JP3422294B2 - Spectrophotometer wavelength calibration method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spectrophotometer equipped with a monochromator that takes out monochromatic light having a specific wavelength, and the difference between the desired wavelength and the actually obtained wavelength. The present invention relates to a wavelength calibration method for solving the problem. In a spectrophotometer such as an ultraviolet-visible light spectrophotometer or an atomic absorption spectrophotometer, a monochromator is used to obtain monochromatic light having a predetermined wavelength. In general, a monochromator includes a wavelength dispersion element such as a diffraction grating or a prism, and a rotation drive mechanism for changing the angle of the wavelength dispersion element with respect to incident light. The wavelength dispersion element is rotated so that monochromatic light having a desired wavelength is extracted through a fixed exit slit. As wavelength dispersion elements, diffraction gratings have been widely used in recent years, and the case where these are used will be described below. In the monochromator, the rotation drive mechanism of the diffraction grating is based on conversion from linear motion to rotational motion by a sine bar mechanism, or by open loop control combining a stepping motor and a reduction gear mechanism.
Or the thing by the closed loop control using a DC servomotor etc. is used for practical use. For example, in a configuration having a rotational drive mechanism in which a stepping motor and a reduction gear mechanism are combined, an optical theoretical formula showing the relationship between the angle of the diffraction grating and the wavelength of the emitted light, and the rotation of the motor shaft of the stepping motor. Based on the design relationship between the angle and the rotation angle that is the output of the gear reduction mechanism, the ideal correspondence between the number of drive pulses, which is the control instruction amount of the motor, and the wavelength of the monochromatic light to be extracted is calculated. Can do. However, it is taken out due to various factors such as a mechanical accuracy error of the rotary drive mechanism, a grating constant of the diffraction grating, a refractive index error of the prism, and an installation error of the optical parts. The accuracy of the wavelength (this is called “wavelength accuracy”) is lost. Many conventional spectrophotometers have adopted measures to relatively reduce the influence of errors by using more precise mechanical parts and optical elements or by increasing the size of the monochromator itself. In such a method, there is a problem that expensive parts must be used and the cost is high, and there is a problem that it is difficult to downsize the monochromator. Recently, by using a spectrum having a known wavelength such as zero-order light and emission line spectrum of a deuterium lamp or several emission line spectra of a mercury lamp, a control instruction value (for example, drive stepping) of a rotary drive mechanism system is used. The amount of deviation actually obtained corresponding to the number of drive pulse signals sent to the motor) is obtained, and wavelength calibration is performed using this. However, the deuterium lamp and the mercury lamp have only about 10 emission line spectra, and the wavelength intervals of these emission line spectra are sparse and uneven. on the other hand,
Since the transmission error of the rotary drive mechanism does not show a uniform increase or decrease with respect to the feed angle, even if wavelength calibration is performed using the bright line spectrum as described above, wavelength calibration is accurately performed over the entire operating wavelength range. In some cases, wavelength accuracy may deteriorate due to wavelength calibration. The present invention has been made in order to solve the above-mentioned problems. The object of the present invention is to achieve high wavelength accuracy over the entire operating wavelength range using inexpensive mechanical parts and an easily available light source. It is to provide a wavelength calibration method of a spectrophotometer that can be achieved. The present invention, which has been made to solve the above-mentioned problems, does not show that the angular transmission error shows a uniform increase or decrease with respect to the feed angle of the motor rotating shaft. A wavelength calibration method for a spectrophotometer equipped with a monochromator that sets the wavelength of monochromatic light to be extracted by a rotational drive mechanism including a motor, comprising: a) measuring an angle accuracy error of the rotational drive mechanism in advance; b ) Bright line group composed of a single light source or a plurality of light sources and having a wavelength interval equal to or less than a wavelength interval corresponding to an angle interval of one half of the minimum period of the angle accuracy error over a wavelength range to be analyzed. C) Corresponding the control instruction value given to the rotation drive mechanism to the true wavelength of the emission line in order to obtain each emission line emitted from the light source unit as monochromatic light from a monochromator. Measure each d) Based on the correspondence between the wavelength and the control instruction value, create wavelength calibration data representing a calibration equation for obtaining the control instruction value corresponding to the desired wavelength, and store this in the storage means. It is characterized by that. In the monochromator, for example, the wavelength of monochromatic light extracted from the exit slit is set by rotating a wavelength dispersion element such as a diffraction grating by the rotation driving mechanism. The angular accuracy error with periodicity that results in wavelength accuracy error depends on the mechanical error of the rotary drive mechanism. Therefore, in the wavelength calibration method according to the present invention, first, the angle accuracy error of the rotation drive mechanism including the motor, that is, the relationship between the feed angle of the motor rotation shaft and the angle transmission error is measured. Then, the minimum cycle of fluctuation of the angle transmission error is found from the waveform indicating this relationship. Here, it is sufficient to obtain the minimum period among the periodic components having a fluctuation amplitude that causes an error exceeding the allowable value of the wavelength accuracy error. If such a minimum period is obtained, a wavelength interval corresponding to an angular interval of a half of the minimum period is set as follows.
Calculation is performed based on a theoretical formula representing the relationship between the angle of the wavelength dispersion element and the wavelength. Here, as described in detail later, when the wavelength calibration is performed at a wavelength step equal to or less than the wavelength interval corresponding to the angle interval of 1/2 or less, the angle interval of 1/2 of the minimum period is set to This is because, as a result of calibration, it is guaranteed that the wavelength shift is reduced as compared with the theoretically derived wavelength at all wavelengths belonging to adjacent wavelength steps. Therefore, a light source unit having a bright line group having a wavelength equal to or shorter than the above wavelength interval over the wavelength range to be analyzed is prepared. As the light source unit, an easily available light source such as a deuterium lamp, a metal element-enclosed discharge tube, or a holo cathode lamp is used.
Or a plurality of combinations can be used. Note that the smaller the wavelength interval of the bright line group, the more effective the calibration and the better the wavelength accuracy, but there are problems such as an increase in the types of necessary light sources and an increase in the time required to acquire calibration data. An appropriate wavelength interval, for example, a wavelength interval corresponding to an angle interval that is a quarter of the minimum period of the angle accuracy error may be set. When obtaining the actual wavelength calibration data,
The light source is turned on, and a control instruction value in the vicinity of the control instruction value theoretically obtained depending on the configuration of the monochromator is sequentially given to the rotation drive mechanism, thereby scanning the wavelength in the vicinity of the target bright line. Then, the control instruction value of the rotation driving mechanism when the emission line spectrum is obtained is associated with the wavelength of the emission line by the photodetector. Such an operation is repeated for all the light sources in the case where a plurality of light sources are used as the target all emission lines in one light source and the light source unit. If a large number of original data consisting of a set of control instruction value and wavelength is obtained in this way, a calibration equation is obtained by using a straight line or an approximate curve based on the original data. This calibration formula is a generalization of the original data obtained from the measurement of bright lines as a representative value. For example, correction due to various error factors is corrected from the control instruction value theoretically obtained corresponding to the wavelength. The calculated control instruction value is calculated. Such a calibration formula can be stored in the storage means as a calculation formula. In this case, at the time of spectroscopic measurement, when a desired wavelength extracted from the monochromator is set, first, the control instruction value is theoretically calculated, and further, the control instruction value is corrected using a calibration equation. Then, the angle of the wavelength dispersion element is appropriately set by giving the control instruction value after the calibration to the rotation drive mechanism. The monochromator generally has a configuration in which the wavelength of monochromatic light extracted from the exit slit is changed by rotating the wavelength dispersive element. In addition, the diffraction grating or the exit slit itself is moved. Therefore, the wavelength may be changed, and the present invention can be applied to all configurations in which the wavelength of monochromatic light is changed by a mechanical driving force using a rotation driving mechanism. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a wavelength calibration method for a spectrophotometer according to the present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration diagram of a main part of a spectrophotometer adopting the wavelength calibration method of the present embodiment. The light source switching unit 1 includes a plurality of lamps 1
a,... can be mounted, and light emitted from one selected lamp is introduced into the monochromator 2.
