laser based sensor for detection of trace gases in the environment m. a. gondal, i. a. bakhtiari and...

LASER BASED SENSOR FOR DETECTION OF TRACE GASES

IN THE ENVIRONMENT

M. A. Gondal, I. A. Bakhtiari and Z. H. YamaniLaser Research Laboratory & Physics Department

King Fahd University of Petroleum & Minerals

Gondal

Bakhtiari

Me!!

Plan

1. Intro.: What is photoacoustic spectroscopy

2. Modifying the PA set up

3. Results for Ethylene

4. Future work

5. Conclusion

Conventional Spectroscopy

Principal:Principal: Energy absorbed is determined by the difference (Io - I) / Io.

DetectorSampleLight Source

Io I

Introduction:What is photoacoustic spectroscopy

Photoacoustic Spectroscopy (PA)

Principal: Principal: EnergyEnergy absorbed by the sample produces sound waves which are detected by sensitive microphone. The name PA is due to “Light Generated Sound”

Advantages:Advantages:Sample may be gaseous, liquid, solid opaque, powdered.Highly sensitive.

Light Source

Chopper Sample

Microphone

LASER RADIATION

EXCITATION

HEAT

(VIBRATIONAL)(ELECTRONIC)EMISSION CHEM. REACTION

RADIATION EXPANSION PHASE TRANSITION

SOUND WAVES

vent

Photoacoustic spectroscopy system using chopper for modulation

Modifying the PA set up

Stepping

Motor

P. Gauge

Valve

Vacuum Pump

Power Meter

Air FilterValve

LensTunable CO2 laser

Beam Splitter

Photoacoustic cell

MicrophoneGrating

`

Spectrum Analyzer

Newport micro stepping motor

grating controller

Variable Gain Low Noise Microphone

Amplifier

CO2 Laser Modulator. Variable

duty cycle (TTL)

Resonant Photoacoustic Cell developed at KFUPM

G a s In le t G a s O u tle t

M icro p h o n eB u ffer V o lu m e

A co u stic F iltersW in d o w s

2122

,, ])()[(2

v

L

n

Rzmn

nmn z

R = 8 mm

L = 160 mm

It turns out that: 0,0,1 ~ 3400 Hz

Don’t worry about higher resonance frequencies, because they are beyond microphone response limit

Noise Level with electronic Modulation

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

2000 2500 3000 3500 4000 4500 5000 5500 6000

Frequency (Hz)

PA

Sig

na

l (a

rb. U

nit

s)

Noise Level recorded with chopper

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

2000 2500 3000 3500 4000 4500 5000 5500 6000

Frequency (Hz)

PA

Sig

na

l In

tes

ity

(a

rb. u

nit

s)

Noise level in PA cell, filled

with pure nitrogen

Results for Ethylene

1 3026 cm-1 CH2 stretch2 1623 cm-1 CC stretch3 1342 cm-1 CH2 scissor4 1023 cm-1 CH2 twist5 3103 cm-1 CH2 stretch6 1236 cm-1 CH2 rock7 949 cm-1 CH2 wag8 943 cm-1 CH2 wag9 3106 cm-1 CH2 stretch10 826 cm-1 CH2 rock11 2989 cm-1 CH2 stretch12 1444 cm-1 CH2 scissor

Fundamental frequenciesfor Ethylene (C2H4)

Molecular weight 28.05 amuFreezing point -169 CBoiling point -104 C

10.2 10.3 10.4 10.5 10.6 10.70

20

40

60

80

100

120

10.6

9610

.675

10.6

5310

.632

10.6

1110

.591

10.5

7110

.551

10.5

3210

.513

10.4

94

10.2

8910

.274

10.2

4710

.233

10.2

2

10.1

95

lase

r po

wer

(re

lativ

e)

wavelenght (m)

Spectral distribution of CO2 linesUse the 10 P(14) of CO2

Time domain PA signal of C2H4

-40

-30

-20

-10

0

10

20

30

40

25487 25487.5 25488 25488.5 25489 25489.5 25490

Time (msec)

Am

plit

ud

e (a

rb. u

nit

)

FFT Spectra Ethlene with mechnical chopper

0

5

10

15

20

25

30

35

3000 3100 3200 3300 3400 3500 3600 3700

Frequency (Hz)

PA

Sig

na

l (a

rb. u

nit

s)

300 ppbV of ethylene under 1000 mbar of

nitrogen

Mechanical (chopper) Modulation

FFT Spectra of Ethlene with electronic moduloation

0

10

20

30

40

50

60

70

80

90

2500 3500 4500

Frequency (Hz)

Sig

na

l in

ten

sit

y (

% f

ull

sc

ale

)

Time domain spectra using electronic modulation

-100

-80

-60

-40

-20

0

20

40

60

80

100

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Time (msec)

Am

pli

tu

de

(a

rb

. u

nit

s)

Electronic Modulation

300 ppbV of ethylene under 1000 mbar of

nitrogen

Chopper Modulated Spectra of Ethylene

0

5

10

15

2800 3000 3200 3400Frequency (Hz)

PA

Sig

nal

(ar

b.

Un

its)

Spectra of Ethylene with Electronic Modulation

0

20

40

60

80

2800 3000 3200 3400Frequncy (Hz)

PA

Sig

nal

(ar

b.

Un

its)

Calibration Curve for Ethylene

R2 = 0.9967

0

5

10

15

20

25

0 50 100 150 200 250 300 350

Concentration (ppbv)

PA

Sig

na

l In

ten

sit

y (

arb

. un

its

)

PA Signal vs. Laser Power

R2 = 0.9979

0

2

4

6

8

10

12

14

16

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Laser Power (watt)

PA

Sig

na

l (arb

. Un

its

)

1- SABIC funded project on detection of ozone.

2- Improve sensitivity.

3- Go out to the field

Future work

This is how the lab looks when we are

there!!!Domestically built CO2 laser

1- The PA system is up and running with good dynamic range and sensitivity for the detection of C2H4.

2- We have a plan for future work, including detection of ozone.

3- We are open for ideas and need your support (esp. graduate students).

Conclusion

KFUPM is gratefully acknowledged for supporting this work

Thank you for your attention

.. بركاته و الله رحمة و عليكم السالم و

Stepping

Motor

P. Gauge

Valve

Vacuum Pump

Power Meter

Air FilterValve

LensTunable CO2 laser

Beam Splitter

Photoacoustic cell

MicrophoneGrating

`

Spectrum Analyzer

Newport micro stepping motor

grating controller

Variable Gain Low Noise Microphone

Amplifier

CO2 Laser Modulator. Variable

duty cycle (TTL)