JP3106093B2 - Infrared spectroscopy and separation method for ionization detection - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ionization detection infrared spectroscopy effective for analysis, separation and purification, isotope separation, etc. of various samples.

[0002]

2. Description of the Related Art As infrared spectroscopy, direct absorption, IR
The dip method and the like are known. In the direct absorption method, light in the infrared region is passed through a sample cell, and transmitted light is separated by a spectroscope to measure the wavelength dependence of absorption intensity, that is, infrared absorption. As the spectroscope, a conventional spectroscope using a dispersive element such as a diffraction grating, an FT-IR spectroscope using light interference, and the like are used. By repeating the integration in conjunction with a computer, the detection accuracy has been dramatically improved, and the measurement of infrared absorption in a dilute gas phase has also become possible. When a high-resolution diode laser is used as the infrared light source, a high-resolution spectrum can be measured without using a spectroscope. However, the direct absorption method detects the presence or absence of absorption as a difference in light intensity before and after entering the sample, and when compared to ionization detection infrared spectroscopy that does not generate a signal when there is no infrared absorption, Detection accuracy is several orders of magnitude lower. Incidentally, Chemical Review Vol. 26, "Laser and Chemical Reaction" (edited by The Chemical Society of Japan) explains that the sensitivity of the direct absorption method is inferior. When a sample containing impurities is measured, the direct absorption method does not have a function of selecting sample molecules, so that the obtained spectrum is a spectrum in which the target substance and the impurities overlap. Therefore, this method can only measure the infrared absorption of a sample and cannot be applied to separation and purification.

On the other hand, in the IR dip method, a tunable infrared laser and an ultraviolet laser are used at the same time, and when infrared light is absorbed by a sample molecule, the number of molecules that are not vibrationally excited (the fractional index of the zero vibration level). ) Decreases.
That is, as shown in FIG. 1, the principle is as follows. First, an infrared laser is applied to a sample molecule, and then an ultraviolet / visible laser beam is made to match the electron transition energy of the sample molecule. As a result, molecules that are not vibrationally excited in the ground electronic state are ionized by a resonant multiphoton process passing through the electronically excited state. FIG. 1 shows resonant two-photon ionization. Therefore, the amount of ions generated by the ultraviolet / visible laser is
It is proportional to the number of molecules in the ground state (zero vibration level). When the first incident infrared laser is not coincident with the vibrationally excited level (FIG. 1a), most of the sample molecules are distributed at the zero vibrational level, and a large amount of ions are generated by resonant two-photon ionization. . On the other hand, when the infrared laser coincides with the vibration level (FIG. 1B), the molecules at the zero vibration level transit to the vibration excitation level, and the number of molecules at the zero vibration level decreases. As a result, the resonance two-photon ionization signal due to the ultraviolet / visible laser light is greatly reduced. Therefore, as shown in FIG. 1c, when the wavelength of the infrared laser is swept while measuring the amount of ions by resonant two-photon ionization, the infrared absorption decreases the amount of ions (dip).
Is detected as In this way, exactly the same IR d
An ip spectrum can also be obtained by measuring the amount of light emitted from an electronically excited state. Again, the resolution is
It depends on the resolution of the laser used.

[0004]

The IR dip method has an advantage that the infrared absorption spectrum of a specific sample can be selectively measured even when applied to a sample in which several kinds of substances are mixed. This is because, since the wavelength of the electronic transition differs for each substance, if the wavelength of the ultraviolet / visible laser is adjusted to the electronic transition of a specific sample molecule, no infrared absorption of the other sample molecules mixed is measured at all. Although selective observation of infrared absorption by electronic transition is a great advantage of the IR dip method, it cannot be measured unless the electronic transition of a sample molecule to be measured is known, and is also a disadvantage. In addition, since the IR dip method measures a change in emission intensity from resonance two-photon ionization or an electronically excited state, even if the output of a light source such as a laser used becomes slightly unstable, fluctuations due to signal and laser intensity fluctuations may occur. Discrimination becomes difficult. Therefore, it is necessary to use a laser whose output is extremely stable in order to obtain good spectra in both the detection sensitivity and the S / N ratio, which is a major problem. In addition, although the infrared absorption spectrum of a specific molecular species can be measured, separation and generation cannot be performed using the difference in infrared absorption of each substance, so that it can be used only as a spectroscopic method. The present invention has been devised to solve such a problem, and takes advantage of the fact that there is a difference in ionization efficiency between a sample vibrationally excited by infrared light and a sample not vibrationally excited. By selectively ionizing and collecting only molecules with ultraviolet and visible light, highly efficient infrared absorption spectra can be measured with high sensitivity. Another object of the present invention is to separate and purify a target substance from a mixed sample.

