JP2012007943A - Fourier transform infrared spectroscopic photometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Fourier transform infrared spectroscopic photometer for detecting transmitted light or reflected light from a sample as an analog signal, converting the detected signal into a digital signal using an A/D converter, and obtaining a spectrum by a Fourier transform of the digital signal, which can efficiently utilize an input range of the A/D converter and prevent an overflow that exceeds the input range of the A/D converter.SOLUTION: A Fourier transform infrared spectroscopic photometer of the present invention is provided with a control/processing unit 30 that predicts a maximum value of an output signal of an A/D converter 28 in the subsequent measurement from a difference value between a maximum value of an output signal of the A/D converter 28 in the current measurement and a maximum value of an output signal of the A/D converter 28 in the previous measurement, and determines an amplification rate of an amplifier 26 in the subsequent measurement based on the predicted value.

Description

  The present invention relates to a Fourier transform infrared spectrophotometer.

  In a Fourier transform infrared spectrophotometer (hereinafter abbreviated as “FTIR”), a Michelson interferometer including a fixed mirror and a moving mirror generates an interference wave whose amplitude varies with time, and irradiates the sample with this interference wave. The transmitted light or reflected light is detected as an interferogram. Then, the detection signal is Fourier transformed to obtain an absorption spectrum in which the horizontal axis represents the wave number and the vertical axis represents the intensity (such as absorbance or transmittance). At this time, one interferogram is generated by one scan of the moving mirror, and an absorption spectrum over the entire predetermined wavelength range can be acquired from the interferogram.

  In such FTIR, an interferogram as transmitted light or reflected light from a sample is detected as an analog signal. Then, after adjusting the gain of the detected analog signal through an amplifier, it is input to an A / D converter and converted into digital data, and an absorption spectrum is obtained by executing Fourier transform on this digital data. (See Patent Documents 1 and 2).

  In FTIR, in order to increase the detection accuracy of transmitted light or reflected light from a sample and obtain a spectrum with less noise, the amplification of the amplifier is performed so that the input signal from the amplifier becomes the maximum value that can be accommodated in the input range of the A / D converter The rate is determined. Therefore, in the conventional FTIR, after starting measurement, an amplification factor is set based on the maximum value of IFG obtained first, and measurement is performed at the same amplification factor until a measurement stop command is received.

JP 2002-22536 A Japanese Patent Laid-Open No. 2003-14543

  However, when continuously measuring the absorption spectrum of a sample whose absorbance changes over time, if measurement is continued at the initially set amplification factor, the measurement must always be performed at an appropriate amplification factor. It may not be. In particular, when the amount of transmitted light or reflected light from the sample increases, an overflow exceeding the input range of the A / D converter occurs and saturates, so accurate measurement cannot be performed. In order to prevent this, an error is reported when the A / D converter is saturated, and as a result, the measurement may be interrupted.

  The problem to be solved by the present invention is to detect transmitted light or reflected light from a sample as an analog signal, convert the detected signal into a digital signal by an A / D converter, and then subject the digital signal to Fourier transform. To provide a Fourier transform infrared spectrophotometer capable of effectively utilizing an input range of an A / D converter and preventing an overflow exceeding the input range of the A / D converter in obtaining a spectrum It is.

The first invention of the present application made to solve the above-described problems includes a light source that generates infrared light, an interferometer that generates interference light from the infrared light, including a fixed mirror and a movable mirror, and the interference A detector that detects light as an analog signal, an amplifier circuit that amplifies the analog signal detected by the detector, an A / D converter that converts the amplified analog signal into a digital signal, and a Fourier transform of the digital signal In a Fourier transform infrared spectrophotometer comprising a processing unit for obtaining a spectrum,
From the maximum value of the output signal of the A / D converter at the time of the current measurement and the maximum value of the output signal of the A / D converter at the time of the previous measurement, the A / D converter at the time of the next measurement The maximum value of the output signal is predicted, and amplification of the amplifier circuit at the next measurement is performed so that the input signal of the A / D converter corresponding to the predicted value is included in the input range of the A / D converter. A control unit for determining the rate is provided.
Here, “previous measurement”, “current measurement”, and “next measurement” are three of a plurality of measurement operations performed after receiving a measurement start command until receiving a measurement stop command. Refers to the measurement operation for one time, for example, the measurement operation for three times in the case where the absorption spectrum of one sample whose light quantity changes with the passage of time is continuously measured a plurality of times. One measurement operation corresponds to, for example, a series of measurement operations performed corresponding to one reciprocation of the movable mirror. In addition, “previous measurement” and “next measurement” may be measurements performed before “this time” and measurements performed in the future, and are not necessarily measured immediately before this measurement. And it is not restricted to the measurement performed immediately after.

