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

Abstract

The invention relates to a method (100) for determining a central wavelength of interest (λc) of a spectral line of interest, comprising the steps of: - A detecting, at a time t1, a first reference measured profile (Ps1ref), • B then detecting, at a time t0, a measured profile of interest (PSech) issued by said sample of interest, • C then detecting, at a time t2, a second reference measured profile (Ps2ref) issued by a reference source (Sref), • D processing said first and second reference measured profiles and processing the measured profile of interest, • E determining a reference position referred to as the intermediate reference position (P0ref) at the time t0 by interpolation, • F determining a value of the central wavelength of interest from a difference between said position of interest (Pech) and said intermediate reference position (P0ref), from said known value of the reference wavelength and from a linear dispersion (DL) of the spectrometer and of the associated detector.

Description

DESCRIPTION TITLE: Method for determining the central wavelength of a spectral line with high precision and associated system FIELD OF THE INVENTION

[0001] The present invention relates to the field of spectroscopy, and more particularly to the determination of the central wavelength of a spectral line with very high precision. STATE OF THE ART

[0002] For certain spectroscopic applications, such as atomic or molecular spectroscopy, or for determining the isotopic abundance of an element in a sample using an optical method (called LIBRIS for Laser-Induced Breakdown Self-Reversal Isotopic Spectrometry, see below), high precision in determining the central wavelength of a spectral line is required. The spectral line to be characterized is produced by a light source and can be an absorption or emission line, atomic or molecular. Typically, an uncertainty of less than 5 pm, or even less than 1 pm, is sought for the central wavelength.

[0003] This problem has not yet arisen in the field of laser ablation plasma spectroscopy (LIBS for Laser Induced Breakdown Spectroscopy, LAMIS for Laser Ablation Molecular Isotopic Spectrometry, etc.), because the width of the observed lines is typically a few tens of micrometers (pm). The wavelength of the lines is therefore usually measured with an uncertainty of a few tens of pm to a few tens of pm, depending on the linear dispersion of the spectrometer used. This uncertainty does not affect these techniques because the analysis is performed based on the intensity of the lines, which is generally integrated over a width of the same order, from a few tens of pm to a few tens of pm.

[0004] Traditionally, the detection system is calibrated in wavelength using a reference source emitting known spectral lines, typically a mercury vapor lamp or a hollow cathode lamp. The position of the The spectral line to be analyzed and the position of the reference line are located in pixels on the detector, and the line to be analyzed is determined from its relative position with respect to that of the reference line. The detector comprises at least N pixels Pi aligned in a line, with i ranging from 1 to N. When it is 2D, integration over all pixels in the same column is performed. For example, the detector is a matrix detector using CCD technology, with 2048x512 pixels.

[0005] Let Δref be the central wavelength of the reference line and 70 the wavelength to be determined, Pref the position of Δref as indicated by detector pixels, and PO the position of 70 on the same detector. The wavelength 7ref is, of course, chosen so that it appears on the detector simultaneously with 70 for the same spectrometer configuration. We have:

[0007] with DL linear dispersion of the detection system, typically in pm / pixel.

[0008] For various reasons (thermal fluctuations, vibrations, etc.), spectrometers and detectors drift very slightly even in the controlled environment of a research laboratory, leading to wavelength drift. This drift is, of course, even more pronounced in out-of-laboratory analysis situations (field, online, using a portable system, etc.). For example, in the case of a 1 m focal length grating spectrometer with a 2400 lines / mm grating, a variation of only 10⁻¹⁰⁵⁻¹¹ minutes can result in a wavelength drift. 3 The degree of the lattice angle causes a wavelength shift of 10 pm, which is unacceptable for LIBRIS analysis, for example of lithium, for which an uncertainty of less than 1 pm is targeted to obtain an acceptable uncertainty on the isotopic abundance. 6 Li.

[0009] The detection of the reference line and the line to be analyzed are performed sequentially. In the most common case, the signal from the sample is transmitted to the detection system via an optical fiber. To perform both measurements, the optical fiber connected to the spectrometer must first be positioned to collect the light flux from the reference source and then that from the emission to be characterized, or vice versa, which takes some time. Typically, these two measurements are separated by a a duration which is on the order of a minute, which is sufficient for such a drift to occur.

[0010] It is therefore impossible to perform accurate LIBRIS measurements without correcting for the wavelength drift of the detection system. The problem arises in the same way in atomic spectroscopy, where the aim is to measure Δ0 accurately by calibrating against a reference source.

[0011] The invention being of particular interest to the LIBRIS method, its principle is recalled below as well as the principle of the LIBS and LAMIS methods.