[0005]

According to the ionization detection infrared spectroscopy of the present invention, in order to achieve the object, a sample is irradiated with infrared rays to generate vibrationally excited molecules, and the vibrationally excited molecules are irradiated with ultraviolet light or visible light. The method is characterized in that the wavelength of infrared light is swept while detecting ions generated by ionization by light irradiation, and the infrared absorption characteristics of the sample molecules are measured as an increase in the amount of ions. When applied to separation / concentration, a mixed sample containing the target substance is vaporized, the generated gas is irradiated with infrared light whose wavelength matches the infrared absorption of the target substance, and the target substance is irradiated with ultraviolet or visible light. Only ionize. The generated target ions are efficiently collected by the electric field.

[0006]

The spectroscopy according to the present invention uses both infrared light and ultraviolet / visible light. Infrared light produces vibrationally excited molecules in a specific vibrational state. The vibrationally excited molecules are selectively ionized and collected by ultraviolet / visible light. That is, since the ions are collected with high efficiency by the electric field, the measurement of the infrared absorption spectrum is performed with high sensitivity. Further, unlike the conventional infrared spectroscopy, the generated ions can be classified by mass, so that information on infrared absorption and mass number can be obtained at the same time.
That is, the substance having infrared absorption can be specified also in terms of mass number, and even if a sample containing impurities is used, only the infrared absorption of the target substance can be selectively measured. Also, using the fact that the infrared absorption wavelength differs for each substance / molecule,
It is also possible to ionize and separate / collect only specific substances in the mixed sample, and it is also applied to separation / purification. The principle of the spectroscopy according to the invention is shown in FIG. The wavelength λ _UV of the visible / ultraviolet light incident together with the infrared light is set slightly lower than the ionization potential IP (ionization limit) of the sample. When the wavelength is set in this manner, the sample that has not been vibrationally excited (the sample molecule has a zero vibration level) does not ionize and emits no signal even when ultraviolet or visible light enters.
On the other hand, since the energy required for ionization is reduced by the energy of the vibration level, the sample vibrationally excited by infrared light is easily ionized by ultraviolet / visible light of the same wavelength, and a signal is detected.

Therefore, when a sample is irradiated with infrared rays to generate vibrationally excited molecules, the vibrationally excited molecules are ionized by ultraviolet irradiation or visible light irradiation, and the wavelength of the infrared light is swept while detecting the generated ions. Can be observed as an increase in the amount of ions. At this time, the resolution is
Depending on the resolution of the tunable infrared light source used, high resolution can be easily obtained when laser light is used as the light source. Further, the method of the present invention can also be applied to separation and purification using a laser as is known from laser enrichment of uranium. For example, when the mixed sample is vaporized and its substrate is irradiated with infrared light whose wavelength matches the infrared absorption of the target substance,
Only the target substance is ionized by ultraviolet / visible light. The generated target ions can be collected with high efficiency by an electric field, and are separated and purified. The wavelength λ _UV of the visible / ultraviolet light incident together with the infrared light is given as follows. Energy E of visible light, when the energy of the vibration excitation level and E _V, is required to satisfy the condition of Equation (1). _{IP>E> IP-E V} ···· (1)

