Plasma-particle Interactions in a Laser-induced Plasma – The Path Toward Quantitative Analysis David W. Hahn Michael Asgill, Prasoon Diwakar University of Florida SPIE Laser Damage Sept. 26, 2012 Laser-Based Diagnostics Laboratory
Laser-induced Plasma Spectroscopy Multi-photon and cascade ionization creates plasma N e ~ 10 18 cm -3 Nd:YAG T ~ 30,000 K (1064-nm) Laser-induced (10-ns) plasma Plasma forms the sample volume, dissociating Atomic emission molecules & fine particles spectroscopy 3000 2500 Relative Intensity (a.u.) 2000 1500 1000 500 Well suited as a rapid, 0 320 340 360 380 400 420 440 460 Wavelength (nm) real-time analytical scheme Czerny-Turner ICCD array 2
Plasma-particle interactions drive analyte signal Analyte Discrete Atoms mass & ions Intensity (a.u.) Vaporization & Dissociation Heat & Mass Transfer Matrix Effects? Plasma 380 385 390 395 400 405 Wavelength (nm) Overview of Talk • Plasma-analyte interactions • Our evolution of understanding • Means to overcome matrix effects 3
Historic perspective: A continuous evolution “Quantitative spectrographic analysis has proved to be impossible.” H. Kayser, Handbuch (1910) “I do not want to arouse exaggerated hopes. The spectrographic analysis has its limitations as have all analytical methods….but the method can compete with other quantitative methods….in the range of low concentrations.” K. Kellerman, Metals and Alloys (1929) V in steel (1929) Courtesy of Ben W. Smith University of Florida
LIBS has demonstrated promising sensitivity LOD ~1.5 fg Ca 10 m m Ca Intensity (a.u.) Ca B. atrophaeous 385 390 395 400 405 410 (d avg ~ 1 m m) Wavelength (nm) Single-shot, Single-spore spectrum of aerosolized B. atrophaeous 5
LIBS has demonstrated promising precision Carbon in Steel L. Barrette and S. Turmel, Spectrochimica Acta B , 56 , 715-723 (2001) 6
Nonetheless, calibration remains an issue Precision & Accuracy? 7
An integrated approach to quantitative LIBS 8
What are the puzzles for quantitative LIBS? Upper Size Limit Other Studies ~5 m m for glucose • Single SiO 2 100 particles. E. Vors & particles Silicon Emission (a.u.) Salmon, Anal. Bioanal. Silicon Emission P/B (a.u.) Chem. (2006) ~7 m m for copper • particles. Gallou et al., 10 Aero. Sci. Tech . (2011) ~2.1 m m 1 1 10 100 Diameter Cubed ( m m 3 ) Diameter Cubed ( m m 3 ) 9 Carranza & Hahn, Anal. Chem . (2002)
Upper size limit: The marble theory Physical Limit Molecule Marble ~5 m m Incomplete Complete Sampling Sampling • Consider the physics: Data suggests a rate limitation rather than an energy limitation 10
Upper size limits vs. residence time Exhaust Nd:YAG ~275 mJ @ 1064 nm Delay generator Nd:YAG Mixing- Spectrometer drying iCCD chamber Delay: 15 to Flow Flow Analyte: 70 m s controller controller (Air) Silica spheres (Argon) 2.47 or 4.09 m m 11
Quantify Si emission Si I 288.16 nm Si 4.09 m m 2.47 m m Emission Intensity (a.u.) 35/5 gate 70/20 gate 284 285 286 287 288 289 290 291 292 Wavelength (nm) 12
Quantify Si emission Expect: Ratio of 4.5 Residence time for complete vaporization does not extend upper size limit Now explore rate limits ( m s) 13
Consider the physical processes: Limiting Rates A) Rate-limited processes: Dissociation & diffusion are slow relative to analytical time-scales Plasma Imaging study B) Infinite rates: Dissociation & diffusion are very fast relative to time- scales for analysis 14
Imaging study of single particles Ca II emission 396.85 nm (0-25,192 cm -1 ) Fiber Optic Spectro- iCCD meter 396.2-nm line filter (3-nm fwhm) iCCD Nd:YAG (1064 nm) Pierced Sample Mirror 1064 mirror with Chamber: UV-grade substrate 6-way cross Aerosol Stream (~2- m m glass particles)
Evidence of finite rates: Atomic calcium cloud 2 m s 4 m s 8 m s 15 m s Plasma Residence Time 600 500 Total Detected Calcium Mass (fg) Total D ~ 0.04 m 2 /s 400 detected 300 calcium ~ 15-20 m s 200 100 for dissociation 0 0 5 10 15 20 25 30 35 Delay Time ( m s)
Localized plasma processes and effects Heat Transfer to Particle Mass Transfer to Plasma Calcium emission from a single particle @ 2 m s Plasma-analyte Finite Rates lead interactions are initially to localized limited to region plasma ~ 1/1000 of total plasma perturbations volume & matrix effects 17
Analysis of matrix effects: Experimental details Exhaust Nd:YAG ~275 mJ @ 1064 nm Delay generator Nd:YAG Mixing- Spectrometer drying iCCD chamber Analyte: Al,Lu,Mn & Delay: Al,Lu,Mn + Na 100 ns to Flow Flow 100 m s controller controller (Air) (Argon) 18
