1	Laser Ablation ICP-MS: Performance, Problems, Pitfalls and Potential Alan Koenig Research Geologist US Geological Survey Laser Ablation ICP-MS Facility Denver, CO
2	Introduction Laser Ablation ICP-MS can be considered the trace element microprobe The sensitivity of the ICP-MS for all trace elements is well known The use of a high energy laser as a sampling device provides direct in-situ measurements of trace elements in any solid (or gels, tar, goo, etc?) material Advances in laser technology, understanding the ablation process and new calibration standards are improving the accuracy, precision and spatial resolution of the technique
3	Laser Ablation (LA) LA-ICP-OES since 1984, LA-ICP-MS first in 1985 The interaction of a material and a high energy laser produces mechanical breaking of the material resulting in an aerosol of particles The exact mechanism of the ablation is complex (involves lots of physics) Advances in laser technology have improved results tremendously since late 80’s Shorter wavelength lasers are now used The ablated material is swept directly into the ICP from the LA system
4	LA Systems Today all systems are comprised of similar key components- The important differences are the laser wavelength Wavelengths Nd:YAG Solid State Lasers 266 nm, 213 nm (+/- 193 nm) Excited Dimer (Excimer) Lasers 193 nm (ArF gas) Significant research has shown shorter wavelength works best
5	LA Basics The spatial resolution (spot size) is from 2 mm to up to 1200 mm – LA system dependant Average spot analysis time is 2-3 minutes Entire 1 inch segment of sample can be scanned in ~ 20 minutes with 50 mm resolution
6	LA Basics (cont.) The material swept into the ICP from the laser should be small and not melted (IR lasers bad) The shorter wavelength lasers (< 200 nm) produce particles small enough to be properly ionized by the ICP For all wavelength lasers, matrix matched calibration standards are important! To account for variations in the transport and ablation of material and internal standard must be used for precise values
7	Benefits No sample preparation, digestion, contamination for prep Fast- up to 150 spot analyses day Spatial (temporal) information Trace element sensitivity Decreased matrix dependence as compared to SIMS or electron microprobe Flexible sample chamber for various sample geometries
8	Why Laser Ablation?
9	LA instead of Spark Small turnings Assessment of small inclusions Composition zoning Failure analyses of small zones or features
10	Limitations Sensitivity limited (sub ppm-ppb) at very small (< 25 mm) Still requires nearly matrix matched standard (not crystalline matched) For some applications there is still methods development evolution happening… Differences in reporting results Different lasers… Different standards…
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12	General Description Important to remember the LA system is not the detector, it is just a fancy way of getting your sample into the plasma We need to get these two systems to work together… LA optimization ICP-MS optimization Data handling
13	Ablation Mechanisms Thermal vaporization
14	Matrix Decomposition
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16	PFA-100 100 mL/min, 2000 ppm Y
17	Particles from Ablated Y₂O₃ Pellet
18	LA Crater in Glass
19	Key Components of LA System Laser Beam delivery system Sample chamber and stages Imaging system Sample transport Software
20	The Laser Solid-state Nd:YAG 1064 nm, 266 nm, 213 nm and 193 nm (soon) Most compact, reliable and robust Most common About Wavelength 213 nm has been proved to be superior for glasses and other transparent materials Not clear about benefits for metals at 213 nm 266 nm provides higher energy and therefore capable of larger beam sizes
21	The Laser Minimization of melting is important Shorter pulse width reduces thermal effects Femtosecond lasers are being tested High energy density is not necessary Energy and beam quality stability are important
22	Beam Delivery An optical attenuator reduces the beam energy without effecting stability Ablation of some metals can occur at energies as low as 5-10% of max energy The beam delivery system removes residual wavelengths from laser beam, focuses (or expands) the beam and images the beam on the sample The beam delivery system is responsible for adjusting the spot size
23	Imaging System The imaging system provides real-time video through the laser objective of the sample The imaging system should provide optical resolution sufficient to resolve details as small (or smaller) than the smallest spot size Most LA systems provide imaging in reflected or transmitted light
24	Sample Chamber The sample chamber provides a seal from ambient atmosphere and is purged with the carrier gas (Ar or He) Chamber design is a critical component to total transport efficiency
25	Sample “Preparation” Virtually any shape or size up to 1.5” or larger Most LA system sample chambers are flexible in geometry/size of sample Flat surfaces are nice but not a requirement Surface contamination can be “cleaned” by the laser (pre-ablation)
26	Transport System Simple tubing system from cell to ICP Automated valves for sample changing and purging (optional) Valves allow automated control and make the system “idiot-proof” Also allow pre-ablation pass to not be sent to torch Still the largest potential for improvement in sensitivity Current transport efficiency of any LA system is no better than 10-20% (!)
