Dynamic reconstruction of a calibration state of an absorption spectrometer

• US 8,711,357 B2
• Filed: 09/16/2011
• Issued: 04/29/2014
• Est. Priority Date: 10/21/2010
• Status: Active Grant

• Current Assignee: Endress+Hauser Optical Analysis Incorporated
• Original Assignee: Spectrasensors, Inc. (Endress+Hauser AG)
• Inventors: Liu, Xiang, Lewison, John, Ji, Wenhai, Feitisch, Alfred
• Primary Examiner(s): Chowdhury, Tarifur
• Assistant Examiner(s): NIXON, OMAR H
• Application Number:
  US13/235,105
• Publication Number:
  US 20120099109A1
• Time in Patent Office: 956 Days
• Field of Search: 356/437
• US Class Current: 356/437
• CPC Class Codes: G01J 3/28 Investigating the spectrum using colour filters G01J3/51
• CPC Class Codes: G01J 3/42 Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry beam switching arrangements G01J3/08
• CPC Class Codes: G01N 2021/399 Diode laser
• CPC Class Codes: G01N 21/031 Multipass arrangements
• CPC Class Codes: G01N 21/274 Calibration, base line adjustment, drift correction
• CPC Class Codes: G01N 21/31 Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry G01N21/72 takes precedence
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis
• CPC Class Codes: G01N 21/3518 Devices using gas filter correlation techniques; Devices using gas pressure modulation techniques
• CPC Class Codes: G01N 21/39 using tunable lasers

Abstract

A reference harmonic absorption curve of a laser absorption spectrometer, which can include a tunable or scannable laser light source and a detector, can have a reference curve shape and can include a first, second, or higher order harmonic signal of a reference signal generated by the detector in response to light passing from the laser light source through a reference gas or gas mixture. The reference gas or gas mixture can include one or more of a target analyte and a background gas expected to be present during analysis of the target analyte. The reference harmonic absorption curve can have been determined for the laser absorption spectrometer in a known or calibrated state. A test harmonic absorption curve having a test curve shape is compared with the reference harmonic absorption curve to detect a difference between the test curve shape and the reference curve shape. Operating and/or analytical parameters of the laser absorption spectrometer are adjusted to correct the test curve shape to reduce the difference between the test curve shape and the reference curve shape.

Claims

1. A method for implementation by one or more data processors forming part of at least one computing system, the method comprising:

retrieving, by at least one data processor, a reference harmonic absorption curve of a laser absorption spectrometer, the laser absorption spectrometer comprising a tunable or scannable laser light source and a detector, the reference harmonic absorption curve having a reference curve shape and comprising a first, second, or higher order harmonic signal of a reference signal generated by the detector in response to light passing from the laser light source through a reference gas or gas mixture, the reference gas or gas mixture comprising one or more of a target analyte and a background gas expected to be present during analysis of the target analyte, the reference harmonic absorption curve having been determined for the laser absorption spectrometer in a known or calibrated state;

comparing, by at least one data processor, a test harmonic absorption curve having a test curve shape with the reference harmonic absorption curve to detect a difference between the test curve shape and the reference curve shape that exceeds a predefined allowed deviation and therefore indicates a change in an output of the laser light source relative to the known or calibrated state, wherein portions of the test harmonic absorption curve are compared with portions of the reference harmonic absorption curve arising due to absorption of the background gases and not by absorption of the target analyte; and

adjusting, by at least one data processor, one or more operating and/or analytical parameters of the laser absorption spectrometer to correct the test curve shape to reduce the difference between the test curve shape and the reference curve shape.

2. A method as in claim 1, further comprising:

promoting, by at least one data processor, a field validation metric of the laser absorption spectrometer, the field validation metric comprising at least one of the difference between the test curve shape and the reference curve shape, an identification of the one or more operating and analytical parameters that were adjusted, and a value by which the one or more operating and analytical parameters were adjusted.

3. A method as in claim 1, wherein the one or more operating and/or analytical parameters of the laser absorption spectrometer comprise at least one of laser light source parameters, detector parameters, and signal conversion parameters used in generating the test harmonic absorption curve from a signal produced by the detector.

4. A method as in claim 1, wherein the laser light source parameters comprise at least one of a temperature, an operating current, a modulation current, a ramp current, and a phase of the laser light source;

the detector parameters comprise at least one of a gain and a phase setting of a detector circuit; and

the signal conversion parameters comprise at least one of a gain and a phase setting of the demodulating device.

5. A method as in claim 1, wherein the output comprises a light intensity as a function of wavelength over a wavelength scan range.

