Infrared gas analysis

• US 4,035,643 A
• Filed: 01/08/1976
• Issued: 07/12/1977
• Est. Priority Date: 06/11/1974
• Status: Expired due to Term

• Current Assignee: Allied Chemical Corporation (Honeywell International Inc.)
• Original Assignee: Allied Chemical Corporation (Honeywell International Inc.)
• Inventors: Barrett, Joseph J.
• Primary Examiner(s): Smith, Alfred E.
• Assistant Examiner(s): Anderson, B. C.
• Application Number:
  US05/647,388
• Time in Patent Office: 551 Days
• Field of Search: 250/339, 250/340, 250/343, 356/106 S
• US Class Current: 250/339.13
• CPC Class Codes: G01J 3/26 using multiple reflection, e.g. Fabry-Perot interferometer, variable interference filters

Abstract

A method and apparatus for detecting and quantitatively measuring a molecular species of gaseous material in a sample to be analyzed are provided. Light containing incoherent infrared radiation is collected, collimated and transmitted by a light conditioning means to a primary filtering means. The primary filtering means selectively transmits light having a frequency range in the region of an absorption band for a molecular species to be detected. A secondary filtering means, adapted to receive the filtered light, transmits light at a plurality of discrete frequencies, providing a detectable signal, through the gaseous material. The intensity of the signal changes in proportion to the concentration of the molecular species. Means are provided for measuring and recording the magnitude of the signal intensity change. The intensity of the detectable signal is not affected by molecular species other than the species appointed for detection, and the intensity differential represents a relatively large change in a small signal. Hence, gaseous constituents are detected in an accurate and economical manner.

Claims

1. Apparatus for detecting and quantitatively measuring a molecular species of gaseous material in a sample to be analyzed, comprising:

(a) light source means for generating incoherent infrared radiation;

(b) light conditioning means for collecting, collimating and transmitting said light;

(c) primary filtering means adapted to receive said light and selectively transmit light having a frequency range in the region of an absorption band for the molecular species to be detected;

(d) secondary filtering means adapted to receive said filtered light and transmit light having a plurality of discrete frequencies forming a plurality of fringes which provide a detectable signal, said secondary filtering means having interference producing means for providing a plurality of transmission windows regularly spaced in frequency, the frequency spacing between adjacent windows being adjusted to equal substantially the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected and the factor n/n'"'"', where n and n'"'"' are integers and n does not equal n'"'"' and scanning means for causing the transmission peaks for adjacent n'"'"' th orders to coincide substantially with the spectral lines of said absorption spectrum, whereby said detectable signal has an intensity substantially equal to the sum of said fringes;

(e) means for transmitting said detectable signal through said gaseous material, whereby the intensity of said detectable signal changes in proportion to the concentration of said molecular species;

(f) signal conditioning means for converting to measurable form said intensity change; and

(g) detecting means for indicating the magnitude of said intensity change.

2. Apparatus as recited in claim 1, wherein said signal conditioning means includes modulating means for modulating the phase difference between interfering rays of said light so as to shift the frequency of each fringe, the modulating range being no greater than the frequency spacing between adjacent orders, and synchronous detection means for detecting the intensity variation of the signal, whereby the magnitude of the signal intensity change can be identified.

3. Apparatus as recited in claim 2, wherein said modulating means has a modulating range of about 1/2 the frequency width of each fringe.

4. Apparatus as recited in claim 3, including indicating and recording means for displaying said signal.

5. Apparatus as recited in claim 3, wherein said modulating means is a piezoelectric cylinder and said synchronous detection means is a phase sensitive detection system.

6. Apparatus as recited in claim 1 wherein said secondary filtering means is a Fabry-Perot interferometer.

7. Apparatus as recited in claim 1 wherein said secondary filtering means is a solid etalon having temperature control means associated therewith for adjusting the optical path length thereof.

8. Apparatus as recited in claim 5 including means for applying to said cylinder a voltage having a square wave form, the limits of said voltage being adjusted so that the intensity of said detectable signal alternates between its maximum and minimum values, means for determining for each cycle of said voltage the difference in amplitude between said maximum and minimum values of said detectable signal to produce an electrical output signal proportional to the maximum and minimum values of the detectable signal.

