AMMONIA AND ETHYLENE MEASUREMENTS
IN PURE NITROGEN
M. PETRUS
Department of Lasers, National Institute for Laser, Plasma and Radiation Physics,
409 Atomistilor St., PO BOX MG-36, 077125, Bucharest, Romania, E-mail: [email protected]
Received September 30, 2015
The present manuscript describes a sensitive system based upon photoacoustic
spectroscopy technology used in the detection of trace gases at very low
concentrations. The detection of gases is very important for a variety of applications,
including medical diagnosis of diseases with the analysis of exhaled gases, the
atmospheric pollutants assessment or analysis of physiological processes at fruits and
vegetables. To fulfill the above applications and many others requiring gas detector
able to have high sensitivity and selectivity. In this work, was used an infrared
photoacoustic spectroscopy able to measure concentration dependent response for
ethylene molecules and for ammonia molecules.
Key words: nitrogen, ammonia molecules, ethylene molecules, CO2 laser
photoacoustic spectroscopy.
1. INTRODUCTION
Ammonia gas is found in trace quantities in the atmosphere, being produced
from the putrefaction of nitrogenous animal and vegetable matter. Ammonia and
ammonium salts are also found in small quantities in rainwater. The kidneys
secrete ammonia to neutralize excess acid. Ammonium salts are found distributed
through fertile soil and in seawater [1].
Ethylene gas is a volatile organic compound generated mainly in the exhaust
of automotive engines powered by fossil fuels and ethanol. Ethylene gas is a
precursor of tropospheric ozone from the reaction with nitrogen oxide and oxygen
in the presence of sunlight. Tropospheric ozone is one of the main compounds of
photochemical smog that has direct implications on human health. One of the
expected effects after exposure to ozone is reduced resistance to infectious diseases
due to destruction of lung tissue. It is believed that chronic exposure to high levels
of urban ozone leads to premature aging of lung tissues [1].
The study of new tools for the detection of gaseous molecules is of great
importance for the development of future societies.
Rom. Journ. Phys., Vol. 61, Nos. 9–10, P. 1539–1546, Bucharest, 2016
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M. Petrus
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Ideally, a sensing tool has to meet important features such as high sensitivity
and selectivity, high accuracy and precision, large dynamic range, multicomponent
capability, none or only minor sample preparation, good temporal resolution,
versatility, reliability, ease of use, and robustness. Spectroscopic systems including
differential optical absorption spectroscopy, Fourier-transform infrared
spectrometers, light detection and ranging (Lidar) systems. Although there is no
ideal instrument that would fulfill all the requirements mentioned above, the
sensing techniques based on photoacoustic principles offers some important
advantages in different applications such as: continuous, sensitive (down to ppb –
10-9 or even sub-ppb concentrations), specific and near real time monitoring of
numerous gases [2–4].
The success of the photoacoustic based trace gas sensing techniques crucially
depends on the availability and the performance of the tunable laser source and of
the detection scheme employed. Lasers offer the advantage of high spectral power
density owing to their intrinsic narrow linewidth in the range of MHz. Since the
laser linewidth is usually much smaller than the molecular absorption linewidth, it
is not an important issue in most measurements.
In this context, we utilized CO2 laser photoacoustic spectroscopy method to
measure the minimum detectable concentrations for ammonia and ethylene
molecules in pure nitrogen.
2. METHOD
The CO2 laser photoacoustic spectroscopy used for the gas content
measurement and presented in this report is schematically shown in Fig. 1 and
described also in other works by [2–26]. In brief, photoacoustic spectroscopy
utilizes a line-tunable CO2 laser and a photoacosutic cell, where the gas is
analyzed. In photoacoustic spectroscopy, the absorbed power is determined directly
via its heat and hence the sound produced in the sample.
The experimental setup consists of a homebuilt, line-tunable and frequency
stabilized CO2 laser. This laser, emitting radiation in the 9.2–10.8 µm infrared
region on 55 different vibrational-rotational lines, has a maximum power of 6.5 W
on the 10P(20) line [5–8].
Our laser beam was modulated by a high quality, low vibration noise and
variable speed (4–4000 Hz) mechanical chopper model DigiRad C-980 or C-995
(30 aperture blade), operated at the appropriate resonant frequency of the cell
(564 Hz).
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Ammonia and ethylene measurements in pure nitrogen
1541
Fig. 1 – Scheme of the CO2 laser photoacoustic spectroscopy experimental setup.
We used a dual-phase, digital lock-in amplifier Stanford Research Systems
model SR 830 with the following characteristics: full scale sensitivity, 2 nV–1 V;
input noise, 6 nV (rms)/ Hz at 1 kHz; dynamic reserve, greater than 100 dB;
frequency range, 1 mHz –102 kHz; time constants, 10 μs–30 s, or up to 30000 s.