[0012] The principle of LIBS technology, illustrated in Figure 1, is to focus a laser pulse onto the surface of a material sample (or the material itself) to generate a transient plasma whose light emission is then analyzed using a spectrometer. By collecting the plasma's light emission and analyzing the spectrum spectrometrically, it is possible to identify the elements present in the plasma and thus determine the material's composition from emission line databases. In LIBS, the intensity is integrated over the entire line width.

[0013] LAMIS technology, for example described in the publication by R. Russo et al., Spectrochim. Acta B 66 (2011) 99, is an alternative derived from LIBS that allows isotopic analysis from the lines of molecules formed by reaction between the ablated material and a constituent of the surrounding environment, or by reaction between two atoms of the ablated material.

[0014] A laser generator L0 produces a laser beam FLO, which is focused onto sample 1 by a first optical system 2. This generates a plasma PI0. The plasma emits light 3, which is collected by an optical system OSO. The focused light is sent to a spectrometer SpecO via an optical fiber FO. The spectrometer SpecO includes (or is associated with) a detector DetO synchronized with the laser generator L0. The spectrometer SpecO records line spectra. Finally, processing means UT0 process the recorded spectra.

[0015] LIBS generates a spectrum, which takes the form of a set of spectral lines corresponding to the emission lines of the elements composing the material. Using available correlation data between emission lines and elements, this spectrum allows the elemental composition of the material sample to be determined. The wavelength X of a line indicates the presence of an element in the material, and the intensity I is related to the concentration of that element.

[0016] LIBS emission spectrometry is also applicable to isotopic analysis because the atomic lines of different isotopes of the same element are at slightly different wavelengths. This spectral shift, called the isotope shift, is due to mass effects (predominant for light elements) and changes in the charge distribution within the nucleus (predominant for heavy elements). If this isotopic analysis is to be performed by LIBS, it is essential to separate the lines of the two isotopes. However, this spectral shift is generally on the order of a fraction of a nanometer or even a few millimeters, as shown in Table I below:

[0017] Table I

[0018] Such a shift is difficult to observe in a plasma generated by laser ablation under normal conditions, because the confinement of the plasma by ambient air at atmospheric pressure results in a high density, and therefore a broadening of the emission lines due to the Stark effect. This broadening commonly reaches several tens or even hundreds of micrometers and consequently masks the isotopic shift, even if the spectrometer used has sufficient spectral resolution to resolve this shift. The limitation here is physical, not instrumental.

[0019] One solution involves performing the analysis at reduced pressure, or even under vacuum. By limiting the plasma's confinement by the surrounding environment, its density is decreased, and sufficient spectral selectivity can be achieved for certain isotopes. A double spectral line is visualized, and the isotopic ratio is determined from the intensity ratio between the two lines associated with the two isotopes. This approach is not applicable to all isotopes and requires a high-resolution, and therefore bulky, spectrometer. A second solution involves sending a second laser beam through the plasma to measure a resonant absorption or fluorescence signal, which is more demanding and complicates the measurement system.

[0020] In the state of the art of atmospheric pressure isotope analysis, the LAMIS technique can also be used, but this requires fulfilling several conditions: 1. Molecules must form in the plasma; 2. They must be sufficiently stable under the temperature / density conditions of the plasma; 3. They must have detectable spectral lines, i.e., lines of sufficient lifetime, sufficiently intense, and within the spectral band of the detection system. In the case of lithium, for example, a LAMIS signal is not detected, probably because the 2 eme The condition is not met.

[0021] The LIBRIS technique is an optical method for determining the isotopic abundance of an element in a sample (solid, liquid, or gas) from the emission spectrum of a laser ablation plasma. This technique is described, for example, in the publication by K. Touchet et al., Spectrochim. Acta B 168 (2020) 105868 and in US document 2019 / 0041336. It is a variant of the LIBS technology and uses the same optical system. The LIBRIS technology overcomes the various drawbacks of the LIBS method by allowing the measurement of an isotopic ratio at atmospheric pressure and without a second laser.

[0022] It is important to remember that electronic transitions of atoms to higher energy levels require an input of energy. This energy can be in the form of photons, in which case the atom absorbs the photons. A special case is that of laser ablation plasma. To simplify, we can Consider that the plasma consists of two distinct parts: the core and the periphery. Photons emitted by the hotter core of the plasma can be absorbed by the cooler periphery. This phenomenon prevents a certain number of emitted photons from leaving the plasma: this is the phenomenon of self-absorption.

