JP4258102B2 - Atomic absorption spectrophotometer - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an atomic absorption spectrophotometer.
[0002]
[Prior art]
In atomic absorption spectroscopic analysis, a component to be measured in a sample is atomized, and the atomic vapor is irradiated with light having one or more emission line spectra from a light source such as a holocathode lamp. At this time, since it is absorbed at a wavelength specific to the atoms constituting the sample, the transmitted light is wavelength-dispersed by a spectrometer, and light having a wavelength specific to the target atom (or element) is selected and detected. Introduced into a vessel (mainly photomultiplier tube). By measuring the difference in light intensity depending on the presence or absence of the sample, the absorbance due to the atomized component to be measured can be obtained.
[0003]
In an atomic absorption spectrophotometer, a light source having an emission line spectrum with a wavelength corresponding to a target element is usually used. Since there may be an error between the wavelength indicated on the operation panel or the like and the wavelength actually selected by the spectroscope, the target bright line is generally adjusted by an adjustment method called line search, for example. The spectroscope is adjusted to match the peak of the spectrum. More specifically, while controlling the spectroscope so as to scan the wavelength in a predetermined wavelength range, a position where the signal intensity from the photodetector is maximized is found, and the spectroscope is fixed at that position. This determines the wavelength. Next, the sensitivity of the photodetector is adjusted so that the signal intensity from the photodetector becomes a predetermined value. Thus, the optimum sensitivity is set such that the detection sensitivity is as high as possible within a range in which the amplifier or the like is not saturated even when the light intensity increases due to frame (flame) light or the like.
[0004]
[Problems to be solved by the invention]
The above adjustment is always performed prior to measurement when the light source is changed to change the target element. However, during the sequential measurement of a plurality of samples after making the above adjustments,
(1) Change in emission intensity of the light source (2) Mechanical fluctuation of the spectroscope due to heat generated by the apparatus itself or heat of the frame (positional deviation of each optical element)
May cause a deviation from the initial optimum state.
[0005]
FIG. 6 is an example of an emission line spectrum. For example, as shown in FIG. 6, in the optimum state, even if the wavelength corresponding to the peak top of the emission line spectrum is selected by the spectroscope, the signal intensity at the peak top changes vertically when the emission intensity of the light source fluctuates. Fluctuates in the axial direction. On the other hand, when the selected wavelength shifts due to mechanical fluctuations of the spectroscope, the wavelength of the monochromatic light introduced into the photodetector shifts in the horizontal axis direction with respect to the peak top. Due to such fluctuation, the signal intensity to be used as a reference fluctuates, and there is a risk that the accuracy of measurement may deteriorate.
[0006]
The present invention has been made in order to solve such problems, and the object of the present invention is to eliminate the effects of fluctuations in the light emission intensity of the light source and mechanical displacement of the spectrometer, and to achieve high accuracy. The object is to provide an atomic absorption spectrophotometer capable of measuring.
[0007]
[Means for Solving the Problems]
In order to solve the above problems, the present invention allows light emitted from a light source to pass through an atomized sample, disperses the light that has passed through a spectrometer, and selects light having a wavelength according to the target component. In the atomic absorption spectrophotometer introduced into the photodetector,
a) a fluctuation detecting means for detecting a signal intensity obtained by the photodetector or a temporal change of a signal value corresponding to the signal intensity at a measurement pause between repeated measurements;
b) wavelength scanning means for controlling the spectrometer to perform wavelength scanning in a predetermined wavelength range with respect to the set wavelength at that time when the variation is detected;
c) wavelength correcting means for resetting the spectroscope to correct the shift of the selected wavelength by the spectroscope based on the signal intensity obtained by the photodetector during the wavelength scanning;
It is characterized by having.
