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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Thank you!

Visit us at booth #4135!

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