P. Perlmutter, S. Shtrikman, and M. Slatkine, "Optoacoustic detection of ethylene in the presence of interfering gases," Appl. Opt. 18, 2267-2274 (1979)
A study of the limitations of optoacoustic detection of ethylene in gas mixtures using a 12C16O2 laser is presented. Particular emphasis is given to the detection of ethylene in urban areas and in fruit storage chambers. Calculations indicate that in most cases of interest the practical minimum detectable and identifiable concentration of ethylene is about 5 ppb. A concentration of 1% of CO2 may increase this limit to 50 ppb. These limits are primarily due to inaccuracy in a priori knowledge of ir spectral signatures of interfering gases. As a practical example of the monitoring of ethylene in a realistic environment, a meausrement with a sensitive resonant optoacoustic cell in an urban area is reported. The same cell is also used to demonstrate the effectiveness of NaOH scrubbers for the elimination of interfering CO2. Measurements of absorption coefficients of ethylene for several 12C16O2 laser transitions are reported and compared with those given in the literature. Data are also given for the isotopic 13C16O2 laser transitions which may be useful in overcoming CO2 interference.
Edward H. Wahl, Sze M. Tan, Sergei Koulikov, Boris Kharlamov, Christopher R. Rella, Eric R. Crosson, Dave Biswell, and Barbara A. Paldus Opt. Express 14(4) 1673-1684 (2006)
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λR(20) = 10.243 μm; λP(14) = 10.532 μm,; λP(16) = 10.548 μm; λP(26) = 10.650 μm.
Taken from Ref. 9.
Taken from Ref. 21.
Taken from Ref. 20.
1 atm−1 cm−1 is an absorption coefficient of medium strength.11
Table II
Minimum Detectable and Identifiable Concentrations for an Additive Noise Limited Analyzer
Calculated using Eq. (6) and Table I.
Optoacoustic cell’s intrinsic additive noise: Δ = 0.3 × 29.1 ppb × atm−1 cm−1 for 1-W radiation at P(14) CO2 laser transition and 1-Hz detection bandwidth.
Notice that the intrinsic minimum detectable concentration of ethylene in an interfering environment is only slightly higher than in a noninterfering one (0.3 ppb). The Background minimum detectable level is 150 times higher, although its absorption coefficient is only 30 times lower than that of ethylene at the P(14) transition.
Table III
Minimum Detectable and Identifiable Concentrations for a Modulation Noise Limited Analyzer
Note: The minimum detectable concentrations are given for various ambient atmospheres; the modulation noise level (instability) is assumed to be 1%.
Checks the influence of a single interfering pollutant on δCi. δCi (C2H4) is approximately that obtained in the additive noise limited analyzer (Table II).
Natural concentrations of H2O and CO2.
High concentration level of CO2.
Table IV
Minimum Detectable and Identifiable Concentrations for Various Inaccuracies in a priori Knowledge of Interfering Spectral Signatures
Interfering gas
Inaccuracy in a priori knowledge of interfering spectral signature (%)
Ethylene dispersed in N2 at concentration levels up to 50 ppm. Temperature 22 ± 2°C; pressure = 1 atm.
Repeatability of results: ±5%.
Notice discrepancies higher than 10% between various results. This discrepancy is typical of many reported ir spectra signatures of various gases.
Table VI
Dependence of Optoacoustic Spectral Signature on Ambient Pressure a
α(λ)/α(λ = 10.5326 μm)
Pressure (mbar)
λ = 10.532 μm [P(14)]
λ = 10.650 μm [P(26)]
λ = 10.629 μm [P(24)]
100 (1 atm)
1
0.08
0.15
800
1
0.09
0.15
600
1
0.10
0.15
400
1
0.12
0.17
200
1
0.27
0.17
100
1
3.70
0.89
80
1
9.60
1.8
60
1
41.3
4.5
The ratio between the absorption coefficients of ethylene at different wavelengths depends on the N2 ambient pressure. This dependence is sharply revealed for P(26) and P(14) laser transitions. In Table VI we present results also for the R(24) transition. This dependence may be used for the detection and identification of ethylene by measuring the optoacoustic signal at reduced pressures. However, the reduction of ambient pressure may change the operating conditions of the optoacoustic cell (like microphone response) and thus decrease the optoacoustic response. The minimum detectable concentration of ethylene may thus be considerably increased.
Table VII
Optoacoustic Spectral Signature of Ethylene at 360 Torr (400 mbara)
Compares optoacoustic measurements of relative ir absorption coefficients with presented in Ref. 12. Temperature = 22 ± 2°C. Measurements repeatability: ±5%. A discrepancy up to 50% at R(18) is observed.