9. Apparatus as recited in claim 7, wherein said solid elaton is composed of optically transparent material selected from the group consisting of potassium bromide, potassium chloride, lithium fluoride, magnesium fluoride, calcium fluoride, cesium bromide, cesium iodide, barium fluoride, sodium chloride and sodium bromide.

10. Apparatus as recited in claim 3, wherein said modulating means is a piezoelectric cylinder and said synchronous detection means is a lock-in amplifier.

11. Apparatus as recited in claim 1, wherein the frequency spacing between adjacent windows of said interference producing means is adjusted to equal substantially the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected and the factor 1/n'"'"' where n'"'"' is an integer greater than one.

12. Apparatus as recited in claim 1, wherein the frequency spacing between adjacent windows of said interference producing means is adjusted to equal substantially the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected and the factor n/n'"'"', where n is an integer greater than one and n'"'"' is equal to one.

13. Apparatus for detecting and quantitatively measuring a molecular species of gaseous material in a sample to be analyzed comprising:

(a) light source means for generating incoherent infrared radiation, said light source means being adapted to emit light having a frequency range in the region of an absorption band for the molecular species to be detected;

(b) light conditioning means for collecting, collimating and transmitting said light;

(c) filtering means adapted to receive said light and transmit light having a plurality of discrete frequencies forming a plurality of fringes which provide a detectable signal, said filtering means having interference producing means for providing a plurality of transmission windows regularly spaced in frequency the frequency spacing between adjacent windows being adjusted to equal substantially the product of frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected and the factor n/n'"'"', where n and n'"'"' are integers and n does not equal n'"'"', and scanning means for causing the transmission peaks for adjacent n'"'"' th orders to coincide substantially with the spectral lines of said absorption spectrum, whereby said detectable signal has an intensity substantially equal to the sum of said fringes;

(d) means for transmitting said detectable signal through said gaseous material, whereby the intensity of said detectable signal changes in proportion to the concentration of said molecular species;

(e) signal conditioning means for converting to measurable form said intensity change; and

(f) detecting means for indicating the magnitude of said intensity change.

14. Apparatus as recited in claim 13 wherein said light source means is a light emitting diode.

15. Apparatus as recited in claim 13, wherein the frequency spacing between adjacent windows of said interference producing means is adjusted to equal substantially the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected and the factor 1/n'"'"', where n'"'"' is an integer greater than one.

16. Apparatus as recited in claim 13, wherein the frequency spacing between adjacent windows of said interference producing means is adjusted to equal substantially the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected and the factor n/n'"'"', where n is an integer greater than one and n'"'"' is equal to one.

17. A method for detecting and quantitatively measuring a molecular species of gaseous material in a sample to be analyzed, comprising the steps of

generating light in the form of incoherent infrared radiation;

collecting, collimating and transmitting the light;

filtering said light so as to selectively transmit light having a frequency range in the region of an absorption band for the molecular species to be detected;

interferometrically filtering said filtered light and transmitting light at a plurality of discrete frequencies to form a plurality of fringes which provide a detectable signal by directing the light through a plurality of transmission windows regularly spaced in frequency, the frequency spacing between adjacent windows being equal substantially to the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected and the factor n/n'"'"', where n and n'"'"' are integers and n does not equal n'"'"', and scanning said light to cause the transmission peaks for adjacent n'"'"' th orders to coincide substantially with the spectral lines of said absorption spectrum, said signal having an intensity substantially equal to the sum of said fringes;

transmitting said detectable signal through said gaseous material, whereby the intensity of said detectable signal changes in proportion to the concentration of said molecular species; and

detecting and indicating the intensity change of said signal.

18. A method as recited in claim 17, including the steps of modulating the phase difference between interfering rays of said light so as to vary the intensity of the signal, the modulating range being no greater than the frequency spacing between adjacent absorption lines of said molecular species.

19. A method as recited in claim 17, wherein the frequency spacing between adjacent windows is adjusted to equal substantially the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected and the factor 1/n'"'"', where n'"'"' is an integer greater than one.

20. A method as recited in claim 17, wherein the frequency spacing between adjacent windows is adjusted to equal substantially the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected and the factor n/n'"'"', where n is an integer greater than one and n'"'"' is equal to one.