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Low concentrations of water vapor within a background of one or more olefin gases may be detected and quantified using a differential absorption spectrometer. A dehydrated sample of the gas is used as a background sample whose absorption spectrum allows elimination of absorption features not due to

Low concentrations of water vapor within a background of one or more olefin gases may be detected and quantified using a differential absorption spectrometer. A dehydrated sample of the gas is used as a background sample whose absorption spectrum allows elimination of absorption features not due to water vapor in the gas. Absorption spectra may recorded using tunable diode lasers as the light source. These lasers may have a wavelength bandwidth that is narrower than the water vapor absorption feature used for the differential absorption spectral analysis.

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What is claimed is: 1. A method comprising: dehydrating a first sample of an olefin gas mixture containing an unknown concentration of water vapor and a varying concentration of one or more olefins to reduce the water vapor concentration; recording a first absorption spectrum of the first sample at

What is claimed is: 1. A method comprising: dehydrating a first sample of an olefin gas mixture containing an unknown concentration of water vapor and a varying concentration of one or more olefins to reduce the water vapor concentration; recording a first absorption spectrum of the first sample at a chosen wavelength, the first absorption spectrum being recorded using a harmonic spectroscopy method; recording a second absorption spectrum of a second sample of the olefin gas mixture, the second absorption spectrum being obtained in parallel or sequentially with the first absorption spectrum, the second absorption spectrum being recorded using the harmonic spectroscopy method; generating a differential absorption spectrum from the first absorption spectrum and the second absorption spectrum; analyzing the differential spectrum to determine the concentration of water vapor in the olefin gas mixture. 2. A method as in claim 1, wherein the recording of the first absorption spectrum comprises: illuminating the first sample with light comprising the chosen wavelength, measuring a first transmitted intensity of light passing through the first sample, and passing the measured intensity to a data analysis device; and the recording of the second absorption spectrum comprises: illuminating the second sample with light comprising the chosen wavelength, measuring a second transmitted intensity of light passing through the second sample, and passing the measured intensity to the data analysis device. 3. A method as in claim 1, wherein the chosen wavelength is one at which water vapor has a resolvably different absorption feature than other components of the olefin gas mixture. 4. A method as in claim 1, wherein the chosen wavelength is absorbed at least approximately 0.001 times as strongly by air with a concentration of 100 ppm of water vapor as by dry air containing an olefin concentration approximately equivalent to that in the olefin gas mixture. 5. A method as in claim 1, wherein the chosen wavelength is selected from 1359.5 nm, 1856.7 nm, 2605.6 nm, 1361.7 nm, 1859.8 nm, 2620.5 nm, 1368.6 nm, 1877.1 nm, 2626.7 nm, 1371.0 nm, 1890.3 nm, 2630.6 nm, 1392.2 nm, 1899.7 nm, 2665.1 nm, 1836.3 nm, 1903.0 nm, 2676.1 nm, 1840.0 nm, 1905.4 nm, 2711.2 nm, 1842.1 nm, 2573.6 nm, 2724.2 nm, 1847.1 nm, 2583.9 nm, 2735.0 nm, 1854.0 nm, 2596.0 nm, and 2740.3 nm. 6. A method as in claim 1, further comprising providing a beam of laser light at the chosen wavelength from a tunable diode laser. 7. A method as in claim 1, wherein the first absorption spectrum and the second absorption spectrum are recorded sequentially in a single sample cell. 8. A method as in claim 1, further comprising maintaining the first and the second sample at a constant temperature. 9. A method as in claim 1, further comprising measuring the concentration of water vapor in the olefin gas mixture using one of a dew point sensor, a piezoelectric adsorption sensor, a phosphorus pentoxide electrolysis sensor, and an aluminum or silicon oxide sensor. 10. A method as in claim 1, wherein: the recording of the first absorption spectrum comprises collecting first intensity data as a function of wavelength for infrared light from a tunable diode laser passing through the first sample, the infrared light from the tunable diode laser being tuned through a chosen wavelength at which water vapor has an absorption feature that is resolvable from other absorption features of other components of the olefin gas mixture at the chosen wavelength, the infrared light having a spectral width that is narrower than the absorption feature; and the recording of the second absorption spectrum comprises collecting second intensity data as a function of wavelength for the infrared light from the tunable diode laser passing through the second sample. 