JP2970697B2 - Background removal Fourier transform spectroscopy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Fourier transform spectroscopy for obtaining an analysis spectrum of a sample by taking out an interferogram using an interferometer and performing Fourier transform on the interferogram. In particular, there is strong background radiation such as fluorescence and infrared rays. The present invention relates to background-removed Fourier transform spectroscopy that performs Fourier transform spectroscopy while removing the background in an environment.

[0002]

2. Description of the Related Art Raman spectroscopy, for example, is a method of irradiating a measurement object (sample) with monochromatic light, mainly laser light, and dispersing Raman light emitted from the sample. However, when the wavelength of the Raman-excited laser light overlaps with the absorption band associated with the electronic transition of the sample, fluorescence is emitted from the sample. The fluorescence often overlaps with the wavelength of the Raman light and has a high intensity, which hinders the measurement of the Raman light.

Several methods have been proposed and implemented to remove such fluorescence. One of them is
There is a method in which a pulse oscillation laser is used for Raman excitation, and the difference in the emission time of Raman light and fluorescence is used to temporally separate by a gate circuit. That is, as shown in FIG.
The Raman light is emitted without delay from the Raman excitation pulsed laser light, but the fluorescence is delayed, so that a part of the fluorescence can be removed by a gate circuit provided in the detection circuit.

Conventionally, background removal by such a gate circuit has been used in a dispersion type spectroscope. However, in a Fourier transform spectroscope, a light source is operated in a pulsed manner, and a detection circuit is thus used. In any case, no background was removed by providing a gate circuit.

Under such circumstances, the present applicant has already used a pulse light source as a light source in a Fourier transform spectroscopy in which an interferogram is taken out using an interferometer and a Fourier transform is performed to obtain an analysis spectrum of the sample. A Fourier transform spectroscopy using a pulsed light source, characterized in that an interferogram consisting of a predetermined frequency component is extracted from the output of the detector and subjected to a Fourier transform to obtain an analysis spectrum of the sample (Japanese Patent Application No. Hei. -82
No. 126, Japanese Patent Application No. 3-66704, Japanese Patent Application No. 3-667
05 (2A89)). In this method, since a pulsed light source is used as a light source, an SOR light source, a pulsed laser-excited Raman sample, or the like can be used as a light source. Therefore, the Fourier transform spectroscopy can be applied to a measurement object affected by light irradiation or a measurement object for which continuous irradiation or the like is not preferable, and the application object is widened.

[0006] Similarly, the applicant has already applied the stimulus periodically to a measurement object that repeatedly shows the same response to the stimulus, sampled the output of a detector using a rapid scan interferometer, and performed the sampling. A time-resolved spectrometry for acquiring an interferogram with respect to a certain delay time, and performing a Fourier transform to measure a reaction state by a spectrum, wherein a repetitive stimulus is given to a measurement object at a cycle longer than twice the reaction cycle, and a detector is provided. The output is sampled by controlling the delay time from the stimulus at a half cycle of the stimulus repetition cycle, and an interferogram consisting of a predetermined frequency component is extracted from the sampled signal.
By taking out the interferogram of the difference between the interferogram obtained from the sample in the excited state and the interferogram obtained from the sample in the normal state, and performing the Fourier transform on the interferogram of the difference, the measurement object at the controlled delay time is obtained. A time-resolved Fourier transform spectroscopic measurement method characterized by obtaining a difference spectrum from a measurement object in a normal state has been proposed (Japanese Patent Application Nos. 1-335748, 3-64623 and 3-64624). This method had the advantage of being unaffected by changes in the state of the device and the measurement environment, which are the advantages of the difference measurement method.

[0007]

However, in the above-described Fourier transform spectroscopy using the pulse light source by the present applicant, no measure is taken to eliminate strong background light or fluorescence in Raman spectroscopy.

The present invention has been made in view of such a situation, and an object of the present invention is to reduce background light such as fluorescence in Fourier transform spectroscopy using a pulsed light source proposed by the present applicant. To remove it.

[0009]

For this purpose, the background-removed Fourier transform spectroscopy of the present invention uses a pulsed light source emitting light at a constant period or a pulsed laser for Raman excitation as a light source and performs detection using a rapid scan interferometer. In the Fourier transform spectroscopy that obtains the interferogram from the detector output and obtains the spectrum of the measurement light by performing the Fourier transform, a signal is extracted from the detector output only while the light source emits light, and a predetermined frequency range of the signal is extracted. This method is characterized in that a component is extracted, sampled, and Fourier-transformed to obtain a spectrum of the measurement light.

