A Fundamental Study of Laser-Induced Breakdown Spectroscopy Using Fiber Optics for Remote Measurements of Trace Metals

1
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

2
LIBS for Elemental Analysis

• Approach
• Fiber optic technology
• Wavelength resolution
• Time resolution
• Accomplishments
• Two operating instruments
• Examining surface morphology
• Studying matrix effects
• Future
• Solutions and slurries

3
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

4
Limiting Factor

• Discriminating analyte atomic emission from
  continuum background emission limits the analysis
• Time
• Wavelength

5
Time-Resolved LIBS Apparatus
6
Fiber-Optic LIBS System Configuration
Pulsed laser
Lens
Delay generator
Detector
Lens
Controller
Fiber-optic LIBS probe
Spectrograph
Computer
7
Fiber-Optic LIBS Probe Design
f/2 Lens
Plasma
Collection Fiber
Excitation Fiber
Sample
Focusing lens
8
Lead in Paint Using Fiber-Optic LIBS Probe
1400
1200
Pb
Ti
Ti
Ti
1000
Solder
800
Intensity
Leaded Paint
600
400
Unleaded Paint
200
0
406.0
404.0
402.0
400.0
398.0
Wavelength (nm)
9
Leaded Paint Calibration Using Fiber-Optic Probe
200
- 4 mJ/pulse, 2 Hz, 532 nm laser, avg. 5
replicate spectra
150
Intensity
100
50
L.O.D. 0.014 Pb (wt/wt) Dry Basis
0
0.10
0.08
0.06
0.04
0.02
0.00
Concentration of Lead ( w/w, Dry Basis)
10
Fiber-Optic Transmission
120
110
1 mm silica-clad 1 mm hard-clad 800 ?m
hard-clad 600 ?m hard-clad
100
90
80
70
Power Out of fiber (mJ)
60
50
fiber breakdown
40
30
20
10
0
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
Power into Fiber (mJ)
11
spectral excit.
10x
Ar
imaging ex. w/GRIN
pellicle
f/8
excitation fiber
LIBS/Raman collection fiber
NdYAG
imaging fiber
HeNe
6x macro lens
imaging fiber
10x
bw CCD
frame grabber
f/7 lens
ICCD
monitor
probe
pulser
controller
spectrograph
12
Video camera
Collection fiber (filtered for Raman)
LIBS excitation fiber (1064 nm) (632 nm pointer)
Filtered Raman excitation fiber (514.5 nm)
Imaging fiber
GRIN lens
Region of interest
Imaged region
Sample
13
5 mm
Fe
Ca
35x103
b
a
Fe
Fe
25
Fe
Intensity
Fe
15
Fe
5
420
416
412
408
404
Wavelength (nm)
Region of Interest
16 x103
d
c
Ca
Sr
14
Sr
Intensity
10
6
2
420
416
412
408
404
Wavelength (nm)
14
Raman spectrum of TiO2
b
200x103
Darkfield image of TiO2 and Sr(NO3)2
on soil
150
Intensity
100
a
50
0
1000
800
600
400
200
Wavenumber (cm-1)
Raman spectrum of Sr(NO3)2
c
200x103
150
Intensity
100
50
1600
1400
1200
1000
800
Wavenumber (cm-1)
15
Raman Images
TiO2 _at_190 cm-1
Darkfield image of TiO2 and Sr(NO3)2 on soil
b
a
Sr(NO3 ) 2 _at_1055cm-1
c
16
Plasma Temperature Profile
Regions
2500
7
7
0
384
382
380
378
376
374
372
370
368
366
2500
6
6
0
384
382
380
378
376
374
372
370
368
366
Graph 5
2500
5
5
0
384
382
380
378
376
374
372
370
368
366
Graph 4
2500
4
4
0
Observed plasma region
384
382
380
378
376
374
372
370
368
366
Graph 3
2500
3
3
0
384
382
380
378
376
374
372
370
368
366
Graph 2
2500
2
2
0
384
382
380
378
376
374
372
370
368
366
Graph 1 (bottom of plasma)
2500
1
1
0
384
382
380
378
376
374
372
370
368
366
7000
6000
Plasma temperature (K)
17
LIBS Imaging Spectrometer
1064 nm mirror
laser
beam stop
ICCD
AOTF
lens
1064 nm mirror
sample
collimating lens
plasma
RF generator
18
Background Subtracted Lead Emission
Repetition Rate 2 Hz, 2000 Shots, 2.5 ?s Delay
722.8 nm Lead Emission Continuum
715.2 nm Continuum Background
Background Subtracted
19
Temporal Dependence of Lead Emission
Background subtracted
Pb emission at 722.8 nm
2.5 mm
2.5 mm
50 ns
675 ns
1. 3 ms
1. 9 ms
2. 5 ms
20
Lead Crater Depth and Plasma Height
0.38 mm
0.38 mm
0.50 mm
2.75
mm
21
Plasma Height vs. Number of Laser Shots
Rep Rate 2 Hz
2.5 ?s delay
2500
1.0 ?s delay
2000
Plasma Height (microns)
1500
1000
2000
1500
1000
500
Number of Laser Shots
22
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