[0008]
Here, the measurement suspension means a state in which light emitted from the light source is not absorbed by the atomic sample and is not affected by light emission other than the light source such as a frame.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Usually, at the initial stage of repeated measurement, the selected wavelength of the spectroscope is set so that light having a peak top wavelength of a predetermined emission line spectrum included in the light source enters the photodetector. Therefore, when the selected wavelength is shifted in the spectroscope, the signal intensity obtained by the photodetector during the measurement pause is reduced. Therefore, when the fluctuation detection unit detects a fluctuation of a predetermined value or more with respect to the initial value, for example, the wavelength scanning unit controls the spectrometer to perform wavelength scanning in a predetermined wavelength range. The wavelength scanning range at this time should just be more than the assumed wavelength shift width. The signal intensity obtained by the photodetector changes corresponding to this wavelength scanning, and it can be estimated that the point where the signal intensity is maximum is the peak top of the original emission line spectrum. Control the spectrometer. As a result, the shift of the selected wavelength of the spectroscope is corrected, and light having the peak top wavelength of the desired emission line spectrum enters the photodetector.
[0010]
Therefore, according to the atomic absorption spectrophotometer according to the present invention, the spectroscope is adjusted so that the measurement is automatically performed in almost the best state without being aware of the operator. This eliminates the influence of the shift of the selected wavelength, and can always perform highly accurate measurement.
[0011]
More preferably, in addition to the correction of the wavelength shift as described above, the light emission intensity fluctuation of the light source can be corrected. That is, even if the wavelength shift does not occur, if the light emission intensity of the light source fluctuates, the signal intensity obtained by the photodetector during the measurement pause is reduced or increased. Therefore, when a fluctuation greater than a predetermined value is detected by the fluctuation detection means, after trying to correct the wavelength shift as described above, the sensitivity adjustment means causes the signal intensity obtained by the photodetector to be a predetermined value. Adjust the sensitivity of the photodetector. According to this, not only the wavelength shift of the spectroscope but also the influence of the fluctuation of the light emission intensity of the light source is corrected, so that more accurate measurement can be performed.
[0012]
As another method, after trying to correct the wavelength shift as described above, a value corresponding to the signal intensity obtained by the photodetector at that time, or a value corresponding to the signal intensity shift amount is set. The same object can be achieved by storing the signal and performing signal processing to correct the absorbance according to the stored value.
[0013]
【Example】
Hereinafter, an atomic absorption spectrophotometer according to an embodiment of the present invention will be described with reference to the drawings.
[0014]
FIG. 1 is an overall configuration diagram of an atomic absorption spectrophotometer according to the present embodiment. In the atomic absorption spectrophotometer, the atomization unit 2 introduces and sprays the sample solution sent from the sample supply unit 3 into the frame formed by the burner, whereby the component to be measured in the sample solution is atomized. It becomes. The light from the light source 1 such as a holocathode lamp is applied to the atomization unit 2 and passes through the atomized vapor of the component to be measured. The light that has passed through the atomic vapor is dispersed by the spectroscope 4 and light having a specific wavelength corresponding to the measurement target component is extracted. The light having the specific wavelength is introduced into the photodetector 5, and a detection signal corresponding to the amount of incident light is extracted and input to the signal processing unit 7. The signal processing unit 7 includes a current amplifier, an A / D converter, a CPU, a memory, and the like, and creates a time change graph of spectrum and absorbance by performing predetermined calculation processing after converting the detection signal into a digital signal. Or perform quantitative analysis based on this graph.
[0015]
The photodetector 5 is a photomultiplier tube including a plurality of stages of dynodes. The incident photoelectrons are brought into contact with the dynodes to emit secondary electrons, and the secondary electrons are introduced into the next stage dynodes. In this way, the electrons are multiplied one after another, and the finally multiplied electrons are detected at the anode. Since the multiplication factor of electrons changes according to the voltage applied from the voltage application unit 6 to the dynode, the detection sensitivity of the photodetector 5 depends on the applied voltage.