The monochromator 2 includes a diffraction grating 3 that rotates in a predetermined angle range, a stepping motor 4, and a reduction gear mechanism 5 that rotationally drives the diffraction grating 3 by decelerating the rotation of the motor shaft. The diffraction grating 3 is set at a predetermined rotation angle corresponding to the number of drive pulses applied to the motor 4. Monochromatic light taken out by the monochromator 2 is detected by a photodetector 6, sampled at an appropriate time interval by an input / output control unit 7 and converted into digital data, and thereafter a personal computer (hereinafter referred to as "personal computer"). (Omitted) is input to 8. For example, in the visible ultraviolet spectrophotometer, a sample to be measured is inserted in the optical path between the monochromator 2 and the photodetector 6, and in the atomic absorption spectrophotometer, the light source switching unit 1 and the monochromator 2 are connected. An atomization unit for atomizing the sample to be measured is disposed in the optical path between them, but these descriptions are omitted in FIG. The personal computer 8 functionally includes a control unit 9 for controlling the operation of each unit and an arithmetic processing unit 10 for performing various data processing. 14 and the display unit 15 are connected.
The storage area of the storage device 11 includes a conversion table data area 12 and a calibration formula data area 13. In the wavelength calibration process as described later, the arithmetic processing unit 10 calculates a calibration equation that can calibrate the wavelength shift based on the data obtained by the photodetector 6,
This is stored in the calibration formula data area 13. Further, the control unit 9 controls the light source switching unit 1 and the motor 4 via the input / output control unit 7 during the wavelength calibration process. The conversion table data area 12 of the storage device 11 contains the number of pulses given to the motor 4 and the monochromator 2.
A conversion table showing a correspondence relationship with the wavelength extracted from is stored in advance. This conversion table was calculated using an optical theoretical formula showing the relationship between the rotation angle and wavelength in the diffraction grating 3 and the design values of the motor 4 and the reduction gear mechanism 5.
It is a table | surface which shows the correspondence of the wavelength and pulse number in an ideal state. An example of this conversion table is shown in FIG. For example, when monochromatic light having a wavelength of 194.1 nm is desired to be extracted by the monochromator 2, if 4337 drive pulse signals are sent to the motor 4, the diffraction grating 3 rotates by a predetermined angle corresponding thereto. In the example of FIG. 3, all the pulse numbers are given in units of 0.1 nm which is the minimum wavelength step of this spectrophotometer. However, the number of pulses is given for each appropriate wavelength interval, and the number of pulses not in the conversion table. May be obtained by calculation from the number of adjacent pulses. Ideally, it should be possible to extract monochromatic light having an accurate wavelength using the data in the above conversion table, but in reality, such as the angular accuracy error of the reduction gear mechanism 5 as described above. Due to various error factors, even if the number of drive pulse signals given in the conversion table is sent to the motor 4, the set angle of the diffraction grating 3 is displaced. Therefore, when the apparatus is adjusted after assembling the monochromator 2 or after repair at the site of use, the wavelength calibration process is executed in the following procedure. FIG. 2 is a flowchart showing the wavelength calibration processing procedure. First, before assembling the monochromator 2, the operator measures an angle transmission error, that is, an angle accuracy error with respect to the feed angle of the motor 4 spindle (step S1). Such a measurement can be performed using a rotary encoder having sufficient accuracy to measure the angular accuracy error. As a result, for example, a graph as shown in FIG. 4 is created. Next, the operator reads the minimum cycle of the angle accuracy error based on the graph (step S2). Strictly speaking, it is desirable to obtain the smallest period among all the periodic components, but a periodic component having an amplitude that causes an angular accuracy error that falls within the allowable value of the wavelength accuracy error in the monochromator 2 is extracted. However, since there is no meaning, the minimum period is extracted by limiting to period components having an amplitude that causes an angle accuracy error exceeding the allowable value. For example, in the example of FIG. 4, A can be the minimum period. Subsequently, on the basis of a theoretical formula representing the relationship between the angle of the diffraction grating 3 and the wavelength, etc., it is half of the minimum period.