  According to a second aspect of the present invention, in the first aspect, the control unit determines the maximum value of the output signal of the A / D converter at the time of the current measurement and the output signal of the A / D converter at the time of the previous measurement. The maximum value of the output signal of the A / D converter at the next measurement is predicted based on the change amount of the maximum value.

A third invention is the first invention or the second invention,
The processing unit is configured to create an interferogram spectrum from an output signal of the A / D converter corresponding to one reciprocation of the movable mirror;
The control unit obtains a maximum value of an output signal of the A / D converter from the interferogram spectrum.

  According to the present invention, the A / D at the next measurement is based on the maximum value of the output signal of the A / D converter obtained by each measurement operation from when the measurement start command is received until the measurement stop command is received. Since the maximum value of the output signal of the converter is predicted and the amplification factor of the amplifier circuit is determined so that the predicted maximum value of the output signal is within the input range of the A / D converter, the input range of the A / D converter Can be effectively utilized, and overflow that exceeds the input range of the A / D converter can be prevented.

The schematic block diagram of FTIR which concerns on one Example of this invention. The flowchart which shows the procedure of the setting process of the amplification factor by a control / processing part. The wave form diagram for demonstrating the operation | movement of FTIR.

  Hereinafter, FTIR according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of the FTIR according to the present embodiment. In the FTIR according to the present embodiment, a main interferometer for obtaining an interferogram, and a timing signal for controlling the sliding speed of the moving mirror and sampling a signal obtained by the photodetector of the main interferometer are generated. A control interferometer is provided. 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, and generates interfering infrared light for performing spectrum measurement. 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 is reciprocally driven back and forth (in the direction of arrow M in FIG. 1) by the movable mirror driving unit 16a, the combined light is interfering light (interferon whose amplitude varies with time). G). 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.

  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.

  The optical components centering on the main interferometer are arranged in the hermetic chamber 10, and the humidity in the hermetic chamber 10 is controlled. This is mainly to protect the beam splitter 14 having KBr having deliquescence as a substrate.

  The light reception signal of the photodetector 20, that is, a laser light interference fringe signal (usually called “fringe signal”) is input to the signal generation unit 29, where a pulse signal for sampling the light reception signal for the infrared interference light is generated. Is done. Further, this laser beam interference fringe signal is also used for performing stable sliding control of the movable mirror 16. As the laser light source 17, a He—Ne laser is generally used, and a pulse signal having the same frequency as the laser light interference fringe signal is generated by the signal generation unit 29.

  The interference light that has passed through the sample 23 arranged in the sample chamber 22 is received by the photodetector 25 as an analog signal. 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 digital data is sent to the control / processing unit 30, and after performing predetermined data processing, a Fourier transform operation is performed to create an absorption spectrum.

  Although the control / processing unit 30 can be a dedicated control / processing device, generally, the entity is a personal computer in which dedicated control / processing software is installed, and performs various input operations. An input unit 41 using a keyboard and a pointing device (such as a mouse) and a monitor 40 for displaying measurement results are connected.

Here, in the control / processing unit 30, the fluctuation range of the analog signal detected by the photodetector 25 is predicted, and the input signal of the A / D converter 28 becomes the input range of the A / D converter 28. The amplification factor of the amplifier 26 is determined so as to be included. Hereinafter, the procedure for determining the amplification factor of the amplifier 26 will be described with reference to FIG.
The control / processing unit 30 includes a memory for storing the digital data sent from the A / D converter 28, and the interferogram spectrum is obtained from the digital data corresponding to one reciprocating motion of the moving mirror stored in the memory. (Hereinafter referred to as “IFG spectrum”), and the maximum value M1 of the IFG signal intensity is obtained from the created IFG spectrum (S1). The obtained maximum value M1 is stored in a memory provided in the control / processing unit 30.

  FIG. 3 shows a general shape of the IFG spectrum after A / D conversion, where the vertical axis represents signal intensity (A / D conversion value) and the horizontal axis represents the position of the moving mirror. As can be seen from FIG. 3, normally, the signal intensity reaches its maximum value when the position of the movable mirror is “0 cm”. Therefore, normally, the maximum value of the IFG signal intensity (vertical axis maximum value) can be obtained by comparing the vertical axis values of about ± 2 cm centered on “0 cm” as the position of the movable mirror.

  Next, the control / processing unit 30 reads the previous IFG maximum value M2 already stored in the memory, and the difference value D1 between the IFG maximum value M2 and the current IFG maximum value M1 (that is, D1 = M1). -M2) is determined. Then, by adding the difference value D1 to the current IFG maximum value M1, a predicted value P1 of the IFG maximum value at the time of the next measurement (that is, P1 = M1 + D1) is calculated (S2).

