Sample Introduction Systems SOLID LASER ABLATION -II

Transcription

1 Sample Introduction Systems SOLID LASER ABLATION -II

2 Laser ablation systems Variable power output control is an absolute must! This is required in order to achieve a suitable sampling rate, which is dependent on the sample matrix and laser beam size. E.g. an analysis using a pit or laser beam diameter of 60 microns, an ablation lasting 60 seconds results in a pit 60 microns (@1 Hz) deep (sampling rate of 1 micron/second)

3 Laser pits Optimal geometry consists of a symmetrical pit and flat bottom These can only result from a homogenized beam energy profile

4 Laser ablation systems In order to maintain reproducibility, energy meters are installed Energy meters are either placed in the beam path, or laser beam is split so that a known fraction is directed towards the meter while the remainder is focused onto the sample

5 Laser ablation systems

6 Laser ablation systems New Wave Research (Division of ESI) UP213 nm UP193 nm

7 Recently developed 193 nm laser systems Analyte 193H Resonetics 193 nm

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18 Laser ablation cells Sample is contained in an air-tight cell through which a flow of gas (He) carries the ablated sample aerosol to the ICP (through tygon tubing). The ultimate goal is to design a cell that attains 100% transport efficiency since at this level laser-induced elemental fractionation (LIEF) processes taking place in the laser cell and transport system will be reduced- hence, improved ion signals and results shall be attained

19 Laser ablation cells use of He carrier gas

20 Laser ablation pits are the pits? Simonetti et al. (2008)

21 Laser ablation pits are the pits? zircon Simonetti et al. (2005)

22 Laser ablation cells Basic requirements (somewhat conflicting) Large enough to house variably sized solid samples Small enough volume in order to attain rapid sample washout and high sensitivity

23 Laser ablation cell Standard Cell of UP213 system

24 Ablate grains in epoxy mounts vs. thin sections?

25 SuperCell Accessory Option 1 2 3

26 Laser ablation cells Supercell Sample stage with slide insert Slide insert removed

27 Laser ablation cells Supercell

28 Laser ablation cells Supercell

29 Simonetti et al. (2008)

30 Analyte 193 nm system Frames Cell The FRAME defines the gas volume, mixing and flow dynamics of the chamber as seen with the teardrop frame. The sample chamber can be fitted with FRAMES that accommodate various shapes, sizes and quantities of samples. The use of FRAMES allows the sample chamber to be reconfigured by the user to with the latest designs that yield optimal signal strength, washout time and signal stability.

31 Resonetics Laurin two volume sample cell Laurin Sample Cell Minimal elemental and isotopic fractionation. Smooth signal down to 1Hz laser repetition while retaining washout performance for optimal depth profiling and spatial resolution when scanning. Sensitivity and fractionation independent of sampling position in the cell

32 Resonetics Laurin two volume sample cell Müller et al. (2009) JAAS

33 Resonetics Laurin two volume sample cell Effective volume of ~1-2 cm 3, funnel-shaped inner cell (hole at the bottom) Funnel is above a sample holder in an air-tight sample cell box with a volume of 380 cm 3 Funnel remains stationary and sample box moves Samples are held and pushed upwards by springs and thus eliminating the need for refocusing of the laser and viewing system

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36 Importance of laser ablation cell design - LIEF Sylvester (2008) GAC Short Course volume 40

37 Importance of laser ablation cell design - LIEF Günther & Koch (2008) GAC Short course volume 40

38 Sylvester (2008) GAC Short Course volume 40

39 Laser ablation analyses Calibration choice of an appropriate standard Ideally you would use a solid sample that is well characterized and homogeneous and has the same (matrix) properties as your sample i.e. matrix-matched (Sylvester, 2008) At times, this can prove somewhat difficult.

