Easily adjustable optical emission spectrometer

Abstract

The present invention relates to an easily adjustable optical emission spectrometer (1) and a method (100) for setting up and operating such an optical emission spectrometer (1), comprising a plasma stand (2) for establishing an optical plasma from a sample material and an optical system (3) for measuring the spectrum of light (L) emitted by the plasma, the optical system (3) comprising at least one light entrance aperture (31), at least one diffraction grating (32) for splitting the light (L) coming from the plasma (A), and one or more detectors (33) for measuring the spectrum of the light (L), the plasma stand (2) and the optical system (3) being directly and fixedly mounted to a plasma stand flange (2B) and an optical system flange (3B), respectively, which are directly and fixedly connected to each other, and the optical emission spectrometer (1) further comprising an analysis unit (34) adapted to analyze the measured spectrum and to compensate for any drift in the spectrum for the detector (33) potentially caused by heat transferred from the plasma stand (2) to the optical system (3), taking into account thermal expansion of the optical system (3).

Description

[Technical field]

[0001] The present invention relates to an easily tunable optical emission spectrometer and to a method for setting up and operating such an apparatus. [Background technology]

[0002] Arc / spark or LIBS optical emission spectroscopy (OES) is an instrument for chemical analysis, used to display and measure the emission spectra of chemical substances. In LIBS, the sample material is vaporized and excited by a laser. In arc / spark, the energy for vaporization and excitation is provided by an electric arc or a series of sparks between an electrode and a material sample acting as a counterelectrode. In both cases, a plasma is generated from (a part of) the sample material. The light emitted by the plasma is transmitted to the optics of the OES, where it is resolved into its individual spectral components. Each chemical element contained in the sample emits light at several characteristic wavelengths (spectral lines) and can therefore be identified. The light intensity of the spectral lines measured in this way corresponds to the concentration of the relevant element in the sample.

[0003] For several reasons, in particular to protect the plasma from ambient air, excitation is usually performed in a so-called spark stand or plasma stand that is argon purged. The optical path from the plasma to the optics may include an optical fiber.

[0004] However, wavelengths shorter than 185 nm are absorbed by optical fibers and the surrounding air, so an argon-purged optical path without optical fibers is required. Such an optical path contains only optical windows, lenses, mirrors, etc., if at all, and is called "direct viewing." To ensure proper direct viewing of the plasma, the optics and plasma stand need to be precisely aligned with each other.

[0005] However, the energy applied to the sample generates a certain amount of heat in the plasma stand. If the heat is transferred from the plasma stand to the optics, the unwanted thermal expansion can cause loss of resolution and performance. Therefore, it is state of the art to implement some insulation between the optics and the plasma stand, for example by providing some kind of gap or placing an insulating material such as nylon in between. The insulation between the plasma stand and the optics can also provide electrical insulation between the plasma stand and the optics to prevent electrical flashover to the optics.

[0006] The mechanical connection is not a rigid one, instead the plasma stand and the optics are designed (more or less) as separate units that can be adjusted individually to obtain the proper alignment with each other. This kind of design results in a complex and critical adjustment process of both units. The adjustment of the units must be done carefully to achieve good optical resolution of the OES. The use of a vacuum or gas-filled optical chamber for observation of vacuum ultraviolet (VUV) wavelengths also makes access to the optics and the plasma stand more difficult, further complicating the adjustment.

[0007] For optical emission spectroscopy (OES) according to the state of the art, the following alignment procedures should be applied: (a) alignment of the position and orientation of the plasma stand as a freely adjustable plasma stand relative to the instrument housing, for example with a plasma stand plate perpendicular to the housing front, (b) alignment of the optics or vacuum chamber in the instrument housing providing an airtight mechanical connection between the optics and the plasma stand, (c) alignment of the optics in the optics / vacuum chamber relative to the plasma stand using a fixture or a laser pointer or images of the plasma at several positions along the beam path, and (d) alignment of a switchable or variable mask in the plasma stand to shield some (hot) parts of the plasma relative to the optics in order to be able to observe only atomic spectral lines in the cooler parts of the plasma without the contribution of ionizing lines coming from the hot parts of the plasma. Adjustment steps (a)-(c) are generally performed visually, so that the accuracy of each step is at most about ±0.1 mm. The masking in step (d) is generally adjusted using the intensity of a spectral line signal, for example Fe 187,6 nm. Assuming that the plasma is perfectly visible without the mask, the mask is adjusted to provide an intensity of 25% of the initial intensity of the spectral line. This procedure results in the masking of the hot parts of the plasma. Some plasma stands use permanent masking, where proper alignment between the plasma stand and the optics becomes particularly important. In other systems, the mask can be switchable or adjustable, for example by a stepping motor, to optimize the masking depending on the substrate (matrix) of the sample. Proper masking and alignment of the optics to the plasma stand shall guarantee good reproducibility of the measurements and good limits of detection if the spectral lines in question are sensitive to masking.