11. A method as in claim 10, wherein a frequency of the infrared light from the tunable diode laser is modulated at a modulation frequency; and the recording of the first absorption spectrum and the second absorption spectrum comprises resolving harmonic signals for the first intensity data and the second intensity data at a multiple of the modulation frequency. 12. A method as in claim 10, further comprising obtaining a zero-absorption baseline by fitting the first intensity data and/or the second intensity data to a low-order polynomial for one or more frequency ranges outside of the chosen wavelength through which the infrared light from the tunable diode laser is tuned; and using the zero-absorption baseline to determine the concentration of water vapor in the olefin gas mixture without the use of calibration data. 13. A method as in claim 1, wherein the recording of the first absorption spectrum comprises: illuminating the first sample with light comprising the chosen wavelength, measuring a first transmitted intensity of light passing through the first sample, and passing the measured intensity to a data analysis device; and the recording of the second absorption spectrum comprises: illuminating the second sample with light comprising the chosen wavelength, measuring a second transmitted intensity of light passing through the second sample, and passing the measured intensity to the data analysis device. 14. A method comprising: dehydrating a first sample of an olefin gas mixture containing an unknown concentration of water vapor and a varying concentration of one or more olefins to reduce the water vapor concentration; recording a first absorption spectrum of the first sample at a chosen wavelength; recording a second absorption spectrum of a second sample of the olefin gas mixture, the second absorption spectrum being obtained in parallel or sequentially with the first absorption spectrum, the first absorption spectrum and the second absorption spectrum being recorded in parallel in first and second sample cells with substantially identical optical path lengths; generating a differential absorption spectrum from the first absorption spectrum and the second absorption spectrum; and analyzing the differential spectrum to determine the concentration of water vapor in the olefin gas mixture. 15. A method as in claim 14, wherein the first absorption spectrum and the second absorption spectrum are recorded using a harmonic spectroscopy method. 16. A method as in claim 14, wherein the chosen wavelength is selected from 1359.5 nm, 1856.7 nm, 2605.6 nm, 1361.7 nm, 1859.8 nm, 2620.5 nm, 1368.6 nm, 1877.1 nm, 2626.7 nm, 1371.0 nm, 1890.3 nm, 2630.6 nm, 1392.2 nm, 1899.7 nm, 2665.1 nm, 1836.3 nm, 1903.0 nm, 2676.1 nm, 1840.0 nm, 1905.4 nm, 2711.2 nm, 1842.1 nm, 2573.6 nm, 2724.2 nm, 1847.1 nm, 2583.9 nm, 2735.0 nm, 1854.0 nm, 2596.0 nm, and 2740.3 nm. 17. An apparatus comprising: a modulated laser light source that emits light comprising a chosen wavelength; a sample cell; a dehydrator that reduces water vapor in a first sample of an olefin gas mixture, the olefin gas mixture containing a varying concentration of one or more olefins and, prior to entering the dehydrator, an unknown concentration of water vapor; one or more valves for alternately and sequentially providing the first sample and a second sample of the olefin gas mixture to the sample cell, the second sample containing the unknown water vapor concentration of the olefin gas mixture; a photodetector positioned to quantify light passing through the sample cell; and a microprocessor that records a first absorption spectrum from the photodetector using a harmonic spectroscopy method when the sample cell contains the first sample, records a second absorption spectrum from the photodetector using the harmonic spectroscopy method when the sample cell contains the second sample, calculates a differential absorption spectrum from the first and second absorption spectra, and calculates the concentration of water vapor in the olefin gas mixture based on the differential absorption spectrum. 18. An apparatus as in claim 17, wherein the laser light source is a tunable diode laser. 19. An apparatus as in claim 17, wherein the laser light source is selected from a vertical cavity surface emitting laser, a horizontal cavity surface emitting laser, a quantum cascade laser, a distributed feedback laser, and a color center laser. 20. An apparatus as in claim 17, wherein the chosen wavelength is absorbed at least approximately 0.001 times as strongly by air with a concentration of 100 ppm of water vapor as by dry air containing an olefin concentration approximately equivalent to that in the olefin gas mixture. 