JP2007010314A - Flame atomic absorption spectrophotometer - Google Patents

Abstract

To improve S / N without saturating a detector signal.
Samples are not introduced into the burner 3, and photomultiplier multiplication is performed in all combinations that can be set for the C ₂ H ₂ flow rate, the flame type, the slit width of the slit in the spectrometer 6 and the wavelength to be dispersed. The signal voltage detected by the tube 8 and input to the A / D converter 11 is stored in the memory 15. At the time of sample measurement, the amplification factor of the photomultiplier tube 8 is adjusted by the negative high voltage regulator 18 according to each measurement condition, or the amplification factor of the detector signal in the amplifier 10 is adjusted to obtain an optimum detector signal voltage. Set and measure. Thereby, even when the signal voltage of the detector increases due to the influence of the radiation light from the flame F on the burner 3, the detector signal voltage does not saturate and measurement can be performed with good S / N. It becomes possible.
[Selection] Figure 1

Description

  The present invention relates to an atomic absorption spectrophotometer that performs quantitative analysis of metal elements contained in various substances, and more particularly to a flame atomic absorption spectrophotometer.

  Depending on the number of atoms (ground state) in the atomic vapor, the sample to be measured is atomized by some method, a vapor layer of atoms is generated in the space, and light of an appropriate wavelength (excitation wavelength) is transmitted. Absorption occurs. An atomic absorption spectrophotometer is a device for obtaining a sample concentration from this absorbance.

  Light emitted from a holocathode lamp that is a light source is dispersed by a spectroscope and reaches a detector. The signal voltage generated by the detector depends on the element type of the holocathode lamp, the characteristics of the spectrometer, and the spectral sensitivity characteristics of the detector.

In a flame atomic absorption spectrophotometer, an atomized liquid sample is mixed into a mixed gas of a fuel gas (acetylene (C ₂ H ₂ )) and an auxiliary combustion gas (air (AIR) or dinitrogen monoxide (N ₂ O)). The sample is further mixed and burned, and heated to a high temperature to atomize the sample. The absorbance is measured by passing measurement light through the flame containing the atoms. During the measurement, the flame itself or the sample itself emits light, so this emitted light also reaches the detector.

  If the signal from the detector containing such radiated light is too large, it may be saturated in the analog circuit or exceed the maximum voltage that can be converted by the A / D converter. Since correct measurement cannot be performed if the maximum voltage is exceeded, the detection signal amplification factor is adjusted in advance before measurement of the sample to set the optimum detection signal voltage (for example, Patent Document 1).

  Usually, this detection signal voltage is set to 50 to 70% of the maximum voltage that can be input to the A / D converter used in the atomic absorption spectrophotometer. The closer to 100%, the better the signal-to-noise ratio (S / N), but considering the noise of analog circuits used in the atomic absorption spectrophotometer and the quantization noise of the A / D converter, radiation from the flame This is because it is necessary to provide a margin to prevent saturation of the detection signal voltage due to light.

Patent Document 2 has means for storing detector signal voltages for combinations of all wavelengths that can be set, slit widths, and atomization temperatures in an atomization section of a furnace type atomic absorption spectrophotometer. There is disclosed an apparatus having an amplification factor control means for controlling an amplifier with an optimum detector signal amplification factor based on a signal voltage stored for a wavelength, slit width, and atomization temperature used for measurement.
JP 2002-71558 A JP 2004-325341 A

As measured by atomic absorption spectrophotometer, radiation from it is flame _{AIR-C 2 H} 2 from the _{N 2 O-C} 2 _H 2 becomes stronger and the higher wavelength is long, as the slit width is wide, The larger the C ₂ H ₂ gas flow rate, the stronger the emitted light from the flame. Therefore, depending on the measurement conditions (wavelength, slit width, flame type, gas flow rate), the signal voltage of the detector may exceed the maximum measurable voltage and accurate measurement may not be possible. In such a state, the operator manually copes with narrowing the slit width. However, if the slit width is narrowed by adjustment, the amount of light incident on the detector decreases, and the S / N deteriorates. That is, with the conventional method, it is not possible to flexibly cope with fluctuations of the radiated light intensity due to the measurement conditions only by giving the radiated light from the flame a certain margin within the range of the normal measurement conditions.