[0008] It is therefore desirable to have available optical emission spectroscopy instruments that require minimal tuning effort yet still provide very good resolution. Summary of the Invention

[0009] It is an object of the present invention to provide an optical emission spectrometer which has a minimum of adjustment effort and also provides very good optical resolution.

[0010] This object is solved by an optical emission spectrometer comprising a plasma stand for generating a luminous plasma from a sample material and an optical system for measuring the spectrum of light emitted by the plasma characteristic of the sample material, the optical system comprising at least one light entrance aperture, at least one diffraction grating for splitting the light coming from the plasma into a spectrum and a detector setup consisting of either a combination of an exit aperture and a PMT detector or a spatially resolving solid-state detector such as a CCD or CMOS line or area sensor for measuring the spectrum, the plasma stand and the optical system being directly and fixedly mounted respectively on a plasma stand flange and an optical system flange which are directly and fixedly connected to each other, the optical emission spectrometer further comprising an analysis unit adapted to analyze the measured spectrum and to compensate for drift of the spectrum relative to the detector caused by heat transferred from the plasma stand to the optical system taking into account thermal expansion of the optical system.

[0011] The plasma can be provided by an electric discharge ignited between two opposing electrodes, one of which is the sample material to be analyzed. Spark or arc atomic emission spectroscopy is used for the analysis of metallic elements in solid samples. An electric arc or spark passes through the sample, heating it to a high temperature to vaporize and excite the atoms in the sample. The excited sample atoms emit light of a characteristic wavelength that can be dispersed by a diffraction grating and quantitatively detected. Quantitative spark analysis can be used for production quality control in foundries and metal casting facilities.

[0012] Alternatively, plasma can be provided by Laser-Induced Breakdown Spectroscopy (LIBS), a type of atomic emission spectroscopy that uses high-energy laser pulses as the excitation source. The laser is focused to form plasma by atomizing and exciting the sample material. Plasma formation begins only when the focused laser achieves a certain threshold of optical breakdown, which generally depends on the environment and the target material. LIBS utilizes optical emission spectroscopy and to this extent is very similar to arc / spark optical emission spectroscopy.

[0013] The emission lines measured may vary from the infrared spectrum to the ultraviolet spectrum. Ambient air has a cut-off wavelength of 185 nm, absorbing shorter wavelengths. If shorter wavelengths are to be detected, the optical emission spectrometer must be operated without ambient air, for example in an argon atmosphere, to avoid undesired absorption along the light path from the plasma stand to the detector in the optical system.

[0014] The performance of the spectroscopic analyzer in general, and the spectral resolution in particular, depends on the settings and adjustments of the optical spectroscopic analyzer, the optical components applied, their arrangement, and the materials used for these components and the base plate. The present invention avoids any degradation of performance due to non-optimal adjustments, since the direct and fixed connection of the optical system and the plasma stand provides a defined and constant adjustment of both components relative to each other, eliminating any manual misalignment.

[0015] The direct and fixed connection between the plasma stand and the optical system leads to a heat transfer from the plasma stand to the optical system, resulting in a temperature rise and a resultant thermal expansion of the optical system. The thermal expansion leads to a thermal drift of the emission line relative to the detector. Since the warm-up of the optical system is a relatively slow process, the resulting drift of the emission line can be continuously compensated by the analysis software installed and executed in the analysis unit, which comprises a processor for executing the software and a data storage device for storing the measurement data. The drift compensation by the plasma stand, which is directly and fixedly connected to the analysis unit and the optical system, avoids alignment errors and alignment drifts during use, both resulting in good spectral resolution and robust performance over time. Furthermore, the avoidance of critical and time-consuming alignment procedures allows a quicker and easier use of the optical emission spectrometer according to the invention with better spectral resolution compared to state-of-the-art devices with complex and fault-sensitive alignment procedures.