[0023] For an observer outside the plasma, and for a measuring instrument, the spectral line profile results from emission and self-absorption at the same wavelength corresponding to the electronic transitions between two energy levels of all the atoms considered, placed along its line of sight. Consequently, the measured intensity is not simply the sum of all the plasma's emissions, as this self-absorption must be taken into account.

[0024] The phenomenon of self-absorption, well known in plasma spectroscopy for elemental analysis, is generally considered undesirable because it leads to distortion of the spectral line profile, and therefore to a non-linearity of the signal with respect to the concentration of the element of interest. LIBRIS exploits this self-absorption effect to deduce information about the isotopes of a given element in a material.

[0025] Figures 2 and 3 illustrate an RS0 line of an element of interest, selected from a spectrum 20, obtained in two cases, depending on the concentration of the element in the material.

[0026] Figure 2 illustrates the case where the concentration of the element in the plasma is lower; the self-absorption phenomenon is weak or even absent. A spectrally broad line profile is obtained, without a central dip. The dashed curves ISO1 and ISO2 represent the emission of the two isotopes. Each line has a width significantly greater than the gap between the two lines, primarily due to the Stark effect in the plasma, which is why they cannot be distinguished individually: the solid line RS0, corresponding to the sum of the two, is detected. The principle of LIBRIS is that the central wavelength of the solid line varies with the isotopic abundance, that is, with the ratio of the amplitudes of the two dashed lines. In this case, we measure the value of the central wavelength Æ0 corresponding to the emission peak, that is, the maximum point or vertex 20 of the observed curve which presents a bell-shaped profile. It is correlated with the ratio between two isotopes Iso1 and Iso2 of the element considered, and it is shifted according to said isotopic ratio.

[0027] Figure 3 illustrates the case where the element is present in high concentration in the plasma, and the self-absorption phenomenon is then pronounced. A line profile with a dip in its center (double bell profile), called an inverted line, is observed, resulting from the superposition of a spectrally broad emission profile with a spectrally narrower absorption profile. In this case, the value of the central wavelength 70, corresponding to the absorption dip, is measured. The central wavelength 70 is measured on the portion of the profile corresponding to absorption, that is, at the minimum point 30 of the observed dip. It is correlated with the ratio between two isotopes, Iso1 and Iso2, of the element in question, and is shifted according to this isotopic ratio. This measurement of the dip wavelength defines the LIBRIS technology.

[0028] Thus, in LIBRIS technology, the isotopic ratio is measured using a very precise measurement of wavelength 70, the maximum of the bell-shaped line or the minimum of the double-bell line, also known as the inverted line. This wavelength A o shifts linearly with isotopic abundance, between A R 1 and A R 2 The indices 1 and 2 refer to two isotopes of the element. R 1 and A R 2 These are physical data available in spectroscopic databases and / or scientific publications. The analytical uncertainty in isotopic abundance is therefore directly related to the uncertainty in determining the wavelength 70.

[0029] In LIBRIS technology, the measurement of 70 directly yields the isotopic ratio. Figure 4 illustrates this evolution of the measured 70 as a function of the isotope proportion. 6Li of Lithium, which has only two isotopes 6 Li and 7 Li. This curve was obtained on an inverted spectral line. The isotopic shift is given by À R 1 - HAS R 2 and corresponds to the measurement range 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 therefore corresponds to the total variation in isotopic abundance. 6 Li from 0% to 100%, the complementary being abundance in 7 Li. Thus, an uncertainty of 1 pm in the determination of the wavelength A R leads to uncertainty about The isotopic abundance of 1 / 15.8 = 6.3%. The measurement accuracy of the isotopic ratio is therefore directly correlated to the measurement accuracy on XO.

[0030] One aim of the present invention is to remedy the aforementioned drawbacks by proposing a method and a system for determining the central wavelength of an absorption or emission line, atomic or molecular, produced by a light source, with sub-picometric accuracy. DESCRIPTION OF THE INVENTION

[0031] The present invention relates to a method for determining the 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 from a sample to be characterized, the spectral line of interest having either a bell-shaped profile, said central wavelength of interest then corresponding to the apex of said bell-shaped profile, or a double-bell-shaped profile, said central wavelength of interest then corresponding to the trough between the two bells, the spectrometer being associated with a detector comprising a plurality of pixels aligned along an X direction, the spectral line of interest being detected on pixels of the detector, the method comprising the steps of: - To detect at time t1 a first measured reference profile from a reference source having a reference spectral line with a central wavelength called the reference wavelength of known value, the reference wavelength being chosen so as to be detected on at least one pixel of the detector, • B Then detect at a time t0 a measured profile of interest from said sample of interest, • C Then detect at time t2 a second measured reference profile from a reference source, • D Process said first and second measured reference profiles, so as to determine a first (P1 ref) and a second reference position of the reference wavelength, and process the measured profile of interest so as to determine a position of interest of the central wavelength, • E Determine an intermediate reference position at time t0 by interpolation, from the first and second reference positions, and from a law of variation of the linear reference position as a function of time between times t1 and t2, • F Determine a value of the central wavelength of interest from a difference between said positions of interest and intermediate reference (POref), of said known value of the reference wavelength and a linear dispersion of the spectrometer and associated detector.

