EXTERNAL/INTERNAL OPTICAL ADAPTER WITH BIASED PHOTODIODES FOR FTIR SPECTROPHOTOMETER

• US 20110079719A1
• Filed: 10/06/2010
• Published: 04/07/2011
• Est. Priority Date: 10/06/2009
• Status: Active Grant

• Current Assignee: Curators of the University of Missouri (University of Missouri)
• Original Assignee: Curators of the University of Missouri (University of Missouri)
• Inventors: Xu, Zhi
• Granted Patent: US 8,830,474 B2
• Time in Patent Office: Days
• US Class Current: 250/339.08
• CPC Class Codes: G01J 3/4535 Devices with moving mirror G01J3/4532 takes precedence

Abstract

A Fourier Transfer Infrared (FTIR) spectrophotometer having reduced baseline noise. The system and method include internal or external optical adapters having a moveable beamsplitter for splitting the source light beam into a reference beam and a sample beam, and may include a variable bandpass filter, variable preamplifier and reversed biased photodiodes.

Claims

1. A method of determining an optical spectrum for a sample substance relative to a reference substance, said method for use with an FTIR spectrophotometry system having a light source generating a source light beam of coherent light, said method comprising:

1) splitting by using a beamsplitter the source beam into a reference beam and a sample beam;

2) directing the reference beam into a reference cell having a substance therein, said substance in the reference cell interacting with the reference beam, said interacting yielding an output reference beam having a direction;

3) directing the sample beam into a sample cell having a substance therein, said substance in the sample cell interacting with the sample beam, said interacting yielding an output sample beam having a direction;

4) detecting by biased photodiodes at least a portion of the output reference beam and at least a portion of the output sample beam;

5) generating a biased reference signal representative of the detected portion of the output reference beam and a biased sample signal representative of the detected portion of the output sample beam;

6) generating a difference signal proportional to the difference between the biased reference signal and the biased sample signal;

7) placing the reference substance in the reference cell and in the sample cell;

8) determining a reference spectrum of the reference substance based on the difference signal;

9) moving the beamsplitter in a direction perpendicular to the propagation of the source beam to determine a minimized noise location of the beamsplitter at which a center burst of an interferogram is minimized;

10) determining a baseline difference spectrum;

11) placing the beamsplitter at the determined minimized noise location;

12) thereafter placing the reference substance in the reference cell and placing the sample substance in the sample cell;

13) determining a sample difference spectrum; and

14) determining the absorption spectrum of the sample based on the reference spectrum, the baseline difference spectrum, and the sample difference spectrum.

2. The method of claim 1 wherein said detecting comprises using a reference photodiode to detect at least a portion of the output reference beam and using a sample photodiode to detect at least a portion of the output sample beam and wherein a switch selectively electrically connects a grounded cathode of the reference photodiode and an anode of the sample photodiode and wherein an anode of the reference photodiode is connected to a cathode of the sample photodiode at a point which provides a current signal, and wherein an amplifier converts the current signal into a reference voltage and a difference voltage;

and further comprising determining a sample difference spectrum and baseline difference spectrum when the switch is closed and a reference spectrum when the switch is open and wherein the spectrum of the sample substance is based on the determined sample difference spectrum, the determined baseline difference spectrum and the determined reference spectrum.

3. The method of claim 2 wherein the amplifier has a variable current-to-voltage conversion factor and wherein a higher gain factor is used to determine a baseline difference spectrum and a sample difference spectrum and a lower gain factor is used to determine a reference spectrum.

4. The method of claim 1 wherein said detecting comprises using a reference photodiode to detect at least a portion of the output reference beam and using a sample photodiode to detect at least a portion of the output sample beam and wherein a cathode of the reference photodiode is electrically connected by a central point to an anode of the sample photodiode, the central point providing a current signal, wherein an anode of the reference photodiode is connected to reverse-biasing battery, wherein a cathode of the sample photodiode is connected to reverse-biasing battery, and an amplifier converting the current signal into a reference voltage and a difference voltage;

and further comprising determining a sample difference spectrum, a baseline difference spectrum and a reference spectrum wherein the spectrum of the sample substance is based on the determined sample difference spectrum, the determined baseline difference spectrum and the determined reference spectrum.

5. The method of claim 4 wherein the amplifier has a variable electronic current-to-voltage conversion factor and wherein a higher gain factor is used to determine a baseline difference spectrum and a sample spectrum.

6. The method of claim 1 wherein the spectrophotometry system comprises an FTIR spectrophotometer with a moving mirror and wherein a variable bandpass filter filters the difference signal, said filter having a variable band center and a variable bandwidth, and further comprising setting the band center and the bandwidth of the filter as a function of a center wavelength of the optical absorption band, a full width at half maximum (FWHM) bandwidth of the optical absorption band and a speed at which the moving mirror of the FTIR spectrophotometer travels.

7. An adapter for use with a spectrophotometry system having a light source generating a source light beam of coherent light, said adapter comprising:

a sealed housing for receiving the source light beam;

an optical system within the housing including a moveable beamsplitter for splitting the source light beam into a reference beam directed in a first path and a sample beam directed in a second path different from the first path wherein the beamsplitter is moveable between the first and second paths;

a reference cell within the housing, said reference cell including a reference for interacting with the reference beam wherein said interacting yields an output reference beam having a direction;

a reference light biased photodetector for detecting at least a portion of the output reference beam based on the direction of the output reference beam and generating a reference signal representative of the detected light;

a sample cell within the housing, said sample cell including a sample for interacting with the sample beam wherein said interacting yields an output sample beam having a direction;

a sample light biased photodetector for detecting at least a portion of the output sample beam based on the direction of the output sample beam and generating a sample signal representative of the detected light;

a detector circuit for producing a difference signal proportional to the difference between the reference signal and the sample signal; and

a processor configured to determine a spectrum of the sample based on the difference signal.

