Ultrasensitive spectrophotometer

• US 20060152726A1
• Filed: 01/13/2005
• Published: 07/13/2006
• Est. Priority Date: 01/13/2005
• 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: Larsen, David W., Xu, Zhi
• Granted Patent: US 7,262,844 B2
• Time in Patent Office: Days
• US Class Current: 356/416
• CPC Class Codes: G01J 2003/425 Reflectance
• CPC Class Codes: G01J 3/02 Details
• CPC Class Codes: G01J 3/0224 using polarising or depolarising elements
• CPC Class Codes: G01J 3/0262 Constructional arrangements for removing stray light
• CPC Class Codes: G01J 3/0286 Constructional arrangements for compensating for fluctuations caused by temperature, humidity or pressure, or using cooling or temperature stabilization of parts of the device; Controlling the atmosphere inside a spectrometer, e.g. vacuum
• CPC Class Codes: G01J 3/0291 Housings; Spectrometer accessories; Spatial arrangement of elements, e.g. folded path arrangements
• CPC Class Codes: G01J 3/42 Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry beam switching arrangements G01J3/08
• CPC Class Codes: G01N 2021/3137 with selection of wavelengths after the sample
• 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 2201/0227 Sealable enclosure
• CPC Class Codes: G01N 2201/0642 Light traps; baffles
• CPC Class Codes: G01N 2201/127 Calibration; base line adjustment; drift compensation

Abstract

The invention concerns measurements in which light interacts with matter giving rise to changes in light intensity, and preferred embodiment spectrophotometer devices of the invention provide for ultrasensitive measurements through a reflection interaction with matter. The level of light source noise in these measurements can be reduced in accordance with the invention. Preferred embodiments of the invention use sealed housings lacking an internal light source, and reflection based sample and reference cells. In some embodiments a substantially solid thermally conductive housing is used. Other features of preferred embodiments include particular reflection based sample and reference cells. A total internal reflection embodiment includes, for example, a prism including an interaction surface, a detector, a lens that focuses a beam output from the prism onto the detector, and a closed interaction volume having an inlet and an outlet for delivering gas or liquid to the interaction surface. In a specular reflection embodiment, a reflective surface is used instead of a prism. In a diffuse reflection embodiment a matte surface is used instead of a prism and the matte surface produces scattering. Aspects of the invention include identification of noise-contributing components in spectrophotometry and the select set of preferred features in a given embodiment, and noise levels very near the shot noise limit may be realized with application of preferred embodiment devices.

Claims

1. A spectrophotometer, comprising:

a sealed housing lacking an internal light source;

a reference cell mounted in said housing;

a sample cell mounted in said housing;

an optical system within the housing to split an externally generated light beam into reference and sample beams and to deliver the reference and sample beams through separate paths, one path passing the reference beam into the reference cell and another path passing the sample beam into said sample cell;

reference cell interaction means for interacting the reference beam with a reference and reflecting at least a portion of the reference beam;

sample cell interaction means for interacting the sample beam with a sample and reflecting at least a portion of the sample beam;

a reference light detector for detecting at least a portion of light reflected by said reference cell interaction means;

a sample light detector for detecting at least a portion of light reflected by said sample cell interaction means;

a light guide for delivering the externally generated light beam into said optical system from a light source external to said sealed housing.

2. The spectrophotometer of claim 1, further comprising a sealed mount mounting said light guide to said housing.

3. The spectrophotometer of claim 1, further comprising:

a light source external to said housing and optically coupled to said light guide.

4. The spectrophotometer according to claim 1, further comprising a selective wavelength device optically coupled to said light guide.

5. The spectrophotometer according to claim 4, wherein said selective device comprises an interference filter.

6. The spectrophotometer of claim 1, wherein said reference cell interaction means and said sample cell interaction means each utilize total internal reflection to respectively interact the reference beam with the reference and the sample beam with the sample.

7. The spectrophotometer of claim 1, wherein said reference cell interaction means and said sample cell interaction means each utilize specular reflection to respectively interact the reference beam with the reference and the sample beam with the sample.

8. The spectrophotometer of claim 1, wherein said reference cell interaction means and said sample cell interaction means each utilize diffuse reflection to respectively interact the reference beam with the reference and the sample beam with the sample.

9. The spectrophotometer according to claim 1, further comprising a dielectric beam splitter for splitting the externally generated light beam into separate sample and reference beams emergent on the two paths.

10. The spectrophotometer of claim 1, further comprising:

a detector circuit receiving a reference current from said reference light detector and a sample current from said sample light detector, said detector circuit producing a difference voltage proportional to the difference between the reference current and the sample current by a current mode subtraction at a summing point having feedback to balance said reference current and said sample current to produce a difference current that is then converted to the difference voltage,

said detector circuit simultaneously producing the difference voltage and at least one of a voltage proportional to said reference current and a voltage proportional to said sample current.

11. The spectrophotometer according to claim 10, further comprising passive thermal stabilization means for promoting internal thermal equilibrium.

12. The spectrophotometer according to claim 10, wherein said reference current is converted to a reference voltage and said sample current is converted to a sample voltage, said reference and sample voltages are subtracted at an input to a difference amplifier, said difference amplifier then producing the difference voltage.

13. The spectrophotometer according to claim 12, wherein said detector circuit further comprises a feedback loop to feed a small fraction of a smaller one of said sample current and said reference current to said input to said difference amplifier.

