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1. (WO2019009720) CALIBRATION OF LASER-INDUCED BREAKDOWN SPECTROSCOPY
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CLAIMS

1. A method of determining unknown concentrations (Cna,Ck,Cca) of constituents (K,Ca,Na) in a composition (X) based on a measurement of the composition (X) by Laser Induced Breakdown Spectroscopy (LIBS), the method comprising

- receiving a LIBS spectrum (Sx) comprising resonance peaks (Rk,Rca,Rna) corresponding to the constituents (K,Ca,Na) in the

composition (X), the resonance peaks comprising spectral amplitudes (Pk,Pca,Dna,Pna) indicative of the unknown concentrations (Cna,Ck,Cca) of the constituents (K,Ca,Na);

- determining a first spectral amplitude (Pk,Pca,Dna) in the LIBS spectrum (Sx) corresponding to the unknown concentration (Ck,Cca,Cna) of a first constituent (K,Ca,Na) to be determined;

- determining a second spectral amplitude (Pna) corresponding to a maximum value of a self-reversed resonance peak (Rna) of the first or another constituents (Na) in the LIBS spectrum (Sx), wherein the maximum value of the self-reversed resonance peak (Rna) is limited by self-absorption reduction during the LIBS measurement;

- calculating an amplitude ratio (Pk/Pna, Pca/Pna, Dna/Pna) between the first spectral amplitude (Pk,Pca,Dna) and the second spectral amplitude (Pna);

- accessing calibration data to match the calculated amplitude ratio (Pk/Pna, Pca/Pna, Dna/Pna) with a predetermined calibration amplitude ratio (Pk'/Pna' Pca'/Pna', Dna'/Pna') as function of a known concentration (Ck',Cca',Cna') of the first constituent (K,Ca,Na); and

- using the known concentration (Ck',Cca',Cna') of the matched calibration amplitude ratio to calculate the unknown concentration

(Ck,Cca,Cna) of the first constituent (K,Ca,Na) in the composition (X).

2. The method according to claim 1, wherein one of the constituents with unknown concentration to be determined is the same as the

constituent (Na) corresponding to the self-reversed resonance peak (Rna), wherein the first spectral amplitude corresponds to a dip value (Dna) parametrizing a dip in the self-reversed resonance peak (Rna) in the LIBS spectrum (Sx).

3. The method according to claim 1 or 2, wherein one of the constituents with unknown concentration to be determined is a distinct constituent (K,Ca) from the constituent (Na) corresponding to the self-reversed resonance peak (Rna), wherein the first spectral amplitude corresponds to a peak value (Pk,Pca) parametrizing a maximum of a resonance peak (Rk,Rca) in the LIBS spectrum (Sx) distinct from the self-reversed resonance peak (Rna).

4. The method according to any of the preceding claims, wherein the unknown concentrations (Cna,Ck,Cca) of multiple constituents are determined including one constituent (Na) corresponding to the self-reversed resonance peak (Rna), and a further one or more constituents (K,Ca) corresponding to another one or more resonance peaks (Rk,Rca) in the LIBS spectrum (Sx) distinct from the self-reversed resonance peak (Rna).

5. The method according to any of the preceding claims, wherein the concentration (Cna) of the first constituent (Na) is determined by matching a ratio of the dip and peak values (Dna/Pna) of the self-reversed resonance peak (Rna) with a corresponding ratio (Dna'/Pna') in the calibration data, wherein the concentration (Cna) of the first constituent (Na) calculated on the basis thereof is used in subsequently selecting calibration data for determining a concentration of another constituent (K,Ca).

6. The method according to any of the preceding claims, wherein the calibration amplitude ratio is based on previous calibration measurements with LIBS spectra comprising corresponding first and second spectral amplitudes at a series of known concentrations (Ck',Cca',Cna') of the first constituent (K,Ca,Na), wherein a concentration (Cna') of the constituent (Na) corresponding to the self-reversed resonance peak (Rna) is sufficiently high to exhibit self-absorption reduction during the LIBS measurement of the calibration measurements.

7. The method according to any of the preceding claims, wherein the calibration data used to determine the unknown concentration of

constituents other than the constituent corresponding to the self-reversed resonance peak (Rna) is based on previous measurements with similar or matching concentrations (Cna) of the constituent (Na) corresponding to the self-reversed resonance peak (Rna), wherein the said concentrations (Cna) determining the calibration and actual measurement data differ no more than a factor two.

8. The method according to any of the preceding claims, wherein the calibration data used to match the calculated amplitude ratio is based on previous measurements using the same light pulse energy (E) as the measurement to determine the unknown concentrations (Ck,Cca,Cna), wherein the pulse energy (E) is the same within one percent.

9. The method according to any of the preceding claims, wherein an indication of light pulse energy (E) of a laser pulse, used during

measurement to generate the LIBS spectrum (Sx), or temperature of the composition (X) during measurement, is calculated based on a comparison of spectral background intensities at different wavelengths (BG240/BG760) of the LIBS spectrum (Sx).

10. The method according to any of the preceding claims, wherein the first spectral amphtude (Pk,Pca,Dna) is correlated to, or covariant with, the second spectral amplitude (Pna), wherein a standard deviation (std) in an average of multiple (N) consecutive measurements of the first and second spectral amplitudes, is lower for the amplitude ratio between the first and second spectral amplitudes (Pk/Pna, Pca/Pna) than the first spectral amplitudes (Pk,Pca) by itself.

11. The method according to any of the preceding claims, wherein the calibration data is stored as an average calibration amplitude ratio (PkVPna' Pca'/Pna', Dna'/Pna') based on the average of multiple amplitude ratios, wherein each of the multiple amplitude ratios is based on the first and second amplitudes of a respective calibration LIBS spectrum.

12. The method according to any of the preceding claims, wherein the composition (X) comprises a biological sample, wherein the concentrations of multiple constituents are determined including sodium (Na), potassium (K), and calcium (Ca), wherein the constituent corresponding to the self-reversed resonance peak (Rna) is sodium (Na).

13. A non-transitory computer readable medium storing instructions that when executed by a computer causes the computer to perform the method according to any of the preceding claims.

14. A LIBS system (100) for determining unknown concentrations (Cna,Ck,Cca) of constituents (K,Ca,Na) in a composition (X), the system comprising

- a sample holder (40) configured to hold the composition (X);

- a light source (30) and/or optics configured to cause Laser Induced Breakdown (LIB) in a sample region of the composition (X);

- a spectrometer (10) configured to receive and spectrally resolve light from the sample region resulting from the Laser Induced Breakdown (LIB);

- a light sensor (20) configured to measure the spectrally resolved light for determining a LIBS spectrum (Sx) of the composition (X); and

- a controller (50) configured to receive the LIBS spectrum (Sx), wherein the controller comprises the computer readable medium of claim 13.

15. A kidney dialysis system (800) comprising the LIBS system (100) according to claim 14.