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

[0033] According to one embodiment, processing step D includes the substep of adjusting values ​​of the measured profile of interest and the first and second measured reference profiles with known mathematical functions so as to determine by interpolation said position of interest and said first and second reference positions with sub-pixel accuracy, the determination of the intermediate reference position then also being determined with sub-pixel accuracy.

[0034] According to one embodiment, the light signal from the sample is impulsive.

[0035] According to one embodiment, the light signal from the sample comes from the emission of a plasma emitted by the sample illuminated by a pulsed laser.

[0036] According to one embodiment, the method according to the invention is adapted to determine an isotopic abundance of an element present in said sample, said central wavelength of interest corresponding to a line resulting from the contributions of two isotopes of said element, said value of the central wavelength of interest allowing to determine said abundance.

[0037] 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 an emission or absorption from a sample to be characterized, the spectral line of interest having either a bell-shaped profile, said central wavelength of interest then corresponding to the apex said bell-shaped profile, or a double-bell-shaped profile, said central wavelength of interest then corresponding to the trough 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 along an X direction, the spectral line of interest being detected on pixels of the detector, - the system being configured so that the detector detects: - at a time t1 a first measured reference profile from a reference source, the reference source having a reference spectral line having a central wavelength called the reference wavelength of known value, the reference wavelength being chosen so as to be detected on at least one pixel of the detector, • then at time tO a measured profile of interest from said sample of interest, • then at time t2 a second measured reference profile from the reference source, - the system further comprising a processing unit configured for: • process the said first and second measured reference profiles, so as to determine a first and a second reference position of the reference wavelength, and process the measured profile of interest so as to determine a position of interest of the central wavelength, • determine an intermediate reference position at time t0 by interpolation, from the first and second reference positions, and from a law of variation of the linear reference position as a function of time between times t1 and t2, • determine a value of the central wavelength of interest from a difference between said positions of interest (Pech) and intermediate reference, said known value of the reference wavelength and a linear dispersion of the detection system.

[0038] According to one embodiment, the measurement system according to the invention is adapted for measuring the isotopic abundance of an element present in the sample, and further comprises: - a pulsed laser configured to illuminate the sample in such a way as to generate a plasma capable of emitting said light signal from the sample, - an optical fiber configured so that the input collects either a light signal from the sample or a light signal from the reference source, and the output is coupled to an input of the spectrometer, the processing unit being configured to synchronize the detector with the laser when detecting the measured profile of interest, said central wavelength of interest corresponding to a line resulting from the contributions of two isotopes of said element, said value of the central wavelength of interest allowing the determination of said isotopic abundance.

[0039] The invention also relates to a computer program comprising instructions which lead the system according to the invention to execute the steps of the process according to the invention.

[0040] The following description presents several embodiments of the device of the invention; these examples are not limiting to the scope of the invention. These embodiments illustrate both the essential features of the invention and additional features related to the embodiments considered.

[0041] The invention will be better understood, and other features, purposes, and advantages thereof will become apparent from the detailed description that follows and with reference to the accompanying drawings, which are given by way of non-limiting examples and on which:

[0042] Figure 1, already cited, illustrates the measurement principle using LIBS, LAMIS and LIBRIS technologies.

[0043] Figure 2, already cited, illustrates a spectral line measured in a case where the element is in low concentration in the plasma; the self-absorption phenomenon is then weak or even negligible.

[0044] Figure 3, already cited, illustrates a spectral line measured in a case where the element is in high concentration in the plasma, the self-absorption phenomenon is then marked.

[0045] Figure 4, already cited, illustrates the evolution of the central wavelength 70 measured as a function of the isotopic abundance of the isotope 6 Lithium (Li) in the sample.

[0046] Figure 5 illustrates the method according to the invention.

[0047] Figure 6 illustrates the measured profile of interest PSech, the first measured reference profile PS1 ref and the second measured reference profile PS2ref, the first theoretical reference profile PSTI ref, the second theoretical reference profile PST2ref and the theoretical profile of interest PSTech.

