METHOD AND APPARATUS FOR ANALYSING SUBSTANCES BY ATOMIC ABSORPTION SPECTROSCOPY

Abstract: In a method for analysing substances by atomic absorption spectroscopy, a sample is atomized to produce free atoms of the substance to be analysed and the absorption of light by the free atoms at a certain wavelength is measured. In the atomization of the sample, the sample brought along a sample line (3) is heated to the temperature required by the atomization by mixing it into a hot auxiliary gas. In the measurement, a magnet (6) is used to produce Zeeman background correction.

Description: METHOD AND APPARATUS FOR ANALYSING SUBSTANCES BY ATOMIC ABSORPTION SPECTROSCOPY The invention relates to a method for analysing substances by atomic absorption spectroscopy. The invention also relates to an apparatus for applying the method. In connection with industrial processes, there is a need to monitor the contents of substances contained in the process, such as the sub- stances passing in different material flows of the process. The contents of the outlet flows from the process, i.e. various emissions into the environment, are particularly interesting. Such emissions are developed, in addition to production processes, also in processes of waste treatment. The disposal of waste by combustion is a promising waste treatment method which, however, has the problem of combustion gases entering the atmosphere, containing e.g. heavy metals hazardous to living nature. For example, the Commission of the European Union has prescribed strict limits for emissions from new refuse incineration plants on gaseous heavy metals present in combustion gases, for example on mercury, cadmium and lead. The above-mentioned requirements mean that it must be possible to analyse the substances flowing out from the process in real time and with high precision. For example particularly in refuse incineration, the composition of the material to be combusted may vary to a very high extent, wherein changes can take place quickly also in the composition of the combustion gas. In order to make it possible to take measures immediately when the contents exceed the allowed limits, combustion gas analyzers must be part of the process control. This is naturally an aim also in all the processes, irrespective of whether the material flow to be monitored and analyzed relates to the production or to the discharge from the process to the environment. One way of analysing the course of the process, such as its material flows, is to take a sample at certain intervals and to analyse it e.g. in an analysis laboratory in connection with the plant. To have continuous control, the sampling frequency should be high. Furthermore, sampling at intervals requires special arrangements, it may be laborious and expensive, and the results always come after a certain delay; that is, the real time requirement is not met. Atomic absorption spectroscopy (AAS) is a generally known method for elementary analysis. The analyses are normally performed at laboratories, of samples brought there. Atomic absorption spectroscopy as a method perse has been under continuous development, and one of the most important phenomena applied in practice is the so-called Zeeman effect, whereby the absorption caused by background (sample matrix) can be measured by using the magnetic field as an aid and thereby to improve the accuracy of the analysis. An atomic absorption spectrometer to measure contents of elements in connection with a process by utilizing the Zeeman effect to improve the accuracy is presented in US patent 3,937,577. Nevertheless, this method will not provide a real-time measurement, because it takes time to prepare the sample. For atomizing the sample, an oven is used in which the atomization must be performed in a separate step; consequently, a continuous monitoring of e.g. the material flow in a process is not possible. The publication EP-A-151 752 discloses an ICP spectroscopic method, in which a plasma gas and a sample gas are mixed together in a preheating zone, in which the heating can be performed up to such a high temperature that the sample is atomized. After this, the measurement can be conducted in a measuring zone by means of atomic absorption spectroscopy. The publication US-3,492,074, in turn, discloses a method for adjusting plasma temperature by changing the mixing ratio of a monatomic and a polyatomic gas. The sample is mixed into this gas mixture when it flows into a plasma generator. In column 4, lines 71 to 74 of the publication, it is said that the sample could be introduced directly into the plasma generator, which would correspond to simultaneous heating of the sample and the gas mixture. It is an aim of the invention to eliminate the above-mentioned drawbacks and to present a method for implementing process control in real time and with high precision. To achieve this aim, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the appended claim 1. The method is based on atomic absorption spectroscopy (AAS) utilizing plasma and preferably the Zeeman effect. There is a direct connection from the process or from a material flow passing in the process, e.g. from a flue gas channel of an incineration plant, along a sample line to an analysis device, in which the sample is mixed into a gas jet heated with plasma to form atoms to be analyzed, and the atoms are analyzed by AAS at a resonance wavelength depending on the substances to be analyzed. A location to be analyzed in a gas jet heated with plasma and containing free atoms is subjected to a magnetic field to produce background correction by means of the Zeeman effect. A gas jet heated with plasma, used as the producer of thermal energy required in atomization in an on-line atomic absorption spectrometer, has several advantages when compared to atomic absorption spectrometers equipped with flame and furnace atomizers. A common feature for both of the above-mentioned atomization methods is that they require either a liquid or solid sample. In this case e.g. a gaseous sample taken from a process material flow should first be led through an absorption solution, in which only part of the atoms contained in the gas is absorbed, and first after this, the liquid sample can be fed into the atomizer. Moreover, flame atomizers involve the problem