The calculation is performed to convert the angular interval of λ into the wavelength interval Λ. Then, a lamp having a plurality of emission line spectra at a wavelength interval smaller than the wavelength interval Λ is selected as a light source (step S3). The reason why bright lines having a wavelength interval equal to or smaller than the angle interval that is a half of the minimum period will be described later. In general, it is almost impossible to obtain such a plurality of emission lines with a single lamp. Therefore, there are a large number of emission lines that are easily available and have high intensity. Combinations with other lamps are useful. For example, by combining a deuterium lamp, a low-pressure mercury lamp and a neon lamp in a visible ultraviolet spectrophotometer, and by combining a deuterium lamp and a mercury holocathode lamp in an atomic absorption spectrophotometer. 20 to 30 bright lines can be obtained, which is often sufficient for such wavelength calibration. Of course, it is sufficient for this group of bright lines to satisfy the above conditions in the wavelength range used by the spectrophotometer, and deviates from the use wavelength range (for example, in many spectrophotometers, the wavelength is less than 180 nm). The bright line in the range is not necessary. Note that the measurement in step S1 needs to be performed by using a special jig called a precise rotary encoder, so that it is necessary to bother the operator. The measurement result of the accuracy error is input to the personal computer 8 through the operation unit 14 such as a keyboard, and is processed by predetermined software to automatically find the minimum cycle. Further, a large number of lamp candidates registered in advance are detected. It is also possible to automatically select one or a plurality of lamps that meet the above conditions and notify the operator. Subsequently, the lamp selected as described above is mounted on the light source switching unit 1 and the personal computer 8 is instructed to execute the wavelength calibration process via the operation unit 14. In response to this, the control unit 9 controls the light source switching unit 1 via the input / output control unit 7 and selects one of the selected lamps (for example, the lamp 1a).
Is turned on (step S4). Further, the control unit 9 converts the number of pulses corresponding to the wavelength range to the conversion table data area 12 in order to perform wavelength scanning within a predetermined range centered on the wavelength of a certain emission line spectrum included in the emitted light from the lamp 1a.
Are sequentially read out, and as many drive pulse signals as the number are sent to the motor 4. As a result, the diffraction grating 3 is rotated by a minute angle, and monochromatic light having a wavelength corresponding thereto is sequentially incident on the photodetector 6 (step S5). The arithmetic processing unit 10 sequentially receives the data obtained by the photodetector 6 through the input / output control unit 7 during the wavelength scanning period, and detects the peak of the bright line spectrum. For example, it is assumed that the spectrum shown in FIG. 5 is obtained by performing wavelength scanning in the range of ± Δλ1 with respect to the wavelength λ1 of the bright line (the number of pulses corresponding to this in the conversion table is c). At this time, in the ideal state, the peak top of the peak of the bright line should appear at the position of the pulse number c, but in the example of FIG. 5, the peak top appears at the position of the pulse number c + Δc. That is, the actual number of pulses corresponding to the wavelength λ1 of the bright line (that is, taking into account a mechanical error) is c + Δ
That is, there is a wavelength shift corresponding to the number of pulses Δc. Accordingly, the number of pulses c is associated with the wavelength λ1.
+ Δc (or pulse amount deviation amount Δc) is temporarily stored in the internal RAM (step S6). If data acquisition for a certain bright line is completed in this way, step S7 is performed.