[0016]
The control unit 8 includes a CPU, a memory, and the like. The CPU operates based on a predetermined program stored in the memory, and the light source 1, the atomization unit 2, the sample supply unit 3, the spectrometer 4, and the voltage application unit. 6 is controlled. Further, the operator can give an instruction regarding the analysis through the operation unit 9 attached to the control unit 8, and further, the analysis condition and the analysis result can be visually confirmed through the display unit 10. In addition, the calculating part and the control part 8 of the signal processing part 7 can be embodied using, for example, the same personal computer.
[0017]
In the above apparatus, a line search is executed in accordance with an instruction from an operator prior to actual measurement. That is, when the operator inputs various conditions such as the wavelength of the target element and the lamp current of the light source 1 from the operation unit 9 and instructs execution of the line search, the control unit 8 turns on the light source 1 in response to this, The spectroscope 4 is controlled to scan a predetermined range before and after the wavelength. Based on the detection signal from the photodetector 5 at that time, the signal processing unit 7 creates a kind of wavelength spectrum called a line search signal as shown in FIG. The control unit 8 sets the spectrometer 4 so that the wavelength that gives the highest signal intensity on the line search signal is selected. Thereby, the spectroscope 4 is set at the optimum position, and light having a wavelength corresponding to the peak top is introduced into the photodetector 5. Further, thereafter, the voltage application unit 6 is controlled to adjust the detection sensitivity of the photodetector 5 so that the peak top signal intensity becomes a predetermined value. Thereby, the photodetector 5 is also set to the optimum sensitivity.
[0018]
In the subsequent measurements, the value corresponding to the signal intensity at the peak top is set as the reference zero level in the time change graph of absorbance, and the absorbance due to atomic vapor is calculated. Here, the signal intensity at the reference zero level is obtained by correcting background absorption and the like. Thus, when a certain sample is measured, for example, as shown in FIG. 5 (a), a graph is obtained in which time is plotted on the horizontal axis and absorbance is plotted on the vertical axis.
[0019]
By the adjustment operation as described above, measurement can be performed in an almost optimum state. However, even if the optimum state is set once, if the light emission intensity of the light source 1 changes with time, the peak top signal intensity changes from S0 to S2, for example, as shown in FIG. Further, when mechanical fluctuations occur in the spectroscope 4 due to the temperature or the like, for example, as shown in FIG. 2A, the center wavelength selected by the spectroscope 4 changes from λ0 to λ1. In either case, the state is shifted from the optimum state. Therefore, in this apparatus, automatic adjustment is performed in the following procedure.
[0020]
FIG. 3 is a flowchart showing this procedure. First, the control unit 8 determines whether or not the measurement is being performed at that time (step S1), and if the measurement is being performed, the process ends. When measurement is not in progress, that is, when there is no absorption due to atomic vapor and no frame is formed, the control unit 8 determines whether the zero level fluctuates by a predetermined value or more (step S2). In the case as shown in FIG. 2 (a), the signal intensity S1 is lower than the initial signal intensity S0, so that it is apparently equivalent to receiving light absorption, and in FIG. 5 (b). As shown by the bold solid line, the zero level on the graph rises. On the contrary, in the case shown in FIG. 2B, the signal intensity S2 is higher than the initial signal intensity S0, so that the zero level decreases as shown by a thick dotted line in FIG. 5B. In other words, it becomes negative. In any case, if such zero level fluctuation is greater than or equal to a predetermined value, readjustment is executed (step S3).
