Laser-Induced Breakdown Spectroscopy for Microanalysis · PDF fileU of A - R. Fedosejevs...
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U of A - R. Fedosejevs 070303 p.1
Laser-Induced Breakdown Spectroscopy for Microanalysis
Robert Fedosejevs, Y. Godwal, M.T. Taschuk, S. L. Lui, Y.Y. Tsui
Department of Electrical and Computer EngineeringUniversity of Alberta, Edmonton, Alberta
Presented at the
3rd INTERNATIONAL CONFERENCE ON THE FRONTIERS OF PLASMA PHYSICS AND TECHNOLOGY
Bangkok, March 5, 2007
Research Funded by:
MPBT/NSERC/UofA Senior Industrial Research Chair
Natural Sciences and Engineering Research Council of Canada
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Outline
• Introduction to LIBS• Scaling of LIBS to µJ Energies• µLIBS Applications
• 2D Surface Microanalysis• Fingerprint Detection & Imaging• Two Pulse Technique to Improve Limit Of Detection• Measurement of Elemental Contaminants in Water• µLIBS in Microfluidic Systems for Lab on a Chip Analysis
• Conclusions
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Overview of LIBS Process
Plasma PlumeExpands
FocussingLens
Target Material397398399400401402
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Wavelength (nm)
Spectrometer
21Laser StrikesTarget Material
3Spectra isobtained.
CharacteristicRadiation
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Overview of LIBS Plasma Expansion
Target Material Target Material Target Material
Shockwave launchedAtomic emission dominatesContinuum decreases
Plasma expands rapidlyContinuum radiation dominates
Laser initiates breakdownPlasma formsPortion of sample taken into plasma
20 ns
wavelength (nm)
Laser pulse
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Typical µLIBS Experimental Set-up
CCD
PD
Sample
Laser Pulse:10 ns, E = 1 - 500 µJI = 0.5 – 250 GW/cm2
50 ps, E = 0.1 - 100 µJI = 0.01 – 10 TW/cm2
100 fs, E = 0.1 - 100 µJI = 0.005 – 5 PW/cm2
~ 5 µm focal spot
OMA
Laser Pulse
Spectrometer/OMAAlignment Camera
Plasma
R = 99%
Dichroic Mirror
300 400 500
1000
10000
Coun
ts
Wavelength (nm)
MicroscopeObjective PM
Filter
Rieger et al., Appl. Spect. 56, 689 (2002)
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1 e + 3
1 e + 4
1 e + 5
3 0 0 3 5 0 4 0 0 4 5 0 5 0 0-1 0 0 0
-8 0 0
-6 0 0
-4 0 0
-2 0 0
0
Inte
nsity
W a ve le n g th (n m )
Delay (ns)
Al+
Mn+
Mn
Al+
AlO-bands
Si
Fe
Mn
AlAl
Time Evolution of an Aluminum Alloy PlasmaAl 3003, 10 µm slit248 nm, 10 ns, TG = 300 ns Eav = 200 µJ
U of A - R. Fedosejevs 070303 p.7Wavelength (nm)
397 398 399 400 401 4020.00.10.20.30.40.50.60.70.80.91.01.11.21.31.41.51.6
Wavelength (nm)
397 398 399 400 401 4020.00.10.20.30.40.50.60.70.80.91.01.11.21.31.41.51.6
SPS 17-4 PHChromiumNominalComposition: 0.6%
ChromiumNominalComposition: 16.5%
Cr I Triplet397.6 nm, 398.3 nm, 399.1 nm
SPS steel,0.6% Chromium
17-4 PH steel,16.5% Chromium
Overview of LIBS :Typical Spectra
Observing elements at less than a single percent concentration is straightforward
Choose spectral window according to the material being observed to maximize information gathered
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• Laser-Induced Breakdown Spectroscopy:• offers rapid analysis• requires no sample preparation• sensitive to all elements• scalable in sample size• requires no contact with the
sample• work in hostile environments
Advantages of LIBS
Laser beam being directed through the lead glass shield window to measure radioactive materials
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LIBS Inspection of Gas Cooled ReactorUsing a fiber optically coupled LIBS system for finding low ductility joints in superheated steam tubes by anomalously high copper content Applied
Photonics
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• Lower laser pulse energies ≤ 100 µJ:• Smaller spot sizes reduces damage to sample• Allows micron scale resolution• Higher repetition rate laser systems can be used• Possibility of portable LIBS systems• LODs achieved are comparable to mJ LIBS
→ New Subfield of µLIBS
• Applications• 3D surface Microanalysis with µm lateral and sub-µm depth
