ELEMENTAL ANALYSIS OF GLASS EXAMINATIONS (PART 2) Module 4

Transcription

1 ELEMENTAL ANALYSIS OF GLASS EXAMINATIONS (PART 2) Module 4 Tatiana Trejos, M.Sc Florida International University Department of Chemistry and Biochemistry International Forensic Research Institute

2 Outline Solid sampling by LA-ICP-MS Instrumentation considerations Results from selected sets Figures of Merit

3 Disadvantages of conventional ICP- MS Destructive Complexity Time consuming High cost ($$)

4 Laser Ablation Ablation is a progressive and superficial destruction of a material by melting, fusion, sublimation, erosion and explosion 100 µm

5 Laser ablation set-up Nd:YAG laser LASER ABLATION UNIT Monitor Optics CCD camera Ar He Power unit He to ICP-MS Ablation chamber Ar Picture sources: www. newwave.com and

6 Advantages of LA vs Solution No chemical sample preparation involved Reduce sources of contamination (reagents, airborne particulates) Reduce poly-atomic interferences Eliminate risk of Hazard HF Not destructive technique, minimal sample amount ( 0.3 µg vs 2-6 mg )

7 Why LA-ICP-MS? Much easier and faster sample preparation!!

8 When a Contamination Can Occur Sample collection Collection techniques Collection devices Sample storage Positive or negative contamination Sample preparation Reagents Laboratory environment Instrument Most of this sources are eliminated using LA Sampling! Sample measurement Sampling introduction system standards

9 Analytical Challenges Quantitative or Qualitative For Quantitative Analysis: Matrix Match Standard Reference Materials Internal standard External calibration (single vs multiple point) Standard addition Quantitation strategy will depend on the matrix and availability of standards

10 Glass analysis Easily recovered from scene. Composition does not varies over time Typical Fragments mm Sample size requirements Sample manipulation Typical content. for elemental profile Low ppm- % in the solid 29 Si, 42 Ca Internal Standard? Forensic Application Sample microheterogeneity ~50 um representative of bulk Available Availability of Solid Standards Variation within population Good discrimination, variation between lots

11 Quantification of Glass by LA-ICPMS Internal Standard (IS) Normalization relative to intensity of other isotope which concentration is known IS could be the lowest abundant isotope of the major element in the sample Should be selected depending of the class material 29 Si (abundance of 4.7%) Correction for drifts Matrix effects Amount of material ablated and transported Internal standard already in the matrix Glass reference standards available

12 Quantification of Glass by LA-ICPMS Single Standard Calibration: Calibration done using a single point standard (NIST 612) Saves time Standards and unknowns should have similar composition (major elements) Calibration curves Reference standard: NIST 612, 610, 614 Glass standards with known element concentration

13 Analysis Sequence SRM 612 (duplicate) ~ 1.5 minutes per sample Control standard (triplicate) sample A (triplicate) sample B (triplicate) Matrix-related std Up to 6 samples per sequence Calibrator standard SRM 612 (duplicate)

14 Ablation modes LA produce a transient signal Single line Single raster Single spot * Depth profile * Single spot is the preferred method for glass CPS Scale Log CPS Which one is the best? Depends on the matrix and the LA system Species Time /s Time Scale Series2 Series3 Si 29 Series7 Ti 49 Sr 88 Series11 Series12 Series13 Series14 Series15 Series16 Series17 Series18 Series19 Series20 Series22 Series23 Series24 Series25 Series26 Series27 Series29 Series30 Series31

15 Forensic Considerations Analytical performance of the technique: Sensitivity Precision Accuracy Ruggedness Discrimination power Sample heterogeneity Reproducibility Quality Control Quantification techniques Sampling Strategies Data Analysis/Statistics Report and testimony

16 Elemental Analysis of Glass Peer reviewed papers: Hickman, D, Glass types identified by chemical analysis, Forensic Science International, 1986, 33(1), Koons, R; Fiedler, C; Rawalt, R, Classification and discrimination of sheet and container glasses by ICP-AES and pattern recognition, Journal of Forensic Sciences, 1988, 33(1), Becker, S; Gunaratnam, L; Hicks, T; Stoecklein, W. and Warman, G, The differentiation of float glass using refractive index and elemental analysis: Comparisons of techniques, Problems of Forensic Science, Vol. XLVII, 2001, T. Trejos, S. Montero, and J.R. Almirall, Analysis and comparison of glass fragments by laser ablation inductively coupled plasma mass spectrometry (LA- ICP-MS), Journal of Analytical and Bioanalytical Chemistry, 2003, 376: Trejos, T and Almirall, J, Effect of fractionation on the elemental analysis of glass using LA-ICP-MS, Analytical Chemistry, 2004, 76(5) Trejos, T and Almirall, J, Sampling strategies for the analysis of glass fragments by LA-ICP-MS.Part I: micro-homogeneity study of glass and its application to the interpretation of forensic evidence, Talanta, 2005, 67(2) Trejos, T and Almirall, J, Sampling strategies for the analysis of glass fragments by LA-ICP-MS. Part II: sample size and sample shape considerations, Talanta, 2005, 67(2) Latkoczy,C; Dücking, M; Becker, S; Günther, D; Hoogewerff J; Almirall, J; Buscaglia, J; Dobney, A; Koons, R; Montero, S; van der Peyl, G; Stoecklein, W; Watling, J; Zdanowicz, V, Evaluation of a standard method for the quantitative elemental analysis of float glass samples by LA-ICP-MS, J. of Forensic Sciences, 2005, 50 (6), (NITECRIME WORK PRODUCT) K. Smith, T. Trejos, R.J. Walting, J.R. Almirall, A guide for the quantitative elemental analysis of glass using laser ablation inductively coupled plasma mass spectrometry, Atomic Spectroscopy 27 (2006)

17 Evaluation of LA-ICP-MS for glass analysis Optimization and method development Analytical Performance of the technique Definition of sampling strategies Fundamental studies Precision, accuracy Reproducibility Homogeneity Fractionation Discrimination Homogeneity Fragment size study Particle size studies Detection limits Ruggedness Single shot study Overall evaluation of its application to forensic science

18 SET OF GLASSES USED FOR STUDY (239) Evaluation Evaluation of of analytical analytical performance (131) Homogeneity studies (108) Homogeneity studies (108) Autowindows (46 ) Containers (45) Headlamps (45 ) Containers 2 six pack bottles (56 fragments) Windshields 2 windshield panes (28 frag.) Windshields (18) Windows (28) Lenses (34) Reflectors (11) Architectural 2 windowpanes (20 fragments) Tempered glass (4)

19 Sample preparation

20 Time of Analysis Solution-ICP MS LA-ICP-MS # of days Sample preparation Instrume ntal Analysis Data Processing Total time steps in the analytical process Saved 80% of analysis time!!!

21 Figures of merit for LA-ICP-MS Element Mg Al Typical Concentration Range [ppm] LOD (ns-la- ICP-MS) [ppm] Mn 85 Rb 88 Sr 90 Zr 137 Ba 139 La 140 Ce 146 Nd 178 Hf FGS02 Certified values (ppm) LA-ICP-MS (ppm) Std. Dev. (ppm) %RSD %Bias Ti * