a fundamental study of laser- induced breakdown spectroscopy using fiber optics for remote...
TRANSCRIPT
A Fundamental Study of Laser-Induced Breakdown
Spectroscopy Using Fiber Optics for Remote
Measurements of Trace Metals
Scott R. Goode and S. Michael Angel
Department of Chemistry and Biochemistry
University of South Carolina
• Approach– Fiber optic technology– Wavelength resolution– Time resolution
• Accomplishments– Two operating instruments– Examining surface morphology– Studying matrix effects
• Future– Solutions and slurries
LIBS for Elemental Analysis
Laser-Induced Breakdown Spectroscopy
• Use laser to vaporize sample
• Laser electric field high enough to cause breakdown
• Monitor emission
• Fiber optics afford capability for remote analysis
Limiting Factor
• Discriminating analyte atomic emission from continuum background emission limits the analysis
– Time
– Wavelength
Time-Resolved LIBS Apparatus
Pulsed Lasermirror
focusinglens
Spectrographplasma
collectionlens
intensifieddetector
TimingControl
1064 nm
Pulsed laserLens
Delaygenerator
Controller
Detector
Computer
Lasertrigger
Spectrograph
Lens
Fiber-opticLIBS probe
Fiber-Optic LIBS System Configuration
SampleFocusing lens
Excitation Fiber
Collection Fiber
f/2 Lens Plasma
Fiber-Optic LIBS Probe Design
Lead in Paint Using Fiber-Optic LIBS Probe
Wavelength (nm)
Ti Ti Ti
140012001000
800
600
400
200
0406.0404.0402.0400.0398.0
Pb
Solder
Leaded Paint
Unleaded Paint
Inte
nsity
Leaded Paint Calibration Using Fiber-Optic Probe
200
150
100
50
0
Inte
nsity
0.100.080.060.040.020.00
Concentration of Lead (% w/w, Dry Basis)
L.O.D.= 0.014% Pb (wt/wt) Dry Basis
- 4 mJ/pulse, 2 Hz, 532 nm laser, avg. 5 replicate spectra
Fiber-Optic Transmission
120
110
100
90
80
70
60
50
40
30
20
10
0
Pow
er O
ut o
f fib
er (
mJ)
150140130120110100908070605040302010
Power into Fiber (mJ)
fiber breakdown
1 mm silica-clad 1 mm hard-clad800 m hard-clad600 m hard-clad
imaging fiberHe:Ne
Nd:YAG
Ar+
pellicle f/8
probe
b&w CCD
6x macrolens 10x imaging fiber
framegrabber
excitationfiber
ICCD
LIBS/Ramancollection
fiber
monitor
pulser
controller spectrograph
f/7 lens
10x
imaging ex. w/GRIN
spectral excit.
Imaged region
Imaging fiber
GRIN lens
Filtered Ramanexcitation fiber(514.5 nm)
LIBS excitationfiber (1064 nm)(632 nm pointer)
Collection fiber(filtered for Raman)
Region of interest
Sample
Videocamera
Inte
nsi
ty
16 x103
Inte
nsi
ty
35x103
25
15
5
420416412408404
FeFe
Fe
Ca
FeFe
Fe
b
14
10
6
2
420416412408404
Sr Ca
Sr
d
Wavelength (nm)
5 mm
Region of InterestWavelength (nm)
a
c
1000800600400200
Darkfield image of TiO2 and Sr(NO3)2
on soil
Raman spectrum of Sr(NO3)2
Raman spectrum of TiO2200x103
150
100
50
0
Inte
nsity
Wavenumber (cm-1)
200x103
150
100
50
1600140012001000800
Wavenumber (cm-1)
Inte
nsity
a
c
b
TiO2 @190 cm-1
Darkfield image of TiO2 and Sr(NO3)2 on soil
Raman Images
Sr(NO3 ) 2 @1055cm-1
a
c
b
Plasma Temperature Profile
2500
0384382380378376374372370368366
Graph 7 (top of plasma)
2500
0384382380378376374372370368366
Graph 6
Graph 52500
0384382380378376374372370368366
Graph 22500
0384382380378376374372370368366
Graph 1 (bottom of plasma)2500
0384382380378376374372370368366
Graph 3
Graph 42500
0384382380378376374372370368366
2500
3843823803783763743723703683660
7
6
5
4
3
2
1
Observed plasm
a region
70006000Plasma temperature (K)
Top
Bottom
Reg
ions
7
6
5
4
3
2
1
LIBS Imaging Spectrometer
sample
ICCD
lens
beam stop
AOTF
RF generator
collimating lens
plasma
laser
1064 nmmirror
1064 nmmirror
Background Subtracted
722.8 nm LeadEmission + Continuum
