JP3765434B2 - Laser ionization mass spectrometer - Google Patents

Description

[0001]
[Technology to which the invention belongs]
The present invention relates to a laser ionization mass spectrometry technique in which sample molecules, which are objects to be measured, are ionized by laser light irradiation, and mass analysis of the sample is performed by measuring the mass spectrum of the ions.
[0002]
[Prior art]
Combustion exhaust gas such as coal and heavy oil, municipal waste and incineration exhaust gas of industrial waste, plastic pyrolysis product gas, etc., although there are traces, nitrogen oxides, sulfur oxides, aromatic compounds, chlorinated organic compounds, Compounds such as chlorinated aromatic compounds and other halogen compounds are contained, and in many cases, a plurality of these compounds exist together, that is, in a mixed state. One of the rapid measurement techniques for these compounds is a method based on laser multiphoton ionization mass spectrometry, which has detection selectivity for the measurement target compound.
[0003]
When measuring a mixed gas sample by laser multiphoton ionization mass spectrometry, an example of the technique is introduced in Analytical Chemistry, Vol. 66, pages 1062 to 1069 (1994). That is, in the laser multi-photon ionization mass spectrometry technique by normal sample introduction, since the peaks corresponding to the respective compounds are wide, the peaks are overlapped and it is difficult to quantify them. Therefore, a gas sample is introduced into a vacuum ionization chamber through a sample introduction valve having a small hole diameter, ionized by irradiation with a laser, and measured with a mass spectrometer. As a result, the gas sample is adiabatically expanded and cooled to near absolute zero, so that the vibration and rotation of each compound molecule are suppressed. Therefore, the peaks corresponding to the respective compounds become sharp and separate, so that quantification is facilitated. This method is sometimes called supersonic molecular beam spectroscopic analysis or supersonic molecular jet spectroscopic analysis because the speed of the introduced molecule is several tens of times the speed of sound.
[0004]
In this supersonic molecular jet sample introduction, in order to create a molecular jet and generally maintain a high vacuum in the ionization chamber, the introduction amount per unit time is reduced when introducing a gas sample continuously or in a pulsed manner. There is a restriction that it must be. For this reason, since the amount of the sample to be measured is extremely small, there is a problem that the sensitivity is lowered as a whole. As a countermeasure, as described in Review Science Instrumentation, Vol. 67, pages 410 to 416 (1996), a slit-shaped nozzle is used so that the sample is increased so that only the portion where the laser beam passes is increased. There is a way to introduce. In this method, a molecular jet ejected in a planar shape is irradiated with laser light from the side on the same plane to increase the space for interaction between the molecule and the laser light, thereby increasing the amount of ions generated.
[0005]
[Problems to be solved by the invention]
However, due to the limitations of the exhaust system capacity, if only a sample flow rate comparable to that of a normal pinhole nozzle can be flowed, even if a slit nozzle is used, an increase in signal can hardly be expected. Since the cooling of the molecular jet is poor, the sensitivity may be lowered. Therefore, unless the sample flow rate is increased, the sensitivity cannot be improved. This reduces the degree of vacuum of the subsequent mass spectrometer, and there is a problem that the safety device for protecting the apparatus is activated and the mass spectrometer is stopped. In particular, when the sample was introduced in a pulsed manner, the pressure fluctuation was large, and this was noticeable when measuring near the limit of the exhaust system capacity with the aim of improving sensitivity.
[0006]
The present invention was made to solve such problems, and in the laser multiphoton ionization mass spectrometry technology for introducing a sample by a supersonic molecular jet, an apparatus that can detect with high sensitivity and can measure stably. The purpose is to provide.