In this case, a signal is extracted from the detector output even at a predetermined delay time after the light source emits light, a component of the signal in a predetermined frequency range is extracted, sampled, and Fourier-transformed to obtain a background signal. By obtaining the spectrum and subtracting this background spectrum from the spectrum obtained from the signal only while the light source is emitting light, the spectrum of the measurement light is obtained, so that the background such as fluorescence can be more reliably obtained. Can be removed.

[0011]

According to the Fourier transform spectroscopy of the present invention, a signal is extracted from a detector output using a rapid scan interferometer only while the pulsed light source or the Raman excitation pulsed laser is emitting light, and a signal within a predetermined frequency range of the signal is obtained. Extract the ingredients,
Since the spectrum of the measurement light is obtained by sampling and performing Fourier transform, strong background light and fluorescence in Raman spectroscopy are removed, and weak Raman light and the like can be accurately measured.

Since a pulsed light source is used as the light source in the same manner as in the premise of the present invention, an SOR light source, a pulsed laser-excited Raman sample, or the like can be used as a light source. Since the time can be shortened and the amount of light to be irradiated can be reduced, Fourier transform spectroscopy can be applied to measurement objects affected by light irradiation or measurement objects where continuous irradiation is not preferable. It spreads.

Moreover, as compared with the method presupposed by the present invention, an apparatus for implementing this method can be constructed with only a simple change of simply adding a gate circuit.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 shows a basic configuration of a Fourier transform spectroscope for performing the method of the first embodiment of the Fourier transform spectroscopy using the pulse light source according to the present invention. This is an improvement of the method of Japanese Patent Application No. 2-82126 described above.
In FIG. 1, 1 is a pulsed laser for Raman excitation (a periodic pulse light source is used for ordinary spectroscopy), 2 is a sample, 3 is an interferometer, 4 is a detector, 5 is a preamplifier, and 6 is a gate circuit. , 7 is a low-pass filter, 8 is a main amplifier,
9 denotes an AD converter and 10 denotes a computer. A trigger signal synchronized with the light emission of the pulse laser 1 is input to the gate circuit 6, so that only the light emission time of the pulse laser 1 is opened. The emission cycle of the pulse laser 1 is
Asynchronously with the sampling cycle of the AD converter 9, the detector 4
Is set to be shorter than twice the reciprocal of the maximum frequency f _max of the interferogram signal obtained from. That is, if the emission cycle of the pulse laser 1 is τ, τ <
1 / 2f _max, or is set to f _max <1 / 2τ. Accordingly, the upper limit of the pass band of the low-pass filter 7 is set to f _max .

The apparatus shown in FIG. 1 is disclosed in Japanese Patent Application No. 2-82126.
1 is basically the same as that of FIG. 1, except that a gate circuit 6 is provided. The operation is basically the same as that of the above-mentioned application, but by providing the gate circuit 6, the fluorescence accompanying the Raman excitation can be removed. This is a feature of the method of the present invention, which will be described with reference to FIG. In the case where a continuous wave laser is used for Raman excitation and there is no fluorescence, the Raman light emitted from the sample 2 is modulated by the interferometer 3, and the detector 4 outputs an interface as shown in FIG. Program is obtained. However, in actuality, since fluorescence is emitted from the sample 2 with a time delay, when Raman excitation is performed from the pulse laser 1 with pulsed light as shown in FIG. 2 (B), it is shown by a solid line in FIG. 2 (C). In addition to the Raman light digital interferogram signal, a signal due to fluorescence as shown by a dotted line is detected. Since a gate circuit 6 is provided next to the preamplifier 5 so as to open only during the emission time of the pulse laser 1, the signal due to the fluorescence indicated by the dotted line in FIG. 2C is cut off, as shown in FIG. Then, an original digital interferogram of Raman light is obtained. By passing this signal through a low-pass filter 7, an intended analog interferogram as shown in FIG. 4E is obtained, which is sampled by an AD converter 9 and Fourier-transformed by a computer 10 to obtain a sample. 2. Spectroscopic analysis of Raman light from 2.