From S4, the relationship between the wavelength and the number of pulses is stored in the internal RAM in the same manner for other bright lines having different wavelengths. Further, when there are a plurality of selected lamps, when the acquisition of data regarding all the bright lines necessary for calibration with respect to a certain lamp is completed, the process returns from step S7 to S4, and the control unit 9 switches to the light source switching unit 1 And the other lamps are turned on, and the relationship between the wavelength and the number of pulses is similarly stored in the internal RAM. When the acquisition of data relating to all necessary bright lines is completed (“Yes” in step S7), the pulse number data given in the conversion table is converted to the wavelength shift based on a large number of discrete data in the wavelength direction. A calibration formula for conversion to pulse number data for obtaining non-monochromatic light is calculated (step S8). Such a calibration formula is a general formula that represents a straight line or an approximate curve interpolating the above-mentioned numerous data, and the simplest calibration formula calculation method is to interpolate between adjacent data with a straight line. It is. Moreover, what is necessary is just to use a high order curve in order to obtain a more accurate calibration formula. The calibration formula calculated in this way is stored in the calibration formula data area 13 of the storage device 11 (step S9). Since the wavelength accuracy error with periodicity is almost dependent on the angle accuracy error of the reduction gear mechanism 5, the angle accuracy error is highly reproducible unless these mechanism parts are changed. The calibration formula can be used continuously. In addition, when it is considered that wavelength recalibration is necessary, such as after adjustment of the apparatus, a new calibration equation may be created according to the above procedure. When performing spectroscopic measurement with this spectrophotometer,
The conversion table data stored in the conversion table data area 12 and the calibration formula stored in the calibration formula data area 13 are used together to calculate the number of pulses corresponding to the desired wavelength. For example, when a desired wavelength scanning range is instructed, the arithmetic processing unit 10 reads the number of pulses corresponding to the minimum wavelength or the maximum wavelength within the wavelength range from the conversion table in the conversion table data area 12 and sets the calibration formula to the calibration formula. The number of pulses after calibration is obtained by reading from the data area 13 and substituting the number of pulses read from the conversion table into the calibration formula. The controller 9 sends a number of drive pulse signals corresponding to the number of pulses after calibration to the motor 4 to set the diffraction grating 3 at a predetermined angle. Next, the number of pulses corresponding to the minimum wavelength or the wavelength separated from the maximum wavelength by a wavelength step within the wavelength scanning range is read from the conversion table and calibrated, and the drive pulse signal corresponding to the number is sent to the motor 4 for diffraction. The grid 3 is further rotated. In this way, when the diffraction grating 3 is sequentially rotated by a minute angle, the monochromatic light whose wavelength shift is corrected can be sequentially extracted from the monochromator 2. Next, referring to FIG. 6, the reason why a bright line having a wavelength equal to or smaller than a wavelength interval corresponding to an angular interval that is a half of the minimum period of the angle accuracy error is used when performing the wavelength calibration process. I will explain. FIG. 6 is a conceptual diagram showing the relationship between the calibration data selection method and the calibration effect when the transmission error has a periodic variation centered on 0 minutes. First, let us consider a case where the above-described calibration equation is calculated by performing linear interpolation using only two points of data obtained based on emission lines of two wavelengths. Assuming that the wavelength of the bright line corresponds to the feed angles 0 ° and 10.5 ° shown in FIG. 6A, the straight line for interpolating the data at two points is L1. If it is determined that the straight line L1 is a calibration line corresponding to the calibration formula, for example, the feed angle 19.5
The transmission error after calibration in degrees is the difference B1 between the value on the straight line L1 and the actual transmission error. In other words, if such a calibration line is used, the transmission error may be larger than the original transmission error D due to calibration. Next, consider a case where a calibration equation is calculated by performing linear interpolation using data obtained based on a plurality of bright lines having the same wavelength interval as the minimum period. Assuming that the feed angle corresponding to the wavelength of the bright line coincides with the feed angle that causes the minimum peak of the periodic fluctuation of the transmission error as shown in FIG. 6B, the polygonal line for interpolating each data is L2. It becomes like this. If it is determined that the broken line L2 is a calibration line corresponding to the calibration formula, the transmission error after calibration at the feed angle of 19.5 degrees is B2. That is, even in this case, the transmission error may be larger than the original transmission error D by calibration for a specific wavelength. Subsequently, a calibration equation is calculated by performing linear interpolation using data obtained based on a plurality of bright lines having a wavelength interval corresponding to an angular interval that is a half of the minimum period. Think about the case. In this case, the worst case is shown in FIG.
