Method for determining the central wavelength of a spectral line with high accuracy, and related system

Abstract

The present invention provides a method (100) for determining a central wavelength of interest (λc) of a spectral line of interest, comprising: a step A of detecting a first reference measurement profile (PS1ref) at a time t1; A step B then detects a measurement profile of interest (PSech) derived from this sample of interest at time t0; a step C of detecting a second reference measurement profile (PS2ref) derived from a reference source (Sref) at time t2; a step D of processing these first and second reference measurement profiles and processing the measurement profile of interest; a step E of determining a reference position, called intermediate reference position (P0ref), at time t0 by interpolation; Step F of determining the value of the central wavelength of interest based on the difference between these positions of interest (Pech) and the intermediate reference position (P0ref), this known value of the reference wavelength, and the linear dispersion (DL) of the spectrometer and associated detector. The present invention relates to a method (100) comprising:

Description

[Technical Field]

[0001] The present invention relates to the field of spectroscopy, and more particularly to determining the central wavelength of a spectral line with very high accuracy. [Background technology]

[0002] For some spectroscopic applications, determining the value of the central wavelength of a spectral line requires high precision, e.g., in atomic or molecular spectroscopy or to determine the isotopic abundance of elements in a sample using an optical method (called LIBRIS (Laser-Induced Breakdown Self-Reversal Isotope Spectroscopy), see below). The spectral line to be characterized is produced by a light source and can be an atomic or molecular absorption or emission line. Uncertainties of less than 5 pm (picometers), or even less than 1 pm, are typically determined for the central wavelength value.

[0003] This problem has not arisen so far in the field of laser ablation plasma spectroscopy (using LIBS (Laser-Induced Breakdown Spectroscopy) or Laser-Induced Plasma Optical Emission Spectroscopy, LAMIS (Laser Ablation Molecular Isotope Spectroscopy) techniques, etc.), since the width of the observed lines is typically a few tens of pm. Therefore, the wavelengths of the lines are usually measured with an uncertainty of about tens to tens of pm, depending on the linear dispersion of the spectrometer used. This uncertainty does not affect these techniques, since the analysis is performed based on the line intensity integrated as a whole over a width of the same order of magnitude, from about tens to tens of pm.

[0004] Conventionally, the detection system is calibrated for wavelength by a reference source that emits a known line, typically a mercury vapor lamp or a hollow cathode lamp. 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 position relative to the position of the reference line. The detector includes at least N pixels Pi (where i varies from 1 to N) aligned in a row. If the detector is two-dimensional, all pixels in the same column are integrated. For example, the detector is a CCD array with 2048 x 512 pixels.

[0005] Let λref be the center wavelength of the reference line, and λ0 be the wavelength to be determined, Pref be the position of λref identified as a pixel on the detector, and P0 be a position on this detector. Naturally, choose the wavelength λ0 so that it appears on the detector at the same time as λ0 for the same configuration of the spectrometer. Then: λ0=λ ref +(P0-P ref ).DL···(1) where DL is the linear dispersion of the detection system, typically in pm / pixel.

[0006] For various reasons (thermal fluctuations, vibrations, etc.), spectrometers and detectors drift very slightly, even in the controlled environment of a laboratory, resulting in wavelength drift. Naturally, this drift is much more pronounced in analytical situations outside the laboratory (in the field, online, using portable systems, etc.). As an example, for a 1 m focal array spectrometer with a 2,400 line / mm grating, 10 -3 A change in grating angle by only degrees causes a wavelength shift of 10 pm, e.g. 6 To obtain acceptable uncertainty in the isotopic abundance of Li, the goal is to obtain uncertainty of less than 1 pm, which is not acceptable for the LIBRIS analysis of lithium.

[0007] 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 by an optical fiber. To perform the two measurements, it is worth positioning the optical fiber connected to the spectrometer so as to first collect the light flux originating from the reference source, which takes a certain time, and then collect the light flux originating from the emission to be characterized (and vice versa). Typically, these two measurements are separated by a duration of the order of one minute, which is sufficient for such drifts to occur.

[0008] Therefore, it is not possible to perform accurate LIBRIS measurements without correcting for wavelength drift in the detection system.Problems arise similarly in atomic spectroscopy, which aims to accurately measure λ by sampling a reference source.

[0009] The present invention is of particular interest to the LIBRIS method, the principles of the LIBRIS method, and the principles of the LIBS and LAMIS methods given below for caution.