Further, when the equation (2) which defines the relationship between the energy of light and the wavelength λ is used, it is understood that the wavelength of ultraviolet / visible light used for ionization should satisfy the condition of the equation (3). E = hc / λ (h is Planck constant, c is the speed of light) (2) hc / (IP−E _V )> λ _UV > hc / IP (3) In the present invention, only vibrationally excited molecules are selectively ionized and detected. This ionization detection is applied to various fields by changing the ionization process. Some examples are described below. (1) When using multiphoton ionization When a laser light source is used for ionizing a sample, ionization by a multiphoton process (hereinafter, referred to as multiphoton ionization) can be caused. By using multiphoton absorption, vibrationally excited molecules can be ionized using light in the visible / near-ultraviolet region having a small one-photon energy (non-resonant multiphoton process). FIG. 3 shows the case where the non-resonant two-photon process, which is the simplest multiphoton process, is used.
Shown in In the case of two-photon ionization, if the wavelength of ultraviolet / visible light used for ionization is set so that the energy of two photons does not exceed the ionization potential, ionization detection infrared spectroscopy can be performed. In this case, the condition of the wavelength is determined as in Expression (5) to Expression (5) in consideration of the relationship between the energy of two photons and IP. (IP / 2)>E> (IP-E _V ) / 2 (4) 2hc / (IP-E _V )> λ _UV > 2hc / IP (5)

In the process of using n photons, the following conditions are similarly obtained. nhc / (IP-E _V )> λ _UV > nhc / IP (6) When the wavelength is set in this manner, molecules that are not vibrationally excited are not ionized by the multiphoton process and vibrate with an ultraviolet light source. Only the excited sample molecules can be selectively multiphoton ionized. Since the generated ions can be mass-selected as described above, the infrared absorption can be measured only for a substance having a specific mass even for a mixed sample. It is also applied to separation and purification. In general, when utilizing multiphoton ionization, the existence of higher-order multiphoton processes is often a problem. For example, when a sample molecule is ionized by using two-photon ionization, molecules that are not vibrationally excited by the three-photon ionization process may be ionized. However, such secondary processes can be suppressed to a negligible level by adjusting the output of a light source such as a laser. This is because, in general, as the number of photons in the multiphoton process increases, the absorption cross section becomes extremely small, and the transition establishment of the multiphoton process (n-photon process) is proportional to the nth power of the photon density I of the irradiated light. is there. For example, the two-photon ionization probability is proportional to the square of I, and the transition probability of the three-photon ionization process is proportional to the third power of I. Therefore, when the photon density of the light used for ionization is appropriately adjusted, a state in which three-photon ionization does not occur even if two-photon ionization occurs can be created as described in the following embodiments.

(2) When the ionization efficiency has wavelength dependence Generally, when a molecule is photoionized, the ionization efficiency is determined by the overlap integral of the vibration wave functions of the initial state (neutral molecule) and the final state (ionized state), that is, Franck-Condo
It is governed by n factors. This is because the ionization efficiency is such that the vibrational quantum numbers (IP ₀ , IP ₁ , IP
₂ ) means that when the wavelength of light used for ionization is swept, the ionization efficiency changes depending on the wavelength. For example, light having a wavelength exceeding the ionization potential IP ₂ generated by vibrationally excited ions (FIG. 4D), as compared with the case where the ionization potential IP ₀ generated by zero vibration level ions is generated (FIG. 4B).
May be much higher in ionization efficiency (left side in FIG. 4). Thus, for samples with ionization efficiency curve shown in FIG. 3, it is not necessary light used for the ionization is slightly lower energy than the ionization potential, as described in FIG. 2, from the IP ₂ also exceeds the ionization potential Ultraviolet / visible light (FIG. 4c) having slightly lower energy may be incident.

Further, depending on the sample molecule, the Frank
-Since the Condon factor changes greatly in the initial vibration state, the ionization efficiency from the zero vibration level (Fig. 4, left)
And the wavelength dependence of the ionization efficiency (right side in FIG. 4) from the vibrationally excited level may greatly differ. For example, although the photo-ionization of zero vibrational level ionization efficiency is increased when the energy of the light is more than IP _2, of lower energy than the IP ₂ is ionized from the vibration excitation level IP ₁
May increase the ionization efficiency. In such a sample, as the ultraviolet / visible light used for ionization, light having a wavelength slightly exceeding IP ₁ from the vibrationally excited level as shown in FIG. 4B may be used. Using such a phenomenon,
The wavelength conditions of ultraviolet and visible light used for ionization can be relaxed to some extent, and it is possible to use a light source such as a YAG laser or a mercury lamp that outputs only a specific wavelength as an ionization light source depending on the substance. Thus, in the spectroscopy / separation / purification method according to the present invention, excitation is performed by infrared absorption using the difference in ionization efficiency between sample molecules that are vibrationally excited by infrared light and sample molecules that are not vibrationally excited. Only the sample molecules obtained are selectively ionized. Therefore, the method of the present invention can be applied to all atoms, and the wavelength dependence of the ionization efficiency assumed in actual sample molecules can be used.