  An object of the present invention made in view of the above problems is to provide a flame atomic absorption spectrophotometer that can improve the S / N at low cost without saturating the detector signal. That is, a light source, an atomization unit that atomizes a sample by a flame in which a mixed gas of fuel gas and auxiliary gas is burned, a spectroscopic unit that extracts a specific wavelength from light from the light source that has passed through the flame, and A detector that detects light from the spectroscopic unit, a signal amplifier that amplifies the signal from the detector, an A / D converter that digitally converts the signal from the amplifier, and a drive voltage of the detector are adjusted. A frame-type atomic absorption spectrophotometer having a voltage adjustment unit, comprising storage means for storing an optimum value of a signal voltage to the A / D converter corresponding to a combination of measurement conditions that can be set simultaneously; The voltage adjusting unit or the signal amplifier is adjusted so that an optimum value of the corresponding signal voltage is input to the A / D converter when a condition is set.

  Further, the signal voltage is set to 50 to 70% of a maximum voltage that can be input to the A / D converter.

  A flame type atomic absorption spectrophotometer according to the present invention made from a second aspect includes a light source, an atomization unit for atomizing a sample by a flame in which a mixed gas of a fuel gas and an auxiliary combustion gas is burned, and the light source. A spectroscopic unit for extracting a specific wavelength from the light of the sample, a sample optical path for guiding the light from the light source through the flame and guiding it to the spectroscopic unit, and a reference optical path for guiding the light to the spectroscopic unit without transmitting the flame An optical system that alternately introduces light from these two optical paths into the spectroscopic unit, a detector that detects light from the spectroscopic unit, a signal amplifier that amplifies the signal from the detector, and In a frame-type atomic absorption spectrophotometer having an A / D converter for digitally converting a signal from an amplifier and a voltage adjusting unit for adjusting a driving voltage of the detector, it corresponds to a combination of measurement conditions that can be set simultaneously. Above Storage means for storing the optimum value of the signal voltage to the A / D converter based on the light from the light path, and when the measurement condition is set, the corresponding optimum value of the signal voltage is the A / D The voltage adjustment unit is adjusted to be input to the D converter, and when the measurement is started, the optimum value of the signal voltage of the detector based on the light from the sample optical path is supplied to the A / D converter. The voltage adjustment unit or the signal amplifier is adjusted so as to be 90 to 100% of the maximum voltage that can be input.

  According to the present invention, A / C corresponding to all combinations of wavelength, slit width, flame type, and gas flow rate that can be set in advance with respect to measurement conditions that affect the magnitude of the signal voltage at the detector before measuring the sample. Since the signal voltage to the D converter is measured and this data is stored in the memory, when measuring the sample, the signal voltage stored in the memory is read in accordance with the measurement conditions and the detector signal is amplified. The rate can be adjusted to optimally set the detector signal voltage. This makes it possible to improve the S / N without saturating the detector signal. When any one (or a plurality) of measurement conditions is changed, the amplification factor is adjusted to the optimum, and the sample can always be measured with the highest S / N. If a person skilled in the operation of the apparatus performs the association of the measurement conditions, the sample can be measured with a good S / N ratio no matter who performs the measurement thereafter.

  In order to realize the function of the present invention, it is possible to cope with only the change of software for controlling the conventional apparatus of this type. Accordingly, there is no significant increase in cost and size of the existing apparatus.

  Embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram of an embodiment of an atomic absorption spectrophotometer according to the present invention. The atomic absorption spectrophotometer of the present invention includes a holocathode lamp 1, a burner 3, an atomizer 4, a spectrometer 6, a photomultiplier tube 8, an amplifier 10, an A / D converter 11, The processing / control unit 13, the memory 15, the operation unit 16, the display unit 17, the negative high pressure regulator 18, and the gas flow rate control unit 19 (19 a, 19 b) are included. A flame F is formed in the upper part of the burner 3 by the combustion of the mixed gas of fuel gas and auxiliary combustion gas. In many cases, atomic absorption spectrophotometers use a photomultiplier tube (PMT) as a detector. Since the amplification factor depends on the voltage (negative high voltage in the present application) applied to the PMT, the amplification factor can be easily adjusted by adjusting the voltage. Therefore, a negative high voltage regulator is used as the amplification factor control means.