In the following embodiment, an apparatus having the structure shown in FIG. 1 is used, but the light emission cycle of the pulse laser 1 is asynchronous with the sampling cycle of the AD converter 9 and the light emission frequency of the pulse laser 1 is 1 / Τ, the interferogram signal obtained from the detector 4, that is, the Raman light modulated by the interferometer 3 is m / 2τ <f <(m + 1)
/ 2τ (m is a positive integer) only in the case where the frequency component f is present. The pass frequency of the low-pass filter 7 is 1
/ 2τ. This is an improvement of the method of Japanese Patent Application No. 3-66705, and differs from the first embodiment in that the signal passed through the low-pass filter 7 is phase-corrected by the computer 10 and the wave number for restoring the wave number. Performing the conversion process. In this case, similarly, Fourier transform spectroscopy is performed by removing fluorescence accompanying Raman excitation. Detail is,
See Japanese Patent Application No. 3-66705.

FIG. 3 shows another embodiment. Also in this case, the emission cycle of the pulse laser 1 is asynchronous with the sampling cycle of the AD converter 9, and when the emission frequency of the pulse laser 1 is 1 / τ, an interferogram signal obtained from the detector 4, ie, interference The Raman light modulated by the total 3 is m / 2τ <f <(m + 1) / 2τ
This is the case where the frequency component f is present only in the range (m is a positive integer). The difference from the apparatus of FIG. 1 is that a band-pass filter 11 is provided instead of the low-pass filter 7. Similarly, Fourier transform spectroscopy is performed by removing the fluorescence accompanying Raman excitation. The pass band of the band-pass filter 11 is set to 1 / 2τ <f <1 / τ. This is an improvement of the method disclosed in Japanese Patent Application No. 3-66704, which is described in detail in the specification.

Incidentally, the aforementioned Japanese Patent Application No. Hei.
In No. 48, as shown in FIG. 9, for a sample 21 that repeatedly emits the same fluorescence upon laser irradiation, an interferogram of a difference between a predetermined delay time and a normal state from the laser irradiation is acquired and the reaction of the measurement target is performed. A time-resolved spectrometry method to obtain the spectral state in the process was proposed. As shown in FIG. 9, the apparatus for this includes a pulse laser 33, a sample 21, an interferometer 22, a detector 23, a timer 24, a frequency divider 25, a variable delay circuit 26, a preamplifier 29, a gate circuit 32, The sample 21 is a measurement target of a reaction cycle τ ′ that repeatedly shows the same response to laser irradiation, and includes a pass filter 30, a lock-in amplifier 31, a main amplifier 33, an AD converter 34, and a computer 35. , A clock signal having a period τ / 2 (≧ reaction period τ ′). The frequency dividing circuit 25 divides the clock signal of the timer 24 by １ ／ and divides the signal of the divided period τ into the pulse laser 33 and the lock-in amplifier 3.
1. The pulse laser 33 stimulates the sample 21 based on the signal generated by the frequency dividing circuit 25, asynchronously with the reference signal of the interferometer 22. The variable delay circuit 26 controls the gate circuit 32 by generating a trigger delayed by a certain time Δτ from the clock signal of the timer 24. Gate circuit 32
Has a gate width sufficiently narrow with respect to the period τ / 2, and passes a signal only while a trigger is present.

With this configuration, an interferogram of a difference between an interferogram obtained from a sample in a state delayed by a certain time Δτ after laser irradiation and an interferogram obtained from a sample in a normal state can be extracted. This is passed through an A / D converter 34 and a computer 35
, And a Fourier transform is performed to obtain a spectrum of a difference between a predetermined delay time and a normal state after laser irradiation. In the case of this figure, the cycle of pulsed laser irradiation is the maximum frequency f of the interferogram signal.
It was assumed that it was shorter than the reciprocal of twice _max . When the period of the pulsed laser radiation and tau, was based on the assumption τ <1 / 2f _max. Further, the pass band of the band-pass filter 30 is set to 1 / 2τ <f <1 / τ.

In the apparatus shown in FIG. 9, by setting the delay time Δτ of the variable delay circuit 26 to zero, the sample 21
It can be seen that the difference spectrum between the fluorescence at the moment of irradiating the excitation pulse laser beam and the laser beam non-irradiation, that is, the spectrum obtained by subtracting the background spectrum at the time of non-irradiation from the spectrum at the time of laser beam irradiation can be obtained. You. In addition, regarding the spectrum at the time of laser light irradiation, since the signal is obtained only at the time of pulse laser light irradiation by the gate circuit 32, the fluorescence with a time delay is cut off.
The background will be removed twice. This apparatus can be used not only for fluorescence spectrum measurement but also for Raman spectroscopy because of its configuration.