The spectroscopy according to the present invention is shown in Table 1 in comparison with the direct absorption method and the IR dip method. With the direct absorption method, the sensitivity is low in principle, there is no mass measurement function, it is difficult to selectively measure the infrared absorption spectrum only for specific materials in a mixed sample, separation and purification The problem is that it cannot be used as a law. IP di
In the p method, although selective measurement in a mixed sample, which is a problem of the direct absorption method, is possible, sensitivity and S / N ratio are inferior, and separation / purification using a difference in infrared absorption is practically used. Cannot be applied to On the other hand, the spectroscopy according to the present invention solves all of these problems and is used in a wide range of fields such as spectroscopy, separation and purification, as will be apparent from the examples described later.

[0013]

[0014]

EXAMPLES In order to demonstrate the effectiveness of infrared spectroscopy with ionization detection, o-fluorophenol was spectrally analyzed using the experimental apparatus shown in FIG. The measurement was performed in a cryogenic isolated molecular state under supersonic jet conditions. This state is a state obtained by ejecting a gas obtained by mixing a sample gas with helium gas into a vacuum chamber maintained at 10 ^-6 Torr, and the sample concentration is very dilute at about 10 ^-5 Torr. is there. An electromagnetic valve was used to eject the sample, and Nd ³⁺ :
The YAG laser was oscillated. The infrared light source is Nd ³⁺ : YA
A pulse wavelength variable infrared laser using a difference frequency generation between a second harmonic (532 nm) of a G laser and a second harmonic excitation wavelength variable dye laser (dye DCM, variable from 620 to 680 nm) was used.
An LNbO single crystal was used as the difference frequency generating crystal. Output is 0.1-0.5mJ / pulse, 4-1.2μm
The wavelength can be changed up to. Further, the second high frequency of the dye laser (dye C540A) excited by the third harmonic (355 nm) of the same Nd ³⁺ : YAG laser was generated by a KDP single crystal and used as an ultraviolet laser.

The wavelength of the ultraviolet laser was fixed at 285 nm, and was set so that vibrationally excited molecules were two-photon ionized. The optical path length of the ultraviolet laser was adjusted so as to reach the sample gas 8 nanoseconds after irradiation with the wavelength variable infrared laser. Ions generated by two-photon ionization were detected by a secondary electron image multiplier for ion detection through a quadrupole mass sorter. The quadrupole mass separator was set to drop only o-fluorophenol (mass number 108). The ion signal was amplified 10 times with a preamplifier, integrated with a boxcar integrator, and recorded on a personal computer as a function of the wavelength of the infrared laser. FIG. 6 shows the ionization detection infrared spectrum measured for o-fluorophenol. The vertical axis in FIG. 6 indicates the ion amount, and the horizontal axis indicates the wavelength of the infrared laser. As can be seen in FIG. 6, the band corresponding to the OH stretching vibration is 36
A strong observation at 34.2 cm ^-1 proved that the method of the present invention was feasible. Further, in this case, the transition to the coupled sound vibration is forbidden, and the absorption coefficient is extremely small as compared with the OH stretching vibration and the like (see Kyoritsu Zensho, “Infrared absorption and Raman effect”, co-authored by Sanichiro Mizushima and Takehiko Shimauchi) Conventionally, observation has been difficult. The clear observation of such a forbidden band indicates that the ionization detection infrared spectroscopy according to the present invention has extremely high sensitivity. Further, the S / N ratio of the obtained spectrum shows a good value for sufficiently identifying such a forbidden band, and can be easily improved by increasing the number of integrations of the boxcar integrator.

The spectrum shown in FIG. 6 is the result of selecting and observing only the o-fluorophenol ion.
It turns out that an infrared spectrum for a specific molecule or a specific substance in the mixed sample can be selectively observed. Therefore, it can be seen from FIG. 6 that the method can be applied as a separation / purification method. In addition, the background is weaker than the signal, and the intensity is adjusted so that a secondary signal due to a higher-order multiphoton process (in this case, a three-photon process) does not cause a problem even when multiphoton ionization is used. You can see what you can do. The ionization detection infrared spectroscopy according to the present invention has the following advantages as compared with the IR dip spectroscopy. That is, the IR dip spectrum of o-fluorophenol was measured using the experimental device of FIG.
It was compared with the ionization detection infrared spectrum. The comparison result
The upper part of FIG. 7 shows an IR dip spectrum, and the lower part shows an ionization detection infrared spectrum. Note that both spectra are shown in FIG. 7 with their horizontal axes aligned.