[0016] Thus, the optical emission spectrometer according to the present invention provides an optical emission spectrometer that has minimal adjustment efforts and also provides very good resolution.

[0017] In another embodiment, the plasma stand flange and the optics flange are provided with non-adjustable attachment means for connecting both flanges together. These attachment means ensure easy and simple (self) alignment of the plasma stand flange and the optics flange with respect to each other. Furthermore, the position and distance of the flanges with respect to each other is fixed by the attachment means. The attachment means used in this embodiment can be screws, pins, etc., also click connections are possible.

[0018] In another preferred embodiment, at least the optics flange and the optical components in the optical path between the entrance slit and the detector are made of the same material, which can create a uniform temperature distribution and prevent unnecessary material wear temperature fluctuations.

[0019] The material may be a metal, preferably a light metal.

[0020] In another embodiment, the plasma stand flange is made of an electrically insulating material. The insulating material prevents the currents induced by the generation of the plasma from flowing through the plasma stand and the plasma stand flange, thus protecting the plasma stand and all components connected to it. As electrically insulating material, sandwich materials such as Trespa or Resopal, or heat-stable plastics such as polyamide (nylon) or polyetheretherketone (PEEK), or ceramic materials are preferably used.

[0021] In another preferred embodiment, the connection between the plasma stand flange and the optics flange is implemented as a rigid toothed connection (e.g., a Hirth joint) to prevent misalignment between both flanges. The toothed connection ensures that the connection points mesh with each other, so that on the one hand the desired easy and simple alignment is achieved and on the other hand misalignment is prevented. Therefore, it is preferred to use a Hirth joint, which can be used to connect the two elements and is characterized by tapered teeth that mesh with each other at the end faces of each element.

[0022] The rigid toothed connection is particularly beneficial when it becomes necessary to disassemble the plasma stand from the optics during maintenance procedures. The auto-adjust setting ensures preservation of an accurate direct view of the plasma.

[0023] In another embodiment, the plasma stand comprises an optical mask fixedly connected to the optical system or the plasma stand and suitably positioned in the optical path between the plasma stand and the optical system to mask at least the hot portion of the plasma.

[0024] In another preferred embodiment, the plasma stand is a spark stand with a spark chamber containing an (e.g. positively charged) electrode. The sample material closes the spark chamber and acts as a counter electrode. The axis of the spark is defined as the connection line between the sample surface and the (e.g. positively charged) electrode tip. There is a large temperature gradient along the axis of the spark in the plasma. The entrance aperture of the optical system directly connected to the plasma stand may be oriented parallel to the axis of the spark or perpendicular to the axis of the spark. Since the entrance aperture acts as a pinhole camera for the beam coming from a direction along the axis of the spark, a perpendicular orientation reduces the width of the spectrometer system and allows the optical mask to be placed anywhere in the beam path between the spark chamber and the entrance aperture. The hot part of the plasma is located close to the sample surface. A parallel orientation reduces the height of the spectrometer system and forces the optical mask closer to the plasma to achieve a sharp masking of at least the hot regions of the plasma.

[0025] In another embodiment, the plasma stand is a Laser Induced Breakdown Spectroscopy (LIBS) stand, where a laser is focused on a sample material as a target material to generate a plasma.

[0026] In order to increase the degree of freedom of measurement with regard to place and time and to ensure easy handling of the optical emission spectrometer, it is preferable that the optical emission spectrometer is a mobile device.

[0027] Furthermore, this object is solved by a method of installing and operating an optical emission spectrometer as described above. The optical emission spectrometer thus comprises a plasma stand which is directly and fixedly mounted on a plasma stand flange and an optics which is directly and fixedly mounted on an optics flange which comprises several optical components. The method of operating these optical emission spectrometers comprises: - directly and fixedly connecting a plasma stand flange and an optics flange to each other; - establishing a light emitting plasma from the sample material; - generating a spectrum of light emitted by the plasma characteristic of the sample material using an optical system comprising at least one entrance aperture and at least one diffraction grating; - measuring a spectrum by one or more detectors; - analyzing the measured spectrum and taking into account the thermal expansion of the optics by the analysis unit, compensating for drift of the spectrum relative to the detector potentially caused by heat transferred from the plasma stand to the optics.