[0048] Figure 7 illustrates the system according to the invention.

[0049] Figure 8 illustrates the system according to the invention adapted for measuring an isotopic ratio of an element present in the sample.

[0050] Figure 9 illustrates the data obtained by repeating the measurement 17 times (measurements i numbered from 1 to 17). For each measurement i, a raw value 7CB(i) (crosses) is determined on the one hand, and a corrected value 7c(i) (points) is determined on the other hand, according to method 100 of the invention.

[0051] Figure 10 shows the mean and standard deviation θ of these 17 measurements in both cases, raw and corrected with 7ref of the HCL lamp, respectively (θ, θ B ) and (7 c m, o c ). DETAILED DESCRIPTION OF THE INVENTION

[0052] The invention relates to a method 100 for measuring the central wavelength of interest 7c of a spectral line of interest RSe measured by a spectrometer, illustrated in Figure 5. The invention also relates to a system 10 for measuring a central wavelength.

[0053] The spectral line of interest corresponds to an emission or absorption from a sample Ech to be characterized and exhibits either a bell-shaped profile, 7c then corresponding to the wavelength of the peak of the bell-shaped profile, or a double bell profile, where Ac corresponds to the wavelength of the trough between the two bells.

[0054] The spectrometer performing the measurement includes (or is associated with) a detector Det comprising a plurality of pixels Pi, i pixel index varying from 1 to N, aligned along a direction X. The spectral line of interest RSe is detected on pixels of the detector Det.

[0055] In a first step A of method 10 according to the invention, a first measured reference profile PSI ref is detected at time t1 from a reference source Sref having a reference spectral line RSref with a central reference wavelength of known value Àref. The reference wavelength is chosen so as to be detected on at least one pixel of the detector Det. To perform the detection and generate the measured spectral profile PSI ref, a light signal SLI ref from the source Sref is injected at the input of the spectrometer Spectro. The reference source is chosen according to the spectral characteristics of the sample to be analyzed.

[0056] The measured spectral profile PSI ref is a set of measurement points indexed according to the pixels Pi of the detector, and to each i is associated a detected intensity I1 i.

[0057] Then, in step B, a measured profile of interest PSech from the sample to be characterized is detected at time t0 (i.e., t0 > t1). To do this, a light signal SLech from the sample Ech is injected at the input of the Spectro spectrometer. The measured spectral profile PSech is a set of measurement points indexed according to the detector pixels, and each j is associated with a detected intensity l0j.

[0058] The reference source Sref is chosen so that the RSref line is detected by the detector without altering the spectrometer settings associated with RSe detection. In one embodiment, the detector pixels detecting the RSe line can be located in a different area of ​​the detector than those detecting the RSref line. In another embodiment, the RSref and RSe lines are located in the same area of ​​the detector. In this case, the reference source should be switched off during sample measurement, or SLref should be made negligible compared to SLech.

[0059] Then, in step C, a second measured reference profile PS2ref from the reference source Sref is detected at time t2 (i.e., t2>t1). To do this, a light signal SL2ref from the source Sref is injected again into the input of the Spectro spectrometer.

[0060] The measured profiles PSI ref and PS2ref correspond to the spectral line RSref measured at two different times t1 and t2, these two times bracketing a measurement of the spectral line of interest RSe. Thus, we have t1 <t0<t2 : le détecteur Det détecte séquentiellement dans le temps le signal issu de la source de référence à t1 , le signal issu de l’échantillon à tO et à nouveau le signal issu de la source de référence à t2.

[0061] In step D, the first and second measured reference profiles PSI ref and PS2ref are processed to determine a first reference position P1 ref of the reference wavelength Xref and a second reference position P2ref of the reference wavelength. These two positions are measured in detector pixels, with the index i representing the abscissa of the detected spectrum. In step D, the measured profile of interest PSech is also processed to determine the position of interest Pech of the central wavelength Xc, again measured in detector pixels.

[0062] The wavelength drift of the detection system [spectrometer + detector] is measured by the difference (P2ref-P1 ref).

[0063] In a step E, the reference position POref at time tO Pref(tO), called intermediate, is determined by interpolation, from P1 ref, P2ref and a law of variation of the linear reference position as a function of time Pref(t) between times t1 and t2.

[0064] Finally, in a step F, a value of the central wavelength of interest ÆC is determined from the difference between Pech and POref, the known value of the reference wavelength Xref and the linear dispersion DL of the detection system [spectrometer + detector] (typically in pm / pixel).

