TRANSMISSION QUANTIFICATION OF OPEN PATH FTIR SPECTRA WITH TEMPERATURE COMPENSATION

• US 20110112772A1
• Filed: 05/14/2009
• Published: 05/12/2011
• Est. Priority Date: 05/16/2008
• Status: Active Grant

• Current Assignee: University of Washington
• Original Assignee: University of Washington
• Inventors: Yost, Michael G., Crampton, Robert S.
• Granted Patent: US 9,194,744 B2
• Time in Patent Office: Days
• US Class Current: 702/24
• 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 2021/3595 using FTIR
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis

Abstract

A transmission quantification approach that is effective at quantifying the concentration of key atmospheric gases, including water vapor and methane, does not require a background spectrum and is immune to changes between background and absorbance spectra. By using local minima and maxima in transmission of a target gas, this approach creates two spectral arrays as long as a single beam input spectra. One of these spectral arrays represents the points in wave-number space that are less absorbing points, and the other represents the more absorbing points. A concentration for a given gas is calculated by determining what reference concentration creates a residual after division by a pure gas spectrum that forces these two arrays to converge.

Claims

1. A method for using a computing device and reference transmittance data for at least one concentration of a gas, to automatically determine an actual concentration of a gas in a path from a sample input signal corresponding to an analytical spectrum of light that has propagated along the path, comprising the steps of:

(a) dividing the analytical spectrum by the reference transmittance data to determine a plurality of residual spectra, including a residual spectrum for each concentration in the reference transmittance data; and

(b) identifying a minimum difference between absorbing and non-absorbing portions of the residual spectra, the concentration in the reference transmittance data that corresponds to the minimum difference indicating the actual concentration of the gas in the path.

2. A method for automatically determining an actual concentration of a gas in an path using an optically derived analytical spectrum for the path, and using transmittance reference data for the gas, wherein the transmittance data comprise a plurality of transmittance spectrum for each of a corresponding plurality of different concentrations of the gas, comprising the steps of:

(a) dividing the analytical spectrum by the transmittance spectrum for each of the plurality of different concentrations of the gas, to determine residual spectra for the different concentrations of the gas;

(b) using mask functions derived from the transmittance reference data, separating the residual spectra into absorbing and non-absorbing residual spectra;

(c) smoothing the absorbing and non-absorbing residual spectra to produce smoothed absorbing and non-absorbing residual spectra; and

(d) determining an actual concentration of the gas in the path by identifying the concentration of the gas for which a difference between the smoothed absorbing and non-absorbing residual spectra is a minimum.

3. The method of claim 2, further comprising the step of determining an absorbing mask function and a non-absorbing mask function from the reference transmittance data.

4. The method of claim 3, wherein the step of determining the absorbing mask function and the non-absorbing mask function comprises the step of comparing each transmittance point of a reference transmittance spectrum to a floating median of transmittance within a window around that transmittance point, wherein the absorbing mask is based on the transmittance points that are on one side of the floating median, and the non-absorbing mask is based on the transmittance points on an opposite side of the floating median.

5. The method of claim 2, wherein the step of using the mask functions comprises the step of multiplying the residual spectra by the mask functions, to produce the absorbing and non-absorbing residual spectra.

6. The method of claim 5, further comprising the step of removing specific values after multiplying the residual spectra by the mask functions, to produce the absorbing and non-absorbing residual spectra.

7. The method of claim 2, wherein the step of determining the actual concentration of the gas in the path by identifying the concentration of the gas for which a difference between the smoothed absorbing and non-absorbing residual spectra is a minimum comprises the step of subtracting the smoothed absorbing residual spectra from the non-absorbing smoothed spectra for each of the different concentrations of the gas to determine a differential intensity.

8. The method of claim 7, wherein the step of determining the actual concentration of the gas in the path by identifying the concentration of the gas for which a difference between the smoothed absorbing and non-absorbing residual spectra is a minimum further comprises the step of selecting the concentration of the gas that minimizes the differential intensity.

9. The method of claim 2, further comprising the step of interpolating the transmittance reference spectra to create an interpolated transmittance spectra for the actual concentration of the gas that was determined.

10. The method of claim 9, further comprising the step of dividing the analytical spectrum by the interpolated transmittance reference spectrum to determine a new residual spectrum that can be used for determining a concentration of a different gas in the path.