The sample including the measurement object is atomized by the atomizer 4. On the other hand, the flow rate of fuel gas (C ₂ H ₂ ) is adjusted by the gas flow rate control unit 19 a, and the auxiliary combustion gas is adjusted by the gas flow rate control unit 19 b, mixed with the atomized sample, and the flame F formed by the burner 3. Is heated and atomized. Whether to select AIR or N ₂ O as the auxiliary combustion gas is switched by a three-way valve (not shown) to select an auxiliary combustion gas source. By switching the three-way valve, the gas flow rate control unit 19b is selected. Is configured such that AIR or N ₂ O used as an auxiliary combustion gas flows in, and it is possible to set which one is selected by an input from the operation unit 16.

  The light including the emission line spectrum emitted from the holocathode lamp 1 passes through the upper part of the burner 3 (flame F) and is introduced into the spectrometer 6. The introduced light passes through the entrance slit in the spectroscope 6, is reflected by the reflecting mirror, is split into a predetermined wavelength by the diffraction grating, passes through the exit slit, and the light having the wavelength to be measured reaches the photomultiplier tube 8. The spectroscope 6 is a Zernitana spectroscope, and includes an entrance slit, a reflecting mirror, a diffraction grating, and an exit slit. Although not shown for convenience, appropriate condensing optical systems are arranged between the holocathode lamp 1 and the burner 3 and between the burner 3 and the spectroscope 6, respectively. The light is introduced into the next stage.

  The light that has reached the photomultiplier tube 8 is photoelectrically converted and the electric signal obtained by the photoelectric conversion is amplified by the amplifier 10, converted to a digital signal by the A / D converter 11, and input to the processing / control unit 13. As described above, the light passing through the flame F at the upper part of the burner 3 is strongly absorbed by light having a wavelength specific to the element contained in the sample. The processing / control unit 13 calculates the ratio between the received light intensity when not receiving such absorption and the received light intensity when receiving absorption, obtains the absorbance, and quantifies the specific component in the sample. The processing / control unit 13 is mainly configured by a computer including a CPU and the like, and performs various arithmetic processes and outputs a control signal for controlling the operation of each unit. The processing / control unit 13 includes a memory 15 for storing analysis conditions and settings necessary for control, and an operation for inputting parameters for the operator of the apparatus to set measurement conditions and adjust the apparatus. A display unit 17 for displaying the unit 16 and the state of the apparatus, analysis results, and the like is connected. The negative high voltage regulator 18 sets the voltage supplied to the photomultiplier tube 8 according to the setting of the processing / control unit 13 and adjusts the amplification factor.

  The signal from the photomultiplier tube 8 is amplified by the amplifier 10. The amplified signal is input to the A / D converter 11 at the subsequent stage and converted into a digital signal. However, the A / D converter 11 has a predetermined resolution (for example, 16 bits for 0 to 10 V), converts the analog input signal range into a digital signal according to the resolution, and outputs the digital signal. For the input signal, only the lower limit value or the upper limit value can be output. In the vicinity of the upper limit value, the upper limit value is exceeded when strong light is generated during actual measurement. Therefore, at the time of sample measurement, the gain of the photomultiplier tube 8 is adjusted by the negative / high pressure regulator 18 according to each measurement condition or The intensity of the signal input to the A / D converter 11 is adjusted by adjusting the amplification factor of the detector signal in the amplifier 10. Therefore, the signal voltage input to the A / D converter 11 corresponding to the combination of measurement conditions that can be set is measured in advance for the measurement conditions that affect the magnitude of the signal voltage in the detector 8 before measuring the sample. If stored, the relationship of the voltage signal input to the A / D converter 11 corresponding to each measurement condition can be obtained. This relationship is stored in the memory 15 in the atomic absorption spectrophotometer.