[0010] The principle of LIBS technology, illustrated in Figure 1, is to focus a laser pulse on the surface of a material sample (or material) to generate a transient plasma whose optical emission is analyzed by a spectrometer. By collecting the optical emission from the plasma and analyzing the spectrum with a spectrometer, it is possible to identify the elements present in the plasma and thus determine the composition of the material based on a transmitted line database. In the case of LIBS, the intensity over the entire width of the line is integrated.

[0011] For example, the LAMIS technique, described in the publication by R. Russo et al., in Spectrochim. Acta B 66 (2011) 99, is an alternative derived from LIBS, which makes it possible to carry out isotopic analyses based on molecular lines formed by reactions between an ablating agent and constituent elements of the surrounding medium or between two atoms of the ablating agent.

[0012] A laser generator L0 generates a laser beam FL0 that is focused onto a sample 1 by a first optical system 2. This generates a plasma P10. The plasma emits an optical emission 3 that is collected by an optical system OS0. The focused optical emission is transmitted to a spectrometer Spec0 by an optical fiber FO. The spectrometer Spec0 comprises (or is associated with) a detector Det0 that is synchronized with the laser generator L0. The spectrometer Spec0 makes it possible to record line spectra. Finally, processing means UT0 are used to process the recorded speckle.

[0013] LIBS makes it possible to generate a spectrum 20 in the form of a set of spectral lines corresponding to the emission lines of the elements forming the material, and the available correlation data between the emission lines and the elements can be used to determine the elemental composition of a material sample. The wavelength λ of the line gives information about the element present in the material, and the intensity I is related to the concentration of this element.

[0014] Furthermore, LIBS emission spectroscopy is applied to isotope analysis, since the atomic lines of various isotopes of the same element are at slightly different wavelengths. The spectral shift, called isotope shift, is due to the mass effect (mainly for light elements) and to changes in the distribution of charge within the nucleus (mainly for heavy elements). When attempting to perform this isotope analysis using LIBS, it is essential to separate the lines of the two isotopes. However, this spectral shift is typically on the order of a fraction of a nm or even a few pm, as shown in Table 1 below.

[0015] [Table 1]

[0016] Such a shift is difficult to observe in plasmas generated by laser ablation under normal conditions because the confinement of the plasma by ambient air at atmospheric pressure leads to high density and therefore broadening of the emission lines due to the Stark effect. This broadening typically reaches tens or even hundreds of pm, thereby masking the isotope shift even when the spectrometer used has sufficient spectral resolution to overcome this shift. In this case, the constraint is physical and not informative.

[0017] The first solution involves performing the analysis at reduced pressure, or even in a vacuum. By limiting the confinement of the plasma by the surrounding medium in this way, the plasma density is reduced, allowing for sufficient spectral selectivity to be found for some isotopes. A doublet is observed, and the isotope ratio is determined based on the intensity ratio between the two lines associated with the two isotopes. This approach is not applicable to all isotopes and requires high resolution, thus requiring a large spectrometer. The second solution involves sending a second laser beam through the plasma to measure the resonance or fluorescence absorption signal, which limits and complicates the measurement system.

[0018] Although the LAMIS technique can be used in prior art isotope analysis at atmospheric pressure, this assumes that several conditions are met: (Condition 1) the molecule must occur in the plasma, (Condition 2) the molecule must be sufficiently stable under the temperature / density conditions of the plasma, and (Condition 3) the molecule must have a sufficiently strong detectable line (i.e., have a sufficient lifetime) in the spectral band of the detection system. In the case of lithium, for example, the LAMIS signal will likely not be detected, since it does not meet the second condition.

[0019] The LIBRIS technique is an optical technique for determining the isotopic abundance of elements in a sample (solid, liquid, or gas) based on the speckle emitted by 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 U.S. Patent Application Publication No. 2019 / 0041336. The LIBRIS technique is a variant of the LIBS technique and uses the same optical system. The LIBRIS technique can overcome various drawbacks of the LIBS method by enabling isotope ratio measurements at atmospheric pressure without using a second laser.

[0020] As a reminder, the electron transition of an atom to a higher energy level requires an energy input. This energy can be in the form of photons, which are absorbed by the atom in this case. A particular case is that of laser ablation plasma. For simplicity, we can think of the plasma as consisting of two distinct parts: a center and a periphery. Photons emitted by the warmer plasma center can be absorbed by the cooler periphery. This phenomenon therefore prevents a certain number of emitted photons from leaving the plasma, a phenomenon known as self-absorption.

