LIBS Detector SystemDesign & Preliminary Testing

David Hahn, Tom Moskal, Ken Iida

Department of Mechanical Engineering

University of Florida

July 9, 2001

Emissioncollection

Laser-Induced Breakdown Spectroscopy (LIBS)Laser-inducedplasma

Pulsedlaser

Sample

Detector

Fiberoptic

Spectrometer

Atomic emission linesprovide species identification

410 415 420 425 430 435 440

Wavelength (nm)

Em

issi

on I

nten

sity

LIBS spectrum of CCA-treated wood

0

2000

4000

6000

8000

10000

12000

14000

410 415 420 425 430 435 440

Em

issi

on

Int

ensi

ty

Wavelength (nm)

Calcium(422.67)

Chromium425.54427.48428.97

Weathered

LIBS detector: Tasks and status

• Acquire laser and spectrometer components

• Interface spectrometer to the computer for data acquisition with external triggering

• Optical configuration design and design of detector package

• Construct prototype detector system

• Synchronize spectrometer data acquisition with firing of laser

LIBS detector: Tasks and status (con’t)

• Interface output trigger signal with PC for firing strobe light upon CCA detection

• Design and build detector mount

• Field test instrument (4-20-01)

• Field testing and evaluation (current)

Detector configuration

LaserTo PC

Data acquisition

Plasma emission

Fiberoptic

Laptop computer

Spectrometer

Ext. trigger from laserData acquisition

and analysis

Strobetrigger

Instrument synchronization

Time0

Laser pulse (10 ns)

Plasma emission (25 us)

Chromium emission (10 us)

Pre-trigger

Detector Delay

Identification of CCA-treated wood

0

2000

4000

6000

8000

10000

12000

14000

410 415 420 425 430 435 440

Em

issi

on In

tens

ity

Wavelength (nm)

Calcium

Chromium

Peak

Base

Field calibration of detector system

• Detect CCA-treated wood by presence of chromium atomic emission

• Non-treated wood also emits light (background emission) at the same spectral region

• Select a threshold for CCA detection

Single laser shots: Untreated wood

P/B (5)

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1

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5

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7

8

9

10

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P/B (5)

LIB

S C

r S

igna

l

Laser Shot Number

Single laser shots: Treated wood

P/B (5)

0

1

2

3

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5

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7

8

9

10

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P/B (5)

LIB

S C

r S

igna

l

Laser Shot Number

Analysis of single laser shots

Treated

Lumber

Untreated

Lumber

100%

accuracy

99%

accuracy

10-shot

average

92%

accuracy

95%

accuracy

Single-shot

analysis

35.069.1 44.198.3 Cr Signal

Output signal for detection of treated wood

• Fire strobe lights when CCA is detected

• Requires firing rate of 2 Hz and TTL trigger logic

• NOVA Electronics built a custom strobe unit with four output channels

Sarasota field test on 7-3-01

Threshold

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4

6

8

10

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Wood Sample Number

Summary of LIBS work to date

• Successfully detect CCA-treated wood by presence of chromium in the field

• Indicator strobe enables real-time, on-line sorting with accuracy approaching 100%

• Continue to assess accuracy of LIBS-based detector and to formulate system improvements