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 Laser mirror focusing lens Spectrograph plasma collection lens intensified detector Timing Control 1064 nm

Pulsed laser Lens Delay generator Controller Detector Computer Laser trigger Spectrograph Lens Fiber-optic LIBS probe Fiber-Optic LIBS System Configuration

SampleFocusing lens Excitation Fiber Collection Fiber f/2 LensPlasma Fiber-Optic LIBS Probe Design

Lead in Paint Using Fiber-Optic LIBS Probe Wavelength (nm) Ti Pb Solder Leaded Paint Unleaded Paint Intensity

Leaded Paint Calibration Using Fiber-Optic Probe Intensity 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 Power Out of fiber (mJ) Power into Fiber (mJ) fiber breakdown 1 mm silica-clad 1 mm hard-clad 800  m hard-clad 600  m hard-clad

imaging fiber He:Ne Nd:YAG Ar + pellicle f/8 probe b&w CCD 6x macro lens 10x imaging fiber frame grabber excitation fiber ICCD LIBS/Raman collection fiber monitor pulser controllerspectrograph f/7 lens 10x imaging ex. w/GRIN spectral excit.

Imaged region Imaging fiber GRIN lens Filtered Raman excitation fiber (514.5 nm) LIBS excitation fiber (1064 nm) (632 nm pointer) Collection fiber (filtered for Raman) Region of interest Sample Video camera

Intensity 16 x10 3 Intensity 35x Fe Ca Fe b Sr Ca Sr d Wavelength (nm) 5 mm Region of Interest Wavelength (nm) a c

Darkfield image of TiO 2 and Sr(NO 3 ) 2 on soil Raman spectrum of Sr(NO 3 ) 2 Raman spectrum of TiO 2 200x Intensity Wavenumber (cm -1 ) 200x Wavenumber (cm -1 ) Intensity a c b

TiO cm -1 Darkfield image of TiO 2 and Sr(NO 3 ) 2 on soil Raman Images Sr(NO 3 ) -1 a c b

Plasma Temperature Profile Graph 7 (top of plasma) Graph 6 Graph Graph Graph 1 (bottom of plasma) Graph 3 Graph Observed plasma region Plasma temperature (K) Top Bottom Regions

LIBS Imaging Spectrometer sample ICCD lens beam stop AOTF RF generator collimating lens plasma laser 1064 nm mirror 1064 nm mirror

Background Subtracted nm Lead Emission + Continuum nm Continuum Background 2.64 mm Repetition Rate: 2 Hz, 2000 Shots, 2.5  s Delay Background Subtracted Lead Emission

Temporal Dependence of Lead Emission 50 ns675 ns 1. 3  s1. 9  s2. 5  s Pb emission at nm Continuum background Background subtracted 2.5 mm

Lead Crater Depth and Plasma Height 0.38 mm 0.50 mm 0.63 mm 1.42 mm 2.75 mm 100 shots 2400 shots 960 shots

Plasma Height vs. Number of Laser Shots Number of Laser Shots Plasma Height (microns) 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 Power 2.86% Zn

Effect of Laser Power 4.18 % Zn

Effect of Laser Power 24.8 % Zn

Effect of Laser Power 34.6 % Zn

Effect of Laser Power 39.7 % Zn

Calibration Plot

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

Zn-to-Cu ratio as a function of focal point 4.18 % Zn

Zn-to-Cu ratio as a function of focal point 8.48 % Zn

Zn-to-Cu ratio as a function of focal point 24.8 % Zn

Zn-to-Cu ratio as a function of focal point 34.6 % Zn

Zn-to-Cu ratio as a function of focal point 39.7 % Zn

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 1064nm mirror plasma sample Pulsed Nd:YAG Timing Control Spectrograph lens ICCD lens Pulser Optical Fiber Colinear Dual-Pulse LIBS Configuration

25x Intensity (arb units) Wavelength (nm) 0  s between lasers 1  1064 nm Laser 1 (100 mJ) Laser 2 (180 mJ) Colinear Dual-Pulse LIBS Enhancement for Copper

Signal-to-Bkg Optimum Delay Between Lasers for Copper Enhancement Time Between Lasers (  s)  Laser 1 = 100 mJ Laser 2 = 180 mJ Colinear Dual-Pulse LIBS

0.38 mm 20  s  T Cu S/B  mm 1  s  T Cu S/B  mm 0  s  T Cu S/B  3 Copper Craters from Colinear Dual-Pulse LIBS

100 Optimum Timing Between Lasers for Lead Enhancement Pb SBR Time Between Lasers (  s)  T Colinear Dual-Pulse LIBS

Comparison of Lead Craters (colinear geometry) 0.60 mm Zero  s  TOne  s  T Pb S/B  6 Pb S/B  2.5

Orthogonal Dual-Pulse LIBS

Controller Nd:YAG plasma Nd:YAG Timing Control Spectrograph ICCD Pulser

Intensity Wavelength (nm) 0  s between lasers -1  s between lasers Orthogonal Dual-Pulse LIBS Enhancement for Cu

Cu Sig-to-bkg Time between lasers (  s) Enhancement of Copper Emission Using Non-Ablating Prespark

150  m 176  m Orthogonal Dual-Pulse LIBS Geometry SEM Craters for Copper

2.86% Zinc at Low Power

2.86% Zinc at High Power

4.18% Zinc at Low Power

4.18% Zinc at High Power

24.8% Zinc at Low Power

24.8% Zinc at High Power

35.6% Zinc at Low Power

34.6% Zinc at High Power

34.6% Zinc at High Power Surface Effect

Targeted DOE Needs ID No: SR : Monitoring Technologies for Effectiveness of Solidification and Stabilization Systems  ID No: SR : Improvements to Physical, Chemical, and Radionuclide Quantification of Solid Waste  ID No: SR : 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