JP2003185498A - Spectrophotometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spectrophotometer that can easily calibrate non-linear properties to a small signal response without requiring any filters for calibration and special hardware mechanisms. <P>SOLUTION: When the start of calibration is instructed, a CPU 15 controls drive circuits 3 and 4, lights up a heavy hydrogen lamp 1, a halogen lamp 2, and at the same time rotates a light path-switching plate 5 so that the translucent mirror of the light path switching plate 5 is arranged on a light path. Light from a light source section passes through a cell holder where no samples exist for entering a photodiode array detector 11. Then, while the charge accumulation time of the photodiode array detector 11 is being changed in steps, detection output is taken in, and an I/O characteristic function (f) is estimated for each wavelength. The I/O characteristic function (f) for each wavelength obtained by the estimation is stored in an EEPROM 18. When measurement is to be carried out, a true photometry value can be obtained by converting output for each wavelength of the detector by the inverse function of the I/O characteristic function (f) corresponding to the wavelength. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to spectrophotometers used as general purpose spectrophotometers or as liquid chromatographic detectors, and more particularly to non-linearity calibration to small signal response.

[0002]

2. Description of the Related Art A spectrophotometer utilizes the characteristic that a substance has a unique absorption coefficient at a specific wavelength.
The substance is specified by the wavelength to be absorbed, and the concentration is measured by the transmittance or the absorbance at that wavelength. In a general-purpose spectrophotometer, a liquid sample is put in a transparent cell, and the sample is analyzed by measuring its transmittance through infrared rays, visible rays, or the like. In a liquid chromatograph, a sample flowing out of a column is passed through a detector, and the detector is irradiated with light to analyze the sample. On the other hand, in spectrum measurement by a spectrophotometer, an optical accessory device using an optical fiber may be used to measure a large sample that cannot be accommodated in a sample chamber or a sample that cannot be moved.

[0003]

Such a spectrophotometer is widely used as a pollution analyzer for quantifying pollutants in water and soil, as well as a pharmaceutical analyzer and a clinical analyzer. It is an important issue to ensure the performance and reliability of the analytical equipment in 1., but when the amount of light input to the photodetector is low, such as when the concentration of the sample solution is high or when the throughput of optical systems such as optical fibers is low. In some cases, non-linearity of a photodetector such as a photodiode or a signal processing circuit causes a measurement error. For this reason, the operator manually inserts an optical filter (photometric value calibration filter) of known absorbance into the measurement optical path to perform correction calculation for the nonlinearity of the spectrophotometer with respect to small signals, and to use the optical filter for spectrophotometric measurement. Calibration is performed as standard equipment on the measuring instrument body. However, when the correction is performed using the calibration filter, the temporal change of the filter itself poses a problem. That is, there is a problem in that the transmittance of the calibration filter changes with time and accurate calibration cannot be performed. Further, in the case where the calibration filter is standardly provided in the main body of the spectrophotometer, there is a problem in that the structure of the apparatus becomes complicated because a mechanism for putting the calibration filter in and out of the measurement optical path is required.

The present invention was devised to solve the above-mentioned problems, and it is possible to easily perform non-linearity calibration with respect to a small signal response without requiring a calibration filter or a special hardware mechanism. An object is to provide a spectrophotometer that can be used.

[0005]

In order to achieve the above object, the spectrophotometer of the present invention comprises a light source, an optical system for guiding the light from the light source to the sample, and a spectroscope for guiding the light transmitted through the sample. In a spectrophotometer including a photodetector on which the light dispersed by the spectroscope is incident, a unit for stepwise changing the integration time of the output signal of the photodetector, for example, the charge storage time of the photodiode array. And a means for obtaining a function representing the input / output characteristics of the device from each detection output when the integrated time is changed stepwise, and the output signal of the detector is corrected using this function.

Since the spectrophotometer of the present invention is constructed as described above, it is always accurate to small signal response without being affected by the change with time of the light transmittance of the optical filter as in the case of using the optical filter. Non-linearity calibration can be performed. Further, since no special calibration hardware other than the standard measurement mechanism included in the ordinary spectrophotometer is required, the configuration can be simplified.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the spectrophotometer of the present invention will be described below with reference to FIGS. FIG. 1 is a schematic configuration diagram of a spectrophotometer, FIG. 2 is an enlarged view of an optical path switching plate, and FIG. 3 is a flowchart showing an operation for calibrating nonlinearity with respect to a small signal. In FIG. 1, 1 is a deuterium lamp, 2 is a halogen lamp, 3 and 4 are lamp driving circuits (DRV), 5 is an optical path switching plate, 6 and 8 are lenses, 7 is a cell holder, 9 is an input slit of a spectroscope, Reference numeral 10 is a grating, 11 is a photodiode array detector (PDA), 12 is a control signal generating circuit for generating a control signal for the photodiode array detector 11, and 13 is a photodiode array detector 1
An amplifier circuit for amplifying the output signal of 1, an A / D converter 14, a CPU 15 and a read only memory (RO).
M), 17 is a random access memory (RAM), 18
Is a non-volatile memory (EEPROM), and 19 is a display.