[0021] For an observer outside the plasma and for the measuring device, the line profile is due to emission and self-absorption at the same wavelength corresponding to the electronic transition between two levels of all possible atoms located in the line of sight. As a result, the measured intensity is not simply the sum of the total plasma emission, since this self-absorption must be taken into account.

[0022] The self-absorption phenomenon, well known in plasma spectroscopy for elemental analysis, is further considered an undesirable phenomenon because it causes distortion of the line profile and therefore nonlinearity of the signal relative to the concentration of the element of interest. LIBRIS uses this self-absorption effect to obtain information about the isotopes of a given element in a material.

[0023] 2 and 3 illustrate the lines RS0 of selected elements of interest from a spectrum 20 obtained in two scenarios depending on the concentration of the element in the material.

[0024] Figure 2 illustrates a scenario where the concentration of elements in the plasma is lower and self-absorption phenomena are barely noticeable or even nonexistent. A spectrally broad line profile is obtained without a valley in the center of the line profile. The dotted curve ISO1 and dashed curve ISO2 represent the emission of the two isotopes. The width of each line is significant, considering the difference between the two lines, mainly due to the Stark effect in the plasma. For this reason, these lines are not individually distinguishable, and the line is detected as the solid line RS0, which corresponds to the sum of the two lines. The principle of LIBRIS is that the central wavelength of a line such as the solid line varies depending on the isotope abundance, i.e., the ratio of the amplitudes of the two dotted and dashed lines. In this case, the value of the central wavelength λ0 corresponding to the emission peak, i.e., the maximum point or apex 20 of the observed curve assuming a Bell profile, is measured. The value of the central wavelength λ0 is correlated with the ratio between the two isotopes ISO1 and ISO2 of the considered element and is shifted depending on this isotope ratio.

[0025] Figure 3 illustrates the case where the concentration of an element is high in the plasma, and the self-absorption phenomenon can then be recognized. A line profile with a valley in the center of the line profile (double bell profile), called an inverse line, is observed and results from the superposition of a spectrally broad emission profile on a spectrally narrow absorption profile. In this case, the value of the central wavelength λ0 corresponding to the absorption valley is measured. In this case, the central wavelength λ0 is measured in the part of the profile corresponding to absorption, i.e., at the minimum point 30 of the observed valley. The central wavelength λ0 is correlated to the ratio between the two isotopes Iso1 and Iso2 of the considered element and is shifted depending on this isotope ratio. It is this wavelength measurement of the valley that defines the LIBRIS technique.

[0026] Therefore, in the LIBRIS technique, isotope ratios are measured based on a very precise measurement of the wavelength λ, the maximum bell line, or the minimum of the line called the inverse line in the double bell. This wavelength λ is:

number

number

[0027] In the LIBRIS technique, the measurement of λ directly yields the isotope ratio. 6 Li and 7 Lithium with only the Li isotope 6 This evolution of λ as a function of the isotope ratio of Li is illustrated in Figure 1. The curve is generated inversely. The isotope shift is

number

[0028] The object of the present invention is to overcome the above-mentioned drawbacks by proposing a method and a system for determining the central wavelength of an atomic or molecular absorption or emission line produced by a light source with sub-picometer accuracy. [Prior art documents] [Patent documents]

[0029] [Patent Document 1] US Patent Application Publication No. 2019 / 0041336 [Non-patent literature]

[0030] [Non-Patent Document 1] R. Russo et al., in Spectrochim. Acta B 66(2011) 99 [Non-patent document 2] K. Touchet et al., in Spectrochim. Acta B 168(2020) 105868 Summary of the Invention [Means for solving the problem]