8. The adapter of claim 7 wherein the reference light biased photodetector comprises a reference photodiode and the sample light biased photodetector comprises a sample photodiode and further comprising a switch selectively electrically connecting a grounded cathode of the reference photodiode and an anode of the sample photodiode and wherein an anode of the reference photodiode is connected to a cathode of the sample photodiode at a point which provides a current signal, and an amplifier converting the current signal into a reference voltage and a difference voltage.

9. The adapter of claim 8 wherein the amplifier has different current-to-voltage conversion factors.

10. The adapter of claim 7 wherein the reference light biased photodetector comprises a reference photodiode and the sample light biased photodetector comprises a sample photodiode and wherein a cathode of the reference photodiode is electrically connected by a central point to an anode of the sample photodiode, the central point providing a current signal, wherein an anode of the reference photodiode is connected to reverse-biasing battery, wherein a cathode of the sample photodiode is connected to reverse-biasing battery, and an amplifier converting the current signal into a reference voltage and a difference voltage.

11. The adapter of claim 10 wherein the amplifier has different current-to-voltage conversion factors.

12. The adapter of claim 7 wherein the beamsplitter is positioned to minimize a center burst of a waveform of the interferogram with a reference in both the reference cell and the sample cell so that the reference signal and sample signal are balanced and at least some source noise is cancelled in the difference signal.

13. The adapter of claim 7 wherein the detector circuit comprises a circuit for selectively producing a reference voltage proportional to the reference signal and a difference voltage proportional to the difference between the reference signal and the sample signal wherein the beamsplitter is moveable along the first and second paths to a position at which noise cancellation is increased and the signal to noise ratio of the difference signal is increased.

14. The adapter of claim 7 further comprising a variable electronic bandpass filter between the detector circuit and the processor for filtering the difference signal and wherein the processor is configured to determine a spectrum of the sample based on the filtered difference signal.

15. The adapter of claim 14 wherein the spectrophotometry system comprises an FTIR spectrophotometer with a moving mirror and wherein the variable electronic bandpass filter has a variable band center and a variable bandwidth and wherein the band center and the bandwidth are a function of a center wavelength of the optical absorption band, a full width at half maximum (FWHM) bandwidth of the optical absorption band and a speed at which the moving mirror of the FTIR spectrophotometer travels.

16. A spectrophotometry system comprising:

a light source generating a light beam;

a lens in a path of the light beam and transmitting the light beam as a coherent source light beam;

an optical system including a moveable beamsplitter for splitting the source light beam into a reference beam directed in a first path and a sample beam directed in a second path different from the first path wherein the beamsplitter is moveable between the first and second paths;

a reference cell including a reference for interacting with the reference beam wherein said interacting yields an output reference beam having a direction;

a reference light biased photodetector for detecting at least a portion of the output reference beam based on the direction of the output reference beam and generating a reference signal representative of the detected light;

a sample cell including a sample for interacting with the sample beam wherein said interacting yields an output sample beam having a direction;

a sample light biased photodetector for detecting at least a portion of the output sample beam based on the direction of the output sample beam and generating a sample signal representative of the detected light;

a detector circuit for producing a difference signal proportional to the difference between the reference signal and the sample signal;

a sealed housing for enclosing the above; and

a processor configured to determine a spectrum of the sample based on the difference signal.

17. The adapter of claim 16 wherein the reference light biased photodetector comprises a reference photodiode and the sample light biased photodetector comprises a sample photodiode and further comprising a switch selectively electrically connecting a grounded cathode of the reference photodiode and an anode of the sample photodiode and wherein an anode of the reference photodiode is connected to a cathode of the sample photodiode at a point which provides a current signal, and an amplifier converting the current signal into a reference voltage and a difference voltage.

18. The adapter of claim 17 wherein the amplifier has different current-to-voltage conversion factors.

19. The adapter of claim 16 wherein the reference light biased photodetector comprises a reference photodiode and the sample light biased photodetector comprises a sample photodiode and wherein a cathode of the reference photodiode is electrically connected by a central point to an anode of the sample photodiode, the central point providing a current signal, wherein an anode of the reference photodiode is connected to reverse-biasing battery, wherein a cathode of the sample photodiode is connected to reverse-biasing battery, and an amplifier converting the current signal into a reference voltage and a difference voltage.

20. The adapter of claim 19 wherein the amplifier has different current-to-voltage conversion factors.

21. The adapter of claim 16 wherein the beamsplitter is positioned to minimize a center burst of a waveform of the interferogram with a reference in both the reference cell and the sample cell so that the reference signal and sample signal are balanced and at least some source noise is cancelled in the difference signal.

22. The adapter of claim 16 wherein the detector circuit comprises a circuit for selectively producing a reference voltage proportional to the reference signal and a difference voltage proportional to the difference between the reference signal and the sample signal wherein the beamsplitter is moveable along the first and second paths to a position at which noise cancellation is increased and the signal to noise ratio of the difference signal is increased.

23. The system of claim 16 further comprising a variable electronic bandpass filter between the detector circuit and the processor for filtering the difference signal and wherein the processor is configured to determine a spectrum of the sample based on the filtered difference signal.

24. The system of claim 23 wherein the spectrophotometry system comprises an FTIR spectrophotometer with a moving mirror and wherein the variable electronic bandpass filter has a variable band center and a variable bandwidth and wherein the band center and the bandwidth are a function of a center wavelength of the optical absorption band, a full width at half maximum (FWHM) bandwidth of the optical absorption band and a speed at which the moving mirror of the FTIR spectrophotometer travels.