Table VIII
Optoacoustic Spectral Signature of Ethylene at Isotopic 13C16O2 Laser Transitions a
13C16O2 laser transition
Wavelength (μm)
α(λ)/α(λ = 10.72 μm)
R(26)
10.719
1
R(22)
10.749
1.97
R(20)
10.765
1.24
R(16)
10.797
1.38
R(14)
10.813
1.43
P(16)
11.103
1
P(18)
11.124
0.72
P(26)
11.213
0.85
P(28)
11.235
1.58
Pressure = 1 atm; temperature = 22 ± 2 °C. Repeatability of results: ±5%. The measured absolute value of the absorption coefficient at λ = 10.719 μm [R(26)] is 0.98 atm−1 cm−1 (±20%). This value reasonably fits Ref. 8 for the P(32) 12C16O2 laser transition (λ = 10.715 μm) and is higher than Ref. 9 by a factor of 2. The general feature of the measured spectra fits Ref. 11, p. 127.
Tables (8)
Table I
Absorption Coefficients (atm−1 cm−1) for Monitoring Ethylene, Using Four CO2 Laser Transitions
λR(20) = 10.243 μm; λP(14) = 10.532 μm,; λP(16) = 10.548 μm; λP(26) = 10.650 μm.
Taken from Ref. 9.
Taken from Ref. 21.
Taken from Ref. 20.
1 atm−1 cm−1 is an absorption coefficient of medium strength.11
Table II
Minimum Detectable and Identifiable Concentrations for an Additive Noise Limited Analyzer
Calculated using Eq. (6) and Table I.
Optoacoustic cell’s intrinsic additive noise: Δ = 0.3 × 29.1 ppb × atm−1 cm−1 for 1-W radiation at P(14) CO2 laser transition and 1-Hz detection bandwidth.
Notice that the intrinsic minimum detectable concentration of ethylene in an interfering environment is only slightly higher than in a noninterfering one (0.3 ppb). The Background minimum detectable level is 150 times higher, although its absorption coefficient is only 30 times lower than that of ethylene at the P(14) transition.
Table III
Minimum Detectable and Identifiable Concentrations for a Modulation Noise Limited Analyzer
Note: The minimum detectable concentrations are given for various ambient atmospheres; the modulation noise level (instability) is assumed to be 1%.
Checks the influence of a single interfering pollutant on δCi. δCi (C2H4) is approximately that obtained in the additive noise limited analyzer (Table II).
Natural concentrations of H2O and CO2.
High concentration level of CO2.
Table IV
Minimum Detectable and Identifiable Concentrations for Various Inaccuracies in a priori Knowledge of Interfering Spectral Signatures
Interfering gas
Inaccuracy in a priori knowledge of interfering spectral signature (%)
Ethylene dispersed in N2 at concentration levels up to 50 ppm. Temperature 22 ± 2°C; pressure = 1 atm.
Repeatability of results: ±5%.
Notice discrepancies higher than 10% between various results. This discrepancy is typical of many reported ir spectra signatures of various gases.
Table VI
Dependence of Optoacoustic Spectral Signature on Ambient Pressure a
α(λ)/α(λ = 10.5326 μm)
Pressure (mbar)
λ = 10.532 μm [P(14)]
λ = 10.650 μm [P(26)]
λ = 10.629 μm [P(24)]
100 (1 atm)
1
0.08
0.15
800
1
0.09
0.15
600
1
0.10
0.15
400
1
0.12
0.17
200
1
0.27
0.17
100
1
3.70
0.89
80
1
9.60
1.8
60
1
41.3
4.5
The ratio between the absorption coefficients of ethylene at different wavelengths depends on the N2 ambient pressure. This dependence is sharply revealed for P(26) and P(14) laser transitions. In Table VI we present results also for the R(24) transition. This dependence may be used for the detection and identification of ethylene by measuring the optoacoustic signal at reduced pressures. However, the reduction of ambient pressure may change the operating conditions of the optoacoustic cell (like microphone response) and thus decrease the optoacoustic response. The minimum detectable concentration of ethylene may thus be considerably increased.
Table VII
Optoacoustic Spectral Signature of Ethylene at 360 Torr (400 mbara)
Compares optoacoustic measurements of relative ir absorption coefficients with presented in Ref. 12. Temperature = 22 ± 2°C. Measurements repeatability: ±5%. A discrepancy up to 50% at R(18) is observed.
Table VIII
Optoacoustic Spectral Signature of Ethylene at Isotopic 13C16O2 Laser Transitions a
13C16O2 laser transition
Wavelength (μm)
α(λ)/α(λ = 10.72 μm)
R(26)
10.719
1
R(22)
10.749
1.97
R(20)
10.765
1.24
R(16)
10.797
1.38
R(14)
10.813
1.43
P(16)
11.103
1
P(18)
11.124
0.72
P(26)
11.213
0.85
P(28)
11.235
1.58
Pressure = 1 atm; temperature = 22 ± 2 °C. Repeatability of results: ±5%. The measured absolute value of the absorption coefficient at λ = 10.719 μm [R(26)] is 0.98 atm−1 cm−1 (±20%). This value reasonably fits Ref. 8 for the P(32) 12C16O2 laser transition (λ = 10.715 μm) and is higher than Ref. 9 by a factor of 2. The general feature of the measured spectra fits Ref. 11, p. 127.
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