[0028] In another embodiment, the method may further include masking at least the hot portion of the plasma with an optical mask positioned in the optical path between the plasma stand and the optical system and suitable to be fixedly connected to the optical system or the plasma stand depending on the orientation of the optical system relative to the axis of the spark.

[0029] The above listed embodiments may be used individually or in any combination to provide the apparatus and process according to the present invention. [Brief description of the drawings]

[0030] These and other aspects of the present invention are illustrated in detail in the following drawings. [Figure 1] FIG. 1 is a schematic diagram of an embodiment of an optical emission spectroscopy apparatus according to the present invention with the plasma stand and optics aligned but not mounted. [Diagram 2] 1 is a schematic diagram of an embodiment of an optical emission spectrometry apparatus according to the present invention with a plasma stand and optics mounted in alignment with each other; [Diagram 3] Schematic diagrams of a plasma stand according to the present invention (a) as an arc / spark stand, (b) as a LIBS stand with a mirror element, (c) as a LIBS stand without a mirror element, and (d) as a sliding spark spectroscopy apparatus. [Figure 4]1 is a flow chart of a method according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] 1 and 2 show an optical emission spectrometer 1 comprising a plasma stand 2 for establishing an emitting plasma from a sample material and an optical system 3 for measuring the spectrum of the light L emitted by the plasma characteristic of the sample material. The optical system 3 comprises at least one light entrance aperture 31, at least one diffraction grating for diffracting the light L coming from the plasma A into a spectrum and one or more detectors 33 for measuring the spectrum of the light L. These components of the optical system 3 and the associated optical system flange 3B are made from the same material, which is metal in order to withstand the temperature. The plasma stand 2 and the plasma stand flange 2B are also made from the same material, which is metal. On the other hand, the plasma stand flange 2B can be made from Trespa in order to electrically insulate the plasma stand 2. However, it is not excluded that the plasma stand flange 2B is made from a suitable plastic, ceramic or Resopal, or a combination thereof. The optical emission spectrometer 1 further comprises an analysis unit 34 adapted to analyze the measured spectrum and to compensate for drifts of the spectrum relative to the detector 33 potentially caused by heat transferred from the plasma stand 2 to the optical system 3, taking into account the thermal expansion of the optical system 3.

[0032] In order to be mounted together, the plasma stand 2 is aligned with its plasma stand flange 2B towards the optical system flange 3B of the optical system 3. The plasma stand 2 shown in FIG. 1 comprises a connection nozzle 21 which is fixedly connected to the plasma stand flange 2B of the plasma stand 2. For better and easier alignment of the plasma stand flange 2B to the optical system flange 3B, the optical system flange 3B provides guiding devices 4. In the embodiment shown in FIG. 1, these guiding devices 4 are pins. However, these guiding devices 4 can be guide rails or similar devices. As a further guiding support for the alignment of the plasma stand 2 and the optical system 3, the entrance opening 31 and the connection nozzle 21 have a complementary shape so that the connection nozzle 21 can be inserted into the entrance opening 31.

[0033] In the mounting state of the plasma stand 2 to the optical system 3, as shown in Fig. 2, the plasma stand 2 and the optical system 3 are directly and fixedly mounted to the plasma stand flange 2B and the optical system flange 3B, respectively, which are directly and fixedly connected to each other. Thereby, the mounting of the plasma stand flange 2B to the optical system flange 3B is performed by non-adjustable mounting means 5, which in this example is a combination of screws and pins 5. Instead of screws, click connections can be used.

[0034] In order to measure the individual spectral components in the optical emission spectrometer 1, it is necessary to transmit the light emitted by the plasma generated in the plasma stand 2. The optical emission spectrometer 1 described here envisages two possibilities for plasma generation from (part of) the sample material. The plasma stand 2 can either be an arc / spark stand 2 (see Fig. 3a) and Fig. 3c)) or a Laser-Induced Breakdown Spectroscopy LIBS stand 2 (see Fig. 3a)).

[0035] FIG. 3a) shows the plasma stand 2 as an arc / spark stand 2, in which a discharge A between two electrodes 22, 23 is established to generate an arc / spark plasma of the sample material. The counter electrode is made of the sample material 23. The axis of the arc / spark AS is arranged essentially perpendicular to the entrance aperture 31. Here, by "entrance aperture" relative to the direction of this aperture is meant the axis of the aperture. However, in the embodiment of FIG. 3a), it is also possible to arrange the axis of the arc / spark AS parallel to the entrance aperture 31. The established arc / spark A evaporates the sample material electrode 23 to generate a plasma that emits light L for spectral analysis in the optical system 3.