[0007]
[Means for Solving the Problems]
The above-described problems include a sample introduction unit having a slit nozzle for forming a molecular jet, a pulsed laser light oscillator, a vacuum ionization chamber having a window through which laser light emitted from the oscillator can pass, or a corresponding portion, A mass spectrometer for analyzing the mass of molecules ionized by laser light is provided, and the slit nozzle and the vacuum ionization chamber are partitioned to prevent the molecular flow around the molecular jet from entering the vacuum ionization chamber. This is solved by a laser ionization mass spectrometer characterized in that a slit skimmer is provided. That is, since the central part of the molecular jet is directional and is a uniform flow of molecules, ionization contributes to the signal (improves sensitivity), but the peripheral molecules do not contribute much to the signal without much directionality. Found. Therefore, unless the molecules on the outer periphery are removed by the skimmer and sent to the mass spectrometer, the load on the exhaust system of the mass spectrometer is reduced even if many samples are introduced.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The sample introduction part used for a commercially available mass spectrometer can be used as it is by simply replacing the nozzle part. The sample introduction format may be either continuous introduction or pulse introduction, but pulse introduction is more preferable in consideration of the burden on the exhaust system such as a pump. The size of the slit is determined so that a supersonic molecular jet can be created, and this depends on the exhaust capacity and the like, but usually the width is about 0.01 to 1.0 mm, especially about 0.1 to 0.8 mm. Is about 5 to 200 mm, particularly about 10 to 30 mm, and the width: length ratio is about 1: 5 to 1: 1000, particularly about 1:10 to 1: 300. As a method for mounting the nozzle, for example, as described in the above-mentioned Review Science Instrumentation, Vol. 67, pages 410 to 416 (1996), a slit nozzle and a valve, that is, a presser with a slit and a code for sealing the slit. Are operated by three commercially available pulse valve drive mechanisms. If the length is not too long, that is, about 30 mm or less, the number of drive mechanisms can be reduced to one.
[0009]
The apparatus of the present invention is characterized in that a slit skimmer is provided for partitioning between the slit nozzle and the vacuum ionization chamber and preventing the molecular flow around the molecular jet from entering the vacuum ionization chamber. The slit of the skimmer is provided so as to pass only the central portion of the molecular jet discharged from the nozzle of the sample introduction portion. Therefore, in principle, the slit of the nozzle and the slit of the skimmer are provided so that the centers thereof substantially coincide with each other. The distance between the discharge port of the slit nozzle that forms the molecular jet and the slit of the slit skimmer is about 3 to 30 mm, particularly about 7 to 25 mm. The slit of the skimmer is preferably set to have a width and length that are greater than or equal to the width and length of the slit nozzle, and is preferably at most twice as long. More preferably, it is 1.2 to 1.5 times. The width of the slit of the skimmer is about 0.01 to 1.2 mm, particularly about 0.1 to 1.0 mm, the length is about 5 to 200 mm, particularly about 10 to 30 mm, and the width: length ratio is 1: 4 to About 1: 1000, especially about 1:10 to 1: 150 is appropriate. The present invention is effective even when the slit of the skimmer is formed in a flat plate skimmer, or the slit protrudes toward the vacuum ionization chamber, but the molecular jet that has passed through the slit does not diffuse, converges or collides. Therefore, it is preferable to form the projection so as not to disturb the flow. The protruding shape is preferably a shape in which both side surfaces of the slit approach a straight or concave shape from the base end toward the slit tip. The angle between the slit tip center and the slit double-sided base ends is preferably about 20 to 70 degrees, particularly preferably about 40 to 50 degrees. The skimmer is partitioned so as not to communicate with the vacuum ionization chamber except for the slit. As the material of the skimmer, SUS, metal such as aluminum, glass, heat-resistant plastic, or the like can be used. Exhaust means are provided so that the molecular jet portion cut by the slit skimmer does not go to the vacuum ionization chamber.