715.2 nmContinuumBackground
2 .64 mm
Repetition Rate: 2 Hz, 2000 Shots, 2.5 s Delay
Background Subtracted Lead Emission
Temporal Dependence of Lead Emission
50 ns 675 ns 1. 3 s 1. 9 s 2. 5 s
Pb emission at 722.8 nm
Continuum background
Background subtracted
2.5 mm
2.5 mm
Lead Crater Depth and Plasma Height
0.38 mm0.38 mm 0.50 mm
0.63mm1.42mm
2.75 mm
100 shots 2400 shots960 shots
Plasma Height vs. Number of Laser Shots
2500
2000
1500
1000
200015001000500
Number of Laser Shots
Pla
sma H
eig
ht
(mic
rons)
2.5 s delay
1.0 s delay
Rep Rate: 2 Hz
Using High Wavelength Resolution
If the major source of noise is the continuum background
– Eliminate the background by time resolution
– Use wavelength resolution to distinguish the atomic lines from the continuum background
Echelle Spectrometer
Matrix effects
• Use binary alloy (brass samples)
• Examine signals from zinc (volatile) and copper (nonvolatile)
• Vary laser power
• Vary focal depth
Studying selective volatilization
• Measure zinc and copper emission from brass standards
• Perform measurements while varying laser power (Q-switch delay)
• See if ratio is independent of power and proportional to concentration
Effect of Laser Power2.86% Zn
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
105 115 125 135 145 155 165 175 185 195
Q-switch delay/ s
Zn/C
u e
mis
sio
n r
atio
5/5/98
5/14/98
Effect of Laser Power4.18 % Zn
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
105 115 125 135 145 155 165 175 185 195
Q-switch delay/ s
Zn/
Cu
em
issi
on
ratio
5/5/98
5/8/98
Effect of Laser Power24.8 % Zn
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
105 115 125 135 145 155 165 175 185 195
Q-switch delay/ s
Zn
/Cu
em
issi
on
ra
tio
5/7/98
5/8/98
Effect of Laser Power34.6 % Zn
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
105 115 125 135 145 155 165 175 185 195
Q-switch delay/s
Cu/
Zn
em
issi
on
ratio
5/7/98
5/8/98
Effect of Laser Power39.7 % Zn
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
105 115 125 135 145 155 165 175 185 195
Q-switch delay/ s
Zn
/Cu
em
issio
n r
atio
5/7/98
5/8/98
Calibration Plot
Brass CRMs
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 0.2 0.4 0.6 0.8
Zn/Cu Concentration Ratio
Zn/
Cu
Em
issi
on
Rat
io
110 s delay
180 s delay
Effect of focus
• Measure Zn-to-Cu emission ratio
– As a function of composition
– As a function of focal point
• Negative: focal point below surface
• Zero: at surface
• Positive: above surface
Zn-to-Cu ratio as a function of focal point 2.86% Zn
0.14
0.16
0.18
0.2
0.22
0.24
0.26
0.28
-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
z/ mm
Zn
/Cu
em
issio
n r
atio
Zn-to-Cu ratio as a function of focal point 4.18 % Zn
0.2
0.22
0.24
0.26
0.28
0.3
0.32
0.34
0.36
0.38
-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
z/ mm
Zn
/Cu
em
issio
n r
atio
Zn-to-Cu ratio as a function of focal point 8.48 % Zn
0.3
0.32
0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
z/ mm
Zn
/Cu
em
issio
n r
atio
Zn-to-Cu ratio as a function of focal point 24.8 % Zn
0.7
0.75
0.8
0.85
0.9
0.95
1
1.05
-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
z/ mm
Zn
/Cu
em
issio
n r
atio
Zn-to-Cu ratio as a function of focal point 34.6 % Zn
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
z/ mm
Zn
/Cu
em
issio
n r
atio
Zn-to-Cu ratio as a function of focal point 39.7 % Zn
0.95
1.05
1.15
1.25
1.35
1.45
1.55
1.65
-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
z/ mm
Zn
/Cu
em
issio
n r
atio
Conclusions
• LIBS is more complex than originally thought.