FIG. 4 shows the configuration of an apparatus to which the present invention is applied for difference spectrum measurement based on the above concept.
The configuration is almost the same as that of FIG. 9, but the variable delay circuit is no longer necessary. In brief, the components are: a Raman excitation pulse laser 1, a sample 2, an interferometer 3, a detector 4, a timer 12, a frequency divider 13, a preamplifier 5, a gate circuit 6, a band pass filter 11, a lock-in amplifier 14. ,
The sample 2 is composed of a main amplifier 8, an AD converter 9, and a computer 10. The sample 2 is an object to be measured for a reaction period τ ′ that repeatedly shows the same response to laser irradiation (actually, this requirement is shown in FIGS. 1 and 3). The timer 12 generates a clock signal having a period τ / 2 (≧ reaction period τ ′). The frequency divider 13 has a timer 12
Is divided into, and a signal of the divided cycle τ is supplied to the pulse laser 1 and the lock-in amplifier 14. The pulse laser 1 excites the sample 2 based on the signal generated by the frequency divider 13 asynchronously with the reference signal of the interferometer 22. Gate circuit 32
Has a gate width sufficiently narrow with respect to the period τ / 2, and allows a signal to pass only during a trigger, that is, a period during which the pulse laser 1 emits light. The period of irradiation of the pulse laser 1 is set shorter than the reciprocal of twice the maximum frequency f _max of the interferogram signal. The pass band of the band-pass filter 11 is set to 1 / 2τ <f <1 / τ. With such a configuration, a spectrum obtained by subtracting the light that becomes the background when no irradiation is performed from the Raman light when the excitation pulse laser light is irradiated can be obtained by removing the fluorescence.

Also in this case, as in the case of FIGS. 1 and 2, when the light emitting cycle of the pulse laser 1 is asynchronous with the sampling cycle of the AD converter 9, and the light emitting frequency of the pulse laser 1 is 1 / τ. This can be applied to a case where the Raman light modulated by the interferometer 3 has the frequency component f only in the range of m / 2τ <f <(m + 1) / 2τ (m is a positive integer). In this case, as shown in FIG. 5, a low-pass filter 7 is used instead of the band-pass filter 11 of FIG.
This is based on the aforementioned Japanese Patent Application Nos. Hei 3-64623 and Hei 3
It is a modification of the method of U.S. Pat. No.-64624, the details of which are incorporated herein by reference. Also in this case, similarly to the case of FIG. 4, the spectrum obtained by subtracting the background light without irradiation from the Raman light when the excitation pulse laser light is irradiated can be obtained by removing the fluorescence. In addition,
In FIG. 5, the lock-in amplifier 14 may be omitted, and instead, the signal passing through the low-pass filter 7 may be subjected to phase correction and wave number conversion processing for restoring the wave number by the computer 10.

A similar difference spectrum measurement can also be configured by providing two measurement channels as shown in FIGS. That is, FIG. 6 shows that the light emission period τ of the pulse laser 1 is the maximum frequency f of the interferogram signal obtained from the detector 4 as in the case of FIG.
1 shows a configuration for measuring a difference spectrum in a case where the difference spectrum is set to be shorter than twice the reciprocal of _max . After the preamplifier 5 in FIG. 1, a gate circuit 6, a low-pass filter 7, a main amplifier 8, an AD converter 9. Computer 10
Gate circuit 6 ', low-pass filter 7', main amplifier 8 ', AD converter 9',
Another channel (computer 10 'is shared with computer 10 in the illustrated case) comprising computer 10' is provided in parallel. The gate circuit 6 'is configured to open only at a predetermined delay time Δτ after the time when the pulse laser 1 emits light by the delay circuit 15. Therefore, from the additional channel measured at the delay time Δτ, a spectrum distribution of fluorescence, background light, and the like when the excitation pulse laser beam is not irradiated can be obtained. By subtracting the non-irradiated spectrum from the measured spectrum at the time, a more accurate Raman spectrum can be obtained. This deduction processing is performed by the computer 10. By the way, with reference to FIG. 8, it has been described that the fluorescent light is emitted with a delay from the Raman-excited pulsed laser light. However, as is apparent from FIG. 8, the fluorescent light is emitted to some extent almost simultaneously with the Raman excitation. Therefore, the measurement value obtained by applying a gate without a delay time includes the simultaneous emission fluorescent light component. Even in the delay time Δτ, fluorescence having an intensity in accordance with the delay time is emitted.To completely remove such fluorescence, if the ratio of the fluorescence intensity at the time of Raman excitation and the delay time Δτ is known in advance. In the subtraction process, the value measured by the delay time Δτ may be subtracted by multiplying the value measured by the reciprocal of the ratio. The point that such a process is possible is shown in FIGS.
It can be said that it is an advantage for the thing. Note that the delay time Δτ
Preferably, the time is adjustable.