[0031] The object of the invention is a method for determining a central wavelength of interest of a spectral line of interest measured by a spectrometer, the spectral line of interest corresponding to an emission or absorption of a sample to be characterized, the spectral line of interest having a bell profile with this central wavelength of interest further corresponding to an apex of the bell profile, or a double bell profile with this central wavelength of interest further corresponding to a valley between two bells, the spectrometer being associated with a detector comprising a plurality of pixels aligned in a direction X, the spectral line of interest being detected at the pixels of the detector, - a step A of detecting, at a time t1, a first reference measurement profile derived from a reference source having a reference spectral line with a central wavelength of known value, referred to as the reference central wavelength, wherein the reference wavelength is selected so as to be detected by at least one pixel of the detector; A step B then detects a measurement profile of interest derived from this sample of interest at time t0; Next, a step C of detecting a second reference measurement profile derived from a reference source at time t2; D. processing the first and second reference measurement profiles to determine first and second reference positions of a reference wavelength, and processing the measurement profile of interest to determine a position of interest of a central wavelength; a step E of determining, by interpolation, a reference position, called intermediate reference position, at time t0 based on the first and second reference positions and on a law of linear variation of the reference positions as a function of time between times t1 and t2; a step F of determining a value of a central wavelength of interest based on the difference between these positions of interest and the intermediate reference position, this known value of the reference wavelength, and the linear dispersion of the spectrometer and associated detector; The method includes:

[0032] According to one embodiment, the variation law is linear.

[0033] According to one embodiment, processing step D includes the sub-step of determining this position of interest and these first and second reference positions by interpolation with an accuracy of less than one pixel, and then adjusting the values ​​of the measurement profile of interest and the first and second reference measurement profiles using known mathematical functions to further determine intermediate reference positions with an accuracy of less than one pixel.

[0034] According to one embodiment, the optical signal emanating from the sample is pulsed.

[0035] According to one embodiment, the optical signal generated from the sample arises from a plasma emission emitted by the sample illuminated by a pulsed laser.

[0036] According to one embodiment, the method according to the invention is suitable for determining the isotopic abundance of an element present in this sample, this central wavelength of interest corresponding to a line resulting from the contribution of two isotopes of this element, this value of the central wavelength of interest allowing this abundance to be determined.

[0037] The present invention further provides a system for measuring a central wavelength of interest of a spectral line of interest measured by a spectrometer, the spectral line of interest corresponding to an emission or absorption of a sample to be characterized, the spectral line of interest having a bell profile with the central wavelength of interest further corresponding to the apex of the bell profile, or a double bell profile with this central wavelength of interest further corresponding to the valley between the two bells, the measurement system comprising: a detection system comprising a spectrometer (Spectro) associated with a detector (Det) comprising a plurality of pixels (Pi) aligned in a direction X, the detection system detecting spectral lines of interest at the pixels of the detector; Including, The system comprises a detector: - detecting a first reference measurement profile at time t1 derived from a reference source, the reference source having a reference spectral line with a center wavelength, referred to as the reference center wavelength, of known value, the reference wavelength being selected to be detected by at least one pixel of the detector; Next, a measurement profile of interest derived from this sample of interest is detected at time t0; Next, a second reference measurement profile derived from the reference source is detected at time t2. It is structured as follows: -The system is processing the first and second reference measurement profiles to determine first and second reference positions of a reference wavelength, and processing the measurement profile of interest to determine a position of interest of a central wavelength; determining a reference position, called intermediate reference position, at time t0 by interpolation based on the first and second reference positions and based on a law of linear variation of the reference positions as a function of time between times t1 and t2; Determine the value of the central wavelength of interest based on the difference between these positions of interest and the intermediate reference position, this known value of the reference wavelength, and the linear dispersion of the detection system. a processing unit configured to: Regarding the system.

[0038] According to one embodiment, the measurement system according to the invention is suitable for determining the isotopic abundance of an element present in a sample, a pulsed laser configured to illuminate the sample to generate a plasma capable of emitting this optical signal originating from the sample; an optical fiber configured to collect an optical signal generated from the sample or an optical signal generated from a reference source, and to couple an output to an input of a spectrometer; Further comprising: The processing unit is configured to synchronize the detector with the laser when detecting a measurement profile of interest, where this central wavelength of interest corresponds to a line resulting from the contribution of two isotopes of this element, and this value of the central wavelength of interest allows this isotope abundance to be determined.

[0039] Finally, the invention relates to a computer program comprising instructions for causing the system according to the invention to carry out the steps of the method according to the invention.

[0040] The following specification describes several embodiments of the device of the present invention, and these examples do not in any way limit the scope of the invention. These embodiments describe both essential features of the invention and additional features related to possible embodiments.

