FI110342B - Method and apparatus for analyzing substances by atomic absorption spectroscopy - Google Patents

Description

1,110,342

Method and apparatus for analyzing substances by atomic absorption spectroscopy

The invention relates to a method for analyzing substances by atomic alpha-5 sorption spectroscopy. The invention also relates to apparatus for carrying out the method.

In the case of industrial processes, there is a need to control the concentrations of the substances contained in the process, such as those passing through the various material streams in the process. The process outflow concentrations, that is, the various emissions to the environment, are of particular interest. Such emissions occur not only in the production process but also in the waste treatment processes. Disposal of waste by incineration is a promising method of waste treatment, but the problem is the flue gases emitted into the atmosphere, which contain e.g. heavy metals hazardous to living nature. For example, the EU Commission has set strict emission limits for heavy gaseous metals in flue gases for new waste incineration plants, eg. mercury, cadmium and lead.

20 The above requirements mean that the outputs from the process must be able to be analyzed in real time and with high precision. six. For example, the composition of the material burned in the incineration process can vary greatly, including flue gas; changes in composition can occur rapidly. In order to be able to deal immediately with concentrations exceeding the permitted limits / 25, flue gas analyzers must be part of the process control. Of course, this is also the goal in all processes, whether the flow of material to be monitored and analyzed is related to production or process removal to the environment.

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One way to analyze the flow of a process, such as its material flows, is to periodically sample and analyze it, for example, in an analytical laboratory at the plant. For continuous monitoring, the sampling frequency should be high. Intermittent sampling requires 35 additional special arrangements, can be laborious and costly, and results will be; ; ' · Algae always with a certain delay, ie no real-time realization

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Atomic absorption spectroscopy (AAS) is a well-known method of elemental analysis. Analyzes are usually performed in laboratories on samples imported there. Atomic absorption spectroscopy as a method itself has been continuously developed, and one of the most important practical applications is the so-called. Zeeman phenomenon, where the absorption of a background (sample matrix) can be measured using a magnetic field to improve the analytical accuracy. An atomic absorption spectrometer for measuring process concentrations of elements using the Zeeman phenomenon to increase accuracy is disclosed in U.S. Patent No. 3,937,577. However, this method does not provide real-time measurement because sample preparation takes time. The furnace is used for atomization of the sample, in which the atomization must be carried out in a separate step, and thus, for example, continuous monitoring of the material flow in the process is not possible.

The object of the invention is to eliminate the above drawbacks and to provide a method for real-time monitoring of processes with high accuracy. To accomplish this purpose, the method according to the invention is essentially characterized in what is set forth in the characterizing part of the appended claim 1. The apparatus 20 according to the invention, on the other hand, is mainly characterized by what is stated in the characterizing part of the appended claim 13.

The method is based on atomic absorption spectroscopy (AAS) which utilizes plasma and preferably the Zeeman phenomenon. From the process or from the flow of material passing through the process, eg combustion plant smoke; · From the gas channel, there is a direct connection along the sample line to the analyzer, where the sample is mixed with a plasma heated gas jet to form the atoms to be analyzed, and the atoms are analyzed by AAS at a resonant wavelength dependent on the analyte.

30 Plasma containing free atoms in the analyte site The moon -: ·. · In a lost gas jet, a magnetic field is applied to effect background correction by means of the Zeeman phenomenon.

As the producer of the thermal energy required to atomize a plasma heated gas jet, the on-line atomic absorption spectrometer has; several advantages over atomic absorption spectrometers with flame and furnace atoms. A common feature of both of these atomization methods is that they require either a liquid or a solid sample, whereby, for example, a gaseous sample from a process stream should first be passed through an absorption solution into which only a portion of the atoms contained in the gas are absorbed. the liquid sample feeds to the atomizer. The problem with flame atom-5 escorts is still a very high dilution factor. Only a very small amount of sample can be mixed with the flame without loss of flame stability. In addition, the problem with flame atoms is the risk of explosion. The oven 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 the exact correct stage. In this case, continuous feed of the sample will fail. The length of the atomization process of the oven atomizer is about 2-4 minutes.

In order to achieve atomization, heating in the invention takes place substantially prior to the addition of the sample, thereby avoiding the aforesaid drawbacks, while providing continuous sample flow from the process material flow and continuous measurement without a separate sample forming and processing period.

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 schematically illustrates prior art sample atomization, 25 Figure 2 schematically illustrates the invention atomization, Figure 3 illustrates the principle of the method, 30 Figure 4 schematically illustrates the Zeeman phenomenon, ... Figure 5 shows the atomization unit included in the apparatus, and:. Figure 6 shows an apparatus for implementing the method.

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Figure 1 illustrates known sample atomization techniques T in AAS, illustrating the process steps of mixing a sample with a gas. Typical for known methods is to mix the sample with the gas either 4,110,342 before heating (oven, premix burner flame) or simultaneously with heating (total consumer burner flame).