[0065] Typically, we have the following formula:

[0066] HAS c = A ref + (P ecfl - POref). DL (1 )

[0067] It is of course necessary to take the value of DL corresponding to the spectral region in which 7ref and 70 are located.

[0068] The method according to the invention allows for a more precise determination of the pixel position of the reference wavelength detector, taking into account the drift of the detection system, by establishing a linear variation law for the reference position over time. This enables a determination of 7c with improved accuracy compared to a measurement that only considers P1 ref (measurement of the reference prior to the measurement of the spectrum of interest) or P2 ref (measurement of the reference after the measurement of the spectrum of interest). With the method according to the invention, a very low uncertainty in the value of 7c is obtained.

[0069] According to a preferred embodiment, the time interval between the two measurements of the Sref spectrum is small, typically on the order of one minute or less. A short time between the two Sref spectrum measurements guarantees a reproducible linear variation between t1 and t2. Indeed, to perform the measurement practically, the input of an optical fiber (FOP), whose output is coupled to the input of the Spectro spectrometer, is moved so as to collect either the light signal from the Sref source (SL1ref for the first measurement and SL2ref for the second measurement), or the SLech light signal from the sample to be characterized (see Figures 7 and 8 below). The time to perform this manipulation, to which the time for recording the spectrum must be added, is typically less than one minute.

[0070] For a linear derivative of Pref(t) we have:

[0072] The parameters a and b are determined from the measurements at times ti and Î2:

[0075] The detector Det sequentially detects in time the first reference signal, the signal of interest, and the second reference signal. This detection generates a first measured reference profile of PSI ref, a measured profile of interest PSech, and a second measured reference profile PS2ref as illustrated in Figure 6. The abscissa of the profiles is the index i of the pixels Pi of the detector and the ordinate is a detected intensity for each pixel, respectively 11 i, lOi and I2i.

[0076] To measure Ac with very high precision, we aim to obtain its Pech position with a precision better than the detector pixel, that is, measured as a fraction of the integer index i. The same applies to the P1 ref and P2 ref positions. According to one embodiment, processing step D includes the substep of adjusting the values ​​of the measured reference profiles PSI ref and PS2 ref and the measured profile of interest PSech with known mathematical functions, so as to determine, by interpolation, the positions of interest and reference with a precision lower than the pixel, as illustrated in Figure 6.

[0077] Thus, theoretical profiles are determined: the first theoretical reference profile PSTI ref, the second theoretical reference profile PST2ref, and the theoretical profile of interest PSTech, also illustrated in Figure 6, which best fit the experimental data points. Typically, the mathematical functions used are chosen from among: Gaussian, Lorentzian, and Voigt.

[0078] Figure 6 shows that without this adjustment, the determined positions would correspond to pixel k of the maximum of the measured spectrum. In this case, the wavelength accuracy cannot be better than the interval between two adjacent pixels. Thanks to these theoretical profiles, the positions P1 ref, P2 ref, and Pech are determined as fractions of pixels (typically with an accuracy to two decimal places), and the accuracy is greatly improved.

[0079] According to one embodiment, the light signal from the SLref sample is pulsed. Preferably, the light signal from the SLech sample originates from the emission of a plasma emitted by the sample illuminated by a pulsed laser.

[0080] According to one embodiment, the method according to the invention is associated with the implementation of LIBRIS technology, that is to say, it is adapted for the precise measurement of an isotopic ratio of an element present in the sample. The SLech light signal originates from the emission of a PI plasma emitted by the sample Ech, illuminated by a pulsed laser L. The central wavelength of interest corresponds to a line resulting from the contributions of two isotopes of the element, and the precise value of the central wavelength of interest 7c allows the isotopic ratio to be determined as explained above. Preferably, in step B, the detector Det is synchronized with the laser L.

[0081] The system 10 according to the invention is illustrated in Figure 7. It comprises a Spectro spectrometer associated with the detector Det, the latter comprising a plurality of pixels Pi aligned along an X direction. Typically, the detector is an intensified CCD type. An example is where the pixel index i varies from 1 to 2048. When the detector is matrix-type, the intensities are preferentially summed in the vertical direction of the columns.

[0082] The system is further configured so that the detector Det detects, at time t1, the first measured reference profile PSI ref from the reference source Sref, then at time t0, the measured profile of interest PSech from said sample of interest, and then at time t2, the second measured reference profile PS2ref from the reference source Sref. To achieve this, according to an embodiment illustrated in Figure 7, the input E of an optical fiber Fop is moved according to the signal to be detected, and then the measurement is performed with the spectrometer.

[0083] The system also includes a processing unit UT configured to implement steps D, E and F.