The photoacoustic cell has a total volume of approximately 1.0 dm3 and it is
made of stainless steel and Teflon to reduce the out gassing problems. The
photoacoustic cell consists of an acoustic resonator tube, windows, gas inlets and
outlets, microphones and an acoustic filter to suppress the window noise. The
photoacoustic cell windows are made of ZnSe and positioned at Brewster angle to
their mounts. The resonant conditions are obtained as longitudinal standing waves
in an open tube (excited in its first longitudinal mode). To achieve a larger signal,
we chose a long absorption path length –300 mm and an inner diameter of the pipe
of 7 mm. The fundamental longitudinal wave, therefore, has a nominal wavelength
of 600 mm and a resonance frequency of 564 Hz.
The two buffer volumes placed near the Brewster windows have a length of
75 mm and a diameter of 57 mm. The inner wall of the stainless steel resonator
tube is highly polished. It is centered inside the outer stainless steel tube with
Teflon spacers. A massive spacer is positioned at one end to prevent bypassing of
gas in the flow system; the other is partially open to avoid the formation of closed
volumes. Gas is admitted and exhausted through two ports located near the ends of
the resonator tube. The perturbation of the acoustic resonator amplitude by the gas
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flow noise is thus minimized. The acoustic waves generated in the photoacoustic
cell are detected by four Knowles electrets miniature microphones (sensitivity
20 mV/Pa each) in series, mounted flush with the wall. They are situated at the
loops of the standing wave pattern, at an angle of 90o to one another. The electrical
output from these microphones is summed and the signal is selectively amplified
by the lock-in amplifier [5–20].
We used a modular software architecture (Keithley TestPoint software)
aimed at controlling the experiments, collecting data, and preprocessing
information. It helps to automate the process of collecting and processing the
experimental results. The software transfers powermeter readings, normalizes data,
and automatically stores files. It allows the user to record parameters such as the
photoacoustic cell responsivity (a constant used to normalize raw data), gas
absorption coefficient, number of averaged samples at every measurement point,
sample acquisition rate, and the total number of measurement points. This software
interfaces the lock-in amplifier, the chopper, the laser powermeter and the gas
flowmeter. It allows the user to set or read input data and instantaneous values for
the photoacoustic voltage, average laser power after chopper, and trace gas
concentration [20–26].
Of great significance in these determinations is the gas handling system due
to its role in ensuring gas purity in the photoacoustic cell. It can be used to pump
out the cell, to introduce the sample gas in the photoacoustic cell at a controlled
flow rate, and monitor the total and partial pressures of gas mixtures. Also, the gas
handling system can perform several functions without necessitating any
disconnections [26].
CO2 laser photoacoustic spectroscopy performs well in terms of sensitive and
selective detection of trace gas and it allows near on-line measurements.
3. RESULTS
Figure 2 shows a graph of the resonant frequency of the photoacoustic cell
(as a function of laser beam chopping frequency).
The acoustic resonator is characterized by the quality factor (Q), which is
defined as the ratio of the resonance frequency to the frequency bandwidth between
half power points. The amplitude of the microphone signal is 1/ 2 of the
maximum amplitude at these points, because the energy of the standing wave is
proportional to the square of the induced pressure.
The quality factor was measured by filling the photoacoustic cell with
1 ppmV of ethylene buffered in nitrogen at a total pressure of 1 atm and by tuning
the modulation frequency in 10 Hz increments (2 Hz increments near the top of the
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Ammonia and ethylene measurements in pure nitrogen
1543
curve) across the resonance profile to estimate the half width, as described above.
For this photoacoustic cell, the profile width at half intensity was 35 Hz,
yielding a quality factor Q = 16.1 at a resonance frequency f = 564 Hz.
Fig. 2 – Resonance curve of the photoacoustic cell.
Figure 3 shows the calibration measurements (concentration dependent
response) for ammonia experimentally determined using commercially prepared,
certified gas mixtures containing 10 ppmV ammonia diluted in pure nitrogen.
Fig. 3 – Calibration curve for ammonia.
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Figure 4 shows the calibration measurements (concentration dependent
response) for ethylene, experimentally determined using commercially prepared,
certified gas mixtures containing 0.96 ppmV ethylene diluted in pure nitrogen.
Fig. 4 – Calibration curve for ethylene.
For calibration we examined this reference mixture at a total pressure of
approximately 1013 mbar and a temperature of 23 oC, using the commonly
accepted values: 30.4 cm-1atm-1 (for ethylene) and 57 cm-1atm-1 (for ammonia).
4. CONCLUSIONS
CO2 laser photoacoustic spectroscopy technique is very important for the
identification and determination of concentrations of trace gases, and has many
applications like medical diagnosis of diseases with the analysis of exhaled gases,
the atmospheric pollutants assessment or analysis of physiological processes at
fruits and vegetables [1–26].
As the gases are present at different concentrations in the exhaled breath, or
in the atmosphere or in fruits and vegetables, monitoring of these gases represent a
scientific and technical challenge.
The results demonstrate that CO2 laser photoacoustic spectroscopy is an
important tool for sensitive and selective detection of ammonia and ethylene
molecules.
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Ammonia and ethylene measurements in pure nitrogen
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