  Subsequently, it is determined whether or not the input signal of the A / D converter 28 corresponding to the obtained predicted value P1 is included in the input range of the A / D converter 28 (S3). The determination is performed by comparing the upper and lower threshold values stored in advance in a memory included in the control / processing unit 30 with the predicted value P1. The upper and lower thresholds are composed of output signals (digital values) corresponding to the input values (analog values) of the A / D converter. Specifically, when the predicted value P1 is equal to or higher than the upper limit threshold, the gain is decreased by one level (S4), and when the predicted value P1 is equal to or lower than the lower limit threshold, the gain is increased by one level (S6). Further, when the predicted value P1 is between the upper limit threshold and the lower limit threshold, the amplification factor is maintained (S5). As described above, when the amplification factor at the next measurement is determined, the control / processing unit 30 changes the amplification factor of the amplifier 26 and stores the current IFG maximum value M1 as M2 in the memory.

For example, in the case of the 4-bit A / D converter 28, since the full scale is “1111”, the upper limit threshold is set to “1110” with a margin. On the other hand, the lower threshold is set to be, for example, half or less of the upper threshold so as to be smaller than the upper threshold even when the amplification factor is increased by one stage. The amplification factor of the amplifier 26 is a numerical value expressed by 2 to the power of n, such as 1 ×, 2 ×, 4 ×, 8 ×, and so on. Means to double. Therefore, when the upper limit threshold is “1110”, it may be set to “110” or less, which is half of the upper limit threshold.
As an example, a case where the predicted value P1 = 101 as a result of measurement with an amplification factor of 2 when the upper limit threshold is set to “1110” and the lower limit threshold is set to “110” will be described. In this case, the predicted value P1 is equal to or lower than the upper threshold value and is equal to or lower than the lower threshold value. For this reason, the control / processing unit 30 increases the amplification factor by one level and sets it to four times.

  If this time is the first IFG acquisition time, the previous IFG maximum value M2 is not stored. Therefore, in this case, measurement is performed with the amplification factor of the amplifier set to the minimum amplification factor (1 time), and the amplification factor is determined at the next measurement from the maximum value of the signal intensity at that time.

  According to the present embodiment, the maximum value of the output signal of the A / D converter 28 is obtained from the IFG spectrum obtained by each measurement operation after receiving the measurement start command and receiving the measurement stop command. Based on this maximum value, the maximum value of the output signal of the A / D converter 28 in the next measurement is predicted, and the input signal corresponding to the predicted maximum value of the output signal is the input range of the A / D converter 28. Therefore, the input range of the A / D converter 28 can be used effectively, and overflow that exceeds the input range of the A / D converter 28 can be prevented. .

  The amplification factor of the amplifier 26 does not need to be updated every time the IFG is acquired, and may be configured to update the amplification factor every time the IFG is acquired a plurality of times. Alternatively, the IFG spectrum may be subjected to integration processing a plurality of times, the IFG maximum value M1 may be obtained from the IFG spectrum after the integration processing, and the amplification factor at the next measurement may be determined.

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 16a ... Moving mirror drive part 17 ... Laser light source 18 ... Mirror 19 ... Mirror 20 ... Light detection 21 ... Parabolic mirror 22 ... Sample chamber 23 ... Sample 24 ... Ellipsoidal mirror 25 ... Photo detector 26 ... Amplifier (amplifier)
28 ... A / D converter 30 ... Control / processing section

Claims (3)

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 as an analog signal, and a detector detected by the detector A Fourier transform infrared spectrophotometer comprising an amplifier circuit that amplifies an analog signal, an A / D converter that converts the amplified analog signal into a digital signal, and a processing unit that obtains a spectrum by Fourier transforming the digital signal. In total
From the maximum value of the output signal of the A / D converter at the time of the current measurement and the maximum value of the output signal of the A / D converter at the time of the previous measurement, the A / D converter at the time of the next measurement The maximum value of the output signal is predicted, and the amplification factor of the amplifier circuit at the next measurement is such that the input signal of the A / D converter corresponding to the predicted value falls within the input range of the A / D converter. A Fourier transform infrared spectrophotometer comprising a control unit for determining   Based on the difference between the maximum value of the output signal of the A / D converter at the current measurement and the maximum value of the output signal of the A / D converter at the previous measurement, the control unit performs the next measurement. The Fourier transform infrared spectrophotometer according to claim 1, wherein the maximum value of the output signal of the A / D converter at the time is predicted. The processing unit is configured to create an interferogram spectrum from an output signal of the A / D converter corresponding to one reciprocation of the movable mirror;
The Fourier transform infrared spectrophotometer according to claim 1, wherein the control unit obtains a maximum value of an output signal of the A / D converter from the interferogram spectrum.

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