[0041] The invention will be better understood and further features, objects and advantages of the invention will become apparent from the following detailed description, given by way of non-limiting example with reference to the accompanying drawings, in which: FIG. [Brief explanation of the drawings]

[0042] [Figure 1] The measurement principles using the already cited LIBS, LAMIS and LIBRIS techniques are illustrated. [Figure 2] The already cited examples illustrate the spectral lines that are measured when there is a low concentration of an element in the plasma, and the self-absorption phenomenon is then barely perceptible or even negligible. [Figure 3]The already cited spectral lines are exemplified below as being measured when there is a high concentration of an element in the plasma and the self-absorption phenomenon is discernible. [Figure 4] Illustrated is the evolution of the central wavelength λ0 measured as a function of the isotopic abundance of the lithium 6Li isotope in the sample, as already cited. [Figure 5] 1 illustrates a method according to the present invention. [Figure 6] Illustrated are a measurement profile of interest PSech, a first reference measurement profile PS1ref and a second reference measurement profile PS2ref, a first reference theoretical profile PST1ref, a second reference theoretical profile PST2ref and a theoretical profile of interest PSTech. [Figure 7] 1 illustrates a system according to the present invention. [Figure 8] 1 illustrates a system according to the present invention that is suitable for measuring the isotope ratios of elements present in a sample. [Figure 9] The data shown is obtained by repeating the measurement 17 times (measurements i numbered 1 to 17). For each measurement i, on the one hand, an approximate value λcB(i) (cross) and, on the other hand, a correction value λc(i) (dot) are determined by the method 100 according to the invention. [Figure 10] The mean and standard deviation σ of these 17 measurements in both the rough and corrected cases with λref (λBm, σB) and (λcm, σc) of the HCL lamp, respectively, are shown. DETAILED DESCRIPTION OF THE INVENTION

[0043] The present invention relates to a method 100 for measuring a central wavelength λc of interest of a spectral line RSe of interest, which is measured by a spectrometer as illustrated in Figure 5. Furthermore, the present invention relates to a system 10 for measuring the central wavelength.

[0044] The spectral line of interest corresponds to the emission or absorption of the sample Ech to be characterized and has a bell profile with λc corresponding to the wavelength of the apex of the bell profile, or a double bell profile with λc corresponding to the wavelength of the valley between the two bells.

[0045] The spectrometer performing the measurements includes (or is associated with) a detector Det that includes a number of pixels Pi (where i is a pixel index varying from 1 to N) aligned in a direction X. A spectral line RSe of interest is detected at a pixel of the detector Det.

[0046] In a first step A of the method 10 according to the invention, a first reference measurement profile PS1ref is detected at a time t1, derived from a reference source Sref having a reference spectral line RSref with a central wavelength, called the reference central wavelength, of known value λref. The reference wavelength is selected so that it is detected by at least one pixel of the detector Det. To detect and generate the measurement spectral profile PS1ref, an optical signal SL1ref originating from the source Sref is injected into the input of the spectrometer Spectro. The reference source is selected depending on the spectral characteristics of the sample to be analyzed.

[0047] The measured spectral profile PS1ref is a set of measurement points indexed according to the pixels Pi of the detector, associating each index i with a detected intensity I1i.

[0048] Next, in step B, a measurement profile PSech of interest derived from the sample to be characterized is detected at time t0 (i.e., t0>t1). For this, an optical signal SLech emanating from the sample Ech is injected into the input of the spectrometer Spectro. The measured spectral profile PSech is a set of measurement points indexed according to the pixels of the detector, associating each index j with a detected intensity I0j.

[0049] Without modifying the adjustment of the spectrometer associated with the detection of RSe, select the source Sref such that the line RSref is detected by the detector. According to one embodiment, the pixels of the detector that detect the line RSe can be located in a zone of a different detector than the detector that detects the line RSref. According to another embodiment, the lines RSref and RSe are located at the same position on the detector. Next, it is desirable for the reference source to switch when measuring the sample or to ensure that SLref can be ignored considering SLech.

[0050] Next, in step C, detect a second reference measurement profile PS2ref derived from the reference source Sref at time t2 (i.e., t2 > t1). For this, re-inject the optical signal SL2ref generated from the source Sref into the input of the spectrometer Spectro.

[0051] The measurement profiles PS1ref and PS2ref correspond to the spectral lines RSref measured at two different instants t1 and t2, and these two instants constitute the measured values of the spectral line RSe of interest. Thus, t1 < t0 < t2, and over time, the detector Det sequentially detects the signal generated from the reference source at t1, the signal generated from the sample at t0, and the signal generated from the reference source again at t2.