of a very high dilution coefficient. Only a very small quantity of the sample can be mixed into the flame without impairing the stability of the flame. Another problem of flame atomizers is the danger of explosion. A furnace atomizer cannot be used in a continuous measuring device, because atomization is a multi-step process in which the optical measurement must be performed at a precisely correct stage. Thus, continuous sample feeding is not possible. In a furnace atomizer, the duration of the atomization process is about 2 to 4 minutes. It is typical for the invention that the sample is mixed into an auxiliary gas which prior to the mixing has been heated to the temperature required by atomization. In practice, this can be performed with an apparatus in which the location of mixing the sample is after the location of heating the auxiliary gas in the direction of flow of the auxiliary gas. Consequently, for atomization, heating takes place in the invention substantially before adding the sample, thus eliminating the above- mentioned drawbacks, and simultaneously a continuous sample flow from the material flow of the process and a continuous measurement can be arranged without a separate sampling and processing step, during which the measurement would be interrupted. In the following, the invention will be described in more detail with reference to the appended drawings, in which Fig. 1 illustrates, in a chart, atomization of a sample of prior art, Fig. 2 illustrates, in a chart, atomization of a sample according to the invention, Fig. 3 illustrates the principle of the method, Fig. 4 illustrates the Zeeman effect in a chart, Fig. 5 illustrates an atomizer unit included in the apparatus, and Fig. 6 illustrates an apparatus for applying the method. Figure 1 shows sample atomization methods known in AAS methods, illustrating the steps of the process in which the sample is mixed into a gas. It is typical for known methods that the sample is mixed into the gas either before the heating (furnace, flame of a pre-mixing type burner) or simultaneously with the heating (flame of a total consumption burner). In the method, a sample is continuously taken from the process, such as from the material flow of the process, along a thin sample conduit, to represent the material composition of the sampling point. At the end of the sample conduit, the sample is thermally atomized by plasma heating in such a way that free atoms are formed of the substances present in the sample to be analysed, light being directed to the atoms in a suitable area from a light source, to be absorbed by unexcited atoms at a certain wavelength when they are excited. The same area can be provided with an effect of a magnetic field to accomplish the Zeeman background correction. The intensity of the light that has passed through the area at this so-called resonance wavelength is measured by known measurement methods, and the results are processed to obtain the actual content data. Figure 2 shows the different steps of atomization in a chart. The flowing auxiliary gas is heated with plasma, and the sample is mixed into the resulting hot auxiliary gas whose temperature is, without the sample, already raised to the level required by atomization. The measurement is performed on an atomized sample existing in a gas jet formed by the hot auxiliary gas. The auxiliary gas will later be also called gas. Figure 3 shows the main principle of the analysis method. The sample is taken from a suitable location in the process, such as the process flow conduit, e.g. a combustion gas channel C of a refuse incineration process, along a sample line 3 to a mixing nozzle 5 where the sample is mixed with a hot gas jet produced by a plasma torch 4. As a result of the mixing, the temperature of the sample gas is raised to the level required for atomization. In the flow direction of the gas, the plasma torch 4 is followed by the mixing nozzle 5, in which the sample line 3 ends and in which the free atoms of the metals to be analyzed are generated in the gas jet coming from the plasma torch 4. The plasma is generated in the plasma torch 4 as direct-current plasma before mixing the sample into the gas jet; that is, the thermal energy required for the atomization is formed substantially before mixing of the sample. The method differs thus from heating furnace and flame absorption methods, in which the thermal energy is produced simultaneously with the mixing at the earliest, or first after mixing of the sample, wherein the sample matrix has a strong effect on the atomization efficiency and the sample quantity in relation to the total volume/flow is limited to be relatively small. When plasma is used for heating the gas and the sample is mixed into the gas first after generating the thermal energy, the mixing ratio can be raised even to a very high level without affecting the stability of the atomization. The sample will not pass through the plasma torch, as a result of which the chemical properties of the sample do not disturb plasma production. As the auxiliary gas to be heated, into which the sample is mixed, it is possible to use a suitable inert carrier gas with good heat transmission properties, such as nitrogen. For applying an accurate AAS measurement, a spectral lamp is used as a light source 1 , wherein background absorption is compensated for in the measurement result with the Zeeman effect. The application is as follows: With respect to the gas jet containing the atoms to be analyzed, a magnet 6 is placed in such a way that a transverse magnetic field is effective over the gas jet, splitting the atomic energy levels according to the Zeeman effect. An energy level remaining in its original place will only absorb light polarized in the direction of the magnetic field, and the shifted levels will only absorb light polarized in a direction perpendicular to the magnetic field. According to polarization of the light to be measured, either absorption caused by the atoms of the metal to be analyzed and the background at the original absorption wavelength (resonance wavelength) or only absorption of the background at the same wavelength is thus obtained. At the absorption wavelength of the metal atom to be examined, light is led from a light-emitting narrow bandwidth spectral lamp used as