In the case of FIG. 7, as in the case of FIG. 3, the Raman light modulated by the interferometer 3 is m / 2τ <f <(m
+1) / 2τ (m is a positive integer) and shows a configuration for measuring a difference spectrum when the frequency spectrum has a frequency component f. The gate circuit 6 and the band-pass filter are provided after the preamplifier 5 in FIG. 11, a channel comprising a gate circuit 6 ', a bandpass filter 11', a main amplifier 8 ', an AD converter 9', and a computer 10 '(similar to the channel comprising the main amplifier 8, the AD converter 9, and the computer 10). , The computer 10 '
Are shared with the computer 10. ) Another one in parallel
One is provided. The operation is clear from the above description, and will not be described.

It should be noted that the present invention is not limited to the above embodiment, and various modifications are possible. The background-eliminated Fourier transform spectroscopy of the present invention can also be applied to time-resolved spectroscopy.

[0026]

As is apparent from the above description, according to the background-eliminated Fourier transform spectroscopy of the present invention,
Extracting a signal from the detector output using a rapid scan interferometer only while the pulsed light source or the pulsed laser for Raman excitation is emitting light, extracting a component of the signal in a predetermined frequency range, sampling, and performing Fourier transform. As a result, since the spectrum of the measurement light is obtained, strong background light and fluorescence in Raman spectroscopy are removed, and weak Raman light and the like can be accurately spectroscopically measured.

Since a pulsed light source is used as the light source as in the premise of the present invention, an SOR light source, a pulsed laser-excited Raman sample, or the like can be used as the light source, and the object to be measured is irradiated with light. Since the time can be shortened and the amount of light to be irradiated can be reduced, Fourier transform spectroscopy can be applied to measurement objects affected by light irradiation or measurement objects where continuous irradiation is not preferable. It spreads.

Moreover, as compared with the method presupposed by the present invention, an apparatus for implementing this method can be constructed with only a simple change of simply adding a gate circuit.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a first or second embodiment of a Fourier transform spectroscope for performing a background removal Fourier transform spectroscopy according to the present invention.

FIG. 2 is a diagram for explaining a background removing operation of the apparatus of FIG. 1;

FIG. 3 is a diagram showing the configuration of another embodiment of an apparatus for performing the method of the present invention.

FIG. 4 is a diagram showing a configuration of still another embodiment.

FIG. 5 is a diagram showing a configuration of another embodiment.

FIG. 6 is a diagram showing a configuration of another embodiment for difference measurement.

FIG. 7 is a diagram showing a configuration of still another embodiment for difference measurement.

FIG. 8 is a diagram showing the relationship between the Raman pump pulsed laser light and the accompanying Raman light, fluorescence, and gate signal.

FIG. 9 is a diagram for explaining one configuration of a time-resolved Fourier transform spectrometer that is a premise of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Pulse laser 2 ... Sample 3 ... Interferometer 4 ... Detector 5 ... Preamplifier 6, 6 '... Gate circuit 7, 7' ... Low-pass filter 8, 8 '... Main amplifier 9, 9' ... AD converter 10 ... Computer 11, 11 ': bandpass filter 12: timer 13, frequency divider 14: lock-in amplifier 15: delay circuit

Claims (2)

(57) [Claims] An interferogram is obtained from a detector output using a rapid scan interferometer, and a spectrum of a measurement light is obtained by performing a Fourier transform using a pulse light source emitting light at a constant period or a pulse laser for Raman excitation as a light source. In Fourier transform spectroscopy, a signal is extracted from a detector output only while the light source is emitting light, a component in a predetermined frequency range of the signal is extracted, sampled, and subjected to Fourier transform to obtain a spectrum of the measurement light. Background removal Fourier transform spectroscopy. 2. A signal of a background is obtained by extracting a signal from a detector output even at a predetermined delay time after the light source emits light, extracting a component of the signal in a predetermined frequency range, sampling and performing Fourier transform. 2. The background removal according to claim 1, wherein the background spectrum is subtracted from a spectrum obtained from a signal only while the light source emits light, thereby obtaining a spectrum of the measurement light. Fourier transform spectroscopy.