[0052] In step D, process the first and second reference measurement profiles PS1ref and PS2ref to determine the first reference position P1ref of the reference wavelength λref and the second reference position P2ref of the reference wavelength. Measure these two positions as pixels of the detector, and the index i forms the abscissa of the detection spectrum. In step D, further process the measurement profile Pech of interest to determine the position Pech of interest of the central wavelength λc, which is also measured as a pixel of the detector.

[0053] Measure the wavelength drift of the detection system [spectrometer + detector] by the difference (P2ref - P1ref).

[0054] In step E, based on P1ref, P2ref and based on the law of linear variation of the reference position as a function of the time Pref(t) between times t1 and t2, the reference position P0ref is determined by interpolation at time t0 Pref(t0), called intermediate time.

[0055] Finally, in step F, the value of the central wavelength of interest λc 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] (typically in pm / pixel).

[0056] Typically, the following formula can be used: λ c =λ ref +(P ech -P0 ref ).DL···(1)

[0057] Naturally, the value DL should be taken to correspond to the spectral region in which λref and λ0 are located.

[0058] Using the method according to the invention, the position of the reference wavelength is determined more accurately as a pixel of the detector, taking into account the drift of the detection system, by establishing a law of linear variation of the reference position as a function of time. This allows λc to be determined with improved accuracy compared to measurements that simply take into account P1ref (measurement of the reference before measuring the spectrum of interest) or P2ref (measurement of the reference after measuring the spectrum of interest). Using the method according to the invention, very low uncertainty is obtained for the value of λc.

[0059] According to a preferred embodiment, the time difference between the two measurements of the spectrum Sref is short, typically on the order of one minute or less. The short time between the two measurements of the spectrum Sref ensures a reproducible linear variation between t1 and t2. In practice, to practically perform the measurements, the input of the optical fiber FOP is moved 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, and the output of the optical fiber FOP is coupled to the input of the spectrometer Spectro (see Figures 7 and 8 below). The time to complete this task, to which the spectral alignment time should be added, is typically less than one minute.

[0060] For a linear drift of Pref(t), P ref (t0)=P0 ref =a.t0+b (2)

[0061] The parameters a and b are determined based on measurements at times t1 and t2.

number

[0062] The detector Det sequentially detects the first reference signal, the signal of interest, and the second reference signal over time, which generates a first reference measurement profile PS1ref, a measurement profile of interest PSech, and a second reference measurement profile PS2ref, as illustrated in Figure 6. The abscissa of the profiles is the index i of the detector pixel Pi, and the ordinates are the detected intensities I1i, I0i, and I2i for each pixel, respectively.

[0063] In order to be able to measure λc with very high accuracy, it is intended to obtain the position Pech measured with an accuracy higher than a pixel of the detector, i.e. as a fraction of the integer index i, similarly for the positions P1ref and P2ref. For this purpose, according to one embodiment, processing step D comprises a sub-step of adjusting the values ​​of the reference measurement profiles PS1ref and PS2ref and the measurement profile of interest PSech using known mathematical functions so as to determine the position of interest and the reference position by interpolation with an accuracy of less than one pixel, as illustrated in FIG.

[0064] Therefore, the theoretical profiles, also illustrated in Figure 6, adjusted at the experimental and possible points, are determined: a first theoretical reference profile PST1ref, a second theoretical reference profile PST2ref and a theoretical profile of interest PSTech. Typically, the mathematical functions used are selected from the following Gaussian, Lorentzian and Voigt functions:

[0065] Figure 6 shows that without this adjustment, the determined position corresponds to the largest pixel k in the measured spectrum. As a result, it is not possible to have a wavelength accuracy better than the spacing between two adjacent pixels. These theoretical profiles allow the positions P1ref, P2ref, and Pech to be determined as fractions of a pixel (typically accurate to two decimal places), greatly improving accuracy.

[0066] According to one embodiment, the optical signal SLref emanating from the sample is pulsed. Preferably, the optical signal SLech emanating from the sample is generated from a plasma emission emitted by the sample illuminated by a pulsed laser.

[0067] According to one embodiment, the method according to the invention relates to an implementation of the LIBRIS technique, i.e., is suitable for accurately measuring the isotope ratio of elements present in a sample ech. The optical signal SLech arises from the emission of plasma PI emitted by the sample ech illuminated by a pulsed laser L. The central wavelength of interest corresponds to the lines resulting from the contributions of the two isotopes of the element, and the precise value of the central wavelength of interest λc allows the isotope ratio to be determined, as described above. Preferably, in step B, the detector Det is synchronized to the laser L.