Since no single light source has a sufficient emission intensity in a wide range from infrared rays, visible rays to ultraviolet rays, a deuterium lamp 1 for producing light mainly in the ultraviolet region is usually used in a spectrophotometer. A halogen lamp 2 for generating infrared and visible light is used. Each of the lamps 1 and 2 is provided with a drive circuit (DRV) 3 and 4 for lighting them.
Reference numeral 4 is connected to the CPU 15.

The deuterium lamp 1 and the halogen lamp 2
Are arranged so that their optical axes intersect, and an optical path switching plate 5 is provided at the intersection. As shown in FIG. 2, the optical path switching plate 5 includes a disc 50 having an opening 51, a semitransparent mirror 52, and a total reflection mirror 53. By arranging these at the intersections of the optical axes of the two lamps, The light of the deuterium lamp 1, the mixed light of both lamps 1 and 2, or the light of the halogen lamp 2 is sent to the cell holder 7 side, respectively. The rotation drive mechanism (not shown) for this optical path switching plate 5 is also C
It is connected to the PU 15. The light emitted from the light source unit is converged by the irradiation lens 6 and irradiated on the sample cell in the cell holder 7. The sample cell absorbs the wavelength component according to the characteristics of the sample, and the light passing through the sample cell passes through the lens 8 and the input slit 9 of the spectroscope and the grating 1
It is incident on 0, is expanded into a spectrum, and the light intensity of each wavelength is detected by the photodiode array detector 11.

The photodiode array detector 11 is one in which a photodiode and a capacitor are connected in parallel, and generally, one in which several hundred elements are integrated on the same silicon substrate is used. In this element, while the read switch is off, the light incident on the photodiode array is converted into electric charge and stored in the capacitor connected in parallel with the photodiode. When the read switch is turned on, the charge is integrated by the integrating circuit. Is converted into voltage. This voltage is sent to the analog / digital (A / D) via the amplifier circuit 13.
It is converted into a digital signal by the converter 14 and taken into the CPU 15.

The CPU 15 controls the operation of each part of the spectrophotometer according to a program stored in the ROM 16 in advance or a program supplied from an external storage device. That is, the lamp drive circuits 3 and 4, the optical path switching plate 5, and the detector control signal generation circuit 12 are controlled according to the analysis conditions set on the display 19, and the output of the A / D converter 14 is fetched, For each wavelength, the ratio between the detection output when there is no sample and the detection output when the sample is in the cell is determined, and the absorbance spectrum is calculated based on this ratio and the measurement result is displayed on the display 19
To display. At this time, the control signal generation circuit 12 turns on the read switch of the photodiode array detector 11,
The charge storage time is controlled by controlling the off state.

Next, the calibration of non-linearity for small signals will be described. CPU for non-linearity calibration
Reference numeral 15 drives the drive circuits 3 and 4 to turn on the deuterium lamp 1 and the halogen lamp 2, and rotates the optical path switching plate 5 so that the semi-transparent mirror 52 is arranged on the optical path. Also,
The CPU 15 controls the control signal generation circuit 12 to sequentially switch the charge accumulation time of the photodiode array detector 11 and capture the detection output. On the other hand, the ROM 16 stores the number N of changes in the storage time of the photodiode array during calibration and the charge storage time c (j) corresponding to each change order j of the storage time.

The operation for calibrating the non-linearity with respect to this small signal will be described in detail below with reference to the flowchart of FIG. When measuring a sample having a high concentration, when the operator gives an instruction to start the calibration by the function button of the software on the display 19 with the sample cell in the cell holder 7, the CPU 15 controls the drive circuits 3 and 4. The deuterium lamp 1 and the halogen lamp 2 are turned on, and the optical path switching plate 5 is rotated so that the semitransparent mirror 52 of the optical path switching plate 5 is arranged on the optical path. Next, the change order j of the accumulation time is set to 0, and the change order 0 is read from the ROM 16.
Read the charge accumulation time c (0) corresponding to. And CP
U15 uses this charge storage time c (0) as the control signal generation circuit 12
, The output of the photodiode array detector 11 detected at the charge accumulation time c (0) is read a plurality of times, the average value for each wavelength is calculated from the output, and stored in the RAM 17. Next, the change order j is set to 1, and this value is compared with the set change count N. Since 1 is smaller than N, the charge storage time c (1) corresponding to j = 1 is read again. , The above operation is repeated. On the other hand, change order j
Becomes equal to the number of changes N, the input / output characteristics of the device are estimated for each wavelength from the obtained value of N times, and then the calibration is completed.

Next, a method of estimating the input / output characteristics of the device will be described with reference to FIG. FIG. 4 is a plot of the measured values in each measurement order j obtained at the position of the estimated value estimated based on the charge accumulation time c (j) corresponding to each change order j, plotted by the X mark, Is created for each wavelength.
Then, the input / output characteristic function f of the device as shown in the figure is estimated for each wavelength from the plotted points. The input / output characteristic function f may be estimated by polygonal line approximation from the plotted values, or may be estimated by polynomial approximation when high accuracy is required.