In the method, a sample representing the material composition of the sampling site is continuously taken from the process, such as the flow of material in the process, through a thin sample passage. At the end of the sample channel, the sample is thermally atomized using plasma heating so that free atoms are formed from the analyte in the sample, and a light is emitted from a light source at a suitable range and absorbed by non-excited atoms at a specific wavelength. A magnetic field may be provided in the same region to effect a Zeeman background correction. The intensity of light passing through the area at this so-called. the resonance wavelength is measured by known measurement methods and the results are processed to obtain the actual concentration data. Figure 2 is a diagram showing the various stages of atomization. The flowing auxiliary gas 15 is heated with plasma and the sample is mixed with the thus obtained hot auxiliary gas, the temperature of which without the sample has already been raised to the level required for atomization. The measurement is carried out on an atomized sample in a gas jet formed by a hot auxiliary gas. The auxiliary gas is later also referred to as gas.

20 ... Figure 3 shows the main principle of the analytical method. The sample is guided from a suitable point in the process, such as a process flow passage, e.g. flue gas channel C of the waste incineration process, along sample line 3 to a mixing nozzle 5 where the sample is mixed with plasma jet 4 with a syn-25 hot gas jet. As a result of mixing, the sample gas temperature rises to the level required for atomization. Plasma flushing 4 is followed, in the gas flow direction, by a mixing nozzle 5, at the end of the sample line 3, in which the free atoms of the metals to be analyzed are generated in the gas jet from the plasma flushing 4. Plasma is formed in plasma jet 4 as DC current before mixing the sample with a gas jet, i.e., the thermal energy required for atomization is generated substantially before mixing the sample. The method '· · thus differs from the heating furnace and flame absorption methods, in which the thermal energy is produced at the earliest by simultaneous mixing 35 or only after mixing the sample, whereby the sample; , * matrix strongly affects atomization efficiency and sample. ·! the amount relative to the total volume / flow is limited to a relatively small amount. By using plasma to heat the gas and mixing the 5 110342 sample with the gas only after the thermal energy has been generated, the mixing ratio can be raised to a very high level without disturbing the stability of the atomization I. The sample does not pass through plasma, so that the chemical properties of the sample do not interfere with plasma formation.

A suitable inert carrier gas having good heat transfer capacity, such as nitrogen, may be used as the auxiliary gas to be heated to mix the sample.

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For accurate AAS measurement, the spectral lamp is used as the light source 1, whereby the absorption of the background is compensated by the Zeeman phenomenon. The realization is as follows: With respect to the gas jet containing the atoms to be studied, a magnet 6 is placed such that a transverse magnetic field is applied over the 15 gas jets, which breaks down the energy states of the atoms according to the Zeeman phenomenon. The energy state retained in its original position absorbs only polarized light in the direction of the magnetic field, and the displaced states only light polarized perpendicular to the magnetic field. The polarization of the light to be measured thus provides either absorption by the atoms and background of the metal being investigated at the original absorption wavelength (resonance wavelength) or only absorption by the background at the same wavelength. At the absorption wavelength of the metal atom under investigation, a light emitting narrowband light source 1 is used to direct light through one or more lenses 2 to a gas jet of magnets 6, N, S, containing atoms to be analyzed. The light that has passed the gas jet is passed through the second lens (s) 2, the rotary polarizer 7 and the monochromator 8 to the light detector 9. The signal obtained for the light detector 9 thus depends on the position of the polarizer 7, i.e. when the 30 polarization directions are parallel to the magnetic field, both the background and the atoms under study absorb light. When the polarization direction is perpendicular to the magnetic field, only the background absorbs light. · From these ratios, the concentration of the atoms to be studied can be calculated using Beer's law, given the range of absorption effect (35 dependent on the atom to be studied) and the absorption distance (proportional to the size of the gas jet). The Zeeman phenomenon is illustrated in Figure 4.

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Figure 5 shows in more detail the formation of free atoms for AAS analysis in an atomic absorption spectrometer. The sample line 3 from the process, such as the flow of material from the process, terminates in the nozzle channel 5a of the mixing nozzle 5, which is perpendicular to the sample line 3. A plasma spray 4 is attached to the mixing nozzle 5, from which a gas jet P is directed to the nozzle channel 5a transversely to the sample line 3. In the dashed circle of optical measurement, the free atoms in the gas jet P are exposed to light from the light source 1 and to the magnetic field 10 generated by the magnet 6 perpendicular to the direction of light from the gas source P containing the sample.

Figure 6 shows an atomic absorption spectrometer apparatus for process flow. An electrically heated sample line 3 is conducted from the flue gas channel C or other flow-through channel 15 through a sample assembly B connected to the channel. The sample line 3 may also be derived from another location where continuous monitoring of the concentrations of the substances is required, such as directly from the process, e.g. By heating the sample line, the sample can be maintained at a temperature of at least the original material-20 flow, such as the flue gas flow, but the temperature may also be raised by heating. Flow along sample line 3 may be continuous or intermittent, with sample line 3 provided with a stopper that only allows a sample to pass into the mixing nozzle 5 for a certain period of time, and the plasma stream 4 operates only briefly during this period for analysis. The sample line 3 ends in the mixing nozzle 5 as described above. The atomic absorption spectrometer is placed as a compact unit close to the flow passage so that the length of the sample line 3 is kept as short as possible. As shown in the figure, the body containing the atomic absorption spectrometer can be attached to sample compound B, for example by flange attachment, but other means of attachment are possible. For optics, the frame contains 1 working-y light source. van spectral lamp, lenses 2, rotary polarizer 7, monochromate. the torch 8 and the light detector 9, and for the sample processing, the plasma beam 4, the mixing nozzle 5, and the DC magnet 6. Atomic absorption spectrometer: for calculating the concentration information and directing the light source 1 to select the correct wavelength, the magnet 7 110342 for adjusting the intensity of the magnetic field 6 and the polarizer 7 for adjusting the speed of rotation of the polarizer.