11. The method of claim 2, further comprising the step of creating the analytical spectrum by directing light along the path and obtaining the analytical spectrum using a spectrometer selected from a group of spectrometers consisting of:

(a) a Fourier transform infrared spectrometer;

(b) an ultraviolet absorption spectrometer;

(c) a visible light spectrometer; and

(d) a Raman spectrometer.

12. A medium including machine readable and executable instructions for carrying out a plurality of functions to automatically determine an actual concentration of a gas in an path using an optically derived analytical spectrum for the path, and using transmittance reference data for the gas, wherein the transmittance data comprise a plurality of transmittance spectrum for each of a corresponding plurality of different concentrations of the gas, said plurality of functions including:

(a) dividing the analytical spectrum by the transmittance spectrum for each of the plurality of different concentrations of the gas, to determine residual spectra for the different concentrations of the gas;

(b) using mask functions derived from the transmittance reference data, separating the residual spectra into absorbing and non-absorbing residual spectra;

(c) smoothing the absorbing and non-absorbing residual spectra to produce smoothed absorbing and non-absorbing residual spectra; and

(d) determining an actual concentration of the gas in the path by identifying the concentration of the gas for which a difference between the smoothed absorbing and non-absorbing residual spectra is a minimum.

13. A system for automatically determining an actual concentration of a gas in a path using an optically derived analytical spectrum for the path, and using transmittance reference data for the gas, wherein the transmittance data comprise a plurality of transmittance spectrum for each of a corresponding plurality of different concentrations of the gas, comprising:

(a) a memory in which are stored the transmittance data, and a plurality of machine executable instructions;

(b) an input port coupled to receive an input signal corresponding to the analytical spectrum; and

(b) a processor that is coupled to the memory and the input port, the processor executing the machine executable instructions to carry out a plurality of functions, including;

(i) dividing the analytical spectrum by the transmittance spectrum for each of the plurality of different concentrations of the gas, to determine residual spectra for the different concentrations of the gas;

(ii) using mask functions derived from the transmittance reference data, separating the residual spectra into absorbing and non-absorbing residual spectra;

(iii) smoothing the absorbing and non-absorbing residual spectra to produce smoothed absorbing and non-absorbing residual spectra; and

(iv) determining an actual concentration of the gas in the path by identifying the concentration of the gas for which a difference between the smoothed absorbing and non-absorbing residual spectra is a minimum.

14. The system of claim 13, wherein the execution of the machine executable instructions by the processor further causes the processor to carry out the function of determining an absorbing mask function and a non-absorbing mask function from the reference transmittance data.

15. The system of claim 14, wherein the execution of the machine executable instructions by the processor further causes the processor to carry out the function of comparing each transmittance point of a reference transmittance spectrum to a floating median of transmittance within a window around that transmittance point, wherein the absorbing mask is based on the transmittance points that are on one side of the floating median, and the non-absorbing mask is based on the transmittance points on an opposite side of the floating median.

16. The system of claim 13, wherein the execution of the machine executable instructions by the processor further causes the processor to carry out the function of multiplying the residual spectra by the mask functions, to produce the absorbing and non-absorbing residual spectra.

17. The system of claim 16, wherein the execution of the machine executable instructions by the processor further causes the processor to carry out the function of removing zeroes after multiplying the residual spectra by the mask functions, to produce the absorbing and non-absorbing residual spectra.

18. The system of claim 13, wherein the execution of the machine executable instructions by the processor further causes the processor to carry out the function of subtracting the smoothed absorbing residual spectra from the non-absorbing smoothed spectra for each of the different concentrations of the gas to determine a differential intensity.

19. The system of claim 18, wherein the execution of the machine executable instructions by the processor further causes the processor to carry out the function of selecting the concentration of the gas that minimizes the differential intensity, to determine the actual concentration of the gas in the path.

20. The system of claim 13, wherein the execution of the machine executable instructions by the processor further causes the processor to carry out the function of interpolating the transmittance reference spectra to create an interpolated transmittance spectra for the actual concentration of the gas that was determined.

21. The system of claim 20, wherein the execution of the machine executable instructions by the processor further causes the processor to carry out the function of dividing the analytical spectrum by the interpolated transmittance reference spectrum to determine a new residual spectrum that can be used for determining a concentration of a different gas in the path.

22. The system of claim 13, further including a spectrometer that produces the input signal corresponding to the analytical spectrum, wherein the spectrometer is selected from a group of spectrometers consisting of:

(a) a Fourier transform infrared spectrometer;

(b) an ultraviolet absorption spectrometer;

(c) a visible light spectrometer; and

(d) a Raman spectrometer.