[0010]
The pulse laser light oscillator is not particularly limited as long as it can oscillate high-power pulse laser light. For example, the following can be used as long as it can oscillate pulse laser light of nanosecond order. . 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 the laser dye. Recently, an optical parametric oscillation laser is commercially available, and can be oscillated using this instead of a dye laser. In addition, the generation region can be expanded up to 220 nm by using double wave generation of dye, mixing, or the like. Femtosecond order laser light can be broadly oscillated by a system comprising a XeCl excimer laser-excited femtosecond pulse dye laser and an amplification KrF excimer laser. This quenches the nanosecond dye laser to further excite the short cavity laser, pass through the saturable absorber, and generate a 9 ps pulse. This light pulse is amplified by a dye amplifier and used as pump light for a distributed feedback dye laser. Eventually, a pulse laser beam having an output of about 20 mJ at the maximum in the ultraviolet region wavelength and femtosecond order can be obtained. In addition, if the oscillation of the femtosecond laser part is interrupted, laser light of nanosecond order can also be oscillated.
[0011]
Condensation of the laser light is not limited at all, and various shapes such as a normal beam cross section or a flat shape using a special lens (cylindrical lens) can be used.
[0012]
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 and the vacuum chamber of the mass spectrometer are connected to each other and there is no partition. In that case, the site where ionization is performed becomes the site in charge of the vacuum ionization chamber.
[0013]
As the mass spectrometer, any type such as a time-of-flight type, a quadrupole type, or a double convergence type can be used.
[0014]
Molecular jet discharge portion partitioned by the slit skimmer, ionization chamber, and an oil rotary pump is a mass spectrometer adjacent thereto, a mechanical booster pump, an oil diffusion pump, by connecting a turbo molecular pump ^{10-6 -} It should be able to hold at about ⁸ torr.
[0015]
Regarding the introduction of the sample, the ionization chamber (or the corresponding part) is usually kept at 10 ^-6 torr or less, so if it is in the form of a gas, a pressure around normal pressure is sufficient, and this becomes the driving force and is introduced. Therefore, there is no need to pressurize, but there is no problem even if a high-pressure sample is directly introduced. Also, as is well known, reducing the pressure increases the density of the molecular jet, which is sometimes preferable because it may improve the sensitivity slightly.
[0016]
For mass ion determination and detection of molecular ions, the mass spectrometer may be operated in a normal operating state, and recording can be performed with a general digital oscilloscope or recorder.
[0017]
【Example】
Example 1
The laser ionization mass spectrometer shown in FIGS. 1 and 2 was produced. Many of the parts used in this apparatus are commercially available products, the sample introduction section 1 is equipped with a pulse valve (PN91-47-900 (85 kg / cm ² )) manufactured by General Valve, and the pulse laser light oscillator 3 is equipped with a Spectra. -The MOPO-730 type laser system manufactured by Physics, the mass spectrometer 5 is a reflectron type time-of-flight type flight tube having a length of 1200 mm, and the detector 56 is F1094 type manufactured by Hamamatsu Photonics. A microchannel plate and a recorder (not shown) were Lecroy 9360 digital oscilloscopes.
[0018]
The slit nozzle 12 of the sample introduction part was made of SUS and the slit opening size was 0.1 mm × 10 mm.
[0019]
Two types of slit skimmer 2 having the cross section shown in FIG. 3 were prepared. The angle between the slit center and the slit base was one 40 ° and the other 50 °. This skimmer 2 is made of aluminum and has a maximum thickness of 1.2 mm, the slit tip is pointed, and the size of the opening of the slit 21 is 0.2 mm × 12 mm. The position of the slit skimmer 2 was 25 mm from the nozzle 12, and the slit 12 was installed so that the major axis directions of the nozzle 12 and the skimmer 2 were the same and the centers were matched.