resolution• On line pollution monitoring of industrial effluents• Monitoring of drinking water standards• Microfluidic point of care medical diagnostic systems
Scaling of LIBS to µJ Energies
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Definition of Limit of Detection
• Noise is evaluated from the pixel to pixel variation on either side of the signal
• LOD (limit of detection) is the point where signal within the full linewidth of the emission line is 3σabove the average noise scaled to the integration width
Signal
Noise
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Single Shot Surface Probe Capability
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Correlation of elements within precipitates - identification
Single Shot µLIBS Aluminum Precipitates
Aluminum 2024 Alloy
Cravetchi et al., Spectrochimica Acta. 59, (2004)
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• Accumulation of 100 0.5 µJ pulses yields useful spectra
• High rep-rate fiber or microchip lasers have potential for LIBS
• Remaining Issues:
• Scaling to sub micron resolution with different materials
• Integration of high rep-rate laser source with ICCD
Al2024, 100 shot average, 0.5 µJ, 266 nm, 130 fs600 l/mm, 100 µm slit, Gate Delay 2.5 ns, Gate Width 100 ns, Pixel Time 16 µs, Gain 275 counts/photoelectron, 9 counts/photon @ 270 nm, 27 counts/photon @ 440 nm
Surface Mapping at sub-µJ Energies with Femtosecond Pulses
0.5 µJ Spectra
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Surface Mapping at sub-µJ Energies
Grey – background matrix, Black – Al2CuMg, White - Al6(Cu,Fe,Mn)
Can build up a map of aluminum alloy surfaces with many single shots2D map of aluminum alloy possible with sub microjoule energiesHowever, a limited number of photons are available at these energies
Al 2024 Alloy, 0.85 µJ, 266 nm, 120fs
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Experimental Setup for µLIBS measurement of fingerprints
LIBS Fingerprint Detection and Imaging
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Nshots = 1, Elaser= 80 µJ, 400 nm 130 fs pulseTdelay = 5 ns, Tgate = 1 µs, Slit = 100 µm, Readout Time = 16 µs1200 lines/mm grating
2D mapping technique can be applied to latent fingerprints
80 µJ, 130 fs fs pulses at 400 nm
Fingerprint Detection Characteristic Spectra
Sample spectra from a fingerprint ridge and gap between fingerprint ridges on silicon wafer
Na 589.2 nm
Si 288.2 nm (2nd order)
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Nshots = 1, Elaser= 80 µJ, 400 nm 130 fs pulseTdelay = 5 ns, Tgate = 1 µs, Slit = 100 µm, Readout Time = 16 µs1200 lines/mm grating
Fingerprint Detection - Line Scan
• Si signal suppressed at locations with a fingerprint ridge
• Femtosecond probe pulses only sensitive to surface layer
Na signal
Si signal
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Nshots = 1, Elaser= 80 µJ, 400 nm 130 fs pulseTdelay = 5 ns, Tgate = 1 µs, Slit = 100 µm, Readout Time = 16 µs1200 lines/mm grating
2D LIBS scan of a 1 mm by 5 mm area of a latent fingerprint from right thumb
Ridge detail is clearly visible in the Na image (upper) and Siimage (lower)
Fingerprint Detection – 2D Scans
Na signal
Si signal
M. Taschuk et al., Applied Spectroscopy 60, pp.1322 –1327 (2006)
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Sodium signals:Original
After 2 cleaning wipes with alcohol soaked lens tissue
After 4 cleaning wipes with alcohol soaked lens tissue
Durability of Na Signature
Nshots = 1, Elaser= 80 µJ, 400 nm 130 fs pulseTdelay = 5 ns, Tgate = 1 µs, Slit = 100 µm, Readout Time = 16 µs1200 lines/mm grating
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• Thus far used 80 µJ 400nm femtosecond pulses• Sample is mostly destroyed using a 50 µm sampling grid
• Try with 5 µJ 266nm femtosecond pulses• stronger UV absorption allows lower pulse energy threshold• Much smaller 10 µm craters• Large surface area preserved for future analysis if necessary• Better suited to lower energy, higher repetition rate laser
Shift to UV Excitation and Lower Energies
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Fingerprint Detection 5 µJ 120 fs 266nm Probe Pulses
Reflective laser focusing and achromatic plasma imaging system
to collect broadband spectrum
SchwarzchildObjective
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Large amount of the surface area remains undamaged by the craters
Craters from Scanning with 5 µJ 266 nm Pulses
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Nshots = 100 shot average, 266 nm, 130 fs pulsesTdelay = 1 - 5 ns, Tgate = 1 µs, Slit = 100 µm, Readout Time = 16 µs600 lines/mm grating
SNR scaling for 3 fingerprints using 266 nm pulses
SNR approaching limit for single shot acquisitions at ~ 3 uJ
SNR scaling with Pulse Energy
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Nshots = 1, Elaser= 5 µJ, 266 nm 130 fs pulseTdelay = 5 ns, Tgate = 1 µs, Slit = 500 µm, Readout Time = 16 µs600 lines/mm grating
Na
Si
2D LIBS scan of a 2 mm by 5 mm area of a latent fingerprint
Ridge detail is clearly visible in the Na image (upper) and Siimage (lower)
Energy requirements reduced to levels easily compatible with fiber or microchip lasers
Portable system at kHz acquisition rate may be possible
Fingerprint Imaging 5 µJ 266 nm Pulses
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Two Pulse LIBS: Laser Ablation - Laser Induced Fluorescence
• Utilizes two pulse technique• One pulse to ablate the sample and create a plume• Second pulse resonantly excites the atomic species of
interest• Improvement of detection limit to ppb from ppm level• Must optimize the parameters for the two laser pulses
• Pulse energies• Inter-pulse temporal separation• Detector efficiency
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Typical LA-LIF Experiment Layout
50 cm lens
2 ω Dye laser wavelength set to 257 nm for Al and 283 for Pb
Waterjet diameter: 1 mm
Probe Pulse
Breakdown Pulse
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Single Pulse µLIBS of 500 ppm Pb
• At shorter gate delays plasma exhibits significant continuum background
• As the plasma cools continuum decreases rapidly
• Optimization of LOD requires optimum gate time
Nshots=100, Elaser=260 µJ, 500 ppm Pb in waterGatewidth=100ns, Slit = 300µm, Detector gain = 255
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LA-LIF of Pb
Ground J=0 3P0
6p1/26p3/2J=2
J=1 7819.263 3P1
10650.327 3P2
6p1/26p1/2
6p1/27s1/2J=1
J=035287.224 3P1
34959.908 3P0
283.389nm
364.061nm
405.895nm
368.451nm
5.8 x 107 s-1
3.4 x 107 s-1
8.9 x 107 s-1
1.5 x 108 s-1
Excitation wavelength
Fluorescencewavelength
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0
0.2
0.4
0.6
350 370 390 410 430
Mill
ions
wavelength (nm)
inte
nsity
(cou
nt)
364nm 368nm
405nm
Nshots=1000, Elaser= 170 µJ, E2pulse= 10 µJ, ∆T=300ns, Slit width = 300 µm, Grating 1200l/mm, [Pb]: 50ppm
LA-LIF spectrum of Pb
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Time Resolved LA-LIF Signal for Pb
• Signal only appears with the probe pulse
• Enhancement is short-lived, on nanosecond time scale
0 200 400 600 800 1000 12000
20000
40000
60000
80000
100000
120000
Sig
nal (
coun
ts)
Time (ns)
single pulse LA-LIF signal
Nshots=100, Elaser=260 µJ, E2nd pulse= 45nJ, 500ppm of Pb in water, ∆T= 700ns, Gatedelay=700ns, Slit = 300µm, Detector gain = 255
FluorescenceSignal
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380 400 420wavelength (nm)
inte
nsity
(cou
nt)
Selective enhancement of LA-LIF
2534735287Upper level (cm-1)
15010Conc (ppm)
AlPb
Al
Pb
2 pulses
1 pulse
* The two spectra have been offset vertically
Nshots=100, Elaser= 170 µJ, E2pulse= 10 µJ, Gatewidth=100ns, ∆T=300ns, Slit = 300 µm, Detector gain = 255
FluorescenceSignal
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Optimization – Pulse Separation
Nshots=1000,Elaser= 170 µJ E2pulse= 8 µJ, Gatewidth=100ns, Slit = 300 µm, Detector gain = 255, [Pb]= 50 ppm in water
Pulse Separation (ns)
0 500 1000 1500 2000 2500 3000 3500
Sig
nal (
coun
t)
0
1e+6
2e+6
3e+6
4e+6
5e+6
6e+6
7e+6
3
4
5
6
7
8
200 400 600 800 1000
4
6
8
1
2
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Scaling with Probe Pulse Energy
Nshots=1000, ∆T= 300ns, Slit = 300 µm, Detector gain = 255, [Pb]= 50 ppm in water
0.00E+00
2.00E+00