14. The spectrophotometer according to claim 13, wherein said small fraction of said smaller one of said sample and said reference current is an amount to produce an approximate 10⁻

3 or less imbalance between said sample current and said reference.

15. The spectrophotometer according to claim 1, further comprising:

a sample light detector chamber to house said sample light detector;

a reference light detector chamber to house said reference light detector; and

light traps in each of said sample light detector chamber and said reference light detector chamber to trap light reflected from said sample light detector and said reference light detector, respectively.

16. The spectrophotometer according to claim 1, wherein said housing comprises a solid unitary metal housing having a hollowed portion shaped to provide for mounting and placement of said reference cell, said sample cell, said optical system, said reference light detector, and said sample light detector.

17. The spectrophotometer according to claim 16, further comprising slots in said solid unitary metal housing for holding components of said optical system.

18. The spectrophotometer according to claim 16, wherein said solid unitary metal housing comprises Aluminum.

19. A spectrophotometer, comprising:

a substantially solid thermally conductive housing;

a hollow portion in said housing defining light communication paths and component locations;

a light entry aperture in optical communication with said hollow portion;

a collimating lens in optical communication with said light entry aperture;

a first beam splitter in optical communication with said collimating lens;

a second beam splitter in optical communication with a transmissive side of said first beam splitter;

a third beam splitter in optical communication with a reflective side of said first beam splitter;

a reference cell reflection interaction interface and reflected beam detection system in optical communication with one of said second and third beam splitters; and

a sample cell reflection interaction interface and reflected beam detection system in optical communication with the other of said second and third beam splitters.

20. The spectrophotometer according to claim 19, wherein said reference and said sample cell interaction interfaces and reflected beam detection systems are in optical communication with a reflective side of said second beam splitter and a transmissive side of said third beam splitter.

21. The spectrophotometer according to claim 19, further comprising light traps formed as part of said hollow portion and disposed to trap light transmitted from the second beam splitter and reflected light from said third beam splitter.

22. The spectrophotometer according to claim 19, further comprising additional light traps to trap reflected light from said reference and sample cell and detection systems.

23. The spectrophotometer according to claim 19, wherein said reference cell reflection interaction interface and reflected beam detection system and said sample cell reflection interaction interface and reflected beam detection system each comprise:

a prism including an interaction surface;

a detector;

a lens that focuses a beam output from said prism onto said detector;

a closed interaction volume having an inlet and an outlet for delivering gas or liquid to the interaction surface.

24. The spectrophotometer according to claim 19, wherein said reference cell reflection interaction interface and reflected beam detection system and said sample cell reflection interaction interface and reflected beam detection system each comprise:

a reflective interaction surface;

a detector;

a lens that focuses a beam output from said reflective interaction surface onto said detector;

a closed interaction volume having an inlet and an outlet for delivering gas or liquid to the reflective interaction surface.

25. The spectrophotometer according to claim 19, wherein said reference cell reflection interaction interface and reflected beam detection system and said sample cell reflection interaction interface and reflected beam detection system each comprise:

a matte interaction surface;

a detector;

a lens or mirror that focuses scattering output from said matte interaction surface onto said detector;

a closed interaction volume having an inlet and an outlet for delivering gas or liquid to the interaction surface.

26. A spectrophotometer, comprising:

a sealed housing lacking an internal light source;

a reference cell mounted in said housing;

a sample cell mounted in said housing;

an optical system within the housing to split an externally generated light beam into reference and sample beams and to deliver the reference and sample beams through separate paths, one path passing the reference beam into the reference cell and another path passing the sample beam into said sample cell;

a reference cell reflection interaction interface and reflected beam detection system in optical communication with the one path passing the reference beam; and

a sample cell reflection interaction interface and reflected beam detection system in optical communication with the another path passing the sample beam;

a reference light detector for detecting at least a portion of light reflected by said reference cell interaction means;

a sample light detector for detecting at least a portion of light reflected by said sample cell interaction means;

a light guide for delivering the externally generated light beam into said optical system from a light source external to said sealed housing.

27. The spectrophotometer according to claim 26, wherein said reference cell reflection interaction interface and reflected beam detection system and said sample cell reflection interaction interface and reflected beam detection system each comprise:

a prism including an interaction surface;

a detector;

a lens or mirror that focuses a beam output from said prism onto said detector;

a closed interaction volume having an inlet and an outlet for delivering gas or liquid to the interaction surface.

28. The spectrophotometer according to claim 26, wherein said reference cell reflection interaction interface and reflected beam detection system and said sample cell reflection interaction interface and reflected beam detection system each comprise:

a reflective interaction surface;

a detector;

a lens that focuses a beam output from said reflective interaction surface onto said detector;

a closed interaction volume having an inlet and an outlet for delivering gas or liquid to the reflective interaction surface.

29. The spectrophotometer according to claim 26, wherein said reference cell reflection interaction interface and reflected beam detection system and said sample cell reflection interaction interface and reflected beam detection system each comprise:

a matte interaction surface;

a detector;

a lens that focuses scattering output from said matte interaction surface onto said detector;

a closed interaction volume having an inlet and an outlet for delivering gas or liquid to the interaction surface.

30. The spectrophotometer according to claim 26, wherein said housing comprises:

a substantially solid thermal conductive housing;

a hollow portion in said housing defining light communication paths and component locations.