Spectral data: Corrected for relative response 19 19
Experimental details: Aerosol generation • ~50-500 nm particles following desolvation Mn/Al/Lu 200 nm Mn/Al/Lu + Na @ 8x mass 200 nm 20
What are the puzzles for quantitative LIBS? Matrix Effects Analyte enhancement & continuum reduction with sodium addition 25 m s 21
Are finite process rates related to matrix effects? Strong matrix effects early in plasma evolution Matrix effects diminish with time Plasma Residence Time ( m s) 22
Are finite process rates related to matrix effects? Break in T decay slope…. Suggests completion of vaporization and equilibration ~20-30 m s Plasma Residence Time ( m s) 23
Understanding plasma / analyte dynamics Early times Later times Bulk plasma T & N e ? Analyte T & N e Equilibration Localized effects between plasma Mass/matrix effects & analyte 24
The path forward for quantitative analysis Must allow sufficient plasma residence time for dissociation, diffusion of heat & mass, and equilibration of analyte species with bulk plasma Diffuse analyte: Local perturbations: Residence Bulk analytical plasma Matrix effects time provides more ideal response 25
What about direct analysis of solids? • LIBS combines the target sampling with the analytical measurement • Different plasma evolution for different materials • Necessitates matrix-matched standards Consider LA-ICP-OES • Uncouples the laser sampling event from the analytical plasma 26
Laser-Ablation LIBS (LA-LIBS) • Separate the laser-ablation process and analytical plasma to uncouple these effects 27
LA-LIBS: Transport efficiency considerations Ablation LIBS Cell Cell Positive Pressure Laser Analytical Ablation Plasma Reduced Pressure • Minimize ablation cell volume • Transport directly to LIBS plasma via carrier gas flow • Vacuum vs. positive pressure? Single-shot crater 28
LA-LIBS: Transport efficiency considerations • Clear maximum in analyte signal • 50% improvement with suction 29
LA-LIBS: Experimental Sample Matrix Sample Fe Mn (%) (%) SM-10 1.96 0.30 Glass 2 Al-alloy 1276-a 0.56 1.01 Cu-Ni alloy 1242 1.80 1.58 Co-Cr alloy Cobalt- Copper- 1297 69.4 7.11 Nickel Chrome Fe-Cr alloy 1761 95.3 0.68 Wide range High Fe Glass 1* 0.64 1.12 of sample Si-K-Ca matrices Glass 2* 0.27 0.66 Si-K-Ca * Courtesy of Anna Matiaske & Ulrich Panne – BAM (Berlin, Germany) 30
LA-LIBS: Sample spectra Mn Mn 31
LA-LIBS: Mn/Fe calibration curve NIST 1242 Co-Cr Cr line interference with Mn line • Al alloy • Cu-Ni • Fe-Cr • High Fe • Glass 32
LA-LIBS: Absolute calibration? • Use LA-LIBS for analysis of soils • Dope with fertilizer to different concentrations of N, P and K • Use a single-laser configuration for total concentration analysis Long-standing interest in LIBS for analysis of soils * In collaboration with Prof. Alejandro Molina and Jhon Pareja University of Colombia - Medellin 33
LA-LIBS: Experimental set-up Laser beam Beam splitter Mirror Pierced Mirror Lens Lens Lens Soil sample Fiber optic Laser Analytical ablation cell LIBS plasma Analytical Laser Ablation Carrier spark gas inlet Plasma Ablation Shaft Split the laser to produce both beams 34
LA-LIBS: Spectral data for soils K K LA-LIBS Direct LIBS 35
LA-LIBS: Calibration results R 2 =0.8843 R 2 =0.6954 LA-LIBS Direct LIBS Superior correlation & near-zero intercept 36
Summary remarks • Particle dissociation, atomic diffusion, & heat transfer have similar, finite time-scales that result in localized plasma perturbations. Temporal considerations are important to minimize matrix effects for quantitative analysis. • LA-LIBS approach can improve LIBS by uncoupling the laser-ablation process from the analytical plasma processes. Moves us closer to the use of non-matrix matched standards. I L S • Understanding of the fundamental processes B will improve LIBS as an analytical method, with implications to the larger analytical community (e.g. ICP-AES & LA-ICP-MS). 37
The future of LIBS: Applications 38
The future of LIBS: Advanced spectral analysis 39
Acknowledgements Graduate students: Michael Asgill Prasoon Diwakar Bret Windom Kris Loper Vince Hohreiter Jorge Carranza Collaborators: Prof. Nico Omenetto Funding: Prof. Kay Niemax • NSF (CHE 0822469) Prof. Ulich Panne • NSF (CBET 0317410) Prof. Alejandro Molina 40
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