27	Commercial Systems
28	Other Systems
29	System Contamination Memory effects Deposits of material in transport system Cone deposits Lens contamination Performance degradation Cone blockage Lens voltages Detector voltages (lifetime issues)
30	"Robust plasma conditions help minimize..." Robust plasma conditions help minimize sample deposition on cones Cones will clog after some period of time
31	Effect of Spot Size Obviously the large the spot, the more material ablated, the more material in the plasma Large spots average heterogeneity at the small scale- more bulk sampling Large spots require high energy to keep the energy density sufficiently high
32	Spot Size and LOD NIST1261 Steel
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34	Homogeneous Standards?
35	How it actually works Always requires method development Currently most ICP-MS instrument software is not designed to handle “transient” and process LA fully Most LA data are processed off-line
36	Quantification …or “so we can blast nice craters, now what??”…
37	Quantification Calibration to an external standard is required (sensitivity factor) Normalization against an internal standard (IS) is required to account for variation in transport to the plasma (variation in ablation yield) Blank subtraction usually involves the gas blank (laser not ablating)
38	Quantification Process
39	Typical Analytical Sequence
40	Quantification LODs are calculated based on sensitivity and 3 or more times the SD of the blank Signal intensities need to be considered Too high saturates the detector Too low provides minimal or no data Matrix matching of standards is important
41	Quantification Choice of internal standard The IS should behave similarly to many of the analytes of interest Multiple ablation suites may be used for multiple IS elements The value of the IS should be well known in both the sample and the external standard
42	Calibration Strategies Numerous calibration strategies exist for LA-ICP-MS data The level of quantification is determined by the application If zoning data is desired it might only be necessary to provide raw data (line scans) or perhaps normalized ratio data If concentration data are required, the accuracy of the data must be considered The calibration strategy should be well understood and tested
43	Types of Calibration Raw data Useful for quick, qualitative assessment of zoning, elemental content and homogeneity Does not account for sensitivity factors, variation in ablation yield, blank contribution or interferences Ratios of raw signal More careful assessment of qualitative changes in concentration Accounts for changes in signal intensity not related to heterogeneity Does not provide quantitative data
44	Types of Calibration External Calibration with complete unknown sample (no internal standard used) Requires an external standard with well known composition and homogeneity Requires the external standard to be at least an approximate matrix match Relates signal intensity to approximate concentration Provides a “ballpark” assessment of concentration and elemental ratios Does not account for differences in ablation yield between standard and unknown (matrix effects) Does not require a known concentration of the sample
45	Types of Calibration External calibration with internal standardization Requirements same as w/o IS plus known concentration of at least one matrix element in sample Provides fully quantitative information to within the accuracy limited by the certainty of the standard and IS value
46	Other Types of Calibration Total mass summed method Requires total mass range information Assumes a total of 100% of composition is accounted for Well suited for TOF-ICP-MS Simultaneous liquid calibration On-line mass bias correction Peizoelectric balance with simultaneous aspiration Liquid standardization Oils, brines and fluid inclusions
47	LA-ICP-MS of Steels
48	Sample Photos The video imaging system provides real-time imaging of the sample during analysis
49	Applications Ni-base Superalloy
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54	High Purity Ni Standards
55	Performance
56	Problems
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58	Depth Profiling
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60	Non Metal Applications
61	Applications Polypropylene
62	Na Calibration Polypropylene
63	Ablation of “Thin Films”
64	LA-ICP-MS Analyses of Ink Industrial and forensic applications of ink both as bulk material (toner) or ink on paper Requires controlled cratering rate, flexible ablation patterns and high sensitivity
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66	New Directions Isotopic Work Ca, Sr, Cu, Zn, Pb, U plus others U-series Standards and Method Development “Routine” Automated Analyses