Easily adjustable optical emission spectrometer

Abstract

The invention relates to an easily adjustable optical emission spectrometer (1), and a method (100) for establishing and operating such a spectrometer (1), the spectrometer (1) comprising a plasma holder (2) for establishing a luminescent plasma from a sample material, and an optical system (3) for measuring the spectrum of light (L) emitted by the plasma, the spectrum being characteristic of the sample material, wherein the optical system (3) comprises at least one light entrance aperture (31), at least one diffraction grating (32) for separating the light (L) coming from the plasma (A) and a method (100) for measuring the spectrum of the light (L). One or more detectors (33) of the optical emission spectrometer (1), wherein the plasma support (2) and the optical system (3) are directly and fixedly mounted on a plasma support flange (2B) and an optical system flange (3B) respectively, which are directly and fixedly connected to each other, and wherein the optical emission spectrometer (1) further comprises an analysis unit (34), which is suitable for analyzing the measured spectrum and compensating for a drift of the spectrum relative to the detector (33) that may be caused by heat transferred from the plasma support (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 optical emission spectrometer which is easily adjustable, and to a method of setting up and operating such a spectrometer. Background Art

[0002] An arc / spark or LIBS optical emission spectrometer (OES) is a device used for chemical analysis and is used to display and measure the emission spectra of chemical substances. With LIBS, the sample material is evaporated and excited by a laser. With arc / spark, the energy for evaporation and excitation is provided by an arc or a series of sparks between an electrode and the material sample, which acts as a counterelectrode. In either case, a plasma is generated from (a portion of) the sample material. The light emitted by the plasma is transmitted to the optical system of the OES, where it is decomposed into its individual spectral components. Each chemical element contained in the sample emits at several characteristic wavelengths (spectral lines) and can therefore be identified. The light intensity of the spectral lines thus measured corresponds to the concentration of the relevant element in the sample.

[0003] For several reasons, in particular to protect the plasma from the ambient air, the excitation usually takes place in a so-called spark stand or plasma stand, which is purged with argon.The light path from the plasma into the optical system may include optical fibers.

[0004] However, wavelengths shorter than 185 nm will be absorbed by the optical fiber and the ambient air, so an argon-purged optical path without optical fiber is required. This optical path only includes the optical windows, lenses, mirrors, etc. (if any), and is called "direct view". To ensure correct direct view of the plasma, the optical system and the plasma holder 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 holder. If this heat is transferred from the plasma holder to the optical system, unwanted thermal expansion can lead to a loss of resolution and performance. Therefore, it is conventional practice to implement some form of thermal insulation between the optical system and the plasma holder, such as by providing a gap or placing a thermally insulating material, such as nylon, between them. This thermal insulation can also provide electrical insulation between the plasma holder and the optical system to prevent electrical flashover to the optical system.

[0006] The mechanical connection is not a rigid connection; instead, the plasma holder and the optical system are designed as (more or less) independent units that can be adjusted individually to achieve proper alignment with each other. This design results in a complex and critical adjustment process for both units. To achieve good optical resolution in OES, the adjustment of the units must be performed carefully. The use of a vacuum or gas-filled optical cavity for observing vacuum ultraviolet (VUV) wavelengths also makes access to the optical system and plasma more difficult, making adjustment even more complicated.

[0007] The following alignment process is used for optical emission spectrometers (OES) according to the prior art: (a) the position and orientation of the plasma holder, which is a freely adjustable plasma holder, is aligned relative to the instrument housing, for example using a plasma holder plate perpendicular to the housing front; (b) the optics or vacuum chamber is aligned within the instrument housing, thereby providing a gas-tight mechanical connection between the optics and the plasma holder; (c) at several locations along the beam path, the optical system within the optics / vacuum chamber is aligned relative to the plasma holder using a fixture, a laser pointer, or an image of the plasma; and (d) a switchable or variable mask is aligned within the plasma holder to shield certain (hot) portions of the plasma from the optics, allowing observation of only the atomic spectral lines in the cooler portion of the plasma without contributing to the electrochemical lines from the hot portion of the plasma. Adjustment steps (a) to (c) are typically performed visually, so the accuracy of each step is preferably around ±0.1 mm. The masking in step (d) is typically adjusted using a spectral line signal, for example the intensity of Fe 187,6 nm. Assuming the plasma is fully visible without the mask, the mask is adjusted to provide an intensity of 25% of the initial intensity of the spectral line. This process results in obscuration of the hot portion of the plasma. Some plasma holders employ permanent obscuration, in which case proper alignment between the plasma holder and the optical system becomes particularly critical. In other systems, the mask can be switchable or adjustable, for example, via a stepper motor, to optimize obscuration depending on the underlying material (matrix) of the sample. If the spectral line in question is sensitive to obscuration, proper obscuration and alignment of the optical system relative to the plasma holder should ensure good repeatability of the measurement and a good detection limit.

[0008] It would therefore be desirable to have an optical emission spectrometer available which has minimal adjustment effort and yet provides very good resolution. Summary of the Invention

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

[0010] This object is achieved by an optical emission spectrometer comprising a plasma holder for generating a luminescent plasma from a sample material and an optical system for measuring a spectrum of light emitted by the plasma, the spectrum being characteristic of the sample material, wherein the optical system comprises at least one light entry aperture, at least one diffraction grating for separating the light from the plasma into a spectrum, and a detector arrangement consisting of a combination of an exit aperture and a PMT detector or a spatially resolved solid-state detector such as a CCD or CMOS line or area sensor, for measuring the spectrum, wherein the plasma holder and the optical system are directly and fixedly mounted on respective plasma holder flanges and optical system flanges which are directly and fixedly connected to each other, and wherein the optical emission spectrometer further comprises an analyzing unit adapted to analyze the measured spectrum and to compensate for drifts of the spectrum relative to the detector caused by heat transferred from the plasma holder to the optical system taking into account thermal expansion of the optical system.

[0011] Plasma can be generated by an electric discharge ignited between two opposing electrodes, one of which is the sample material to be analyzed. Spark or arc atomic emission spectrometry is used to analyze metallic elements in solid samples. An arc or spark passes through the sample, heating it to a high temperature, vaporizing it and exciting the atoms within. The excited sample atoms emit light of a characteristic wavelength, which can be scattered by a diffraction grating and quantitatively detected. Quantitative spark analysis is used for production quality control of foundries and metal casting equipment.

[0012] Alternatively, the plasma can be generated 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 a plasma by atomizing and exciting the sample material. Plasma formation only begins when the focused laser reaches a specific threshold of optical breakdown, which typically depends on the environment and the target material. LIBS utilizes optical emission spectroscopy and, to this extent, is very similar to arc / spark emission spectroscopy.

[0013] The emission lines to be measured can range from the infrared spectrum to the ultraviolet spectrum. Ambient air has a cutoff wavelength of 185 nm and absorbs shorter wavelengths. If shorter wavelengths are to be detected, the optical emission spectrometer must be operated without ambient air (e.g., in an argon atmosphere) to avoid unwanted absorption in the optical system along the light path from the plasma holder to the detector.

[0014] The performance of conventional spectrometers, in particular the spectral resolution, depends on the spectrometer's setup and adjustment, the applied optical components, their arrangement, and the materials used for these components and for the substrate. The present invention avoids any degradation of performance due to non-optimal adjustment, since the direct and fixed connection of the optical system and the plasma support provides a defined and constant adjustment of the two components relative to each other, eliminating any manual misalignment.

[0015] The direct and fixed connection between the plasma support and the optical system results in heat transfer from the plasma support to the optical system, leading to a temperature increase and ultimately thermal expansion of the optical system. This thermal expansion causes thermal drift of the emission lines relative to the detector. Since warm-up of the optical system is a relatively slow process, any emission line drift that occurs can be continuously compensated by analysis software installed and executed on an analysis unit, which includes a processor for executing the software and a data memory for storing measurement data. Drift compensation in the analysis unit and the direct and fixed connection of the plasma to the optical system avoid alignment errors and alignment drift during use, both of which result in good spectral resolution and robust performance over time. Furthermore, avoiding a critical and time-consuming alignment process allows the optical emission spectrometer according to the present invention to be faster and easier to use, resulting in better spectral resolution than devices according to the prior art that require complex and error-prone alignment procedures.

[0016] The optical emission spectrometer according to the invention therefore provides an optical emission spectrometer which has minimal adjustment effort and also provides very good resolution.

[0017] In another embodiment, the plasma support flange and the optics flange include non-adjustable mounting means for connecting the two flanges together. These mounting means ensure easy and simple (self-)alignment of the plasma support flange and the optics flange relative to each other. Furthermore, the position and distance of the flanges relative to each other are fixed by the mounting means. The mounting means used in this embodiment may be screws, pins, or the like, or may be a snap-fit ​​connection.

[0018] In another preferred embodiment, at least the optical system flange and the optical components in the optical path between the entrance slit and the detector are made of the same material. This creates a uniform temperature distribution and prevents temperature fluctuations that could cause unnecessary material wear.

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

[0020] In another embodiment, the plasma support flange is made of an electrically insulating material. The insulating material prevents current caused by the generation of plasma from flowing through the plasma support and the plasma support flange, thereby protecting the plasma support and all components connected thereto. Preferably, the electrically insulating material is a sandwich material such as Trespa or Resopal, or a thermally stable plastic such as polyamide (nylon) or polyetheretherketone (PEEK), or a ceramic material.

[0021] In another preferred embodiment, the connection between the plasma support flange and the optics flange is implemented as a rigid toothed connection (e.g., a Hirth joint) to prevent misalignment between the two flanges. The toothed connection ensures that the connection points mesh with each other, thereby achieving the desired easy and simple alignment on the one hand and preventing misalignment on the other. Therefore, a Hirth joint is preferably used, which can be used to connect two components and is characterized by meshing tapered teeth on the end faces of each component.

[0022] The rigid toothed connection is particularly advantageous when the plasma holder needs to be removed from the optical system during maintenance. The self-adjusting device will ensure an accurate direct view of the protective plasma.

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

[0024] In another preferred embodiment, the plasma holder is a spark holder having a spark chamber containing a (e.g., positively charged) electrode. The sample material encloses the spark chamber and serves as a counter-electrode. The axis of the spark is defined as the connecting line between the sample surface and the (e.g., positively charged) electrode tip. A large temperature gradient exists along the axis of the spark within the plasma. The entrance aperture of the optical system, which is directly connected to the plasma holder, can be oriented parallel to the axis of the spark or perpendicular to the axis of the spark. The 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, as the entrance aperture acts as a pinhole camera with respect to the beam from a direction along the spark axis. The hot part of the plasma is located near the sample surface. The parallel orientation reduces the height of the spectrometer system and forces the optical mask to be close to the plasma to achieve a sharp shielding of at least the hot zone of the plasma.

[0025] In another embodiment, the plasma holder is a laser induced breakdown spectroscopy (LIBS) holder, wherein a laser is focused on a sample material as a target material to generate plasma.

[0026] In order to increase the flexibility of the measurement with respect to position and time and to ensure easy operation of the optical emission spectrometer, it is preferred that the optical emission spectrometer is a mobile device.

[0027] Furthermore, this object is achieved by a method for arranging and operating an optical emission spectrometer as described above. Thus, the optical emission spectrometer comprises a plasma support fixedly mounted directly on a flange of the plasma support and an optical system including several optical components fixedly mounted directly on a flange of the optical system. The method for operating these optical emission spectrometers comprises the following steps:

[0028] directly and fixedly connecting the plasma support flange and the optical system flange to each other;

[0029] establishing a luminescent plasma from a sample material;

[0030] generating a spectrum of light emitted by the plasma using an optical system including at least one entrance aperture and at least one diffraction grating, the spectrum being characteristic of the sample material;

[0031] measuring the spectrum with one or more detectors; and

[0032] An analysis unit analyzes the measured spectrum and, taking into account thermal expansion of the optical system, compensates for drifts of the spectrum relative to the detection that may be caused by heat transferred from the plasma support to the optical system.

[0033] In another embodiment, the method may further comprise the step of shielding at least the hot portion of the plasma by an optical mask adapted to be arranged in the optical path between the plasma support and the optical system and fixedly connected to the optical system or the plasma support depending on the orientation of the optical system relative to the spark axis.

[0034] The embodiments listed above may be used alone or in any combination to provide apparatus and methods according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] These and other aspects of the invention are described in detail in the following figures.

[0036] Figure 1 : A schematic diagram of an embodiment of an optical emission spectrometer according to the present invention, wherein the plasma support and the optical system are aligned but not mounted;

[0037] Figure 2 : A schematic diagram of an embodiment of an optical emission spectrometer according to the present invention, wherein the plasma holder and the optical system are aligned and mounted to each other;

[0038] FIG3 : Schematic diagram of a plasma stand according to the present invention, the plasma stand serving as, an arc / spark stand (a), a LIBS stand with a reflector element (b), and a LIBS stand without a reflector element (c), and (d) a sliding spark spectrometer; and

[0039] Figure 4 : Flowchart of the method according to the present invention. DETAILED DESCRIPTION

[0040] Figure 1 and Figure 2 An optical emission spectrometer 1 is shown, comprising a plasma holder 2 for establishing a luminescent plasma from a sample material, and an optical system 3 for measuring the spectrum of light L emitted by the plasma, the spectrum of light L being characteristic of the sample material. The optical system 3 comprises at least one light entry aperture 31, at least one diffraction grating for diffracting light L from the plasma A into a spectrum, and one or more detectors 33 for measuring the spectrum of light L. These components of the optical system 3 and the associated optical system flange 3B are made of the same material, which is metal to withstand temperatures. The plasma holder 2 and plasma holder flange 2B are also made of the same material, which is also metal. Alternatively, the plasma holder flange 2B can be made of Trespa to electrically insulate the plasma holder 2. However, it is not excluded that the plasma holder flange 2B be made of a suitable plastic, ceramic, or Resopal, or a combination thereof. The optical emission spectrometer 1 also comprises an analysis unit 34 adapted to analyze the measured spectrum and, taking into account thermal expansion of the optical system 3, compensate for any drift in the spectrum relative to the detector 33 that may be caused by heat transferred from the plasma holder 2 to the optical system 3.

[0041] For assembly, the plasma support 2 and its plasma support flange 2B are aligned toward the optical system flange 3B of the optical system 3. Figure 1 The plasma support 2 shown comprises a connecting nozzle 21 which is fixedly connected to the plasma support flange 2B of the plasma support 2. In order to better and more easily align the plasma support flange 2B with the optical system flange 3B, the optical system flange 3B is provided with a guide 4. Figure 1 In the embodiment shown, the guide means 4 are pins. However, the guide means 4 may be rails or similar devices. As a further guide support for aligning the plasma holder 2 and the optical system 3, the entrance aperture 31 and the connecting nozzle 21 have complementary geometries, so that the connecting nozzle 21 can be inserted into the entrance aperture 31.

[0042] When the plasma support 2 is mounted on the optical system 3, Figure 2As shown, the plasma support 2 and the optical system 3 are directly and fixedly mounted on the plasma support flange 2B and the optical system flange 3B, respectively, which are directly and fixedly connected to each other. Therefore, the plasma support flange 2B is mounted to the optical system flange 3B using a non-adjustable mounting device 5, which in this example is a combination of screws and pins 5. Instead of screws, a snap-fit ​​connection can be used.

[0043] In order to measure the individual spectral components in the optical emission spectrometer 1, the light emitted by the plasma generated in the plasma holder 2 must be transmitted. The optical emission spectrometer 1 described here assumes two possibilities for generating the plasma from (a portion of) the sample material. The plasma holder 2 can be an arc / spark holder 2, see Figure 3a ) and Figure 3c), or Laser Induced Breakdown Spectroscopy LIBS Stand 2, see Figure 3a ).

[0044] Figure 3a ) shows a plasma stand 2 as an arc / spark stand 2, wherein an electric discharge A is established between two electrodes 22, 23 to generate an arc / spark plasma of a sample material.

[0045] The counter electrode is made of sample material 23. The axis of the arc / spark AS is arranged substantially perpendicular to the entrance aperture 31. Here, "entrance aperture" relative to the direction of the aperture means the axis of the aperture.

[0046] However, in Figure 3a ), the axis of the arc / spark AS can also be arranged parallel to the incident aperture 31. The established arc / spark A evaporates the sample material electrode 23, generating a plasma that emits light L for spectral analysis in the optical system 3.

[0047] Figure 3b ) and FIG. 3 c) each show a plasma holder 2 as used in laser induced breakdown spectroscopy (LIBS). In LIBS, a sample material 23 is evaporated and excited by a laser LA. The sample material 23 is arranged on a sample holder 24 below the laser LA so that the laser LA is focused on the sample material 23 as a target material to generate plasma. Figure 3b ), the generated plasma is respectively reflected by the light L emitted by the plasma and then focused by the optical device 25, in particular, by the reflector element and the focusing lens in the plasma support 2, so as to transmit the light L to the incident aperture 31 of the optical system 3. This allows for an improvement in the focusing and accuracy of the light L. In the embodiment of FIG. 3 c), the generated plasma respectively transmits the light L emitted by the plasma directly to the incident aperture 31. This embodiment is simpler and allows for spatial shielding of the plasma and a reduction in size.

[0048] Figure 3d ) shows a schematic visualization of a sliding spark spectrometer. An arc A is ignited between two electrodes 22 and slides along a non-conductive sample 23 positioned between the electrodes 22, vaporizing the sample material and converting it into a plasma state. The sliding spark plasma emits characteristic spectral lines of the sample material. The direct light optics can be mounted for self-alignment with the spark holder.

[0049] Figure 4 A method 100 for setting up and operating an optical emission spectrometer 1 as described above is shown. To this end, the plasma support 2, which is directly and fixedly mounted on the plasma support flange 2B, and the optical system 3, which includes a plurality of optical devices 32, 33, which are directly and fixedly mounted on the optical system flange 3B, are initially provided as separate components, such as Figure 2 As shown. After the plasma holder 2 is connected to the optical system flange 3B, if the axis of the entrance aperture and the spark are oriented parallel to each other, an optical mask is appropriately arranged in the optical path of the plasma holder 2 and the optical system 3 and fixedly attached, for example, to the plasma holder 2 to shield at least the hot portion of the plasma. In a second step 120, light L emitted from the plasma by the sample material 23 is generated, thereby generating a spectrum of the light L emitted by the plasma that is characteristic of the sample material 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 detectors 33 of the optical system 3. After the measurement, a fourth step 140 is performed in which the measured spectrum of the light L is analyzed and any drift of the spectrum of the light L relative to the detectors 33 that may be caused by heat transferred from the plasma holder 2 to the optical system 3 is compensated by an analysis unit. In another embodiment, the method may further comprise a step 150 of shielding at least one hot portion of the plasma by means of an optical mask adapted to be arranged in the optical path between the plasma support 2 and the optical system 3 and fixedly connected to the optical system 3 or to the plasma support 2 depending on the orientation of the optical system 3 relative to the axis of the spark AS.

[0050] Reference Number List

[0051] 1. Optical emission spectrometer according to the present invention

[0052] 2 Plasma bracket

[0053] 2B plasma support flange

[0054] 21 Connecting nozzle

[0055] 22 Plasma support electrode

[0056] 23 sample materials

[0057] 24 sample holders

[0058] 3 Optical system

[0059] 3B optical system flange

[0060] 31Entry aperture, such as an entrance slit

[0061] 32 diffraction grating

[0062] 33 detector settings

[0063] 34 control unit

[0064] 4 guide device

[0065] 5. Installation

[0066] 100 Method for establishing and operating an optical emission spectrometer according to the present invention

[0067] 110 Directly and securely connect the plasma support flange and the optical system flange together

[0068] 120 Establishing (120) luminescent plasma from sample material

[0069] 130Measure the spectrum of light emitted by the plasma

[0070] 140 Analyzes the measured spectrum and compensates for spectral drift relative to the detector

[0071] 150 Shielding at least the hot portion of the plasma by an optical mask

[0072] A arc

[0073] AS Axis of the arc / spark

[0074] LA Laser

[0075] L light

Claims

1. An optical emission spectrometer (1), characterized in that: The optical emission spectrometer (1) comprises a plasma holder (2) for establishing a luminescent plasma from a sample material, and an optical system (3) for measuring a spectrum of light (L) emitted by the plasma, the spectrum of the light (L) being characteristic of the sample material, wherein the optical system (3) comprises at least one light entrance aperture (31), at least one diffraction grating for diffracting the light (L) from the plasma into a spectrum, and one or more detectors (33) for measuring the spectrum of the light (L), wherein the plasma holder (2) and the optical system (3) are respectively directly and fixedly mounted on a plasma holder flange (2B) and an optical system flange (3B) which are directly and fixedly connected to each other, and wherein the optical emission spectrometer (1) further comprises an analyzing unit (34) adapted to analyze the measured spectrum and to compensate for a drift of the spectrum relative to the detector (33) caused by heat transferred from the plasma holder (2) to the optical system (3) taking into account thermal expansion of the optical system (3).

2. The optical emission spectrometer (1) according to claim 1, characterized in that , The plasma support flange (2B) and the optical system flange (3B) include non-adjustable mounting means (5) connecting the two flanges (2B, 3B) together.

3. The optical emission spectrometer (1) according to claim 2, characterized in that At least one optical system substrate and optical components in the optical path between the incident aperture (31) and the detector (33) are made of the same material.

4. The optical emission spectrometer (1) according to claim 3, characterized in that At least the optical system flange (3B) is made of metal.

5. The optical emission spectrometer (1) according to claim 4, characterized in that The plasma support flange (2B) is made of an electrically insulating material.

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

7. The optical emission spectrometer (1) according to claim 6, characterized in that The connection between the plasma support flange (2B) and the optical system flange (3B) is realized as a rigid tooth connection to prevent misalignment between the two flanges (2B, 3B).

8. The optical emission spectrometer (1) according to claim 7, characterized in that The plasma support (2) comprises an optical mask which is fixedly connected to the optical system (3) or the plasma support (2) and is suitably arranged in the optical path between the plasma support (2) and the optical system (3) to shield at least a hot part of the plasma.

9. The optical emission spectrometer (1) according to claim 8, characterized in that The plasma holder (2) is a spark holder for establishing an electric discharge between two electrodes (22) to generate an arc / spark plasma of the sample material, wherein one electrode is made of the sample material (23) evaporated in the arc / spark, wherein the axis (AS) of the arc / spark is arranged perpendicular to the incident aperture (31).

10. The optical emission spectrometer (1) according to claim 8, characterized in that The plasma holder (2) is a spark holder established between two electrodes (22) to generate arc / spark plasma of the sample material, wherein one electrode is made of the sample material (23) evaporated in the arc / spark, wherein the axis (AS) of the arc / spark is arranged parallel to the entrance aperture (31).

11. The optical emission spectrometer (1) according to claim 10, characterized in that The plasma holder (2) is a laser induced breakdown spectroscopy (LIBS) holder, in which laser light (LA) is focused on the sample material (23) as a target material to generate the plasma.

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

13. A method (100) for setting up and operating an optical emission spectrometer (1) as claimed in any one of the preceding claims, characterized in that The optical emission spectrometer (1) comprises a plasma support directly and fixedly mounted on a plasma support flange (2B) and an optical system comprising a plurality of optical components directly and fixedly mounted on an optical system flange (3B). The method comprises the following steps: directly and fixedly connecting the plasma support flange (2B) and the optical system flange (3B) to each other (110); establishing (120) a luminescent plasma from a sample material; generating a spectrum of light emitted by the plasma using an optical system comprising at least one entrance aperture (31) and at least one diffraction grating, the spectrum being characteristic of the sample material; measuring (130) the spectrum by one or more detectors (33); The spectrum measured is analyzed (140) by an analysis unit (34) and a drift of the spectrum relative to the detector (33) caused by heat transferred from the plasma support (2) to the optical system (3) is compensated taking into account thermal expansion of the optical system (3).

14. The method according to claim 13, wherein Also included is shielding (150) at least a hot portion of the plasma by an optical mask (21), the optical mask (21) being adapted to be arranged in the optical path between the plasma support (2) and the optical system (3) and fixedly connected to the optical system (3) or the plasma support (2) depending on the orientation of the entrance aperture relative to the axis of the spark / arc.

Images