JPS622258B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical emission spectrometer using a plasma flame as a light source.

In emission spectroscopy using high-frequency inductively coupled plasma as a light source, analysis results such as detection sensitivity, accuracy, interference, and stability vary depending on various factors, making it difficult to find the optimal analysis conditions for each element to be detected. It's not easy.

The present invention simplifies the method for setting analysis conditions by clarifying the factors that change the evaluation criteria of the analyzer, such as sensitivity and accuracy, in emission analysis using the above-mentioned high-frequency inductively coupled plasma as a light source. The purpose is to enable the user to easily set analysis conditions according to the purpose of analysis and obtain good analysis results.

In optical emission spectrometry analysis using a plasma flame as a light source, a plasma flame of argon gas or other gas is formed using a plasma torch, a sample is mixed with carrier gas and introduced into the plasma flame, and the sample is heated by the plasma and removed inside the plasma. The atoms of the sample components are excited by the bombardment of electrons, causing them to emit spectral lines unique to each element, which are detected and photometered through a spectrometer.

First, factors that influence the quality of the analysis results will be explained. Figure 1 shows how the spectral line intensity changes depending on the carrier flow rate for three types of samples, with the horizontal axis representing the flow rate of the carrier gas feeding the sample into the plasma flame and the vertical axis representing the spectral line intensity. It is something. Generally, while the carrier flow rate is low, the strength increases linearly with the carrier flow rate, but it eventually reaches a maximum and after that, as the carrier flow rate increases, the strength actually decreases. This can be explained by saying that while the carrier flow rate is low, the spectral intensity is proportional to the amount of sample introduced. However, as the carrier flow rate increases, the cooling effect cannot be ignored, and the spectral line intensity reaches saturation, and as the carrier is further increased, the intensity decreases. Of the three curves, the one with the peak on the far left is when the sample is a difficult-to-dissociate element, and in this case, a higher temperature is required to excite the atoms of the sample element than in the case of other types of samples. However, the effect of plasma cooling by the carrier appears quickly. In this case, the same level of spectral line intensity cannot be obtained unless the sample concentration in the carrier is higher than that of other types of samples. Figure 1 shows the following:

(1) The spectral line intensity reaches its maximum at a certain value of the carrier gas flow rate. In areas where the carrier flow rate is lower or higher than that, the spectral line intensity fluctuates due to fluctuations in the carrier flow rate. That is, stability is low. In the vicinity of the carrier flow rate where the spectral line intensity is maximum, stability is improved and detection sensitivity is also improved.

(2) Depending on the dissociation difficulty of sample component atoms, the more difficult the element is to dissociate, the lower the carrier flow rate to obtain the maximum spectral intensity must be set, and conversely, the sample concentration in the carrier must be set higher as the element is more difficult to dissociate. be.

FIG. 2 shows how the position of the light source relative to the spectrometer affects the detection limit of the target element. The position of the light source relative to the spectrometer here refers to the height of the flame from which the light is introduced into the spectrometer, since the plasma flame of the light source extends upward. In the diagram drawn in the upper left of the same figure, 1 is the plasma torch, 2 is the high frequency coil, and 3 is the plasma torch.
is a plasma flame, and the height is measured with the upper edge of the high-frequency coil 2 as height 0, and the horizontal axis of the graph in Figure 2 represents the height, and when the height is aligned with the optical axis of the spectrometer, That's what I'm saying. A carrier gas containing sample mist is sent from below to the plasma center.
It rises upward in the plasma. The sample rises in the plasma along with the flow of this carrier, and is excited by the plasma to emit light. This emission intensity varies depending on the temperature, and since the temperature in the plasma flame changes in the height direction, the emission intensity of the sample atoms varies depending on the height within the plasma. On the other hand, the gas that forms the plasma (for example, argon) itself emits light within the plasma, and this light becomes noise in the spectral line of the element to be detected, and the intensity of this noise also differs depending on the position in the height direction of the plasma. Since the target spectral line is mixed with this noise, there is a limit sample concentration at which the target spectral line is buried in the noise and cannot be detected, and the vertical axis in FIG. 2 shows this limit. In other words, Figure 2 shows how the S/N ratio changes depending on the position of the light source in the height direction, and Figure 2 shows that this S/N ratio has the maximum value. ing. By appropriately selecting the position of the light source relative to the spectrometer in the height direction depending on the target element, it is possible to obtain both the maximum sensitivity and the stability of the results. When trying to detect ion spectral lines, it is better to collect light from a higher position above the plasma flame than when detecting neutral spectral lines. Generally, the reason why the S/N ratio deteriorates near the base of a plasma flame is thought to be because the lower part of the plasma flame is at a higher temperature and the noise caused by the plasma's own light emission is greater. Furthermore, the S/N ratio decreases even in the upper part of the plasma flame because the plasma temperature is low, so the amount of light emitted by the atoms of the sample components decreases, and the sample diffuses as it ascends through the flame, expanding the light emitting region of the sample, which can be detected by the spectrometer. This is thought to be due to a decrease in the incidence efficiency.