[0036] 3b) and 3c) respectively show a plasma stand 2 used in laser-induced breakdown spectroscopy (LIBS). The LIBS sample material 23 is evaporated and excited by the laser LA. The sample material 23 is placed on a sample stage 24 below the laser LA, which is focused on the target material, the sample material 23, to generate a plasma. In the embodiment of FIG. 3b), the light L emitted by each of the generated plasmas is then reflected and focused by an optical device 25 in the plasma stand 2, in particular mirror elements and focusing lenses, which transmit the light L to the entrance aperture 31 of the optical system 3. This allows focusing of the light L and can increase the accuracy. In the embodiment of FIG. 3c), the generated plasmas, respectively, transmit the light L emitted by the plasma directly to the entrance aperture 31. This embodiment is simpler and allows spatial masking and size reduction of the plasmas.

[0037] Figure 3d) shows a schematic visualization of a sliding spark spectroscopy setup. An arc A is ignited between two electrodes 22 and slides along a non-conductive sample 23 placed between the electrodes 22, thereby vaporizing the sample material and converting it into a plasma state. The sliding spark plasma emits characteristic spectral lines of the sample material. Direct light optics may be mounted in a self-aligning manner on a spark stand.

[0038] FIG. 4 shows a method 100 for setting up and operating the optical emission spectrometry device 1 as described above. For this, the plasma stand 2, which is directly and fixedly mounted on the plasma stand flange 2B, and the optical system 3, which comprises several optical elements 32, 33, which are directly and fixedly mounted on the optical system flange 3B, are first supplied as individual parts, as shown in FIG. 2. After connecting the plasma stand 2 to the optical system flange 3B, an optical mask is suitably placed in the optical path of the plasma stand 2 and the optical system 3, for example, fixedly connected to the plasma stand 2, where the entrance aperture and the axis of the spark are oriented parallel to each other in order to mask at least the hot part of the plasma. In a second step 120, light L emitting the plasma from the sample material 23 is established, whereby a spectrum of the light L emitted by the plasma characteristic of the sample material is generated using an optical system comprising at least one entrance aperture 31 and at least one diffraction grating 32. Subsequently, in a third step 130, the spectrum of the light L is measured by one or more of the detectors 33 of the optical system 3. After the measurement, a fourth step 140 is performed, in which the spectrum of the measured light L is analyzed and a drift of the spectrum of the light L relative to the detector 33, potentially caused by heat transferred from the plasma stand 2 to the optical system 3, is compensated by the analysis unit. In another embodiment, the method may further comprise a step 150 of masking at least the hot part of the plasma by an optical mask, suitable to be placed in the optical path between the plasma stand 2 and the optical system 3 and fixedly connected to the optical system 3 or to the plasma stand 2 depending on the orientation of the optical system 3 with respect to the axis of the spark AS. [Explanation of symbols]

[0039] 1. Optical emission spectroscopic analysis device according to the present invention 2 Plasma Stand 2B Plasma Stand Flange 21 Connection nozzle 22 Plasma stand electrode 23 Sample Materials 24 Sample stage 3 Optical system 3B Optical flange 31 entrance aperture, e.g. entrance slit 32 Diffraction Grating 33 Detector Setup 34 Control Unit 4 Guidance device 5. Mounting Means 100 Method of Setting Up and Operating an Optical Emission Spectroscopy Apparatus According to the Present Invention 110 The plasma stand flange and the optical system flange are directly and fixedly connected to each other. 120 Establishing a light-emitting plasma from the sample material (120) 130 Measure the optical spectrum of the light emitted by the plasma 140 Analyze the measured spectrum and compensate for the drift of the spectrum relative to the detector 150 Mask at least the hot part of the plasma with an optical mask A Arc AS Arc / Spark Axis LA Laser L light

Claims

1. An optical emission spectrometer (1) comprising a plasma stand (2) for establishing an emitting plasma from a sample material and an optical system (3) for measuring a spectrum of light (L) emitted by the plasma, which is characteristic of the sample material, the optical system (3) comprising at least one light entrance aperture (31), at least one diffraction grating (32) for diffracting the light (L) coming from the plasma (A) into a spectrum and one or more detectors (33) for measuring the spectrum of the light (L), the plasma stand (2) and the optical system (3) being directly and fixedly mounted respectively on a plasma stand flange (2B) and an optical system flange (3B) which are directly and fixedly connected to each other, the optical emission spectrometer (1) analyzing the measured spectrum and measuring the spectrum of the light (L) emitted by the plasma (A), ... and an analysis unit (34) adapted to compensate for drift of the spectrum relative to the detector (33) potentially caused by heat transferred from the plasma stand (2) to the optical system (3) taking into account thermal expansion of the plasma stand (2).

2. said plasma stand flange (2B) and said optical system flange (3B) are provided with non-adjustable mounting means (5) for connecting both flanges (2B, 3B) together; Characterized in that An optical emission spectrometer (1) according to claim 1.

3. At least the optical base plate and the optical components (32) in the optical path between the entrance slit (31) and the detector (33) are made of the same material. Characterized in that An optical emission spectrometer (1) according to claim 1 or 2.

4. At least the optics flange (3B) is made of metal; Characterized in that An optical emission spectrometer (1) according to any one of claims 1 to 3.

5. The plasma stand flange (2B) is made of an electrically insulating material; Characterized in that An optical emission spectrometer (1) according to any one of the preceding claims.

6. The electrically insulating material is a sandwich material or a thermally stable plastic or ceramic material; Characterized in that An optical emission spectrometer (1) according to claim 5.

7. the connection between the plasma stand flange (2B) and the optical system flange (3B) is implemented as a rigid toothed connection to prevent misalignment between both flanges (2B, 3B); Characterized in that An optical emission spectrometer (1) according to any one of the preceding claims.

8. the plasma stand (21) is provided with an optical mask fixedly connected to the optical system (3) or to the plasma stand (2) and suitably arranged in the optical path between the plasma stand (2) and the optical system (3) for masking at least a hot part of the plasma, Characterized in that An optical emission spectrometer (1) according to any one of the preceding claims.

9. said plasma stand (2) being a spark stand (2) for establishing a discharge (A) between two electrodes (22, 23) for generating an arc / spark plasma of said sample material, one of said electrodes being made from said sample material (23) to be evaporated in said arc / spark (A), the axis of said arc / spark (AS) being arranged perpendicular to said entrance aperture (31); Characterized in that An optical emission spectrometer (1) according to any one of the preceding claims.

10. said plasma stand (2) being a spark stand (2) for establishing a discharge (A) between two electrodes (22, 23) for generating an arc / spark plasma of said sample material, one of said electrodes being made from said sample material (23) to be evaporated in said arc / spark (A), the axis of said arc / spark (AS) being arranged parallel to said entrance aperture (31); Characterized in that An optical emission spectrometer (1) according to any one of the preceding claims.

11. The plasma stand (2) is a laser-induced breakdown spectroscopy (LIBS) stand (2) in which a laser (LA) is focused on the sample material (24) as a target material to generate the plasma; Characterized in that An optical emission spectrometer (1) according to any one of the preceding claims.

12. The optical emission spectrometer (1) is a mobile device. Characterized in that An optical emission spectrometer (1) according to any one of the preceding claims.

13. 13. A method (100) for setting up and operating an optical emission spectrometry apparatus (1) according to any one of claims 1 to 12, comprising a plasma stand mounted directly and fixedly on a plasma stand flange (2B) and an optical system mounted directly and fixedly on an optical system flange (3B) comprising several optical components, comprising: - directly and fixedly connecting (110) said plasma stand flange (2B) and said optics flange (3B) to each other; - establishing (120) a light emitting plasma from the sample material; - generating a spectrum of light emitted by said plasma characteristic of said sample material using an optical system comprising at least one entrance aperture (31) and at least one diffraction grating (32); - measuring (130) said spectrum by one or more detectors (33); - analyzing (140) the measured spectrum and compensating for drift of the spectrum relative to the detector (33) potentially caused by heat transferred from the plasma stand (2) to the optical system (3) taking into account thermal expansion of the optical system (3) by an analysis unit (34).

14. The method (100) of claim 13, further comprising a step (150) of masking at least a high temperature portion of the plasma by an optical mask (21) positioned in the optical path between the plasma stand (2) and an optical system (3) and suitable for fixedly connecting to the optical system (3) or the plasma stand (2), depending on the orientation of the entrance opening relative to the axis of the spark / arc.

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