[0068] A system 10 according to the invention is illustrated in Figure 7. The system 10 includes a spectrometer Spectro associated with a detector Det that includes a plurality of pixels Pi aligned in a direction X. Typically, the detector is an intensified CCD type detector. An example is a pixel index i that varies from 1 to 2,048. Preferably, if the detector is an array, the intensities are summed vertically in columns.

[0069] Furthermore, the system is configured such that the detector Det detects a first reference measurement profile PS1ref derived from the reference source Sref at time t1, then detects a measurement profile of interest PSech derived from this 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 an embodiment illustrated in Figure 7, the input E of the optical fiber FOp is moved according to the signal intended to be detected, and then a measurement is performed using the spectrometer.

[0070] Furthermore, the system includes a processing unit UT configured to perform steps D, E and F.

[0071] The source Sref is selected according to the wavelength λc of interest; in fact, λref must be close enough to λc so that the two wavelengths can be detected by the detector without changing the spectrometer tuning. Typically, Sref is a hollow cathode lamp that continuously emits a small number of photons. The signal originating from the sample is typically strong and short, in which case the collection parameters of the detection system are different for the detection of the two signals (originating from the reference and the sample).

[0072] These parameters are the delay of the measurement relative to the laser shot (if only the sample signal), the width of the collection time gate, the number and speed of accumulations, the detector gain, and the signal averaging (a non-exhaustive list).

[0073] These parameters are, for example, as follows:

[0074] [Table 2]

[0075] According to one embodiment, a system 10 according to the present invention is suitable for measuring a sample signal originating from a plasma, as illustrated in Figure 8. According to one embodiment, the system further comprises a pulsed laser L configured to illuminate the sample to generate a plasma PI capable of emitting an optical signal SLech originating from the sample. The system further comprises an optical fiber FOp, the input of which is configured to collect the optical signal originating from the sample or the optical signal originating from the reference source, and the output S of which is coupled to the input of a spectrometer Spectro.

[0076] The processing unit UT is configured to synchronize the detector Det with the laser L upon detection of a measurement profile of interest.

[0077] Preferably, in addition to the laser L, the system 10 includes an optical component 2 for focusing the laser beam onto the sample, and an optical system SO configured to inject a portion of the optical signal emanating from the sample into the input E of the optical fiber.

[0078] According to one embodiment, the system 10 according to the invention is suitable for measuring the isotope ratio of an element present in a sample. The central wavelength of interest then corresponds to a line resulting from the contribution of two isotopes of the element, and the value of the central wavelength of interest allows the isotopic abundance to be determined.

[0079] Below we briefly provide results illustrating the benefits of the framing correction method. A Jobin Yvon THR1000 spectrometer with a 2,400 line / mm grating centered at 670 nm is used. The detector is an Andor iStar 2048 x 512 pixel intensified camera with a linear dispersion DL of 2.774 pm / pixel at 670 nm.

[0080] The lines of the mercury vapor lamp are measured in the presence of spectrometer drift, and the lines of the mercury vapor source are the spectral lines of interest.

[0081] Before and after detection of the spectral line of interest, the line of a reference source formed by a lithium hollow cathode lamp (HCL) is measured and known precisely as equal to λref=670.776 nm.

[0082] The spectrometer drift is then corrected by a method 100 according to the present invention.

[0083] The acquisition parameters are given in Table 3 below.

[0084] [Table 3]

[0085] The graph in FIG. 9 illustrates data obtained by repeating the measurement 17 times (measurement i numbered 1 to 17). For each measurement i, on the other hand, the approximate value λc B(i) (cross), on the other hand, the correction values ​​λc(i) (dots) are determined by the method 100 according to the invention. The approximate values ​​are obtained by direct measurements using the detection system. The variance in the approximate data is evident and is due to the drift of the spectrometer. The variance of the correction values ​​of λc is very small across the 17 measurements.

[0086] Figure 10 shows the λref (λ B m, σ B ) and (λ c m, σ c The mean and standard deviation σ of these 17 measurements are shown for both the rough and corrected cases with λ. The reference value (known very precisely) for the wavelength of a mercury vapor lamp is λ lvm = 671.643 nm. This value is also plotted in Figure 11 and allows the relevance of the method according to the invention to be examined. c m is the value λ B λ is higher than m lvm It can be seen that the measurement method according to the present invention significantly improves the accuracy and precision of the measured wavelength.