23
Echelle Spectrometer
24
Matrix effects

• Use binary alloy (brass samples)
• Examine signals from zinc (volatile) and copper
  (nonvolatile)
• Vary laser power
• Vary focal depth

25
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

26
Effect of Laser Power2.86 Zn
27
Effect of Laser Power4.18 Zn
28
Effect of Laser Power24.8 Zn
29
Effect of Laser Power34.6 Zn
30
Effect of Laser Power39.7 Zn
31
Calibration Plot
32
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

33
Zn-to-Cu ratio as a function of focal point
2.86 Zn
34
Zn-to-Cu ratio as a function of focal point 4.18
Zn
35
Zn-to-Cu ratio as a function of focal point 8.48
Zn
36
Zn-to-Cu ratio as a function of focal point 24.8
Zn
37
Zn-to-Cu ratio as a function of focal point 34.6
Zn
38
Zn-to-Cu ratio as a function of focal point 39.7
Zn
39
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.

40
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

41
Spectrometer

• High Spectral Resolution (7500)
• High Time Resolution (5 ns)
• Delivery?

42
Alternative Excitation

• Use laser system to vaporize solid sample.
• Direct vapor into microwave-excited plasma.
• Use emission from microwave plasma for chemical
  analysis.

43
Colinear Dual-Pulse LIBS Configuration
Pulser
ICCD
Controller
Pulsed NdYAG
Optical Fiber
Spectrograph
lens
Timing
Control
1064nm mirror
lens
Pulsed NdYAG
plasma
sample
44
Colinear Dual-Pulse LIBS Enhancement for Copper
3
0
?
s between lasers
25x10

1
?
s between lasers
20
15
Intensity (arb units)
10
5
530
525
520
515
510
505
500
Wavelength (nm)
45
Optimum Delay Between Lasers for Copper
Enhancement
16
Colinear Dual-Pulse LIBS
14
12
Laser 1 100 mJ Laser 2 180 mJ
Signal-to-Bkg
10
8
6
4
2
500
400
300
200
100
0
46
Copper Craters from Colinear Dual-Pulse LIBS
20 ?s ?T
1 ?s ?T
0 ?s ?T
0.38 mm
0.38 mm
0.38 mm
Cu S/B ? 15
Cu S/B ? 14
Cu S/B ? 3
47
Optimum Timing Between Lasers for Lead Enhancement
Colinear Dual-Pulse LIBS
4.0
3.5
Pb SBR
3.0
2.5
100
80
60
40
20
0
Time Between Lasers (? s) ?T
48
Comparison of Lead Craters (colinear geometry)
Zero ?s ?T
One ?s ?T
0.60 mm
0.60 mm
Pb S/B ? 6
Pb S/B ?2.5
49
Orthogonal Dual-Pulse LIBS
50
Orthogonal Dual-Pulse LIBS
NdYAG
Pulser
ICCD
Controller
Timing
Control
Spectrograph
plasma
NdYAG
51
Orthogonal Dual-Pulse LIBS Enhancement for Cu
0 ?s between lasers
10
-1 ?s between lasers
8
Intensity
6
4
2
0
530
525
520
515
510
505
500
Wavelength (nm)
52
Enhancement of Copper Emission Using Non-Ablating
Prespark
14
12
10
8
Cu Sig-to-bkg
6
4
2
0
-5
-4
-3
-2
-1
0
Time between lasers (?s)
53
Orthogonal Dual-Pulse LIBS Geometry SEM Craters
for Copper
150 ?m
150 ?m
176 ?m
54
2.86 Zinc at Low Power
141.2
56.3
144.4
120.8
36.4
55
2.86 Zinc at High Power
110.3
111.8
259.9
86.6
56
4.18 Zinc at Low Power
88.9
133.9
95.0
101.2
124.9
90.5
57
4.18 Zinc at High Power
57.8
97.8
71.7
89.1
91.0
60.6
93.8
58
24.8 Zinc at Low Power
88.0
75.4
62.0
7.8
130.0
59
24.8 Zinc at High Power
101.3
89.1
57.9
93.3
100.0
106.7
100.8
60
35.6 Zinc at Low Power
70.9
101.6
92.5
90.2
79.1
61
34.6 Zinc at High Power
173.9
119.6
126.3
85.4
119.1
84.4
108.8
109.6
62
34.6 Zinc at High Power Surface Effect
99.4
110.5
63
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

64
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