6. A method as in claim 1, wherein

the light source comprises at least one of a tunable diode laser, a tunable semiconductor laser, a quantum cascade laser, a vertical cavity surface emitting laser (VCSEL), a horizontal cavity surface emitting laser (HCSEL), a distributed feedback laser, a light emitting diode (LED), a super-luminescent diode, an amplified spontaneous emission (ASE) source, a gas discharge laser, a liquid laser, a solid state laser, a fiber laser, a color center laser, an incandescent lamp, a discharge lamp, a thermal emitter, and a device capable of generating frequency tunable light through nonlinear optical interactions;

and

the detector comprises at least one of an indium gallium arsenide (InGaAs) detector, an indium arsenide (InAs) detector, an indium phosphide (InP) detector, a silicon (Si) detector, a silicon germanium (SiGe) detector, a germanium (Ge) detector, a mercury cadmium telluride detector (HgCdTe or MCT), a lead sulfide (PbS) detector, a lead selenide (PbSe) detector, a thermopile detector, a multi-element array detector, a single element detector, and a photo-multiplier.

7. A method as in claim 1, wherein the comparing further comprises:

applying, by at least one data processor, a curve fitting algorithm to quantify the difference between the test curve shape and the reference curve shape.

8. A method as in claim 1, wherein the comparing further comprises:

applying, by at least one data processor, at least one of subtracting, dividing, cross correlation, curve fitting, and multivariable regression for one or more parts or the entire of the test curve and the reference curve, and computing one or more of the difference, the ratio, the mean square error (MSE), the coefficient of determination (R²), the cross correlation function/integral and the regression coefficients in the light intensity (i.e., the y-axis) and/or the wavelength (i.e. the x-axis) domain to quantify the difference between the test curve shape and the reference curve shape.

9. A method as in claim 8, wherein the reference curve comprises at least one of a calibration reference curve stored during calibration of the laser absorption spectrometer and a curve constructed using one or more stored calibration reference curves selected according to a composition of the background gas and/or the concentration of the target analyte.

10. A method as in claim 1, further comprising obtaining the test harmonic absorption curve, the obtaining comprising:

passing light from the light source through a volume of gas, the light comprising a scanned wavelength range and an imposed modulation frequency;

converting a signal from the detector to the test harmonic curve, the converting comprising demodulating the signal at the first, the second or the higher order harmonic frequency of the imposed modulation frequency.

11. A method as in claim 1 wherein the volume of gas comprises one of a sample cell filled with a test gas comprising a known or unknown concentration of the target analyte, the sample cell filled with the background gas, a reference gas volume disposed in a path of the light and containing the test gas with the known or unknown concentration of the target analyte, and the reference gas volume disposed in the path of the light and containing only the background gas or gas mixture.

12. A method as in claim 11, wherein the sample cell comprises at least one of a multiple-pass configuration in which the light is reflected between one or more optically reflective mirrors while the light remains inside the sample cell;

a multiple-pass configuration in which the light is refracted by one or more optical elements while the light remains inside the sample cell;

a Herriot Cell;

an off-axis optical resonator;

a White cell;

an optical cavity;

a hollow core light guide; and

a single pass configuration in which the light is not being reflected while the light remains inside the sample cell.

13. A method as in claim 1, further comprising checking fidelity of the laser absorption spectrometer by one or more of analyzing a premixed calibration sample from a premixed container, a prepared calibration sample generated using a permeation device, a non absorbing zero gas, and a non absorbing zero gas contained in a reference volume.

14. A method as in claim 1, wherein the laser absorption spectrometer further comprises a signal detection circuit that allows at least one of adjustment of a signal phase with respect to a modulation current phase of the laser light source, adjustment of frequency filters, and adjustment of DC and AC gain.

15. An apparatus comprising:

a non-transitory machine-readable storage medium storing instructions that, when executed by at least one processor, cause the at least one processor to perform operations comprising;

retrieving a reference harmonic absorption curve of a laser absorption spectrometer, the laser absorption spectrometer comprising a tunable or scannable laser light source and a detector, the reference harmonic absorption curve having a reference curve shape and comprising a first, second, or higher order harmonic signal of a reference signal generated by the detector in response to light passing from the laser light source through a reference gas or gas mixture, the reference gas or gas mixture comprising one or more of a target analyte and a background gas expected to be present during analysis of the target analyte;

the reference harmonic absorption curve having been determined for the laser absorption spectrometer in a known or calibrated state;

comparing a test harmonic absorption curve having a test curve shape with the reference harmonic absorption curve to detect a difference between the test curve shape and the reference curve shape that exceeds a predefined allowed deviation and therefore indicates a change in an output of the laser light source relative to the known or calibrated state, wherein portions of the test harmonic absorption curve are compared with portions of the reference harmonic absorption curve arising due to absorption of the background gases and not by absorption of the target analyte; and

adjusting one or more operating and/or analytical parameters of the laser absorption spectrometer to correct the test curve shape to reduce the difference between the test curve shape and the reference curve shape.