the light source 1 through one or several lenses 2 into a gas jet located between magnetic poles N, S of the magnet 6 and containing atoms to be analyzed. Light passed through the gas jet is led through a second lense/lenses 2, a rotatable polarizer 7 and a monochromator 8 to a light detector 9. A signal received at the light detector 9 is thus dependent on the position of the polarizer 7; in other words, when the polarization direction is parallel to the magnetic field, both the background and the atoms to be examined absorb light. When the polarization direction is perpendicular to the magnetic field, only the background will absorb light. From the ratio of these signals, it is possible to calculate the content of the atoms to be analyzed, by using Beer's law, when the absorption cross-sectional area (dependent on the atom to be analyzed) and the absorption length (proportional to the size of the gas jet) are known. The Zeeman effect is illustrated in Fig. 4. Figure 5 shows, in more detail, the production of free atoms for AAS analysis in an atomic absorption spectrometer. A sample line 3 coming from the process, such as the process material flow, ends in a nozzle channel 5a of a mixing nozzle 5, extending in a direction perpendicular to the sample line 3. To the mixing nozzle 5 is fixed a plasma torch 4, from which a gas jet P is directed to the nozzle conduit 5a transversely to the sample line 3. In an area of optical measurement, encircled with a broken line, free atoms in the gas jet P are exposed to light coming from the light source 1 as well as the magnetic field produced by the magnet 6. The magnetic field is perpendicular to the direction of the flow of the gas jet P containing the sample and to the light coming from the light source 1. Figure 6 shows an atomic absorption spectrometer in connection with a process material flow. From a flue gas channel C or another flow channel, an electrically heated sample line 3 is led through a sampling connector B connected to the channel. The sample line 3 can also be led from another location, in which continuous monitoring of contents of substances is required, such as directly from the process, for example at the end of a combustion furnace. By heating the sample line, the sample can be kept at least at the temperature of the original material flow, such as a combustion gas flow, but the temperature can also be raised from the initial temperature by heating. The flow along the sample line 3 can be continuous or periodical, wherein the sample line 3 is equipped with a shut-off valve which lets the sample into the mixing nozzle 5 only for a certain time, and the plasma torch 4 only operates for a short time during this phase for the analysis. The sample line 3 ends in the mixing nozzle 5 in the above-described manner. The atomic absorption spectrometer is placed as a compact unit close to the flow channel in such a way that the length of the sample line 3 is kept as short as possible. As shown in the figure, the frame containing the atomic absorption spectrometer can be fixed to the samplling connector B, for example by flange mounting, but also other fixing methods are possible. As for the optics, the frame comprises a spectral lamp used as the light source 1 , lenses 2, the rotatable polarizer 7, the monochromator 8, and the light detector 9, and for the sample processing, the plasma torch 4, the mixing nozzle 5, and the DC magnet 6. The atomic absorption spectrometer is coupled by means of a connecting cable 11 to a control and data collecting unit 10 located in the control room of the plant. In this unit 10, signals coming from the light detector 9 are processed to calculate actual content data, and this unit 10 controls the light source 1 to select the correct wavelength, the magnet 6 to adjust the strength of the magnetic field, and the polarizer 7 to adjust the rotational speed of the polarizer. The auxiliary gas is heated by an electric discharge produced by the plasma torch 4. In the plasma torch 4, direct-current plasma or alternating-current plasma is used to heat the auxiliary gas, but also induction plasma is possible, if the magnetic field used therein does not induce stray fields or they do not have a harmful effect on the analysis. The auxiliary gas is preferably heated with plasma, but instead of a plasma torch also other ways are feasible to heat the gas before mixing the sample into it, for example a burner. Also in this case the sample is mixed into the hot combustion gases first after the combustion. As the magnet 6, preferably a direct-current magnet is used. As direct- current magnets, which produce a magnetic field which is constant with respect to time, an electromagnet or a permanent magnet can be used. The magnetic field is effective all the time in the gas jet, and a change in the signal for the Zeeman background correction is achieved with the polarizer 7, which can be between the light source 1 and the gas jet P, or between the gas jet P and the light detector 9. The term light refers in this context to wavelengths normally used in atomic absorption spectroscopy, and it does not restrict the invention to visible light only. The invention is not limited to the above-presented embodiment, but it can be modified within the scope of the inventive idea presented in the claims. In particular, the invention is suitable for analyzing the heavy metals Hg, Pb and Cd from various processes and material flows, par- ticularly process discharge, such as flue gas conduits of incineration processes. Mercury, lead and cadmium all follow the normal Zeeman distribution. The invention is also suitable for the analysis of other elements, such as elements following the normal Zeeman distribution, from different material flows. By means of the invention, it is possible to analyze samples taken from other processes as well. The invention is not limited solely to the analysis of substances taken from gaseous material flows, but it can also be used to analyse samples in liquid form, if the liquid is sprayed in small aerosol particles into the mixing nozzle. Finally, the invention is not limited solely to methods utilizing Zeeman background correction, but Zeeman background correction is a preferred alternative, because it can be used to improve the measurement accuracy.

