Optical Absorption Spectrometer and Method for Measuring Concentration of a Substance

Abstract

An optical absorption spectrometer is provided for determining the concentration of a substance within a sample. The optical absorption spectrometer comprises a first radiation source for supplying radiation to the sample to be measured; at least one cavity for containing the sample during measurement; and a detector assembly for detecting radiation transmitted along first and second optical paths through the sample, the length of the first optical path being greater than that of the second optical path.

1 . An optical absorption spectrometer for determining the concentration: of a substance within a sample, comprising:
 a first radiation source for supplying radiation to the sample to be measured;   at least one cavity for containing the sample during measurement;   a detector assembly for simultaneously detecting radiation transmitted along first and second optical paths through the sample, the length of the first optical path within the sample being greater than that of the second optical path within the sample, whereby the radiation detector receives the sum of the radiation transmitted along each of the first and second optical paths, and outputs a radiation signal corresponding thereto; and   a processor adapted to generate a measurement of the concentration of the substance in the sample from the radiation signal output by the detector.   
   
   
       2 . An optical absorption spectrometer according to  claim 1  wherein the at least one cavity has a dimension co-axial with the first optical path which is greater than a dimension of the at least one cavity co-axial with the second optical path. 
   
   
       3 . An optical absorption spectrometer according to  claim 2  wherein the first and second optical paths are parallel, and the dimension of the cavity parallel to the first and second optical paths varies in a direction perpendicular to the axes of the first and second optical paths. 
   
   
       4 . An optical absorption spectrometer according to  claim 3  wherein the dimension of the cavity parallel to the first and second optical paths varies in a step-wise manner. 
   
   
       5 . An optical absorption spectrometer according to  claim 3  wherein the dimension of the cavity parallel to the first and second optical paths varies continuously. 
   
   
       6 . An optical absorption spectrometer according to  claim 2  wherein the cavity is defined within a chamber, the chamber comprising cavity varying means for varying the cavity dimensions. 
   
   
       7 . An optical absorption spectrometer according to  claim 6  wherein the cavity varying means comprise at least one chamber wall for dividing the cavity from a sample-free volume. 
   
   
       8 . An optical absorption spectrometer according to  claim 7  wherein the sample-free volume contains a radiation-transparent material. 
   
   
       9 . An optical absorption spectrometer according to  claim 6  wherein the cavity varying means comprise chamber walls for defining the cavity therewithin, the chamber walls varying in thickness. 
   
   
       10 . An optical absorption spectrometer according to  claim 1  wherein the cavity is provided with a first radiation guiding assembly for guiding radiation along the first optical path, and a second radiation guiding assembly for guiding radiation along the second optical path. 
   
   
       11 . An optical absorption spectrometer according to  claim 10  wherein the first and second radiation guiding assemblies comprise a plurality of reflective elements. 
   
   
       12 . An optical absorption spectrometer according to  claim 11  wherein the first and second radiation guiding assemblies further comprise one or more beam splitters for dividing radiation between the first and second optical paths. 
   
   
       13 . An optical absorption. spectrometer according to  claim 10  wherein the cavity dimensions are adjustable such that the length of the first and/or second optical paths can be adjusted. 
   
   
       14 . An optical absorption spectrometer according to  claim 1  further comprising a second radiation source for supplying radiation to the sample to be measured, the first radiation source being arranged to supply radiation along the first optical path, and the second radiation source being arranged to supply radiation along the second optical path. 
   
   
       15 . An optical absorption spectrometer according to  claim 1  wherein the detector assembly comprises first and second detector elements for detecting radiation transmitted through the sample, the first detector element being arranged to detect radiation on the first optical path, and the second detector element being arranged to detect radiation on the second optical path. 
   
   
       16 . An optical absorption spectrometer according to  claim 1  adapted for the detection of methane. 
   
   
       17 . An optical absorption spectrometer according to  claim 1  wherein the optical absorption spectrometer is a correlated interference polarization spectrometer and further comprises:
 a filter for filtering radiation transmitted by the sample, the filter having a number of pass bands at wavelengths corresponding to absorption peaks in the absorption spectrum of the substance to be detected, the filter being responsive to an applied signal to modulate the wavelengths of the pass bands;   wherein the detector assembly comprises a processor for determining the difference in the minimum and maximum intensities of the detected radiation to thereby determine the concentration of the substance in the sample.   
   
   
       18 . A method of determining the concentration of a substance within a sample, comprising:
 transmitting radiation through the sample to be measured along a first optical path and a second optical path, the length of the first optical path within the sample being greater than that of the second optical path within the sample;   filtering the radiation transmitted through the sample using a filter having a number of pass bands at wavelengths corresponding to absorption peaks in the absorption spectrum of the substance to be detected, the filter being responsive to an applied signal to modulate the wavelengths of the pass bands;   simultaneously detecting the sum of the filtered radiation on each of the first and second optical paths to determine the difference in its maximum and minimum intensities to thereby determine the concentration of the substance in the sample.   
   
   
       19 . An optical absorption spectrometer according to  claim 5  wherein the dimension of the cavity parallel to the first and second optical paths varies linearly. 
   
   
       20 . An optical absorption spectrometer according to  claim 8  wherein the radiation-transparent material is nitrogen. 
   
   
       21 . An optical absorption spectrometer according to  claim 11  wherein the plurality of reflective elements are selected from a group consisting of mirrors and prisms.