16. An apparatus as in claim 15, further comprising the at least one processor.

17. An apparatus as in claim 15, further comprising the laser absorption spectrometer.

18. An apparatus as in claim 17, wherein

the light source comprises at least one of a tunable diode laser, a tunable semiconductor laser, a quantum cascade laser, a vertical cavity surface emitting laser (VCSEL), a horizontal cavity surface emitting laser (HCSEL), a distributed feedback laser, a light emitting diode (LED), a super-luminescent diode, an amplified spontaneous emission (ASE) source, a gas discharge laser, a liquid laser, a solid state laser, a fiber laser, a color center laser, an incandescent lamp, a discharge lamp, a thermal emitter, and a device capable of generating frequency tunable light through nonlinear optical interactions;

and

the detector comprises at least one of an indium gallium arsenide (InGaAs) detector, an indium arsenide (InAs) detector, an indium phosphide (InP) detector, a silicon (Si) detector, a silicon germanium (SiGe) detector, a germanium (Ge) detector, a mercury cadmium telluride detector (HgCdTe or MCT), a lead sulfide (PbS) detector, a lead selenide (PbSe) detector, a thermopile detector, a multi-element array detector, a single element detector, and a photo-multiplier.

19. An apparatus as in claim 17, wherein the volume of gas comprises one of a sample cell filled with a test gas comprising a known or unknown concentration of the target analyte, the sample cell filled with the background gas, a reference gas volume disposed in a path of the light and containing the test gas with the known or unknown concentration of the target analyte, and the reference gas volume disposed in the path of the light and containing only the background gas or gas mixture.

20. An apparatus as in claim 19, wherein the sample cell comprises at least one of a multiple-pass configuration in which the light is reflected between one or more optically reflective mirrors while the light remains inside the sample cell;

a multiple-pass configuration in which the light is refracted by one or more optical elements while the light remains inside the sample cell;

a Herriot Cell;

an off-axis optical resonator;

a White cell;

an optical cavity;

a hollow core light guide; and

a single pass configuration in which the light is not being reflected while the light remains inside the sample cell.

21. An apparatus as in claim 17, wherein the laser absorption spectrometer further comprises a signal detection circuit that allows at least one of adjustment of a signal phase with respect to a modulation current phase of the laser light source, adjustment of frequency filters, and adjustment of DC and AC gain.

22. An apparatus as in claim 15, wherein the one or more operating and/or analytical parameters of the laser absorption spectrometer comprise at least one of laser light source parameters, detector parameters, and signal conversion parameters used in generating the test harmonic absorption curve from a signal produced by the detector.

23. An apparatus as in claim 22, wherein the laser light source parameters comprise at least one of a temperature, an operating current, a modulation current, a ramp current, and a phase of the laser light source;

the detector parameters comprise at least one of a gain and a phase setting of a detector circuit; and

the signal conversion parameters comprise at least one of a gain and a phase setting of the demodulating device.

24. An apparatus as in claim 15, wherein the output comprises a light intensity as a function of wavelength over a wavelength scan range.

25. An apparatus as in claim 15 further comprising:

means for generating the test harmonic absorption curve.

26. A method for implementation by one or more data processors forming part of at least one computing system, the method comprising:

retrieving, by at least one data processor, a reference harmonic absorption curve of a laser absorption spectrometer, the laser absorption spectrometer comprising a tunable or scannable laser light source and a detector, the reference harmonic absorption curve having a reference curve shape and comprising a first, second, or higher order harmonic signal of a reference signal generated by the detector in response to light passing from the laser light source through a reference gas or gas mixture, the reference gas or gas mixture comprising one or more of a target analyte and a background gas expected to be present during analysis of the target analyte, the reference harmonic absorption curve having been determined for the laser absorption spectrometer in a known or calibrated state;

continually comparing, by at least one data processor, a test harmonic absorption curve having a test curve shape with the reference harmonic absorption curve to detect a difference between the test curve shape and the reference curve shape that exceeds a predefined allowed deviation and therefore indicates a change in an output of the laser light source relative to the known or calibrated state;

wherein portions of the test harmonic absorption curve are compared with portions of the reference harmonic absorption curve arising due to absorption of the background gases and not by absorption of the target analyte; and

iteratively adjusting, by at least one data processor, one or more operating and/or analytical parameters of the laser absorption spectrometer to correct the test curve shape until the difference between the test curve shape and the reference curve shape is below a pre-determined threshold.

27. An apparatus comprising:

means for retrieving a reference harmonic absorption curve of a laser absorption spectrometer, the laser absorption spectrometer comprising a tunable or scannable laser light source and a detector, the reference harmonic absorption curve having a reference curve shape and comprising a first, second, or higher order harmonic signal of a reference signal generated by the detector in response to light passing from the laser light source through a reference gas or gas mixture, the reference gas or gas mixture comprising one or more of a target analyte and a background gas expected to be present during analysis of the target analyte, the reference harmonic absorption curve having been determined for the laser absorption spectrometer in a known or calibrated state;

means for comparing a test harmonic absorption curve having a test curve shape with the reference harmonic absorption curve to detect a difference between the test curve shape and the reference curve shape that exceeds a predefined allowed deviation and therefore indicates a change in an output of the laser light source relative to the known or calibrated state, wherein portions of the test harmonic absorption curve are compared with portions of the reference harmonic absorption curve arising due to absorption of the background gases and not by absorption of the target analyte; and

means for adjusting one or more operating and/or analytical parameters of the laser absorption spectrometer to correct the test curve shape to reduce the difference between the test curve shape and the reference curve shape.