• Much of the data are consistent with a low-power heating mechanism and a high power dielectric vaporization mechanism.
• Can design experiments to decouple excitation and vaporization.
Segregate excitation effects from vaporization effects
• Brass samples, known composition
• Laser ablation into solution
• Dissolution
• Chemical analysis by ICP-MS
• Determine if materials vaporized in proportion to concentration
• Determine factors that affect selective and nonselective vaporization
Spectrometer
• High Spectral Resolution (7500)
• High Time Resolution (5 ns)
• Delivery?
Alternative Excitation
• Use laser system to vaporize solid sample.
• Direct vapor into microwave-excited plasma.
• Use emission from microwave plasma for chemical analysis.
Pulsed Nd:YAG
Controller
1064nmmirror
plasmasample
Pulsed Nd:YAG
TimingControl
Spectrograph
lens
ICCD
lens
Pulser
Optical Fiber
Colinear Dual-Pulse LIBS Configuration
25x103
20
15
10
5
Inte
nsit
y (a
rb u
nits
)
530525520515510505500
Wavelength (nm)
0 s between lasers
1 s between lasers
1064 nm
Laser 1 (100 mJ) Laser 2 (180 mJ)
Colinear Dual-Pulse LIBS Enhancement for Copper
Sig
nal-
to-B
kg
Optimum Delay Between Lasers for Copper Enhancement
16
14
12
10
8
6
4
2
5004003002001000
Time Between Lasers (s)
Laser 1 = 100 mJLaser 2 = 180 mJ
Colinear Dual-Pulse LIBS
0.38 mm
20 s T
Cu S/B 15
0.38 mm
1 s T
Cu S/B 14
0.38 mm
0 s T
Cu S/B 3
Copper Craters from Colinear Dual-Pulse LIBS
100
Optimum Timing Between Lasers for Lead Enhancement
4.0
3.5
3.0
2.5
806040200
Pb
SB
R
Time Between Lasers (s) T
Colinear Dual-Pulse LIBS
Comparison of Lead Craters (colinear geometry)
0.60 mm 0.60 mm
Zero s T One s T
Pb S/B 6Pb S/B 2.5
Orthogonal Dual-Pulse LIBS
Orthogonal Dual-Pulse LIBS
Controller
Nd:YAG
plasma
Nd:YAG
TimingControl Spectrograph
ICCD
Pulser
10
8
6
4
2
0
Inte
nsity
530525520515510505500
Wavelength (nm)
0 s between lasers -1 s between lasers
Orthogonal Dual-Pulse LIBS Enhancement for Cu
14
12
10
8
6
4
2
0
Cu
Sig
-to-
bkg
-5 -4 -3 -2 -1 0
Time between lasers (s)
Enhancement of Copper Emission Using Non-Ablating Prespark
150 m 150 m 176 m
Orthogonal Dual-Pulse LIBS GeometrySEM Craters for Copper
2.86% Zinc at Low Power
36.4120.8
144.456.3 141.2
2.86% Zinc at High Power
86.6
259.9111.8
110.3
4.18% Zinc at Low Power
88.9133.9
124.9
101.2
90.5
95.0
4.18% Zinc at High Power
89.1 91.0
97.8
60.6
93.8
71.7
57.8
24.8% Zinc at Low Power
130.07.8
62.075.4 88.0
24.8% Zinc at High Power
101.3
89.1
57.9
93.3
100.0
106.7100.8
35.6% Zinc at Low Power
70.9
92.5
90.2
79.1
101.6
34.6% Zinc at High Power
108.8109.6
85.4
173.9126.3
119.6
119.1
84.4
34.6% Zinc at High Power Surface Effect
110.5
99.4
Targeted DOE Needs
• ID No: SR99-3025: Monitoring Technologies for Effectiveness of Solidification and Stabilization Systems
ID No: SR99-1003: Improvements to Physical, Chemical, and Radionuclide Quantification of Solid Waste
ID No: SR99-1004: Need for Continuous Emissions Monitors for Measurement of Hazardous Compound Concentrations in Incinerator Stack Gas
Targeted DOE Needs
ID No. RL-SS06 Improved, Real-Time, In-Situ Detection of Hexavalent Chromium in Groundwater
ID No. RL-DD038: Liquids Characterization for CDI
ID No. RL-SS15: Improved, In Situ Characterization to Determine the Extent of Soil Contamination of One or More of the Following Heavy Metals: Hexavalent Chromium, Mercury, and Lead