[0020]
The pulsed laser light 32 emitted from the oscillator 3 is collected by the lens 31 and enters the vacuum ionization chamber 4 through the window 41. On the other hand, the sample gas is intermittently introduced by the pulse valve 11 of the sample introduction unit 1 and ejected from the slit nozzle 12 to form a molecular jet 13. The molecular jet 13 collides with the slit skimmer 2, and only the central portion thereof passes through the slit 21 of the skimmer 2, and only the substantially parallel flow enters the vacuum ionization chamber 4. Therefore, this molecular jet 22 is irradiated with a laser beam 32 and ionized, and enters the mass spectrometer 5. In the mass spectrometer 5, the direction of the molecular jet 22 is first bent by 90 degrees by the repeller electrode 52, and then accelerated by the high voltage acceleration electrode 53. Thereafter, the ions pass through the ion passage hole 54, are further reflected by the ion reflector 55, and each ion is detected by the ion detector 56. This detection signal is measured by a digital oscilloscope. An exhaust system is connected to each of the front chamber 23, the vacuum ionization chamber 4, and the vacuum chamber 50 of the mass spectrometer partitioned by the slit skimmer 2, and the inside is maintained in a vacuum state.
[0021]
Mass spectrometry was performed using chlorobenzene as the sample gas. The laser light energy at this time was 2 mJ, the irradiation time was 5 ns, and the wavelength was 269.8 nm. Chlorobenzene was introduced into the high-vacuum ionization chamber 4 together with argon gas by a supersonic jet having a constant concentration. The generated ions were detected with a microchannel plate and integrated 10 times with a digital oscilloscope to obtain a spectrum. The results are shown in FIG.
[0022]
Comparative Example 1
The same experiment was performed using the same apparatus as in Example 1 except that no slit skimmer was added. The molecular jet at this time was ejected as shown by the dotted line in FIG. 2, and the result obtained in FIG. 4 is shown.
[0023]
It can be seen from FIG. 4 that the signal intensity hardly decreases even when the slit skimmer is used. It is clear that when the slit skimmer is used, the molecules in the outer peripheral portion of the molecular jet, that is, the extra molecules that do not contribute to the signal stop, and do not proceed to the ionization chamber and the mass spectrometer. Therefore, this data suppresses fluctuations in the degree of vacuum of the mass spectrometer and improves the degree of vacuum in the case of introducing the same amount of sample without impairing the sensitivity by adding a slit skimmer. If the vacuum of the mass spectrometer is kept the same with or without the slit skimmer, the addition of the slit skimmer can introduce a large amount of samples with a uniform molecular flow that contributes to the signal, and the sensitivity can be improved. Means.
[0024]
【The invention's effect】
As described above, according to the technology of the present invention, since a large amount of sample can be introduced without increasing the capacity of the mass spectrometer, that is, with an inexpensive and compact mass spectrometer, the sensitivity can be improved while the laser ionization mass spectrometer is compact. There is an effect that it can be planned.
[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 cross-sectional view of a sample introduction portion, a slit skimmer, a pulse laser beam oscillator, and a vacuum ionization chamber portion of this apparatus.
FIG. 3 is a sectional view of a slit portion of a two-type slit skimmer used in the above apparatus.
FIG. 4 is a graph showing a mass spectrum of chlorobenzene obtained using the above apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ......... Sample introduction part 11 ... Pulse valve 12 ... Slit nozzle 13 ... Molecular jet 2 ......... Slit skimmer 21 ... Slit 22 ... Molecular jet 23 ... Front chamber 3 ......... Pulse laser light oscillator 31 ... Lens 32 ... Pulse Laser beam 4 ......... Vacuum ionization chamber 41 ... Window 5 ......... Mass spectrometer 50 ... Vacuum chamber 51 ... Partition wall 52 ... Repeller electrode 53 ... Acceleration electrode 54 ... Ion passage hole 55 ... Ion reflector 56 ... Detector 61 ... Exhaust System (oil rotary pump or oil diffusion pump)
62 ... Exhaust system (oil rotary pump or oil diffusion pump)
63 ... Exhaust system (turbo molecular pump)

Claims (1)

Sample introduction part equipped with slit nozzle for forming molecular jet, pulsed laser light oscillator, vacuum ionization chamber or corresponding part with window through which laser light emitted from the oscillator can pass, ionized by the laser light And a slit skimmer for partitioning between the slit nozzle and the vacuum ionization chamber to prevent the molecular flow around the molecular jet from entering the vacuum ionization chamber. Laser ionization mass spectrometer characterized in that

Images