[0084] The reference source (Sref) is chosen based on the wavelength 7c of interest: 7ref must be sufficiently close to 7c so that both wavelengths can be detected by the detector without changing the spectrometer settings. Typically, Sref is a hollow-cathode lamp, which emits a small number of photons continuously. Since the signal from the sample is typically intense and short-lived, the acquisition parameters of the detection system are different for detecting the two signals (from the reference and the sample).

[0085] These parameters are (non-exhaustive list): measurement delay relative to laser firing (for the sample signal only), acquisition time gate width, number and accumulation rate, detector gain, signal averaging.

[0086] These parameters include, for example, the following:

[0088] According to one embodiment, the system 10 of the invention is adapted for measuring a sample signal from a plasma, as illustrated in Figure 8. In another embodiment, the system further comprises a pulsed laser L configured to illuminate the sample so as to generate the plasma PI capable of emitting the light signal SLech from the sample. It also comprises an optical fiber FOp configured such that its input collects either a light signal from the sample or a light signal from the reference source, and its output S is coupled to an input of the Spectro spectrometer.

[0089] The UT processing unit is configured to synchronize the Det detector with the L laser when detecting the measured profile of interest.

[0090] Preferably the system 10 includes, in addition to the laser L, an optics 2 which focuses the laser beam onto the sample and an optical system SO configured to inject a part of the light signal from the sample into the input E of the optical fiber.

[0091] According to one embodiment, the system 10 according to the invention is adapted for measuring an isotopic ratio of an element present in the sample. The central wavelength of interest then corresponds to a line resulting from the contributions of two isotopes of the element, the value of the central wavelength of interest allowing the determination of the isotopic abundance.

[0092] The following are brief results illustrating the advantages of the framing correction method. A Jobin Yvon THR1000 spectrometer equipped with a 2400 lines / mm grating centered at 670 nm was used. The detector was an Andor iStar intensified camera with 2048x512 pixels and a linear dispersion DL of 2.774 pm / pixel at 670 nm.

[0093] We measure a line from a mercury vapor lamp in the presence of spectrometer drift; the line from the mercury vapor source constitutes the spectral line of interest.

[0094] Before and after detection of the spectral line of interest, the line of the reference source consisting of a hollow cathode (HCL) lithium lamp is measured, which is known with precision, equal to Aref = 670.776 nm.

[0095] The drift of the spectrometer is then corrected according to method 100 according to the invention.

[0096] The acquisition parameters are given in Table III below.

[0097] Table III

[0098] The graph in Figure 9 illustrates the data obtained by repeating the measurement 17 times (measurements i numbered from 1 to 17). For each measurement i, a raw value ACB(i) (crosses) and a corrected value Xc(i) (dots) are determined according to method 100 of the invention. The raw values ​​are obtained by direct measurement with the detection system. The dispersion of the raw data is evident and results from spectrometer drift. The corrected values ​​of 7c are very slightly dispersed across the 17 measurements.

[0099] Figure 10 shows the mean and standard deviation of these 17 measurements in both cases, raw and corrected with HCL lamp reference, respectively (n, OB) and (7 c m, o c )- The reference value, moreover, is known in a very The precise wavelength of the mercury vapor lamp is X| Vm = 671.643 nm. This value is also shown in Figure 11 and allows testing the suitability of the method according to the invention. The value X c m is much closer to X| Vm that the value X B m. It follows that the measurement method according to the invention significantly improves the accuracy and fidelity of the measured wavelength.

[0100] The bias is determined for both measurements, namely:

[0101] Bias (raw measurements) = X| Vm - 7 B m = 35 pm

[0102] Bias (corrected measurements) = A| V m -7 c m = -3 pm

[0103] And we deduce the uncertainties I of the two measurements by applying the formula:

[0105] The uncertainty in the wavelength measurement thus decreases from 38 pm for the raw measurement to 4 pm for the corrected measurement.

[0106] Table IV below specifies, on an example of measurement (No. 5), the times t0, t1, t2, the positions measured in pixels P1 ref, Pech, P2ref by fitting the experimental data with theoretical curves, the coefficients a and b determined with formulas (3) and (4), the interpolated intermediate position POref, the wavelength of the reference source Àref, and that of the measured source Àc, before and after correction by bracketing. ;0107] Table IV