The auxiliary gas is heated by an electrical discharge of 5 provided by a plasma torch 4.

Plasma flusher 4 uses DC plasma or AC plasma to heat the auxiliary gas, but induction plasma is also possible if the magnetic field used therein does not produce diffuse fields or they do not adversely affect the analysis.

The auxiliary gas is preferably heated with plasma, but other ways of heating the gas before mixing the sample with it, e.g. Here again, the sample is mixed with the hot combustion gases only after combustion.

The magnet 6 is preferably a DC magnet. As DC magnets, which provide a time constant magnetic field, an electromagnet or permanent magnet can be used.

20 The magnetic field is constantly affected by the gas jet, and the change in sig-. , in the case for the Zeeman background correction, polarizer 7 is obtained which may be located between the light source 1 and the gas jet P or the gas jet P and the light detector 9.

'The term light, as used herein, refers to wavelengths normally used in atomic absorption spectroscopy and is not limited to visible light.

The invention is not limited to the above embodiment, but may be modified within the scope of the inventive idea of the claims. The invention is particularly suitable for the analysis of heavy metals Hg, Pb and Cd from various processes and material streams, in particular process effluent streams such as flue gas ducts from combustion processes. Mercury, lead and cadmium all follow the normal Zeeman-35 distribution. The invention is also applicable to other elements such as normal

For the analysis of elements that follow the Zeeman distribution from different substance streams. The invention can also be used to analyze samples taken from other processes. The invention is not limited to the analysis of substances taken from gaseous material streams, but can also be used to analyze samples in liquid form if the liquid is sprayed as small aerosol particles into the mixing nozzle. Finally, the invention is not limited solely to methods employing Zeeman background correction, but Zeeman background correction is a preferred option because it can improve measurement accuracy.

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Claims (18)

9 110342 A method for analyzing substances by atomic absorption spectroscopy, wherein the sample is atomized to obtain the free atoms of the analyte and the absorption of light by the free atoms at a given wavelength is measured, characterized in that the sample is Method according to claim 1, characterized in that the sample is atomized in a heated gas jet (P). Method according to claim 1 or 2, characterized in that the auxiliary gas is heated by direct current, alternating current or induction plasma. Method according to claim 2 or 3, characterized in that the spectroscopic measurement is carried out in a gas jet (P). A method according to any one of the preceding claims, characterized in that the sample is conducted as a continuous sample stream to the atomization and the measurement from the atomized sample is continuous. A method according to any one of the preceding claims 1 to 4, characterized in that the sample is subjected to periodic atomization and, ·. the measurement of the atomized sample is periodic. ::; A method according to any one of the preceding claims, characterized in that the sample is heated while being introduced into the atomization. A method according to any one of the preceding claims, known as V; the use of the Zeeman phenomenon caused by the magnetic field in the background correction of the measurement. 35 Method according to claim 8, characterized in that the absorbances of the analyte and the background are measured in the light detector (9): by changing the polarization of the incident light. 10 110342 Method according to Claim 9, characterized in that the polarization is changed by a rotary polarizer (7). 11. A method according to any one of the preceding claims, characterized in that the sample is taken from a process, such as a process stream, e.g., a flue gas duct of the combustion process. A method according to any one of the preceding claims, characterized in that the sample is analyzed for one or more of the following metals: Hg, Cd and Pb. 13. Apparatus for analyzing substances by atomic absorption spectroscopy comprising a line of samples (3), a line of samples (3) for atomizing a sample, a light source (1) and an optics arranged to pass a light beam through the sample and a light detector (9) passing through the sample. for receiving a beam of light, characterized in that in the atomizer the sample line (3) enters a mixing point which is separated from the heating point so that the sample reaches the temperature required for atomization when mixed with the auxiliary gas previously heated at the heating point. Apparatus according to claim 13, characterized in that the atomization device comprises a mixing nozzle (5) terminating at the sample line (3) and directed to a gas jet (P) • · from the heating point. .t to mix and atomize the sample. Apparatus according to Claim 13 or 14, characterized in that the heating point is an electric discharge caused by a plasma beacon (4). Apparatus according to any one of claims 13 to 15, characterized in that it comprises a magnet (6) for effecting a Zeeman background correction. ; . 35 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). 5 11 110342 Apparatus according to one of the preceding claims 13 to 17, characterized in that the sample line (3) is introduced from a process, such as a process material flow channel, e.g. a flue gas channel (C). 12 110342 I Patent krav: