Method for determining central wavelength of spectral line with high accuracy and associated system

Abstract

The invention relates to a method (100) for determining a central wavelength of interest ([lambda] c) of a spectral line of interest, comprising the following steps: A, detecting a first reference measurement profile (PS1ref) at time t1; b, then detecting a measurement profile of interest (PSeck) derived from the sample of interest at a time t0; c, then detecting a second reference measurement profile (PS2ref) derived from the reference source (Sref) at time t2; d, processing the first reference measurement profile and the second reference measurement profile, and processing the measurement profile of interest; e, determining said reference position at time t0, referred to as an intermediate reference position (P0ref), by interpolation; f, determining the value of the central wavelength of interest based on the difference between the position of interest (Peck) and the intermediate reference position (P0ref), the known value of the reference wavelength and the linear dispersion (DL) of the spectrometer and associated detector.

Description

Field of the Invention

[0001] The present invention relates to the field of spectroscopy and, more particularly, to determining the central wavelength of spectral lines with very high precision. Background Art

[0002] For some spectroscopy applications, for example, in atomic or molecular spectroscopy, or for determining the isotopic abundances of elements in a sample using an optical method (referred to as LIBRIS (laser-induced breakdown self-reversal isotope spectroscopy), see below), high precision is required when determining the value of the central wavelength of a spectral line. The spectral line to be characterized is generated by a light source and can be an atomic or molecular absorption or emission line. For the value of the central wavelength, an uncertainty of less than 5 pm or even less than 1 pm is typically sought.

[0003] So far, this problem has not arisen in the field of laser ablation plasma spectroscopy (using techniques such as LIBS (laser-induced breakdown spectroscopy) or laser-induced plasma optical emission spectroscopy, LAMIS (laser ablation molecular isotope spectroscopy), etc.) because the width of the observed lines is typically several tens of pm. Therefore, depending on the linear dispersion of the spectrometer used, the wavelength of the line is typically measured with an uncertainty of about 10 pm to several tens of pm. This uncertainty does not affect these techniques because the analysis is performed based on the intensity of the line integrated over a width of the same order of magnitude, typically from about ten to several tens of pm.

[0004] Conventionally, the detection system is calibrated in terms of wavelength by means of a reference source (usually a mercury vapor lamp or a hollow cathode lamp) that emits a known line. The positions of the line to be analyzed and the reference line are identified as pixels on the detector, and the line to be analyzed is determined based on its relative position with respect to the position of the reference line. The detector includes at least N pixels Pi aligned in a row, where i varies from 1 to N. When it is 2D, all pixels in the same column are integrated. For example, the detector is a CCD array with 2048×512 pixels.

[0005] Assume that λref is the central wavelength of the reference line, λ0 is the wavelength to be determined, Pref is the position of λref identified as a pixel of the detector, and P0 is the position of λ0 on the detector. Of course, the wavelength λ0 is chosen such that for the same configuration of the spectrometer, it appears at λ0 on the detector simultaneously. Then:

[0006] λ 0 = λ ref +(P 0 -P ref ).DL (1),

[0007] where DL is the linear dispersion of the detection system, usually in pm / pixel.

[0008] Due to various reasons (thermal fluctuations, vibrations, etc.), spectrometers and detectors drift very slightly even in a controlled environment in a research laboratory, which leads to wavelength drift. Of course, this drift is even more pronounced in analytical situations outside the laboratory (in the field, online, using portable systems, etc.). For example, in the case of a 1m focal array spectrometer with a grating of 2400 lines / mm, a change in the grating angle of only 10 -3 degrees causes a wavelength shift of 10 pm, which is unacceptable for LIBRIS analysis of lithium, for example, where the goal is an uncertainty of less than 1 pm in order to obtain 6 an acceptable uncertainty in the isotopic abundance of Li.

[0009] The detection of the reference line and the detection of the line to be analyzed are carried out sequentially over time. In the most common case, the signal originating from the sample is routed to the detection system via an optical fiber. In order to perform two measurements, it is then worth first positioning the optical fiber connected to the spectrometer in order to collect the light flux originating from the reference source, and then collecting the light flux originating from the emission to be characterized, and vice versa, which requires a certain amount of time. Usually, these two measurements are separated by a duration of approximately one minute, which is sufficient for this drift to occur.

[0010] Therefore, it is impossible to perform accurate LIBRIS measurements without correcting for the wavelength drift of the detection system. This problem occurs in the same way in atomic spectroscopy where it is intended to accurately measure λ0 by sampling a reference source.

[0011] The present invention is particularly concerned with the LIBRIS method, the principles of which, as well as the principles of the LIBS and LAMIS methods, are provided below as a reminder.

[0012] Figure 1 The principle of the LIBS technique shown is to focus a laser pulse on the surface of a material sample (or material) in order to generate a transient plasma, the optical emission of which is analyzed by a spectrometer. By collecting the light emission from the plasma and analyzing the spectrum using spectroscopy, the elements present in the plasma can be identified and thus the composition of the material can be determined based on a transmission line database. For LIBS, the intensity is integrated over the entire width of the line.

[0013] For example, the LAMIS technique was described by R. Russo et al. in a publication in Spectrochim. Acta B66(2011)99 is an alternative derived from LIBS that allows isotopic analysis based on lines of molecules formed by reactions between the constituent elements of the ablated material and the ambient medium or by reactions between two atoms of the ablated material.

[0014] The laser generator L0 generates a laser beam FL0, which is focused on the sample 1 by means of the first optical system 2. This generates a plasma PI0. The plasma emits an optical emission 3 that is collected by the optical system OS0. The focused optical emission is sent to the spectrometer Spec0 via the optical fiber FO. The spectrometer Spec0 includes a detector Det0 that is synchronized with (or associated with) the laser generator L0. The spectrometer Spec0 allows the recording of a line spectrum. Finally, a processing device UT0 is used to process the recorded spectrum.

[0015] LIBS allows the generation of a spectrum 20 in the form of a set of spectral lines corresponding to the emission lines of the elements forming the material, and allows the use of the available correlation data between the emission lines and the elements to determine the elemental composition of the material sample. The wavelength λ of the line provides information about the elements present in the material, and the intensity I is related to the concentration of the element.

[0016] LIBS emission spectrometry is also applicable to isotope analysis because the atomic lines of various isotopes of the same element are at slightly different wavelengths. This spectral shift, called the isotope shift, is due to the mass effect (mainly for light elements) and the change in the charge distribution within the core (mainly for heavy elements). When intending to use LIBS for such isotope analysis, the lines of the two isotopes must be separated. However, this spectral shift is typically on the order of a fraction of a nm or even a few pm, as shown in Table I below:

[0017] isotope emission line isotope shift 7Li→6Li 670.775 nm +15.8 pm 10B→11B 208.891 nm -2.5 pm 238U→235U 424.437 nm +25 pm 239Pu→240Pu 594.522 nm +13 pm

[0018] Table I

[0019] It is difficult to observe this shift in the plasma generated by laser ablation under normal conditions because the confinement of the ambient air to the plasma at atmospheric pressure results in a high density, and thus the emission lines are broadened due to the Stark effect. Even if the spectrometer used has sufficient spectral resolution to overcome this shift, this broadening typically reaches several tens or even hundreds of pm, and thus masks the isotope shift. In this case, the limitation is physical and not instrumental.

[0020] The first solution involves performing the analysis at reduced pressure or even in a vacuum. Thus, by limiting the confinement of the ambient medium to the plasma, its density is reduced, and sufficient spectral selectivity can be found for some isotopes. Doublet lines are seen, and the isotope ratio is determined based on the intensity ratio between the two lines associated with the two isotopes. This method is not applicable to all isotopes and requires high resolution, and thus requires a bulky spectrometer. The second solution involves sending a second laser beam through the plasma in order to measure the resonance or fluorescence absorption signal, thus limiting the measurement system and complicating it.

[0021] In the prior art of isotope analysis at atmospheric pressure, the LAMIS technique can also be used, but this assumes that several conditions are met: 1. Molecules must be formed in the plasma; 2. They must be stable enough under the temperature / density conditions of the plasma; 3. They must have detectable lines, i.e., have a sufficient lifetime, be strong enough, and be in the spectral band of the detection system. In the case of lithium, for example, no LAMIS signal was detected, probably because the second condition was not met.

[0022] The LIBRIS technique is an optical technique for determining the isotope abundances of elements in a sample (solid, liquid, or gas) based on the emission spectrum of a laser ablation plasma. This technique is described, for example, in the publication by K. Touchet et al. in Spectrochim. Acta B 168 (2020) 105868 and in the document US2019 / 0041336. It is a variant of the LIBS technique and uses the same optical system. The LIBRIS technique allows various drawbacks of the LIBS method to be overcome by allowing the measurement of isotope ratios at atmospheric pressure and without the need for a second laser.

[0023] As a reminder, the electronic transition of an atom to a higher energy level requires an energy input. This energy can be in the form of a photon, in which case the photon is absorbed by the atom. A particular case is that of a laser ablation plasma. For the sake of simplicity, it can be considered that the plasma consists of two distinct parts, a core and a periphery. Photons emitted by the hotter core of the plasma can be absorbed by the colder periphery. Thus, this phenomenon prevents a certain number of emitted photons from leaving the plasma: this is the self-absorption phenomenon.

[0024] For an observer outside the plasma, and for the measuring device, the profile of the line is produced by the emission and self-absorption at the same wavelength corresponding to the electronic transition between the two energy levels of all the considered atoms placed on its line of sight. Thus, the measured intensity is not only the sum of all the emissions of the plasma, as this self-absorption needs to be taken into account.

[0025] The self-absorption phenomenon (which is well known in plasma spectroscopy for elemental analysis) is more considered an undesirable phenomenon because it causes a distortion of the line profile and thus a non-linearity of the signal with respect to the concentration of the element of interest. LIBRIS uses this self-absorption effect to derive information about the isotopes of a given element in a material.

[0026] Figure 2 and Figure 3 Shows the line RS0 of the element of interest selected from the spectrum 20 obtained in two scenarios depending on the concentration of the element in the material.

[0027] Figure 2Shows a scenario where the concentration of elements in the plasma is low and the self - absorption phenomenon is scarcely distinguishable or even non - existent. A spectral - wide line profile is obtained, with no valley at its center. The dotted and dashed curves ISO 1 and ISO 2 represent the emissions of two isotopes. Considering the difference between the two lines, the width of each line is significant, mainly due to the Stark effect in the plasma, and for this reason, they are not separately distinguishable: the line is detected as the solid line RS0, which corresponds to the sum of the two. The principle of LIBRIS is that the center wavelength of the line as a solid line varies with the isotope abundance, i.e., with the ratio of the amplitudes of the two dotted and dashed lines. In this case, the value of the center wavelength λ0 corresponding to the emission peak is measured, i.e., the maximum point or vertex 20 of the observed curve presenting a bell - shaped profile. It is related to the ratio between the two isotopes Iso 1 and Iso 2 of the element considered, and it shifts as a function of the said isotope ratio.

[0028] Figure 3 Shows the case where the element is highly concentrated in the plasma, and then the self - absorption phenomenon is distinguishable. A line profile with a valley at its center (double - bell - shaped profile), called an inverted line, can be seen, which is produced by covering the spectral - wide emission profile with a spectral - narrow absorption profile. In this case, the value of the center wavelength λ0 corresponding to the absorption valley is measured. In this case, on the part of the profile corresponding to the absorption, i.e., at the minimum point 30 of the observed valley, the center wavelength λ0 is measured. It is related to the ratio between the two isotopes Iso 1 and Iso 2 of the element considered, and it shifts as a function of the said isotope ratio. It is this wavelength measurement of the valley that defines the LIBRIS technique.

[0029] Therefore, in the LIBRIS technique, the isotope ratio is measured based on the maximum bell - shaped line in the double - bell, the minimum of the line (called the inverted line), or a very precise measurement of the wavelength λ0. This wavelength λ0 shifts linearly between λ R 1 and λ R 2 where the indices 1 and 2 refer to the two isotopes of the element. λ R 1 and λ R 2 are physical data available in spectral databases and / or scientific publications. Therefore, the analytical uncertainty regarding the isotope abundance is directly related to the uncertainty in determining the wavelength λ0.

[0030] In the LIBRIS technique, measuring λ0 directly yields the isotope ratio.Figure 4 shows this evolution of λ0 measured as a function of the ratio of the two 6 Li and 7 Li isotopes of lithium- 6 Li isotope. This curve has been generated on an inverted line. The isotope shift is given by λ R 1 -λ R 2 and corresponds to the degree of measurement of the technique for a given line. In the case of lithium and for the 670.778 nm line used in LIBRIS, this shift is 15.8 ± 0.3 pm and thus corresponds to 6 the total variation of the isotope abundance of 7 Li from 0% to 100%, where any complement is R the abundance of R Li. Thus, an uncertainty of 1 pm in determining the wavelength λ

[0031] The object of the present invention is to overcome the above disadvantages by proposing a method and a system for determining the central wavelength of an atomic or molecular absorption or emission line generated by a light source with sub-picometer accuracy. Summary of the Invention

[0032] The object of the present invention is a method for determining the central wavelength of interest of a spectral line of interest measured by a spectrometer, said spectral line of interest corresponding to the emission or absorption of a sample to be characterized, said spectral line of interest having: a bell-shaped profile, wherein the central wavelength of interest then corresponds to the vertex of the bell-shaped profile; or a double-bell-shaped profile, wherein the central wavelength of interest then corresponds to the valley between the two bells, the spectrometer being associated with a detector, the detector comprising a plurality of pixels aligned in the direction X, the spectral line of interest being detected on the pixels of the detector, the method comprising the following steps:

[0033] -A. Detecting, at time t1, a first reference measurement profile derived from a reference source having a reference spectral line having a central wavelength with a known value called the reference central wavelength, the reference wavelength being selected so as to be detected on at least one pixel of the detector;

[0034] ●B. Then detecting, at time t0, an interesting measurement profile derived from the sample of interest;

[0035] ●C. Then detecting, at time t2, a second reference measurement profile derived from the reference source;

[0036] ●D. Process the first reference measurement profile and the second reference measurement profile to determine a first reference position and a second reference position of the reference wavelength, and process the measurement profile of interest to determine a position of interest of the central wavelength;

[0037] ●E. Based on the first reference position and the second reference position, and based on a linear variation law of the reference position as a function of time between times t1 and t2, determine by interpolation the reference position called the intermediate reference position at time t0;

[0038] ●F. Determine the value of the central wavelength of interest based on the difference between the position of interest and the intermediate reference position, the known value of the reference wavelength, and the linear dispersion of the spectrometer and the associated detector.

[0039] According to one embodiment, the variation law is linear.

[0040] According to one embodiment, the processing step D includes the following sub-steps: adjust the values of the measurement profile of interest and the first and second reference measurement profiles with a known mathematical function to determine the position of interest and the first and second reference positions by interpolation with a precision less than one pixel, and then also determine the intermediate reference position with a precision less than one pixel.

[0041] According to one embodiment, the optical signal originating from the sample is pulsed.

[0042] According to one embodiment, the optical signal source originating from the sample is the emission of a plasma emitted by the sample irradiated with a pulsed laser.

[0043] According to one embodiment, the method according to the invention is adapted to determine the isotope abundance of elements present in the sample, the central wavelength of interest corresponding to a line produced by the contributions of two isotopes of the element, wherein the value of the central wavelength of interest allows the determination of the abundance.

[0044] The invention also relates to a system for measuring the central wavelength of interest of a spectral line of interest measured by a spectrometer, the spectral line of interest corresponding to the emission or absorption of a sample to be characterized, the spectral line of interest having: a bell-shaped profile, wherein the central wavelength of interest then corresponds to the vertex of the bell-shaped profile; or a double-bell-shaped profile, wherein the central wavelength of interest then corresponds to the valley between the two bells, the measurement system comprising:

[0045] - Detection system, including a spectrometer, the spectrometer being associated with a detector, the detector including a plurality of pixels aligned in direction X, the spectral line of interest being detected on the pixels of the detector;

[0046] - The system is configured such that the detector:

[0047] - Detects a first reference measurement profile derived from a reference source at time t1, the reference source having a reference spectral line with a central wavelength having a known value called the reference center wavelength, the reference wavelength being selected to be detected on at least one pixel of the detector;

[0048] o Then detects an interesting measurement profile derived from the sample of interest at time t0;

[0049] o Then detects a second reference measurement profile derived from the reference source at time t2;

[0050] - The system further includes a processing unit, the processing unit being configured to:

[0051] ● Process the first reference measurement profile and the second reference measurement profile to determine a first reference position and a second reference position of the reference wavelength, and process the interesting measurement profile to determine an interesting position of the central wavelength;

[0052] ● Based on the first reference position and the second reference position, and based on the linear variation law of the reference position as a function of the time between t1 and t2, determine the reference position called the intermediate reference position at time t0 by interpolation;

[0053] ● Based on the difference between the interesting position and the intermediate reference position, the known value of the reference wavelength, and the linear dispersion of the detection system, determine the value of the interesting central wavelength.

[0054] According to one embodiment, the measurement system according to the present invention is adapted to measure the isotope abundance of elements present in the sample, and further includes:

[0055] - A pulsed laser, the pulsed laser being configured to irradiate the sample to generate a plasma, the plasma enabling the emission of the optical signal originating from the sample;

[0056] - An optical fiber, configured such that the input collects the optical signal originating from the sample or the optical signal originating from the reference source, and the output is coupled to the input of the spectrometer;

[0057] The processing unit is configured to synchronize the detector with the laser when detecting the measurement profile of interest, the central wavelength of interest corresponding to a line resulting from the contributions of two isotopes of the element, the value of the central wavelength of interest allowing determination of the isotope abundance.

[0058] Finally, the present invention relates to a computer program comprising instructions which cause a system according to the present invention to execute the steps of a method according to the present invention.

[0059] The following description describes several embodiments of the device of the present invention: these examples in no way limit the scope of the present invention. These embodiments describe the basic features of the present invention and additional features related to the considered embodiments. Description of the Drawings

[0060] The present invention will be better understood from the following detailed description and with reference to the drawings, and its further features, objects and advantages will become apparent, the drawings being provided by way of non-limiting example, and in which:

[0061] Already cited Figure 1 Shows the measurement principles using LIBS, LAMIS and LIBRIS techniques;

[0062] Already cited Figure 2 Shows the spectral lines measured in the presence of a low-concentration element in the plasma, the self-absorption phenomenon then being barely distinguishable or even negligible;

[0063] Already cited Figure 3 Shows the spectral lines measured in the presence of a high-concentration element in the plasma, the self-absorption phenomenon then being distinguishable;

[0064] Already cited Figure 4 Shows as a function of the isotope abundance of lithium- 6 Li isotopes in the sample, the evolution of the central wavelength λ0 measured;

[0065] Figure 5 Shows the method according to the present invention;

[0066] Figure 6 Shows the measurement profile of interest PSech, the first reference measurement profile PS1ref and the second reference measurement profile PS2ref, the first reference theoretical profile PST1ref, the second reference theoretical profile PST2ref and the theoretical profile of interest PSTech;

[0067] Figure 7 Shows the system according to the present invention;

[0068] Figure 8Shows a system suitable for measuring the isotope ratio of elements present in a sample according to the present invention;

[0069] Figure 9 Shows data obtained by repeating the measurement 17 times (measurement i numbered from 1 to 17). For each measurement i, on the one hand, the rough value λc B (i) (cross) is determined by the method 100 according to the present invention, and on the other hand, the corrected value λc(i) (dot) is determined;

[0070] Figure 10 Shows the average value and standard deviation σ of these 17 measured values in two cases, which are (λ B m, σ B ) and (λ c m, σ c ), which are rough and corrected with λref of the HCL lamp. Detailed implementation mode

[0071] The present invention relates to a method 100 for measuring the central wavelength λc of an interesting spectral line RSe measured by a spectrometer, as Figure 5 shown. The present invention also relates to a system 10 for measuring the central wavelength.

[0072] The interesting spectral line corresponds to the emission or absorption of the sample Ech to be characterized and has: a bell-shaped profile, where λc then corresponds to the wavelength of the vertex of the bell-shaped profile; or a double-bell-shaped profile, where λc then corresponds to the wavelength of the valley between the two bells.

[0073] The spectrometer for performing the measurement includes a detector Det (or associated with the detector Det), and the detector Det includes a plurality of pixels Pi aligned in the direction X, where i is a pixel index that varies from 1 to N. The interesting spectral line RSe is detected on the pixels of the detector Det.

[0074] In the first step A of the method 10 according to the present invention, at time t1, a first reference measurement profile PS1ref derived from a reference source Sref having a reference spectral line RSref is detected. The reference spectral line RSref has a central wavelength called the reference central wavelength with a known value λref. The reference wavelength is selected so as to be detected on at least one pixel of the detector Det. In order to detect and generate the measured spectral profile PS1ref, an optical signal SL1ref derived from the source Sref is injected at the input of the spectrometer Spectro. The reference source is selected according to the spectral characteristics of the sample to be analyzed.

[0075] Measuring the spectral profile PS1ref is a set of measurement points indexed by the pixels Pi of the detector, and each index i is associated with a detection intensity I1i.

[0076] Then, in step B, at time t0 (i.e., t0 > t1), a measurement profile of interest PSech derived from the sample to be characterized is detected. For this purpose, the optical signal SLech originating from the sample Ech is injected into the input of the spectrometer Spectro. The measured spectral profile PSech is a set of measurement points according to the detector pixel indices, and each index j is associated with a detection intensity l0j.

[0077] A source Sref is selected such that the detector detects the line RSref without modifying the adjustment of the spectrometer associated with the detection of RSe. According to one embodiment, the pixels of the detector for the detection line RSe can be located in a region of the detector different from the region of the detection line RSref. According to another embodiment, the lines RSref and RSe are located at the same position on the detector. Then it is recommended to turn off the reference source when measuring the sample, or to ensure that SLref is negligible in view of SLech.

[0078] Then, in step C, at time t2 (i.e., t2 > t1), a second reference measurement profile PS2ref derived from the reference source Sref is detected. For this purpose, the optical signal SL2ref originating from the source Sref is again injected into the input of the spectrometer Spectro.

[0079] The measurement profiles PS1ref and PS2ref correspond to the spectral lines RSref measured at two different times t1 and t2, which frame the measurement of the spectral line RSe of interest. Thus, t1 < t0 < t2: The detector Det sequentially detects over time the signal originating from the reference source at t1, the signal originating from the sample at t0, and the signal originating from the reference source again at t2.

[0080] In step D, the first reference measurement profile PS1ref and the second reference measurement profile PS2ref are processed to determine a first reference position P1ref of the reference wavelength λref and a second reference position P2ref of the reference wavelength. These two positions are measured as pixels of the detector, where the index i forms the abscissa of the detected spectrum. In step D, the measurement profile of interest PSech is also processed in order to determine an interesting position Pech of the central wavelength λc, still measured as a pixel of the detector.

[0081] The wavelength drift of the detection system [spectrometer + detector] is measured by the difference (P2ref - P1ref).

[0082] In step E, based on P1ref, P2ref and on a linear variation law of the reference position Pref(t) as a function of the time between times t1 and t2, the reference position P0ref at a time t0 called the intermediate time is determined by interpolation.

[0083] Finally, in step F, the value of the central wavelength λc of interest is determined based on the difference between Pech and P0ref, the known value of the reference wavelength λref and the linear dispersion DL of the detection system [spectrometer + detector] (usually in pm / pixel).

[0084] Generally, the following formula is available:

[0085] λ c = λ ref +(P ech - P0 ref ).DL (1)

[0086] Of course, the value DL corresponding to the spectral region in which λref and λ0 are located should be adopted.

[0087] Using the method according to the invention, by establishing a linear variation law of the reference position as a function of time and taking into account the drift of the detection system, the position of the reference wavelength is determined more precisely as a pixel of the detector. This allows λc to be determined with improved precision compared to measurements considering only P1ref (measuring the reference before measuring the spectrum of interest) or P2ref (measuring the reference after measuring the spectrum of interest). Using the method according to the invention, a very low uncertainty about the value of λc is obtained.

[0088] According to a preferred embodiment, the time difference between two measurements of the spectrum of Sref is low, typically on the order of one minute or less. The low time between two measurements of the spectrum Sref guarantees a reproducible linear variation between t1 and t2. In fact, to actually perform the measurement, the input of the optical fiber FOp is moved, and the output of the optical fiber FOp is coupled to the input of the spectrometer Spectro in order to collect the optical signal originating from the source Sref (SL1ref for the first measurement and SL2ref for the second measurement) or the optical signal SLech originating from the sample to be characterized (see Figure 7 and Figure 8 below). The time to complete this task (to which the spectral registration time will be added) is typically less than one minute.

[0089] For the linear drift of Pref(t), then:

[0090] P ref (t 0 ) = P0 ref = a.t 0+b (2)

[0091] Based on the measurement at time t 1 and t 2 the parameters a and b are determined as follows:

[0092]

[0093] b = P ref (t 2 ) - a·t 2 (4)

[0094] The detector Det sequentially detects the first reference signal, the signal of interest, and the second reference signal over time. This detection generates a first reference measurement profile PS1ref, a measurement profile of interest PSech, and a second reference measurement profile PS2ref, as Figure 6 shown. The abscissa of the profiles is the index i of the pixels Pi of the detector, and the ordinate is the intensity detected for each pixel I1i, I0i, and I2i respectively.

[0095] In order to be able to measure λc with very high precision, it is intended to obtain its position Pech with a precision better than that of the pixels of the detector, i.e., as a fractional measurement of the integer index i; similarly for the positions P1ref and P2ref. For this purpose, according to one embodiment, the processing step D includes the following sub-steps: adjusting the values of the reference measurement profiles PS1ref and PS2ref and the measurement profile of interest PSech using known mathematical functions in order to determine the position of interest and the reference positions with a precision less than one pixel by interpolation, as Figure 6 shown.

[0096] Thus, the theoretical profiles, a first theoretical reference profile PST1ref, a second theoretical reference profile PST2ref, and a theoretical profile of interest PSTech, are also shown in Figure 6 and they are adjusted to the experimental points as much as possible. Generally, the mathematical functions used are selected from the following: Gaussian, Lorentzian, Voigt.

[0097] Figure 6 It is shown that without this adjustment, the determined position would correspond to the pixel k of the maximum of the measured spectrum. Thus, it would not be possible to have a wavelength precision better than the interval between two adjacent pixels. With the aid of these theoretical profiles, the positions P1ref, P2ref, and Pech are determined as fractions of pixels (usually with a precision up to two decimal places), and the precision is greatly improved.

[0098] According to one embodiment, the optical signal originating from the sample SLref is pulsed. Preferably, the optical signal source originating from the sample SLech is the emission of the plasma emitted by the sample irradiated by a pulsed laser.

[0099] According to one embodiment, the method according to the present invention is associated with an implementation of LIBRIS technology, i.e., it is adapted to accurately measure the isotope ratios of the elements present in the sample Ech. The optical signal SLech originates from the emission of the plasma PI emitted by the sample Ech irradiated by the pulsed laser L. The central wavelength of interest corresponds to the line produced by the contributions of two isotopes of the element, and the exact value of the central wavelength λc of interest allows the determination of the isotope ratio, as described above. Preferably, in step B, the detector Det is synchronized with the laser L.

[0100] The system 10 according to the present invention is shown in Figure 7 which. It includes a spectrometer Spectro associated with a detector Det, and the detector Det includes a plurality of pixels Pi aligned in the direction X. Generally, the detector is of the intensified CCD type. An example is i, where i is a pixel index that varies from 1 to 2048. When the detector is preferably an array, the intensities are summed in the vertical direction of the columns.

[0101] The system is also configured such that the detector Det detects a first reference measurement profile PS1ref derived from a reference source Sref at time t1, then detects an interested measurement profile PSech derived from the sample of interest at time t0, and then detects a second reference measurement profile PS2ref derived from the reference source Sref at time t2. To this end, according to Figure 7 an embodiment shown, the input E of the optical fiber FOp is moved according to the signal to be detected, and then the measurement is performed with the spectrometer.

[0102] The system also includes a processing unit UT configured to implement steps D, E, and F.

[0103] The source Sref is selected according to the wavelength λc of interest: in fact, λref needs to be close enough to λc such that the two wavelengths can be detected on the detector without changing the adjustment of the spectrometer. Generally, Sref is a hollow cathode lamp that continuously emits several photons. In the case where the signal from the sample is usually strong and short, in this case, the acquisition parameters of the detection system are different for the detection of the two signals (derived from the reference and the sample).

[0104] These parameters are (non-exhaustive list): the delay of the measurement relative to the laser emission (only for the sample signal), the width of the acquisition time gate, the number and rate of accumulation, the gain of the detector, the averaging of the signal.

[0105] These parameters are for example as follows:

[0106] sample reference measurement delay relative to laser emission 1 μs not applicable width of acquisition time gate 500 ns 200 ms number of accumulations and rate 20 to 20 Hz 10 to 3 Hz detector gain 3000 3000 signal averaging on... 10 acquisitions 1 acquisition

[0107] Table II

[0108] According to one embodiment, the system 10 according to the present invention is adapted to measure a sample signal originating from a plasma, as Figure 8 shown. According to one embodiment, the system further includes a pulsed laser 1 configured to irradiate a sample to generate a plasma PI capable of emitting an optical signal SLech originating from the sample. It also includes an optical fiber FOp configured such that its input collects an optical signal originating from the sample or an optical signal originating from a reference source, and its output S is coupled to the input of a spectrometer Spectro.

[0109] The processing unit UT is configured to synchronize the detector Det with the laser L when a measurement profile of interest is detected.

[0110] Preferably, in addition to the laser L, the system 10 further includes an optical device 2 for focusing the laser beam onto the sample, and an optical system SO configured to inject a portion of the optical signal originating from the sample into the input end E of the optical fiber.

[0111] According to one embodiment, the system 10 according to the present invention is adapted to measure the isotope ratio of elements present in a sample. Then, the central wavelength of interest corresponds to a line generated by the contributions of two isotopes of the element, and the value of the central wavelength of interest allows determination of the isotope abundance.

[0112] Results demonstrating the advantages of the framing correction method are briefly provided below. A Jobin Yvon THR1000 spectrometer with a 2400 lines / mm grating centered at 670 nm was used. The detector is an Andor iStar 2048×512 pixel intensified camera with a linear dispersion DL of 2.774 pm / pixel at 670 nm.

[0113] The lines of a mercury vapor lamp were measured in the presence of drift in the spectrometer, and the lines of the mercury vapor source are the spectral lines of interest.

[0114] Before and after detecting the spectral lines of interest, the lines of a reference source formed by a lithium hollow cathode lamp (HCL) were measured, which is precisely known and equal to λref = 670.776 nm.

[0115] Then, the drift of the spectrometer was corrected according to the method 100 according to the present invention.

[0116] The acquisition parameters are provided in Table III below.

[0117] width of acquisition time gate 50 ms number of accumulations and rate 100 to 18 Hz detector gain 4000

[0118] Table III

[0119] Figure 9The curve diagram shows the data obtained by repeating the measurement 17 times (measurement i is numbered from 1 to 17). For each measurement value i, on the one hand, the rough value λc B (i) (cross) is determined according to the method 100 of the present invention, and on the other hand, the correction value λc(i) (dot) is determined. The rough value is obtained by using the direct measurement of the detection system. The dispersion of the rough data is obvious and is caused by the drift of the spectrometer. The correction value of λc is very slightly dispersed in the 17 measurements.

[0120] Figure 10 Shows the average value and standard deviation σ of these 17 measurement values in two cases, which are (λ B m, σ B ) and (λ c m, σ c ), which are rough and corrected with λref of the HCL lamp. The reference value of the wavelength of the mercury vapor lamp (which is also very precisely known) is λ lvm = 671.643 nm. This value is also mentioned in Figure 11 and allows testing the relevance of the method according to the present invention. The value of λ c m ratio λ B m is closer to λ lvm . Thus, it can be seen that the measurement method according to the present invention significantly improves the accuracy and precision of measuring the wavelength.

[0121] Determine the deviation between the two measurements, that is:

[0122] Deviation (rough measurement) = λ lvm - λ B m = 35 pm;

[0123] Deviation (corrected measurement value) = λ lvm - λ c m = -3 pm.

[0124] Then, two measurement uncertainties I are derived from it by applying the following formula:

[0125] Therefore, the uncertainty regarding the wavelength measurement changes from 38 pm for the rough measurement to 4 pm for the corrected measurement.

[0126] In the measurement example (No. 5), Table IV below specifies the time t0, t1, t2, the measurement positions P1ref, Pech, P2ref in the pixels obtained by adjusting the experimental data with the theoretical curve, the coefficients a and b determined by formulas (3) and (4), the interpolated intermediate position P0ref, the wavelength of the reference source λref, and the wavelengths of the measurement source λc before and after the framing correction.

[0127]

[0128] Table IV

Claims

1. A method (100) for determining the central wavelength of interest (λc) of a spectral line of interest (RSe) measured by a spectrometer, the spectral line of interest corresponding to the emission or absorption of a sample (Ech) to be characterized, the spectral line of interest having: a bell-shaped profile, where the central wavelength of interest then corresponds to the vertex of the bell-shaped profile; or a double-bell-shaped profile, where the central wavelength of interest then corresponds to the valley between the two bells, the spectrometer being associated with a detector (Det), the detector (Det) including a plurality of pixels (Pi) aligned in the X direction, the spectral line of interest being detected on the pixels of the detector, the method comprises the following steps: -A. Detecting, at time t1, a first reference measurement profile (PS1ref) derived from a reference source (Sref) having a reference spectral line (RSref), the reference spectral line having a central wavelength with a known value (λref) called the reference central wavelength, the reference wavelength being selected to be detected on at least one pixel of the detector; ●B. Then detecting, at time t0, an interest measurement profile (PSech) derived from the sample of interest; ●C. Then detecting, at time t2, a second reference measurement profile (PS2ref) derived from the reference source (Sref); ●D. Processing the first reference measurement profile and the second reference measurement profile to determine a first reference position (P1ref) and a second reference position (P2ref) of the reference wavelength, and processing the interest measurement profile to determine an interest position (Pech) of the central wavelength; ●E. Determining, by interpolation, the reference position called the intermediate reference position (P0ref) at time t0 based on the first reference position and the second reference position, and based on a linear variation law of the reference position as a function of the time between time t1 and t2; ●F. Determining the value of the central wavelength of interest based on the difference between the interest position (Pech) and the intermediate reference position (P0ref), the known value of the reference wavelength, and the linear dispersion (DL) of the spectrometer and the associated detector.

2. The method according to any one of the preceding claims, wherein, the processing step D comprises the following sub-steps: adjusting the values of the interest measurement profile and the first reference measurement profile and the second reference measurement profile with a known mathematical function so as to determine the interest position and the first reference position and the second reference position by interpolation with a precision less than one pixel, and then also determining the intermediate reference position with a precision less than one pixel.

3. The method according to any one of the preceding claims, wherein, the optical signal originating from the sample is pulsed.

4. The method according to the preceding claim, wherein, the optical signal source originating from the sample is the emission of a plasma emitted by the sample irradiated by a pulsed laser.

5. The method according to the preceding claim, suitable for determining the isotopic abundances of the elements present in the sample, wherein the central wavelength of interest corresponds to a line resulting from the contributions of two isotopes of the element, and the value of the central wavelength of interest allows the determination of the abundances.

6. A system (10) for measuring the central wavelength of interest (λc) of a spectral line of interest (RSe) measured by a spectrometer, the spectral line of interest corresponding to the emission or absorption of a sample (Ech) to be characterized, the spectral line of interest having: a bell-shaped profile, wherein the central wavelength of interest then corresponds to the vertex of the bell-shaped profile ； or a double-bell-shaped profile, wherein the central wavelength of interest then corresponds to the valley between the two bells, the measurement system comprising: - A detection system including a spectrometer (Spectro) associated with a detector (Det), the detector (Det) including a plurality of pixels (Pi) aligned in the X direction, the spectral line of interest being detected on the pixels of the detector, the system being configured such that the detector: o Detects at time t1 a first reference measurement profile (PS1ref) derived from a reference source (Sref), the reference source (Sref) having a reference spectral line (RSref) with a central wavelength having a known value (λref) called the reference central wavelength, the reference wavelength being selected to be detected on at least one pixel of the detector; o Then detects at time t0 an interesting measurement profile (PSech) derived from the sample of interest; o Then detects at time t2 a second reference measurement profile (PS2ref) derived from the reference source (Sref); - The system further includes a processing unit (UT), the processing unit (UT) being configured to: ● Process the first reference measurement profile and the second reference measurement profile to determine a first reference position (P1ref) and a second reference position (P2ref) of the reference wavelength, and process the interesting measurement profile to determine an interesting position (Pech) of the central wavelength; ● Based on the first reference position and the second reference position, and based on the linear variation law of the reference position as a function of the time between t1 and t2, determine by interpolation the reference position called the intermediate reference position (P0ref) at time t0; ● Determine the value of the central wavelength of interest based on the difference between the interesting position (Pech) and the intermediate reference position (P0ref), the known value of the reference wavelength, and the linear dispersion (DL) of the detection system.

7. The measurement system according to the preceding claim, further comprising: - A pulsed laser (L), the pulsed laser (L) being configured to irradiate the sample to generate a plasma (PI), the plasma (PI) enabling the emission of the optical signal originating from the sample; - An optical fiber (FOp), configured to collect an input optical signal from the sample or from the reference source, and output-coupled to the input of the spectrometer; The processing unit is configured to synchronize the detector with the laser when detecting the measurement profile of interest.

8. The measurement system according to the preceding claim, adapted to measure the isotopic abundances of elements present in the sample, the central wavelength of interest corresponding to a line produced by the contributions of two isotopes of the element, the value of the central wavelength of interest allowing determination of the isotopic abundances.

9. A computer program comprising instructions which cause a system according to any one of claims 6 to 8 to perform the steps of a method according to any one of claims 1 to 5.

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