobenzene obtained in Example 1.

FIG. 3 is a graph showing changes in the molecular ion peak intensity of chlorobenzene obtained in Example 2.

FIG. 4 is a graph showing changes in the molecular ion peak intensity of bromobenzene obtained in Example 2.

FIG. 5 is a graph showing changes in the molecular ion peak intensity of iodobenzene obtained in Example 2.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Sample introduction part 11 ... Pulse valve 12 ... Nozzle 13 ... Molecular jet 2 ... Pulse laser light oscillator 21 ... Lens 22 ... Pulse laser light 3 ... Vacuum ionization chamber 31 ... Window 4 ... Mass analysis Total 40 ... Vacuum chamber 41 ... Partition wall 42 ... Repeller electrode 43 ... Acceleration electrode 44 ... Ion passage hole 45 ... Detector 51 ... Exhaust system (oil diffusion pump, oil rotary pump or turbo molecular pump) 52 ... Exhaust system (turbo molecular pump) )

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. Laser ionization comprising a mass spectrometer for analyzing the mass of molecules ionized by the laser light, wherein the pulse laser oscillator is capable of oscillating an ultrashort pulse laser light having a peak output of 1 MW or more. Mass spectrometer 2. A sample gas is injected into a vacuum ionization chamber or a corresponding part by a pulse valve capable of forming a molecular jet to form a pulsed molecular jet, and the molecular jet has an ultrashort pulse having a peak output of 1 MW or more. A mass spectrometric method comprising irradiating a laser beam to be ionized, and analyzing a mass of a molecule ionized by the laser beam. 3. The energy of a pulse laser beam is 5 mJ.
The irradiation time is 3 to less than the excitation lifetime of the molecule to be measured.
3. The method according to claim 2, wherein the ratio is 1/10000.