40 Laser ablation analyses Internal Standardization in general, the signal from the laser is dependent upon the amount of material ablated However, the amount of material ablated van vary due to: variations in laser-sample coupling (how effectively laser ablates sample), variations in laser power, variations in transport efficiency

41 Laser ablation analyses Internal Standardization Typically, use an element contained in your standard material ( external standard ) that has been accurately determined by another in-situ, high resolution method of analysis (e.g. electron microprobe) this is then referred to as the internal standard (IS) In minerals, we often use CaO (wt%) or SiO 2 (wt%) obtained from electron microprobe analysis (but can use any other element) as long as it is well characterized and doesn t overlap with major ICP-MS spectral interferences This calibration process allows for any variations in ablation, transport, and laser power to be accounted for

42 Laser ablation analyses Theory behind trace element abundance calculations: where: conc ni = (cpsn ij /abundance j ) / (yield ni ) conc ni = the concentration of element i in analysis n cpsn ij = the mean count rate (background-subtracted) of isotope j of i in analysis n abundance j = natural abundance of isotope j yield ni = cps per ppm of element i in analysis n

43 Laser ablation analyses The yield of element i in analysis n is determined by: where: yield ni = yield ns * Int(yield ni / yield ns ) std yield ns = cps per ppm of the internal standard s in analysis n Int(yield ni / yield ns ) std = the ratio of the yield of element i in analysis n to the yield of the internal standard s in analysis n, interpolated over the standard analyses.

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45 GLITTER On-line, interactive data reduction software package for laser ablation-icp- MS instrumentation GLITTER - GEMOC Laser ICPMS Total Trace Element Reduction software package

46 Laser ablation STANDARDS element abundance determinations For silicate minerals, typically use NIST (National Institute Standard Technology; index.html) fused glass beads (Standard Reference Material SRM) SRM NIST 610 elements ~500 ppm SRM NIST 612 elements ~50 ppm SRM NIST 614 elements ~1 ppm

47 Laser ablation STANDARDS - element abundance determinations APATITE Durango (e.g. Trotter & Eggins, 2006, Chem. Geol.; Simonetti et al., 2008, Archaeometry); GARNET PN2 (Canil et al., 2003, CJES) ZIRCON 91500, Mudtank, BR266, GJ-1 (Wiedenbeck et al., 1995; Geostand. Newsletter; Hoskin & Schaltegger, 2003, Mineral. Society Reviews; Jackson et al., 2004)

48 Laser ablation STANDARDS isotopic analysis Common Pb: SRM NIST 610, 612 U-Pb dating: ZIRCON 91500, BR266, Mudtank, Temora, Plešovice, GJ-1 TITANITE Khan APATITE Madagascar, Emerald Lake, and Kovdor - Hf isotope measurement: - ZIRCON BR266, Plešovice

49 Laser ablation STANDARDS The GLITTER software is loaded with several commonly used standard reference materials, e.g. SRM NIST glasses However, new external standards can be uploaded within the software

50 Laser ablation Laser ablation sample cell should house the unknown sample and standard material simultaneously Typically, a sequence of laser ablation analyses of unknowns are bracketed (before and after) by analyses of standard material referred to as Standard- Sample Bracketing technique

51 Laser ablation Time-resolved spectra (time vs. ion signal intensity cps) Analysis includes an interval prior to the start of the laser ablation process for measurement of the background or baseline (cps) usually in the order of 45 to 60 seconds

52 Laser ablation Background measurement is followed by ablation of sample for a duration of ~60 seconds or less for most applications The duration of the ablation experiment is a function of: Abundances of elements under investigation Coupling efficiency of material being ablated Thickness of your sample section

53 Laser ablation In GLITTER, time-resolved spectra is accessed via the Review Signal Selection option In this window, you select your background, sample ion signal plateau, and the isotope to monitor

54 Laser ablation GLITTER

55 Laser ablation Lack a well characterized standard? One option is to report elemental ratios and not absolute abundances However, when doing so, user is assuming that elemental ionization is identical for all elements during the ablation process (in cell), transport and within the plasma itself This assumption holds true or is within the typical analytical uncertainty associated with laser ablation analyses (i.e. 5 to 10%) when Mass difference between elements being investigated is kept to a minimum!

56 Laser ablation sampling strategy? Single spot vs. Rastering? LIEF (laser induced elemental fractionation) is a progressive change in the ratios of measured signals of certain element pairs with the increasing number of laser pulses applied to the sample LIEF may occur at: Ablation site (result of evaporation/condensation processes) During transport to the plasma In the plasma itself (incomplete volatilization of particles)

57 Laser ablation Previous studies have indicated that complete atomization and ionization of laser ablated particles is subject to their size and composition, and temperature (plasma) and particle trajectory Overall, LIEF consists of two components: constant bias resulting from non-stoichiometric ablation; time-dependent fractionation that results in a change in the particle composition, number and size during the laser ablation process

58 Laser ablation particle size distribution Several previous studies have shown that the size distribution of particles produced during laser ablation exert a significant control on the nature and size of elemental and isotopic fractionation Major factors affecting the size distribution of particles making their way to the ICP source are: the fluence wavelength pulse duration of the laser the aspect ratio of pit composition of the sample carrier gas the size-dependent transport efficiency of the ablated particles.