As shown in (c), the feed angle corresponding to the wavelength of the bright line is 3
In the case of degrees, 6 degrees, 9 degrees,..., The straight line L3 interpolating each data coincides with a transmission error of 0 minutes. This means that the error cannot be improved even by calibration. In other words, the error does not increase by calibration even in the worst case. In practice, since it is rare that this worst condition is reached, the wavelength accuracy error is significantly improved in almost all cases. Thus, the minimum cycle of angle accuracy error is 2
If a measured data point is obtained with an angular interval of 1 or less and wavelength calibration is performed using a calibration formula calculated using this data point, the motor 4 is driven based on a theoretically derived conversion table. The wavelength accuracy error can be surely reduced as compared with the case of doing so. It should be noted that the above embodiment is merely an example, and it is obvious that modifications and corrections can be made as appropriate within the scope of the present invention. For example, in the above embodiment, the calibration formula is used to convert the number of pulses. However, a new conversion table showing the correspondence between the calibrated number of pulses and the wavelength is created, and this conversion table is stored in the storage device. It may be stored and used for spectroscopic measurement.
Moreover, it is good also as a structure which is not equipped with the light source switching part 1, and an operator replaces | exchanges a lamp himself. As described above, according to the wavelength calibration method of the spectrophotometer according to the present invention, even if there is an angle accuracy error in the rotation drive mechanism, the error is eliminated by the calibration process. Monochromatic light with high wavelength accuracy can be extracted. An inexpensive mechanism component having a relatively large angle accuracy error can be used. Further, it is not necessary to increase the size of the whole in order to increase the accuracy of the mechanical parts. Therefore, the cost and size of the spectrophotometer can be reduced. Further, since it is not necessary to use a special lamp for wavelength calibration, calibration can be easily performed at the site where the spectrophotometer is used.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration diagram of a main part of a spectrophotometer adopting a wavelength calibration method according to an embodiment of the present invention. FIG. 2 is a flowchart of wavelength calibration processing procedures. FIG. 3 is a diagram showing an example of a conversion table showing the relationship between the wavelength and the number of pulses. FIG. 4 is a graph showing a relationship between a feed angle of a rotational drive mechanism and a transmission error. FIG. 5 is an explanatory diagram when measuring a wavelength error. FIG. 6 is a conceptual diagram showing a relationship between how to select calibration data and a calibration effect when a transmission error has a periodic variation. DESCRIPTION OF SYMBOLS 1 ... Light source switching part 1a ... Lamp 2 ... Monochromator 3 ... Diffraction grating 4 ... Stepping motor 5 ... Reduction gear mechanism 6 ... Photo detector 7 ... Input / output control part 8 ... Personal computer 9 ... Control part 10 ... Arithmetic processing unit 11 ... storage device 12 ... conversion table data area 13 ... calibration formula data area

─────────────────────────────────────────────────── ─── Continued on the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01J 3/06 G01J 3/18 G01J 3/26 G01D 3/00-4/18 G01N 21/00-21 / 01 G01N 21/17-21/64

Claims (1)

(57) [Claims] [Claim 1] Monochromatic light extracted by a rotational drive mechanism including a motor, the angle transmission error of which does not show a uniform increase or decrease with respect to the feed angle of the motor rotation shaft A wavelength calibration method for a spectrophotometer equipped with a monochromator for setting the wavelength of a), wherein a) the angle accuracy error of the rotational drive mechanism is measured in advance, and b) a single or a plurality of light sources and analyzed. Using a light source unit that emits an emission line group having a wavelength interval equal to or less than a wavelength interval corresponding to an angular interval that is a half of the minimum period of the angular accuracy error over a target wavelength range, and c) the light source A control instruction value given to the rotational drive mechanism to obtain each emission line emitted from the monochromator as monochromatic light from a monochromator, corresponding to the true wavelength of the emission line, and d) the wavelength and the control instruction value Based on the correspondence with Create a data wavelength calibration representing the calibration equation for obtaining a control instruction value corresponding to the length and stored it in the storage unit, the wavelength calibration method of the spectrophotometer, characterized in that.