[0021]
In the readjustment, first, the control unit 8 controls the spectrometer 4 to perform wavelength scanning by setting a predetermined wavelength range with respect to the wavelength setting position at that time (step S4). For example, as shown in FIG. 4, scanning is performed in the range of λ2 to λ3 with λ1 as the center wavelength. Usually, the wavelength shift is not so large, so the wavelength range at this time can be much smaller than the wavelength scanning range at the time of the first line search. Therefore, the time required for wavelength scanning can be short. Based on the detection signal obtained from the photodetector 5 during wavelength scanning, the signal processing unit 7 creates a curve corresponding to the line search signal as shown in FIG. The controller 8 resets the spectroscope 4 so that the wavelength that gives the highest signal intensity on this curve is selected (step S5). Thereby, the spectroscope 4 is set again at the optimum position, and light having a wavelength corresponding to the peak top is introduced into the photodetector 5. Further, the control unit 8 controls the voltage application unit 6 to readjust the detection sensitivity of the photodetector 5 so that the peak top signal intensity becomes a predetermined value (step S6). Thereby, the photodetector 5 is also set to the optimum sensitivity again.
[0022]
If the processes in steps S1 to S6 are always repeated, the measurement can be performed immediately before the adjustment to the optimum state. In addition, a button for instructing such readjustment may be prepared in the operation unit 9 and the processing of steps S2 to S6 may be executed when the button is operated.
[0023]
Next, another embodiment of the present invention will be described. In the above embodiment, after resetting the spectrometer 4 in step S5, the detection sensitivity of the photodetector 5 is also reset in step S6. Since the measurement cannot be started during the series of adjustment operations, the measurement interval may be limited by the adjustment time. Usually, it takes more time to find the optimum value of detection sensitivity than the time required for wavelength scanning. Therefore, in this embodiment, instead of resetting the detection sensitivity as described above, the influence of the fluctuation of the reference signal intensity is eliminated by signal processing.
[0024]
That is, after resetting the spectrometer 4 in step S5, the signal processing unit 7 calculates the difference between the zero level at that time or the zero level at that time and the zero level at the time of the first line search. Store in memory. For example, if the zero level is shifted as shown by the thick dotted line in FIG. 5B due to an increase in the light emission intensity of the light source 1, the zero level is shifted by Δz from the time of the first line search at time t1. ing. Therefore, this difference Δz is stored in the memory, and in the subsequent measurements, the absorbance is offset by this difference Δz when calculating the absorbance. Thereby, it is possible to eliminate the influence of the fluctuation of the light emission intensity of the light source 1 on the graph of the absorbance change with time without resetting the detection sensitivity.
[0025]
The above-described embodiment is merely an example, and it is apparent that changes and modifications can be made as appropriate within the scope of the present invention.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram of an atomic absorption spectrophotometer according to an embodiment of the present invention.
FIG. 2 is a diagram showing an example of a spectrum when there is a variation in light emission intensity of a light source and a wavelength shift of a spectroscope.
FIG. 3 is a flowchart showing an adjustment procedure according to the present embodiment.
FIG. 4 is a diagram showing an example of a spectrum obtained during wavelength scanning in adjustment according to the present embodiment.
FIG. 5 is a graph showing an example of a change in absorbance with time.
FIG. 6 is a diagram showing an example of a spectrum obtained during a line search before measurement.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Light source 2 ... Atomization part 3 ... Sample supply part 4 ... Spectroscope 5 ... Photodetector (photomultiplier tube)
6 ... Voltage application unit 7 ... Signal processing unit 8 ... Control unit 9 ... Operation unit 10 ... Display unit

Claims (1)

In an atomic absorption spectrophotometer that allows light emitted from a light source to pass through an atomized sample, disperses the wavelength of the light that has passed through a spectrometer, and selects light having a wavelength according to the target component and introduces it to a photodetector. ,
a) a fluctuation detecting means for detecting a signal intensity obtained by the photodetector or a temporal change of a signal value corresponding to the signal intensity at a measurement pause between repeated measurements;
b) wavelength scanning means for controlling the spectrometer to perform wavelength scanning in a predetermined wavelength range with respect to the set wavelength at that time when the variation is detected;
c) wavelength correcting means for resetting the spectroscope to correct the shift of the selected wavelength by the spectroscope based on the signal intensity obtained by the photodetector during the wavelength scanning;
An atomic absorption spectrophotometer comprising:

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