[0087] The bias for the two measurements was determined, namely: Bias (approximate measurement value) = λ lvm -λ B m=35pm Bias (corrected measurement) = λ lvm -λ c m=-3pm

[0088] The uncertainty of the two measurements is then estimated by applying the following formula:

number

[0089] Therefore, the uncertainty on the wavelength measurement changes from 38 pm for the coarse measurement to 4 pm for the corrected measurement.

[0090] Table 4 below specifies the times t0, t1, t2, pixel measurement positions P1ref, Pech, and P2ref in the measurement example (number 5) by adjusting the experimental data using the theoretical curve, the coefficients a and b determined in equations (3) and (4), the interpolated intermediate position P0ref, the wavelength λref of the reference source, and the wavelength λc of the measurement source before and after framing correction.

[0091] [Table 4]

Claims

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

2. 2. The method of claim 1, wherein processing step D includes the substeps of determining the location of interest and the first and second reference locations by interpolation with an accuracy of less than one pixel, and then adjusting values ​​of the measurement profile of interest and the first and second reference measurement profiles using known mathematical functions to further determine the intermediate reference location with an accuracy of less than one pixel.

3. 3. The method of claim 1, wherein the optical signal generated from the sample is pulsed.

4. The method according to any one of claims 1 to 3, wherein the optical signal originating from the sample originates from the emission of plasma emitted by the sample illuminated by a pulsed laser.

5. 5. The method of any one of claims 1 to 4, suitable for determining the isotopic abundance of an element present in the sample, wherein the central wavelength of interest corresponds to a line due to contributions from two isotopes of the element, and wherein the value of the central wavelength of interest allows the abundance to be determined.

6. A system (10) for measuring a central wavelength of interest (λc) of a spectral line of interest (RSe) measured by a spectrometer, said spectral line of interest corresponding to an emission or absorption of a sample (Ech) to be characterized, said spectral line of interest having a bell profile with said central wavelength of interest further corresponding to the apex of the bell profile or a double bell profile with said central wavelength of interest further corresponding to the valley between two bells, said measurement system comprising: a detection system comprising a spectrometer (Spectro) associated with a detector (Det) comprising a plurality of pixels (Pi) aligned in a direction X, said spectral lines of interest being detected by the pixels of said detector; Including, The system further comprises: detecting, at time t1, a first reference measurement profile (PS1ref) derived from a reference source (Sref), said reference source (Sref) having a reference spectral line (RSref) with a central wavelength called the reference central wavelength of a known value (λref), said reference wavelength being selected to be detected in at least one pixel of the detector; Next, a measurement profile of interest (PSec) derived from said sample of interest is detected at time t0, Next, a second reference measurement profile (PS2ref) derived from the reference source (Sref) is detected at time t2. It is structured as follows: - said system comprising: - processing the first and second reference measurement profiles to determine a first reference position (P1ref) and a second reference position (P2ref) of the reference wavelength, and processing the measurement profile of interest to determine a position of interest (Pech) of the central wavelength; determining a reference position, called intermediate reference position (P0ref), at said time t0 by interpolation based on said first and second reference positions and based on a linear variation law of said reference positions as a function of time between said times t1 and t2, Determine the value of the central wavelength of interest based on the difference between the position of interest (Pech) and an intermediate reference position (P0ref), the known value of the reference wavelength, and the linear dispersion (DL) of the detection system. a processing unit (UT) configured to: System (10).

7. a pulsed laser (L) configured to illuminate the sample so as to generate a plasma (PI) capable of emitting the optical signal emanating from the sample; an optical fiber (FOp) whose input collects the optical signal originating from the sample or the optical signal originating from the reference source and whose output is configured to be coupled to the input of the spectrometer; Further comprising: The measurement system of claim 6 , wherein the processing unit is configured to synchronize the detector to the laser when the processing unit detects the measurement profile of interest.

8. 8. The measurement system of claim 6 or 7, suitable for measuring the isotopic abundance of an element present in the sample, wherein the central wavelength of interest corresponds to a line due to contributions from two isotopes of the element, and wherein the value of the central wavelength of interest allows the isotopic abundance to be determined.

9. A computer program comprising instructions for causing the system of any one of claims 6 to 8 to carry out the steps of the method of any one of claims 1 to 5.

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