FIG. 3 shows the relationship between the measurement bandwidth of the spectrometer and the detection limit, which is the same as the vertical axis in FIG. The measurement bandwidth is the width of the exit slit of the spectrometer expressed in terms of the wavelength range (Å) of the spectral image on the exit slit surface. The atoms of the sample components move in the plasma flame at the speed of gas molecules at the heating temperature, and the wavelengths of the spectral lines emitted by the atoms shift due to the Doppler effect, so the spectral lines spread over a certain wavelength range. It's stiff. If only the desired spectral line exists, it is best to leave the spectrometer's exit slit open to the same width as the spread range of the spectral line mentioned above, from the point of view of the efficiency of introducing light into the photodetector. If there is other interference noise light, widening the slit width means that more noise light will be taken in, resulting in a decrease in the S/N ratio. Therefore, as shown in FIG. 3, the injection slit width has a width that provides the highest S/N ratio, and the detection limit increases even if it is wider or narrower than that. In FIG. 3, the curve on the left side shows an element with a small Doppler effect, and the curve on the right side shows an element with a large Doppler effect.

From the above, it has become clear how the three factors of carrier gas flow rate, light source position, and spectrometer exit slit width affect the analytical results. These factors give the maximum detection sensitivity or the minimum S/N ratio at a certain set value, so setting these factors to the maximum sensitivity will provide the maximum sensitivity and stability of the measurement results at the same time. As a result, analysis accuracy can be improved.

In the present invention, a carrier gas flow rate adjustment means, a plasma torch vertical position adjustment means, and a spectrometer injection slit width adjustment means are provided, and the setting positions of each of the adjustment means are determined for each element in advance, and for the element to be detected and quantified, The present invention provides a plasma light source emission spectrometer in which each adjusting means is adjusted to the predetermined set positions.

FIG. 4 shows an embodiment of the present invention. Reference numeral 1 denotes a plasma torch, to which argon gas and ground gas, which are plasma forming gases, are supplied from the bottom at constant flow rates. 2 is a high frequency coil wound around the plasma torch 1, and 3 is a plasma flame formed on the plasma torch. Reference numeral 4 denotes a carrier gas reservoir, and the carrier gas passes through a flow meter 5, passes through a flow rate regulating throttle 6, and is ejected into a spray chamber 7. A sample suction tube 8 is inserted into a carrier gas nozzle in the spray chamber 7, and a sample solution is sucked up from the sample container 9 and sprayed into the spray chamber 7. A portion of the atomized sample solution dries into an aerosol and flows out into the center of the plasma flame through a tube inserted into the plasma torch together with the carrier gas. Excess sample liquid sprayed into the spray chamber is drained from the drain discharge pipe 1.
It is discharged through 9. The plasma torch 1 is held by a holder 11 that is movable along a vertical guide 10, and the holder 11 is moved up and down by a screw 12. 13 is a drive mechanism for the screw 12. L is a condensing lens that captures the image of the center line of plasma flame 3 using spectroscope 1.
It is formed on the entrance slit S1 of No. 4. M1 is a collimator concave mirror, G is a reflection diffraction grating, and M2 is a telemeter concave mirror, which converges the light diffracted by the grating G onto the exit slit S2. 15 is injection slit S2
This is a slit width adjustment mechanism. The CPU is a microcomputer that stores appropriate values for the carrier gas flow rate, plasma torch height position, and slit width for each element, and when the element to be detected is specified, the above parameters for that element are read out and the aperture 6 is adjusted. Mechanism 16, drive mechanism 13 for screw 12, and width adjustment mechanism 1 for injection slit S2
5, the carrier gas flow rate, plasma torch height position, and spectrometer exit slit width are compared with the parameters read above, and when the parameters match, the driving of each adjusting means is stopped. In this way, optimum adjustment can be automatically made for the element to be detected with a single operation.

The spectroscopic analyzer of the present invention has the above-described configuration to clarify the factors that affect analytical sensitivity and stability, and how they affect them, and to obtain stability in measurement results by adjusting the sensitivity to the best possible value. As a result, measurement accuracy can be improved, and the adjustment operation can be simplified.

[Brief explanation of the drawing]

Figure 1 is a graph of the relationship between carrier gas flow rate and spectral line detection intensity, Figure 2 is a graph of the relationship between light source position and detection limit, and Figure 3 is a graph of measurement bandwidth (exit slit width of spectrometer) and detection limit. relationship graph. FIG. 4 is a block diagram of an apparatus according to an embodiment of the present invention. 1... Plasma torch, 4... Carrier gas reservoir,
6...Flow rate adjustment aperture, 7...Spray chamber, 9...Sample container,
13... Plasma torch vertical drive mechanism, 15... Injection slit width adjustment mechanism, 16... Aperture adjustment mechanism.

Claims (1)

[Claims] 1 Plasma torch vertical position adjustment means, carrier gas flow rate adjustment means, spectrometer exit slit width adjustment means, and each of the above adjustments in which the optimal plasma torch height, carrier gas flow rate, and spectrometer exit slit width are stored for each type of measurement object. A plasma light source emission spectrometer comprising a means control device.