JP4206618B2 - Fourier transform infrared spectrophotometer - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a Fourier transform infrared spectrophotometer.
[0002]
[Prior art]
In a Fourier transform infrared spectrophotometer (hereinafter referred to as “FTIR”), a Michelson interferometer including a fixed mirror and a moving mirror generates an interference wave (interferogram) whose amplitude varies with time, and this is used as a sample. And the transmitted light or reflected light is detected. Then, by performing Fourier transform on this detection signal, an absorption spectrum having the wave number on the horizontal axis and the intensity (absorbance) on the vertical axis can be obtained.
[0003]
In such FTIR transmission measurement, an absorption spectrum is created by measuring the target sample, and a background spectrum is created by performing blank measurement without mounting the sample with the same optical system, and the difference between the two is obtained. A process of calculating the transmittance spectrum of the sample by calculation is performed.
[0004]
[Problems to be solved by the invention]
Conventionally, in FTIR, the wave number resolution can be set from the outside according to the measurement purpose and the like. When calculating the transmittance spectrum as described above, the wave number resolution of the absorption spectrum of the target sample and the background spectrum must be the same, so when the wave number resolution is changed, the wave number resolution at the changed wave number resolution is It was necessary to perform a blank measurement and create a background spectrum based on it. In general, in FTIR, a calculation process is performed in which repeated measurement is performed many times in order to improve the SN ratio, and the measurement results are integrated. For this reason, it takes a few seconds at the minimum to measure a single sample and obtain a spectrum. Since the same number of times of integration is performed even in the blank measurement, the total measurement time is considerably long. Become. Thus, performing blank measurement for each sample measurement has been a major obstacle to improving measurement efficiency.
[0005]
The present invention has been made to solve the above-mentioned problems, and the object of the present invention is to eliminate the blank measurement that is performed every time the sample is measured regardless of the wave number resolution. An object of the present invention is to provide a Fourier transform infrared spectrophotometer that can improve the above.
[0006]
[Means for Solving the Problems]
The blank measurement as described above needs to be performed under almost the same conditions as the measurement of the target sample. In general, however, in FTIR, care is taken to stabilize the device including the optical system, such as when the power is not used. Has been paid. Therefore, the blank measurement may not be performed immediately before or immediately after the measurement of the target sample as long as the change over time such as a decrease in the light emission intensity of the light source is negligible. Therefore, in the Fourier infrared spectrophotometer according to the present invention, blank measurement is performed in advance with the highest wave number resolution selectable by the apparatus, data is acquired, and this is stored as basic data of the background spectrum. When wavenumber resolution is selected at the time of sample measurement, the sample is measured with the wavenumber resolution, and only the data corresponding to the wavenumber resolution is selected from the basic data of the background spectrum, and the background spectrum is determined based on the data. create. This eliminates the need for actual blank measurement during sample measurement.
[0007]
That is, a Fourier transform infrared spectrophotometer according to the present invention includes a light source that generates infrared light, a fixed mirror and a moving mirror, an interferometer that generates interference light from the infrared light, and the interference light. In a Fourier transform infrared spectrophotometer comprising a detector for detecting, and a processing unit for Fourier-transforming the detection signal to obtain a spectrum,
a) Collect the digitized data of the interferogram obtained by the detector by performing a blank measurement that does not pass through the sample with the highest wave number resolution selectable by the device, and store it in the storage means A blank measuring means to be placed;
b) a selection means for selecting data from the storage means according to the selected wave number resolution when measuring the target sample;
c) a spectrum generating means for calculating a spectrum of the target sample using a background based on the selected data;
It is characterized by having.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, FTIR according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of the main part of the present FTIR, FIG. 2 is a functional block diagram of the main part of the data processing unit 29 in FIG. 1, and FIG. 3 is a waveform diagram for explaining the operation of the present FTIR.
