Cantilever enhanced tunable diode laser photoacoustic analysis of nitrous oxide in automotive application Juho Uotila, Sauli Sinisalo, Jussi Raittila, Ismo Kauppinen ¹Gasera Ltd., Tykistökatu 4, 20520 Turku, Finland Pittcon 2012, Orlando
Cantilever enhanced tunable diode laser photoacoustic
analysis of nitrous oxide in automotive application
Juho Uotila, Sauli Sinisalo, Jussi Raittila, Ismo Kauppinen ¹Gasera Ltd., Tykistökatu 4, 20520 Turku, Finland
Pittcon 2012, Orlando
Photoacoustic technology with
cantilever pressure sensor
Photoacoustics is proved to be extremely sensitive
technique in gas analysis – there is a long tradition of
very sensitive measurements with gas lasers.
Gasera is offering a choice for enhancing the
microphone sensitivity by using an optical microphone
with a cantilever pressure sensor.
Cantilever is made out of silicon and has dimensions in
the level of: length 5 mm, width 1.2 mm, thickness 10
um.
Because the cantilever has very low spring constant (1
N/m), it reacts to extremely low pressure variations.
The cantilever movement is measured optically with a
compact laser interferometer, which allows wide
dynamic range for the measurement of movements
from below 1 pm to over 10 µm.
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Diode laser
Angle mirror
CMOS array
Cantilever and frame
Interference
pattern
Laser
Photoacoustic cell Microphone
Cantilever pressure sensor
combined to laser sources
The cantilever enhanced photoacoustic cell
performs extremely well with laser sources.
The best ever normalized noise equivalent
absorption coefficient (NNEA) value 1.7 x 10-10
cm−1W/√Hz for the photoacoustic cell was
measured for cantilever enhanced cell by Koskinen
et. al. This is more than ten times better than
reported e.g. with tuning fork QEPAS 5.4×10−9
cm−1W/√Hz.
In a test made by Lindley et. al. the three
photoacoustic cells were compared by measuring
the detection limit for acetylene: a resonant cell containing a single microphone – 650ppb,
a differential cell with dual microphone – 440 ppb,
a cantilever pressure sensor –14 ppb.
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1. V. Koskinen, J. Fonsen, K. Roth, J. Kauppinen, "Cantilever enhanced photoacoustic detection of carbon dioxide using a tunable diode laser source", Appl.
Phys. B 86, 451 - 454 (2007), Rapid Communications.
2. R. E. Lindley, A. M. Parkes, K. A. Keen, E. D. McNaghten, A. J. Orr-Ewing, “A sensitivity comparison of three photoacoustic cells containing a single
microphone, a differential dual microphone or a cantilever pressure sensor” Appl. Phys. B 86, 707 – 713, (2007).
Reference cell 1
Resonant cell
Reference cell 2
Differential cell
Cantilever enhanced
photoacoustic cell
LP1 photoacoustic gas analyzer
Tunable laser photoacoustic spectroscopy with cantilever enhanced optical microphone
Gas cell stabilized to 50°Celsius temperature
Patented ultra-sensitive optical microphone based on a MEMS cantilever sensor coupled with a laser interferometer
to measure microscopic movement of the cantilever sensor
19” 3U housing for both table stand and rack mount installation
Built in PC computer with 5,7” color VGA display in the front
User interface of setting the alarm levels for concentrations of gases under monitoring
Data storage capacity of approx. 2 GB. Sufficient for more than a year of continuous monitoring of 2 gases with the
shortest sampling interval.
Transfer of measurement results to memory stick via USB or to PC via USB, Ethernet or serial ports.
Three gas connections in the rear. The two incoming gas lines, sample and purge gas line, are equipped with filters
for dust and small particles.
Compensation of the fluctuations of temperature and pressure within the operational conditions
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Measurement setup
Optical
microphone
DSP unit
Laser driver
TEC controller
Readout interferometer
Photoacoustic cell:
Length 100 mm,
Diameter 4 mm
Cantilever
DFB diode laser
Aspheric lens Laser beam
Beam dump
Gas IN Gas OUT Balance cell
Tunable diode laser spectroscopy with LP1
Wavelength modulation
Signal measured at second harmonic frequency.
No background signal.
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Frequency [Hz]
Time [a.u.]
Laser current modulation signal Photoacoustic signal
Laser line
Gas absorption line
Nitrous oxide measurement in car
tailpipe emission monitoring Nitrous oxide (N2O) is a strong greenhouse gas. For this reason there is a wide interest in the N2O concentration
measurement.
New regulations e.g. in the EU and USA limit the allowed car emission rate of N2O.
N2O monitoring from the car emission is a challenging task for IR spectroscopy, because of the high concentration of
interfering molecules and relatively low concentrations (330 ppb in ambient air).
LP1 tunable diode laser based analyzer is promising for this application because of the high sensitivity (cantilever
sensor), selectivity (low sample pressure) and wide dynamic range (short absorption path length).
LP1 can be used for the measurement from bag samples or bypass from the constant flow.
Measurement conditions:
Detection limit requirement is about 10 times lower than ambient concentration (10 ppb – 50 ppb).
Typical background gas concentrations in the emission monitoring are shown in table below
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Gas Typical concentrations [ppm]
N2O 0.350
H2O 20 000
CO 200
CO2 25 000
CH4 0-200
Selection of the laser wavelength
Spectrum of N2O (350 ppb) in the near infrared range.
Sensitivity requirement forced to use the band at 2.9 µm.
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1.5 µm 3.5 µm
N2O line
Selection of the laser wavelength
Spectrum of N2O (350 ppb), CO2 (2.5 %), H2O (2 %), CO (200 ppm), and CH4 (200 ppm) mixture.
The reality in the tailpipe emission sample shows that there are 5 orders of magnitude higher absorptions.
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1.5 µm 3.5 µm
N2O line
Selection of the laser wavelength
Absorbance spectrum of N2O (350 ppb), CO2 (2.5 %), H2O (2 %), CO (200 ppm), and CH4 (200 ppm) mixture in the
atmospheric pressure with 10 cm absorption path length.
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2895 nm 2898 nm
N2O line
Effect of pressure
Absorbance spectrum of N2O (350 ppb), CO2 (2.5 %), H2O (2 %), CO (200 ppm), and CH4 (200 ppm) mixture in the
300 mbar pressure with 10 cm absorption path length.
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2895 nm 2898 nm
N2O line
Measurement of N2O spectrum
N2O line at 2896 nm seems to fulfill the selectivity and sensitivity requirement in the 300 mbar pressure with typical
car tailpipe sample.
Photoacoustic cantilever technology allows the sensitive measurement also in the low pressure.
The sensitivity is even enhanced due to the lower pressure.
Laser power was 4 mW for the target wavelength.
Second derivative spectrum of N2O (103 ppm) is shown in the figure below in the 1024 mbar and 306 mbar
pressures, measurement time for single point was 1 second and scanning step was 0.01 nm.
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Diode laser mounting and optics for
2896 nm wavelength. Measured second derivative N2O spectrum of 103 ppm
in 1024 mbar and 306 mbar pressures.
Measurement of N2O spectrum in room air
At 300 mbar pressure the N2O lines are separated easily from H2O lines. CO2 does not have significant absorption
lines present in the concentrations below 500 ppm.
Second derivative spectrum of N2O (~380 ppb) in laboratory air is shown in the figure below (1 s integration time).
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N2O
H2O
Measured laboratory air spectrum and simulated HITRAN spectrum.
Measurement of N2O signal in
different concentrations Signal of 0 ppm, 330 ppb, 4 ppm, 30 ppm, and 100 ppm measured with 1 s integration time.
Detection limit (2 x RMS) with 1 s integration time is 80 ppb and with 1 minute integration time 20 ppb.
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103 ppm
30 ppm
330 ppb 0 ppm 330 ppb 4 ppm
0 ppb 330 ppb N2 and
laboratory air
with 30 s
integration time
Measurement of N2O in realistic
sample mixtures Spectra of other interfering gases were measured at 300 mbar pressure.
The spectra with following concentrations were measured: N2O (4 ppm), CO2 (1 %), H2O (2 %), CO (100 ppm), and
CH4 (99 ppm).
H2O or CH4 did not interfere with N2O.
The only interfering component was CO2, which has a rather weak line at the position of smaller N2O, but also even
weaker line in the middle of N2O line at 2896.5 nm.
This small interference can be overcome by measuring signal at CO2 line and N2O line wavelengths. This roughly
doubles the response time, but on the other hand also CO2 concentration is measured.
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CO2
N2O N2O H2O
Measurement of N2O signal from car
exhaust gas sample (petrol engine)
Sample was taken into a syringe from the petrol engine
tailpipe right after ignition and after short warm-up time.
Laser spectrum of the sample indicate that there was 16 ppm
of N2O in the first sample and 380 ppb after the motor was
warmed up.
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N2O
N2O
Measurement of N2O signal from car
exhaust gas sample (diesel engine)
Sample was taken into a syringe from the diesel engine
tailpipe right after short warm-up time (76°C).
Laser spectrum of the sample indicate that there was 950 ppb
of N2O in the sample.
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N2O
Conclusions
Cantilever enhanced tunable diode laser spectroscopy provides an attractive solution to
the automotive N2O measurement application due to the sensitivity (cantilever sensor),
selectivity (low sample pressure) and wide dynamic range (short absorption path
length).
The detection limit of the proposed system for N2O is 20 ppb (@ 1 min) even in the
presence of high amount of CO2 (up to 20%) and water vapor (up to 50 000 ppm).
Suitable also for N2O bag sampling measurement (low sample volume).
Cantilever enhanced photoacoustics is the only technique that has at the same time
short absorption path length, low sample volume, low pressure, and high sensitivity with
NIR laser sources.
Proposed technology is highly suitable to other greenhouse gas applications such as
environmental monitoring of H2O, CH4, N2O and CO2.
Simultaneous measurement of several gases is possible with the same instrument.
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