59 Laser ablation It has also been shown that particle size distribution changes during a single analysis subject to the parameters listed above but it also depends on absorption of laser radiation by the sample (Horn et al. 2001) and the sampling strategy, i.e. scanning (raster) or stationary (single spot) ablation.

60 Laser ablation sampling strategy Sampling strategies using different types of laser. SEM images of static (left image) and scanning (right image) ablation craters in NiS produced with a nonhomogenized 266 nm Nd:YAG laser (Kosler, 2008)

61 Laser ablation sampling strategy Optical microscope images of laser craters produced by static ablation (side view, crater diameters are 30 and 60 µm) and box raster laser scanning in zircon ablated by a non-homogenized 266 nm Nd:YAG laser (modified from Košler & Sylvester 2003),

62 Laser ablation sampling strategy SEM images of box raster and single laser pits produced by a nonhomogenized 196 nm femtosecond laser in zircon (left image) and a corresponding detail of the bottom of the laser crater (right image).

63 Laser ablation sampling strategy The major difference between stationary vs. rastering sampling modes is a rapid decrease in particle size and signal intensity for ablation with stationary beam compared to more steady signal and less change in particle size distribution while the laser beam scans across the sample surface.

64 Temporal variations of mass concentrations of particles determined by a TSI DustTrak 8520 laser photometer which has a capability to detect particles >0.3 µm (data from Košler et al. 2005a). The aerosol was generated by ablating silicate NIST-612 glass in He atmosphere with a 266 nm Nd:YAG laser fired at 10 Hz repetition rate.

65 Laser ablation sampling strategy Scanning Almandine Static Pyrope

66 Laser ablation sampling strategy Stationary vs. Scanning (rastering) of almandine and pyrope garnet: The laser ablation conditions were similar to those used for trace element analysis in silicate minerals The laser was a 213 nm Nd:YAG, 10 Hz repetition rate and 3 J/ cm 2 for 90 seconds (total of 900 pulses) Produced round laser pits that were 25 and 40 µm in diameter in the garnets 150x25 and 150x40 µm trenches were produced by using identical ablation conditions while moving the stage under stationary laser beam at a speed of 10 µm/s. Volume of ablated material calculated from dimensions of craters The results suggest that the volume of ablated material while using laser scanning mode is ca 15% larger compared to the volume of material removed during stationary ablation, irrespective of laser beam diameter and color of the garnet.

67 Laser ablation sampling strategy However, The total number of counts obtained from laser scanning during ablation of different sample matrices and for different lasers and ablation conditions is % larger compared to signal intensities obtained from stationary ablation sampling The ablation time required for achieving comparable precisions by the two sampling strategies is at least 20% shorter for laser scanning and accordingly, the spatial resolution of laser scanning ablation is similar, or better, compared to the stationary ablation.

68 Laser ablation sampling strategy & imaging prerequisite SEM photo of ablated zircon Cathodoluminescence photograph of same zircon grain

69 Laser ablation - summary Laser ablation is an excellent tool for semiquantitative to quantitative analysis of many geological/solid samples Realistically, you should expect anywhere between 5 to 10% RSD (relative standard 2 sigma level) for precision on elemental abundance determinations Benefits of spatial resolution outweigh any drawbacks Choice of appropriate, matrix-matched external standard material, optimization of instrument parameters (both laser and ICP-MS instrument), and selection of sampling strategy are critical parameters for accuracy and precision

70 Laser ablation data GLITTER Elemental abundances are reported in an excel compatible.csv file One Sigma Error - The one sigma error estimates use N counting statistics on the signal and background counts, propagated through the equations. An assumed 1% uncertainty (relative) on the elemental concentrations of the reference material, and a 3% uncertainty (relative) on the values of the internal standard is propagated throughout the calculations. These values can be changed, or turned off completely by the user via the Options window.

71 Laser ablation data - GLITTER The Minimum Detection Limit - The detection limit (MDL) at the 99% confidence level is determined by Poisson counting statistics: MDL = 2.3* (2B) where B is the total counts in the background interval.