[0009]
In FIG. 1, the main interferometer includes an infrared light source 11, a condensing mirror 12, a collimator mirror 13, a beam splitter 14, a fixed mirror 15, a moving mirror 16, and the like. generate. That is, the infrared light emitted from the infrared light source 11 is applied to the beam splitter 14 through the condensing mirror 12 and the collimator mirror 13, and is divided into two directions, a fixed mirror 15 and a moving mirror 16. The lights reflected by the fixed mirror 15 and the movable mirror 16 are combined again by the beam splitter 14 and sent to the optical path toward the parabolic mirror 21. At this time, since the movable mirror 16 reciprocates back and forth (in the direction of the arrow in FIG. 1), the combined light becomes interference light (interferogram) whose amplitude varies with time. The light collected by the parabolic mirror 21 is irradiated into the sample chamber 22, and the light that has passed through the sample 23 disposed in the sample chamber 22 is collected by the ellipsoidal mirror 24 onto the photodetector 25.
[0010]
On the other hand, the control interferometer includes a laser light source 17, a mirror 18, a beam splitter 14, a fixed mirror 15, a movable mirror 16, and the like, and generates laser interference light for obtaining an interference fringe signal. That is, the light emitted from the laser light source 17 is applied to the beam splitter 14 through the mirror 18 and is transmitted to the parabolic mirror 21 as interference light similarly to the infrared light. Since this laser interference light travels as a light beam having a very small diameter, it is reflected by the mirror 19 inserted in the optical path and introduced into the photodetector 20.
[0011]
The optical components centering on the main interferometer are disposed in the hermetic chamber 10, and the humidity is controlled in the hermetic chamber 10. This is mainly to protect the beam splitter 14 having KBr having deliquescence as a substrate.
[0012]
The light reception signal of the light detector 20, that is, the laser light interference fringe signal is input to the signal generation unit 30, where a pulse signal for sampling the light reception signal for the infrared interference light is generated. The received light signal obtained by the photodetector 25 is amplified by the amplifier 26, sampled by the sample and hold circuit (S / H) 27 at the timing of the pulse signal, and then by the A / D converter (A / D) 28. Converted to digital data. The data processing unit 29 executes Fourier transform on this data to create an absorption spectrum, and further creates a transmittance spectrum using background data as will be described later. The series of measurement operations are executed under the control of the control unit 31.
[0013]
In this FTIR, blank measurement without a sample is performed before measuring a target sample. Various methods for performing the blank measurement are conceivable, but most simply, the sample 23 is not arranged in the sample chamber 22, so to speak, the light passing through the sample chamber 22 is measured. When the target sample is a liquid sample or the like, a cell that stores the liquid sample is placed in the sample chamber 22 without the sample being placed, and light passing through the cell is measured. Alternatively, a sample changer capable of exchanging a plurality of samples may be installed in the sample chamber 22, one of them may be set as a blank sample, and this may be selected and measured.
[0014]
At the time of this blank measurement, the wave number resolution with the highest resolution is selected in this apparatus. In this FTIR, assuming that the size of the infrared light source 11 is an ideal state, the wave number resolution depends on the optical path difference. That is, the required optical path difference is different for each wave number resolution. For example, in this apparatus, the wave number resolution is assumed to be six types of 0.5, ¹ , 2, 4, 8, and 16 cm ⁻¹ . Now, the most resolution is that it is necessary optical path difference 2cm to high wavenumber resolution 0.5 cm ^-1, relative to the wavenumber resolution ^{1 cm -1} is sufficient optical path difference half 1 cm. At lower wavenumber resolution, the optical path difference may be shorter.
[0015]
Assuming that the interferogram obtained by the photodetector 25 as a result of the blank measurement is as shown in FIG. 3A, the data digitized by the sample hold circuit 27 and the A / D converter 28 is shown in FIG. As shown in b). In such an interferogram, the horizontal axis represents the optical path difference (that is, the moving distance of the movable mirror 16). As described above, the wave number resolution depends on the optical path difference. The lower the wave number resolution, the smaller the optical path difference. Therefore, the measurement data for other wave number resolutions other than the highest wave number resolution are all included in the measurement data for the highest wave number resolution. Is included. This is the reason for selecting the highest wave number resolution during blank measurement. The data as shown in FIG. 3B is stored in a memory provided in the data processing unit 29.