A method for creating the relationship between the measurement conditions and the optimum value of the signal voltage to the A / D converter 11 will be described. The sample is not introduced into the burner 3, for example, the flow rate of C ₂ H ₂ is 2.0 L / min, the flame type is AIR-C ₂ H ₂ , the slit width is 0.2 nm, and the wavelength to be dispersed by the diffraction grating is 185 nm. The value of 50 to 70% of the signal voltage detected by the photomultiplier tube 8 under this measurement condition is stored in the memory 15. The reason why the value stored in the memory 15 is set to 50 to 70% of the actual measurement result is to allow for various fluctuations such as the light emission of the sample itself. Similarly, a sample is not introduced into the burner 3, and the value of 50 to 70% of the signal voltage to the corresponding A / D converter 11 is set for all combinations of wavelength, slit width, flame type and gas flow rate. Store in the memory 15. The input voltage to the A / D converter 11 in FIG. 2 is classified according to the wavelength, slit width, flame type, and C ₂ H ₂ flow rate. FIG. 2 is an example of the optimum detection signal voltage corresponding to the combination of each wavelength, slit width, flame type, and C ₂ H ₂ flow rate in the present embodiment.

  Since the measurement conditions are clear when measuring the sample, the relationship between the measurement conditions, the measurement conditions stored in the memory 15 and the signal voltage to the A / D converter 11 is applied to the corresponding A / D converter 11. Measurement is started after adjusting the amplification factor of the signal of the detector so that the signal voltage becomes an optimum value and setting the signal voltage of the above-mentioned optimum detector. This makes it possible to improve the S / N without saturating the detector signal. When any one (or a plurality) of measurement conditions is changed, the amplification factor is adjusted to the optimum, and the sample can always be measured with the highest S / N. If the expert regarding the operation of the apparatus performs the association of the measurement conditions as shown in FIG. 2, thereafter, the sample measurement can be performed with a good S / N ratio regardless of who performs the measurement.

  The correspondence between the measurement conditions shown in FIG. 2 and the signal voltage to the A / D converter 11 is created assuming alkali metal measurement. When measuring alkali metals, the light emitted by the sample itself is stronger than that measured by other elements, and the absorption by the alkali metal is 588.995 nm for sodium and potassium. There are characteristic absorptions at 766.491 nm, lithium at 570.784 nm, and cesium at 852.110 nm, and the wavelength for measuring any element is 550 nm or more. Measuring in a wavelength region of 550 nm or more means measuring an alkali metal. Therefore, if the wavelength to be measured is set to 550 nm or more, light emission from the sample itself is expected to be strong. Therefore, the signal voltage to the A / D converter 11 should be set low even when the measurement conditions are the same. ing.

  This setting is stored in the memory 15, and at the time of sample measurement, the amplification factor of the photomultiplier tube 8 is adjusted by the negative high voltage regulator 18 according to each measurement condition, or the signal of the detector in the amplifier 10 is adjusted. The measurement is performed after the amplification factor is adjusted and set to the optimum signal voltage of the A / D converter 11 of FIG. Thereby, even when the signal voltage of the detector increases due to the influence of the radiation light from the flame on the burner 3, the signal voltage to the A / D converter 11 is not saturated, and a good S / N ratio is obtained. It becomes possible to measure with.

  Further, in the case of a frame type double beam atomic absorption spectrophotometer, the signal voltage to the A / D converter 11 that is actually output is measured after the above-described means, and the signal voltage is output to the A / D converter 11. It is possible to measure the sample with better S / N by adjusting the gain again so that the signal voltage becomes the optimum voltage.

In the case of the double beam method, as shown in FIG. 3, a sample optical path that transmits the flame F and a reference optical path that does not transmit the flame F are configured by the beam splitter BS and mirrors M1 and M2, and which light enters the spectrometer 6. The sector mirror SM is selected to control. In the case of the single beam method, light is incident from the light source 1 on the flame not including the sample, and the incident intensity (I _R ) of the light in the detector 8 and light is incident from the light source 1 on the flame including the sample for detection. I mean that determine the absorbance from the ratio of the incident light intensity (I _S) in the vessel 8, after measuring the I _R, before measuring the I _S is not possible to change the amplification factor of the detector signals . On the other hand, in the double-beam method, since measuring the I _R which also serves as a reference while the sample is being heated by the flame F, Changing the amplification factor of the detector signals, for both I _S and I _R The amplification factor will be changed equally. In the single beam method, the optimum value of the signal voltage to the A / D converter 11 is 50 to 70% of the signal voltage that can be input to the A / D converter 11 in consideration of various fluctuation margins such as light emission of the sample itself. However, in the case of the double beam system, this margin is almost unnecessary when the flame F is ignited and the sample is introduced and the detector voltage is increased by the radiation of the flame itself and the sample itself. Therefore, in the double beam method, if the amplification signal is adjusted so that the optimum detection signal voltage is 90 to 100% of the input to the A / D converter 11, the sample can be measured with a better S / N. Is possible. In the flame type atomic absorption spectrophotometer, the measurement time is longer than that of the furnace type and is several seconds to several tens of seconds. Further, since the time until the sample is atomized can be predicted to some extent, in the double beam method, the amplification factor can be adjusted during the measurement time.