The IR dip spectroscopy is a method of detecting infrared absorption by reducing a multiphoton ionization signal which resonates with an excited state of a sample molecule when absorption by an infrared laser occurs. Therefore, it is necessary to cut off the infrared laser before and after the measurement and measure the amount of resonant multiphoton ions using only the ultraviolet laser. This ion signal amount is shown as infrared off on both sides of the IR dip spectrum. The zero point in the figure indicates zero resonance multiphoton ion amount, that is, 100% absorption.
In the IR dip spectrum, it can be seen that the ion amount was greatly reduced at 3634.2 cm ⁻¹ , and the OH stretching vibration of the sample was observed as in the ionization detection infrared spectrum. However, as seen in the spectrum of FIG. 7, the base line vibrated violently in a short cycle and a long cycle, and it was extremely difficult to distinguish other bands. Also, no coupled sound vibration band clearly observed by ionization detection infrared spectroscopy could be observed. As described above, the IR dip spectroscopy is found to be an extremely problematic spectroscopy even though it has an advantage that sensitivity and S / N ratio are poor and that selective observation in a mixed sample can be performed using electronic transition. . On the other hand, the ionization detection infrared spectroscopy according to the present invention has good sensitivity and S / N ratio, and can be performed by conventional direct absorption methods such as mass measurement and selective observation in a mixed sample. Since it has functions that cannot be obtained, it has been proved that it is a very useful spectroscopic / separation method.

[0018]

As described above, the ionization detection infrared spectroscopy of the present invention is an infrared absorption measurement method with extremely high sensitivity, and the ultra-high sensitivity infrared spectroscopy uses a much smaller amount of sample than before. However, an infrared absorption spectrum can be measured. Furthermore,
Since no spectroscopic method capable of simultaneously obtaining information on infrared absorption and mass number has existed, development in a wide range of fields can be expected as a novel analytical method. For example, conventionally, an infrared absorption spectrum is usually measured after a mixture is once separated and purified using chromatography or the like. However, when a spectroscopic method according to the present invention is used, separation and purification are not required at all. . Conventionally, mass measurement was performed independently of infrared absorption. However, according to the present invention, infrared absorption and a mass spectrum can be obtained simultaneously. Furthermore, in the conventional laser isotope separation method represented by the laser enrichment of nuclear fuel, only the infrared laser is used. However, in the case of the present invention, the infrared laser and the ultraviolet / visible laser are used to separate and separate the light with high efficiency. Purification can be performed.

[Brief description of the drawings]

FIG. 1 is a principle diagram showing a conventional IR dip method

FIG. 2 is a diagram illustrating the principle of ionization detection infrared spectroscopy according to the present invention.

FIG. 3 shows infrared spectroscopy with ionization detection using a two-photon ionization process according to the present invention.

FIG. 4 Ionization detection infrared spectroscopy when the ionization efficiency has wavelength dependence

FIG. 5 is an experimental apparatus used in an embodiment of the present invention.

FIG. 6: Infrared spectrum of ionization detection of o-fluorphenol

FIG. 7 is a graph comparing an ionization detection infrared spectrum of o-fluorphenol with an IRdip spectrum.

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G01N 21/00-21/01 G01N 21/17-21/74 G01N 27/62-27/70 JICST file ( JOIS)

Claims (2)

(57) [Claims] (1) Irradiating a sample with infrared light to generate vibrationally excited molecules.
And irradiate the vibrationally excited molecule with ultraviolet light or visible light.
Infrared while detecting ions generated by ionization
Sweeps the wavelength of light to determine the infrared absorption characteristics of sample molecules
Ionization detection spectroscopy characterized by measuring as an increase in 2. A mixed sample containing the target substance is vaporized, and the generated gas is irradiated with infrared light whose wavelength matches the infrared absorption of the target substance, and only the target substance is ionized by ultraviolet or visible light, An ionization detection infrared separation method characterized by collecting generated target ions.