[0016]
In addition, when performing a blank measurement, it is preferable to do as follows. In FTIR, it is known that the S / N ratio can be improved by repeating the measurement a plurality of times and integrating the data obtained as a result. It is desirable to perform such integration also in the blank measurement. The greater the number of integrations, the better. However, there are time restrictions during normal sample measurement. So, for example, at night or in the daytime when the device is not used for a relatively long time, the blank measurement is repeated automatically or manually many times, and the results are integrated to obtain the final data. To do. In this way, even if the integration is performed many times, the sample measurement is not disturbed, and a high S / N ratio can be achieved.
[0017]
In FIG. 2, a background data storage unit 40 is a memory in which data obtained at the time of blank measurement as described above is stored. When integration processing is performed, the stored data is data that has been integrated.
[0018]
The operation of the present FTIR when measuring the target sample 23 is as follows. A wave number resolution is instructed to the data processing unit 29 by an operator via an operation unit (not shown). The data selection unit 41 selects and reads data from the background data storage unit 40 according to the instructed wave number resolution. For example, when the maximum wave number resolution is instructed, all data is read out. If the wave number resolution is lower than that, data having a reduced number is read according to the corresponding optical path difference as described above. The background spectrum creating unit 42 creates a background spectrum by performing Fourier transform on the selected data.
[0019]
On the other hand, the above-described measurement is performed on the sample 23 arranged in the sample chamber 22, and the data acquired at this time is input to the sample spectrum creating unit 43. The sample spectrum creation unit 43 creates an absorption spectrum by performing Fourier transform on the measurement data. The difference spectrum calculation unit 44 calculates the difference between the two spectra and calculates the transmittance spectrum for the sample.
[0020]
In this FTIR, when sample measurement with different wave number resolution is performed, it is not necessary to perform blank measurement at that wave number resolution one by one, and all or part of the already stored background data is selected. All you have to do is Therefore, it is not necessary to devote time to blank measurement at the time of sample measurement, and the measurement can proceed very efficiently. In addition, since blank measurement can be performed when sufficient time can be secured, the number of measurements can be greatly increased, the number of data integration is increased, and the SN ratio of the background spectrum itself is improved. The SN ratio of the transmission spectrum is also improved.
[0021]
Note that the above embodiment is an example of the present invention, and it is apparent that modifications and changes can be made as appropriate within the scope of the present invention.
[0022]
【The invention's effect】
According to the Fourier transform infrared spectrophotometer according to the present invention, it is not necessary to perform a blank measurement according to the wave number resolution at the time of each sample measurement. A corrected spectrum can be obtained. Therefore, the measurement efficiency is greatly improved.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a main part of an FTIR according to an embodiment of the present invention.
FIG. 2 is a functional block diagram of a main part in a data processing unit in FIG.
FIG. 3 is a waveform diagram for explaining the operation of FTIR according to the present embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Airtight chamber 11 ... Infrared light source 12 ... Condensing mirror 13 ... Collimator mirror 14 ... Beam splitter 15 ... Fixed mirror 16 ... Moving mirror 17 ... Laser light source 18, 19 ... Mirror 20, 25 ... Photodetector 21 ... Parabolic Surface mirror 22 ... Sample chamber 23 ... Sample 24 ... Ellipsoidal mirror 26 ... Amplifier 27 ... Sample hold circuit 28 ... A / D converter 29 ... Data processing unit 30 ... Signal generation unit 31 ... Control unit 40 ... Background data storage unit 41 ... Data selection section (selection means)
42 ... Background spectrum creation unit 43 ... Sample spectrum creation unit 44 ... Difference spectrum calculation unit

Claims (1)

A light source that generates infrared light, a fixed mirror and a moving mirror, an interferometer that generates interference light from the infrared light, a detector that detects the interference light, and a spectrum obtained by Fourier transforming the detection signal In a Fourier transform infrared spectrophotometer comprising a processing unit for obtaining
a) Collect the digitized data of the interferogram obtained by the detector by performing a blank measurement that does not pass through the sample with the highest wave number resolution selectable by the device, and store it in the storage means A blank measuring means to be placed;
b) a selection means for selecting data from the storage means according to the selected wave number resolution when measuring the target sample;
c) a spectrum generating means for calculating a spectrum of the target sample using a background based on the selected data;
A Fourier transform infrared spectrophotometer.

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