4.00E+00
6.00E+00
8.00E+00
1.00E+01
0 2 4 6 8 10 12 14Second Pulse Energy (µJ)
Sign
al (c
ount
)
First Pulse Energy
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380 400 420Wavelength (nm)
Inte
nsity
(cou
nt)
LA-LIF spectrum of 100 ppb Pb sample: 1000 shot
SNR~5
Nshots=1000, Elaser= 170 µJ E2pulse= 10 µJ, ∆T= 300ns, Slit = 300 µm, Detector gain = 255, Gate width = 300 ns
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Pb spectrum of 100 ppb Pb sample: 10000 shots
380 400 420Wavelength (nm)
Inte
nsity
(cou
nt)
SNR~12
Nshots=10000, Elaser= 170 µJ E2pulse= 10 µJ, ∆T= 300ns, Slit = 300 µm, Detector gain = 255, Gate width = 300 s
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100 shot LOD for Pb in Water
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
0.1 1 10 100 1000Pb Concentration (ppm)
Cou
nts
normalized signal3σ noise floor
200 ppb
20Gate width (ns)
300∆T (ns)
200 ppbLoD (3σ)
102nd Pulse (µJ)
170Ablation pulse (µJ)
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1000 shot LOD for Pb in Water
20Gate width (ns)
300∆T (ns)
73 ppbLoD (3σ)
102nd Pulse (µJ)
170Ablation pulse (µJ)
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
0.01 0.1 1 10 100 1000
Pb Concentration (ppm)
coun
t
normalized signal3σ noise floor
73 ppb
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Comparison with Previous Pb LOD Reports
All values scaled to 100 shot equivalents
[1] R. Kopp, et. al. Fresenius’ J. Anal. Chem. 355, 16 (1996).
[2] G. Arca, et. al. IGARSS 96, Vol 1, 27-31 May 1996, 520-522.
[3] M. Taschuk, et. al. EMSLIBS 2003, Heraklion, Crete October 1st, 2003
[4] K. M. Lo, et. al. Appl. Spectrocopy, vol. 56, Number 6, 2002
[5] X.Y.Pu, Appl. Spectroscopy 57,5,
[6] Le Bihan et. al. Annal. Bioanal. Chem 2003 Le Bihan et. al.20030.3ppt (20 shots)ETA-LEAF
http://pyrite.chem.northwestern.edu/
1.5ppbICP-AES
http://servant.geol.cf.ac.uk/icppage.htm
0.01-0.1ppbICP-MS
sourceLimit of detectionmethod
Detection limit of Pb using other techniques
µLIBS
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282 284 286 288 290
Wavelength nm
0
1
2
3
x10 4
Cou
nts (
Bg
Cor
rect
ed)
Apply LIBS in microfluidic systemDetection of single cell contentsLab-on-a-chip application – micro Total Analytic Systems (µTAS)
Drop-on-demand actuator (thermal or piezoelectric)
microchannel ~50µm
orifice, ~ few µm
1-10µm droplet
µLIBS in Microfluidic Systems
LIBS probe
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Rapid thermal heater
Piezoelectric pulser
µs-pulse
Microdroplet Generation
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microheater
The orifice, the channel, and the reservoir are all machined by laser-micromachining
microheater
orifice
microchannel
Prototype Thermal Droplet Ejector
Micro-Heater Element
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• Development of µLIBS• High resolution and small probe spot size demonstrated• LODs in ppm range demonstrated
• Initial µLIBS Applications: • Surface mapping of alloys for quality control of metal
manufacturing• Micron scale size resolution• Fingerprint detection both by Na and substrate lines
• Overcomes fluorescence masking for some materials• µJ energies with fs uv pulses leaves large surface area
for further investigation or as evidence• Can increase acquisition speed to multi-kHz repetition rates• High resolution 3D scans possible – µm lateral and sub-µm
depth
Conclusions
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• LA-LIF• µJ energies sufficient for excitation and resonant probing• Increase sensitivity to ppb levels
• Initial LA-LIF Applications: • Monitoring of water quality• 25 ppb detection of Pb in water with 10000 shots
• High repetition rate lasers (10-100 kHz) would allow 2.5 ppb sensitivity in 10 - 100 second measurement times
• i.e. real time water quality monitoring
• Can be scaled to portable systems using upconverted fiber lasers with fiber Bragg gratings to generate the exact probe wavelengths
• Future applications in lab on a chip for medical diagnostics in the doctors office
Conclusions
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The End