  The correspondence between the measurement conditions and the detection signal voltage is not limited to that shown in FIG. 2, and can be further classified. In such a case, the amplification factor of the amplifier 10 can be adjusted more precisely during actual sample measurement, and good S / N can be obtained even under various measurement conditions. Needless to say, it is possible to make a simple division.

  As mentioned above, although the Example of this invention was described, this invention is not limited to the said Example, A various change can be made within the range of the summary of this invention described in the claim. .

It is a schematic block diagram of one Example of the atomic absorption spectrophotometer of this invention. It is an example of the optimal detection signal voltage in each wavelength, slit width, flame type, and gas flow rate. It is a schematic block diagram of one Example of the atomic absorption spectrophotometer of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Holo cathode lamp 3 ... Burner 4 ... Atomizer 6 ... Spectroscope 8 ... Photomultiplier tube 10 ... Amplifier 11 ... A / D converter 13 ... Processing / control unit 15: Memory 18: Negative and high pressure regulators 19a, 19b ... Gas flow rate control unit F: Flame M1, M2 ... Mirror BS ... Beam splitter SM ... Sector mirror

Claims (3)

A light source;
An atomization unit for atomizing a sample with a flame obtained by burning a mixed gas of a fuel gas and an auxiliary combustion gas;
A spectroscopic unit for extracting a specific wavelength from light from the light source that has passed through the flame;
A detector for detecting light from the spectroscopic unit;
A signal amplifier for amplifying the signal from the detector;
An A / D converter for digitally converting a signal from the amplifier;
In a frame type atomic absorption spectrophotometer having a voltage adjustment unit for adjusting the driving voltage of the detector,
Storage means for storing an optimum value of the signal voltage to the A / D converter corresponding to a combination of measurement conditions that can be set simultaneously;
The frame-type atomic absorption is characterized in that the voltage adjustment unit or the signal amplifier is adjusted so that an optimum value of the corresponding signal voltage is input to the A / D converter when the measurement condition is set. Spectrophotometer. The flame atomic absorption spectrophotometer according to claim 1,
The optimum value of the signal voltage is set to 50 to 70% of the maximum voltage that can be input to the A / D converter, and is a flame atomic absorption spectrophotometer. A light source;
An atomization unit for atomizing a sample with a flame obtained by burning a mixed gas of a fuel gas and an auxiliary combustion gas;
A spectroscopic unit for extracting a specific wavelength from the light from the light source;
A sample optical path for guiding the light from the light source through the flame and guiding it to the spectroscopic unit and a reference optical path for guiding the light to the spectroscopic unit without passing through the flame are formed, and the light from these two optical paths is alternately formed. An optical system to be introduced into the spectroscopic unit;
A detector for detecting light from the spectroscopic unit;
A signal amplifier for amplifying the signal from the detector;
An A / D converter for digitally converting a signal from the amplifier;
In a frame type atomic absorption spectrophotometer having a voltage adjustment unit for adjusting the driving voltage of the detector,
Storage means for storing an optimum value of a signal voltage to the A / D converter based on light from the sample optical path corresponding to a combination of measurement conditions that can be set simultaneously;
When the measurement conditions are set, the voltage adjustment unit is adjusted so that the optimum value of the corresponding signal voltage is input to the A / D converter,
Further, when the measurement is started, the voltage is set so that the optimum value of the signal voltage of the detector based on the light from the sample optical path is 90 to 100% of the maximum voltage that can be input to the A / D converter. A frame-type atomic absorption spectrophotometer characterized by adjusting an adjustment unit or the signal amplifier.

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