21. An apparatus as in claim 17, wherein the chosen wavelength is selected from 1359.5 nm, 1856.7 nm, 2605.6 nm, 1361.7 nm, 1859.8 nm, 2620.5 nm, 1368.6 nm, 1877.1 nm, 2626.7 nm, 1371.0 nm, 1890.3 nm, 2630.6 nm, 1392.2 nm, 1899.7 nm, 2665.1 nm, 1836.3 nm, 1903.0 nm, 2676.1 nm, 1840.0 nm, 1905.4 nm, 2711.2 nm, 1842.1 nm, 2573.6 nm, 2724.2 nm, 1847.1 nm, 2583.9 nm, 2735.0 nm, 1854.0 nm, 2596.0 nm, and 2740.3 nm. 22. An apparatus as in claim 17, further comprising a thermally controlled chamber that encloses one or more of the laser light source, the photodetector, and the sample cell. 23. An apparatus as in claim 17, further comprising: an additional water vapor concentration analyzer selected from a dew point measurement device, a piezoelectric adsorption device, a phosphorus pentoxide electrolysis device, and an aluminum or silicon oxide sensor. 24. An apparatus comprising: a laser light source that emits light comprising a chosen wavelength; a dehydrator to reduce water vapor in a first sample of an olefin gas mixture, the olefin gas mixture containing an unknown concentration of water vapor and a varying concentration of one or more olefins; a first sample cell for containing the first sample; a second sample cell for containing a second sample of the olefin gas mixture, wherein the second sample cell has a substantially identical path length to the first sample cell; a gas flow divider that directs the first sample to the first sample cell and the second sample to the second sample cell for parallel analysis; optical components for splitting the beam between the first sample cell and the second sample cell; a first photodetector positioned to quantify light passing through the first sample cell; a second photodetector positioned to quantify light passing through the second sample cell; and a microprocessor that records a first absorption spectrum from the first photodetector, records a second absorption spectrum from the second photodetector, calculates a differential absorption spectrum from the first and second absorption spectra, and calculates the concentration of water vapor in the olefin gas mixture based on the differential absorption spectrum. 25. An apparatus as in claim 24, wherein the laser light source is a tunable diode laser. 26. An apparatus as in claim 25, wherein the laser source is modulated and the first and the second absorption spectra are harmonic absorption spectra. 27. An apparatus as in claim 25, wherein the laser source is modulated and the first and the second absorption spectra are direct absorption spectra. 28. An apparatus as in claim 24, wherein the laser light source is selected from a vertical cavity surface emitting laser, a horizontal cavity surface emitting laser, a quantum cascade laser, a distributed feedback laser, and a color center laser. 29. An apparatus as in claim 24, wherein the chosen wavelength is absorbed at least approximately 0.001 times as strongly by air with a concentration of 100 ppm of water vapor as by dry air containing an olefin concentration approximately equivalent to that in the olefin gas mixture. 30. An apparatus as in claim 24, wherein the chosen wavelength is selected from 1359.5 nm, 1856.7 nm, 2605.6 nm, 1361.7 nm, 1859.8 nm, 2620.5 nm, 1368.6 nm, 1877.1 nm, 2626.7 nm, 1371.0 nm, 1890.3 nm, 2630.6 nm, 1392.2 nm, 1899.7 nm, 2665.1 nm, 1836.3 nm, 1903.0 nm, 2676.1 nm, 1840.0 nm, 1905.4 nm, 2711.2 nm, 1842.1 nm, 2573.6 nm, 2724.2 nm, 1847.1 nm, 2583.9 nm, 2735.0 nm, 1854.0 nm, 2596.0 nm, and 2740.3 nm. 31. An apparatus as in claim 24, further comprising: a thermally controlled chamber that encloses one or more of the laser source, the first photodetector, the second photodetector, the first sample cell and the second sample cell. 32. An apparatus as in claim 24, further comprising: an additional water vapor concentration analyzer selected from a dew point measurement device, a piezoelectric adsorption device, a phosphorus pentoxide electrolysis device, and an aluminum or silicon oxide sensor. 33. An apparatus comprising: means for generating a beam of light at a wavelength where water molecules and other components of an olefin gas mixture have different absorbances, the olefin gas mixture containing an unknown water vapor concentration and varying concentrations of two or more olefins; means for reducing water vapor in a first sample of the olefin gas mixture; means for illuminating the first sample of the gas with the beam and recording a first absorbance spectrum; means for illuminating a second sample of the olefin gas mixture with the beam and recording a second absorbance spectrum, the second sample being illuminated in parallel or sequentially with the first sample; processing means for generating and analyzing a differential absorbance spectrum from the first absorbance spectrum and the second absorbance spectrum to determine the concentration of water vapor in the olefin gas mixture.