The input / output characteristic function f for each wavelength obtained by the estimation is stored in the EEPROM 18, and in the case of analysis, the output for each wavelength of the detector is input / output characteristic function corresponding to that wavelength. A true photometric value can be obtained by converting the inverse function of f. At this time, as described above, when calculating the absorbance spectrum,
Since the ratio of the detection output in the absence of the sample and the detection output in the state where the sample is contained in the cell is obtained for each wavelength, the detection output obtained in each case is nonlinearly corrected.

In the above embodiment, the case where the spectrophotometer of the present invention is applied to a general-purpose spectrophotometer having a cell holder has been described, but the spectrophotometer of the present invention is also applied to a detector of a liquid chromatograph. be able to. Further, in the above embodiment, an example in which a photodiode array detector is used as a detector has been described. However, a single detector is used, and a wavelength is also scanned in a spectrophotometer of a type that performs wavelength scanning by rotating a grating. The present invention can be applied, and in this case, the present invention can be implemented by changing the integration time of the circuit that integrates the output of a single detector according to the calibration order.

Further, in the above embodiment, the number N of times of storage time change and each charge storage time c (j) are read from the ROM 16, but they can be set by an input device such as a keyboard attached to the display 19. If the calibration data is acquired in the vicinity of the actual absorbance of the measurement object, more accurate calibration can be realized. Further, in the above embodiment,
Although the optical path switching plate 5 was rotated so that the semi-transparent mirror 52 was arranged on the optical path, when only the light of the deuterium lamp 1 or the halogen lamp 2 was used for the actual measurement of the sample, the calibration measurement was performed. Also in this case, the calibration measurement can be performed after rotating the optical path switching plate 5 so that the same position (the opening 51 or the total reflection mirror 53) as in the actual sample measurement is arranged on the optical path. In this case, since the calibration measurement is performed in a state close to the measurement of the sample, more accurate calibration can be realized.

In the above embodiment, the function button of the software is used to instruct the start of the calibration of the nonlinearity with respect to the small signal, but the calibration may be automatically started when the power of the apparatus is turned on. Yes, again
The calibration may be automatically performed periodically at an appropriate cycle. If the automatic calibration is performed in this manner, accurate analysis can always be performed even if the detector and the optical system change with time. Further, instead of performing the calibration by the function button of the software, if the apparatus is provided with a calibration start button, the calibration can be performed by hardware.

Further, in the above embodiment, the start of the calibration is separately instructed. However, when the measurement is performed, if the calibration is always performed before the start of the measurement, it is possible to reduce the influence of the temperature fluctuation in a short time. Fluctuations can also be prevented. Further, in the above example, the approximate curve was created from the plot of the values obtained by the calibration, but if the equation representing the input / output characteristics of the detector is theoretically or experimentally known in advance, the equation It is also possible to determine the characteristic parameters included in the above by using an optimization method or the like.

[0020]

The spectrophotometer of the present invention is configured as described above, and the integration time of the output signal of the photodetector, for example,
By changing the charge storage time of the photodiode array in stages and obtaining the function that represents the input / output characteristics of the device from each detection output when the integration time is changed, the output signal of the detector is corrected using this function. As in the case of using an optical filter, the nonlinearity for a small signal response can always be accurately calibrated without being affected by the change of the optical transmittance of the optical filter with time, and high accuracy can be achieved over a wide range of the input light amount. A measurement can be made. Further, since no special calibration hardware other than the standard measurement mechanism included in the ordinary spectrophotometer is required, the configuration can be simplified.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of a spectrophotometer of the present invention.

FIG. 2 is an enlarged view of an optical path switching plate used in the spectrophotometer of the present invention.

FIG. 3 is a flowchart showing an operation when calibrating the spectrophotometer of FIG.

FIG. 4 is a diagram plotting a relationship between an actual measurement value and an estimated value obtained by calibration.

[Explanation of symbols]

1 Deuterium lamp 2 Halogen lamp 3, 4 lamp drive circuit 5 Optical path switching plate 6 irradiation lens 7 Cell holder 8 lenses 9 input slits 10 grating 11 Photodiode array detector 12 Detector control signal generation circuit 13 Amplifier circuit 14 A / D converter 15 CPU 16 ROM 17 RAM 18 EEPROM 19 Display

Claims (2)

[Claims] 1. A light source, an optical system for guiding the light from the light source to the sample, a spectroscope for guiding the light transmitted through the sample, and a photodetector on which the light dispersed by the spectroscope enters. In the spectrophotometer, means for stepwise changing the integrated time of the output signal of the photodetector and means for obtaining a function representing the input / output characteristics of the device from each detection output when the integrated time is changed stepwise. A spectrophotometer, characterized in that the output signal of the photodetector is corrected using this function. 2. The spectrophotometer according to claim 1, wherein the photodetector is a photodiode array, and the integration time of the output signal is changed by changing the charge storage time of the photodiode array. Total.