Claims: Claims: 1. A method for analysing substances by atomic absorption spectroscopy, in which a sample is atomized to produce free atoms of the substance to be analyzed and the absorption of light by the free atoms at a certain wavelength is measured, characterized in that in the atomization of the sample, the sample is heated to the temperature required by the atomization by mixing it into a hot auxiliary gas. 2. The method according to claim 1 , characterized in that the atomization of the sample takes place in a heated gas jet (P). 3. The method according to claim 1 or 2, characterized in that the auxiliary gas is heated by direct-current, alternating-current or induction plasma. 4. The method according to claim 2 or 3, characterized in that the spectroscopic measurement takes place in the gas jet (P). 5. The method according to any of the preceding claims, characterized in that the sample is led in a continuous sample flow to atomization and the measurement from the atomized sample is continuous. 6. The method according to any of the preceding claims 1 to 4, char- acterized in that the sample is led periodically to atomization and the measurement from the atomized sample is periodical. 7. The method according to any of the preceding claims, characterized in that the sample is heated simultaneously with passing it to the atomization. 8. The method according to any of the preceding claims, characterized in that the Zeeman effect induced by a magnetic field is utilized in the background correction of the measurement. 9. The method according to claim 8, characterized in that the absorptions of the substance to be analyzed and of the background are measured by changing the polarization of light entering a light detector (9)- 10. The method according to claim 9, characterized in that the polarization is changed by a rotatable polarizer (7). 11. The method according to any of the preceding claims, characterized in that the sample is taken from a process, such as the material flow of a process, for example from a flue gas channel of an incineration process. 12. The method according to any of the preceding claims, characterized in that from the sample, one or several of the following metals are analyzed: Hg, Cd and Pb. 13. An apparatus for analyzing substances by atomic absorption spectroscopy, which apparatus comprises a sample line (3), a device for atomizing the sample at the end of the sample line (3), a light source (1), and optics arranged to convey a beam of light through the sample, as well as a light detector (9) for receiving the beam of light passed through the sample, characterized in that in the atomizing device, the sample line (3) enters a mixing point which is located separately from the point of heating and after the point of heating, in such a way that the sample reaches the temperature required for the atomization when it is mixed into an auxiliary gas heated previously in the heating point. 14. The apparatus according to claim 13, characterized in that the atomizing device comprises a mixing nozzle (5), in which the sample line (3) ends and to which a gas jet (P) is directed from the heating point to mix and atomize the sample. 15. The apparatus according to claim 13 or 14, characterized in that the heating point is an electric discharge produced with a plasma torch (4). 16. The apparatus according to any of the preceding claims 13 to 15, characterized in that it comprises a magnet (6) to produce Zeeman background correction. 17. The apparatus according to claim 16, characterized in that it comprises a polarizer (7) between the light source (1) and the light detector (9), to change the polarization of the light entering the light detector (9). 18. The apparatus according to any of the preceding claims 13 to 17, characterized in that the sample line (3) is brought from a process, such as a material flow channel of a process, for example from a flue gas channel (C).

Inventors: HAEYRINEN VILLE, HERNBERG ROLF, OIKARI RISTO, HAEYRINEN, VILLE, HERNBERG, ROLF, OIKARI, RISTO
Applicants: HAEYRINEN VILLE, HERNBERG ROLF, OIKARI RISTO, OSEIR OY, HAEYRINEN, VILLE, HERNBERG, ROLF, OIKARI, RISTO
Application year: 1999
Main classification: G01N21
IPC codes: G01N21