Claims

CLAIMS Method (100) for determining a central wavelength of interest (Xc) of a spectral line of interest (RSe) measured by a spectrometer, the spectral line of interest corresponding to an emission or an absorption of a sample (Ech) to be characterized, the spectral line of interest having either a bell-shaped profile, said central wavelength of interest then corresponding to the peak of said bell-shaped profile, or a double-bell profile, said central wavelength of interest then corresponding to the trough between the two bells, the spectrometer being associated with a detector (Det) comprising a plurality of pixels (Pi) aligned in a direction X, the spectral line of interest being detected on pixels of the detector, the method comprising the steps of: - A Detect at a time t1 a first measured reference profile (PSI ref) from a reference source (Sref) having a reference spectral line (RSref) having a central wavelength called the reference wavelength of known value (Xref), the reference wavelength being chosen so as to be detected on at least one pixel of the detector, • B Then detect at a time tO a measured profile of interest (PSech) from said sample of interest, • C Then detect at a time t2 a second measured reference profile (PS2ref) from a reference source (Sref), • D Process said first and second measured reference profiles, so as to determine a first (P1 ref) and a second (P2ref) reference position of the reference wavelength, and process the measured profile of interest so as to determine a position of interest (Pech) of the central wavelength, • E Determine a so-called intermediate reference position (POref) at time t0 by interpolation, from the first and second reference positions, and from a law of variation of the linear reference position as a function of time between times t1 and t2, • F Determine a value of the central wavelength of interest from a difference between said positions of interest (Pech) and intermediate reference (POref), of said known value of the wavelength of reference and a linear dispersion (DL) of the spectrometer and the associated detector.

2. Method according to one of the preceding claims in which the processing step D comprises the sub-step of adjusting values ​​of the measured profile of interest and of the first and second measured reference profiles with known mathematical functions so as to determine by interpolation said position of interest and said first and second reference positions with sub-pixel precision, the determination of the intermediate reference position then also being determined with sub-pixel precision.

3. Method according to one of the preceding claims in which the light signal from the sample is pulsed.

4. Method according to the preceding claim in which the light signal from the sample comes from an emission of a plasma emitted by the sample illuminated by a pulsed laser.

5. Method according to the preceding claim adapted to determine an isotopic abundance of an element present in said sample, said central wavelength of interest corresponding to a line resulting from the contributions of two isotopes of said element, said value of the central wavelength of interest making it possible to determine said abundance.

6. System (10) for measuring a central wavelength of interest (Xc) of a spectral line of interest (RSe) measured by a spectrometer, the spectral line of interest corresponding to an emission or an absorption of a sample (Ech) to be characterized, the spectral line of interest having either a bell-shaped profile, said central wavelength of interest then corresponding to the peak of said bell-shaped profile, or a double-bell profile, said central wavelength of interest then corresponding to the hollow between the two bells, the measuring system comprising: - a detection system comprising a spectrometer (Spectro) associated with a detector (Det) comprising a plurality of pixels (Pi) aligned along a direction X, the spectral line of interest being detected on pixels of the detector, the system being configured so that the detector detects: o at a time t1 a first measured reference profile (PSI ref) from a reference source (Sref), the reference source (Sref) having a reference spectral line (RSref) having a central wavelength called the reference wavelength of known value (Xref), the reference wavelength being chosen so as to be detected on at least one pixel of the detector, o then at a time t0 a measured profile of interest (PSech) from said sample of interest, o then at a time t2 a second measured reference profile (PS2ref) from the reference source (Sref), - the system further comprising a processing unit (UT) configured to: • process said first and second measured reference profiles, so as to determine a first (P1 ref) and a second (P2ref) reference position of the reference wavelength, and process the measured profile of interest so as to determine a position of interest (Pech) of the central wavelength, • determine a so-called intermediate reference position (POref) at time t0 by interpolation, from the first and second reference positions, and from a law of variation of the linear reference position as a function of time between times t1 and t2, • determine a value of the central wavelength of interest from a difference between said positions of interest (Pech) and intermediate reference (POref), from said known value of the reference wavelength and from a linear dispersion (DL) of the detection system. Measurement system according to the preceding claim further comprising: - a pulsed laser (L) configured to illuminate the sample so as to generate a plasma (PI) capable of emitting said light signal from the sample, - an optical fiber (FOp) configured so that the input collects either a light signal from the sample or a light signal from the reference source, and the output is coupled to an input of the spectrometer, the processing unit being configured to synchronize the detector with the laser when detecting the measured profile of interest.

8. Measurement system according to the preceding claim adapted for measuring the isotopic abundance of an element present in the sample, said central wavelength of interest corresponding to a line resulting from the contributions of two isotopes of said element, said value of the central wavelength of interest making it possible to determine said isotopic abundance.

9. Computer program comprising instructions which cause the system of one of claims 6 to 8 to execute the steps of the method according to one of claims 1 to 5.

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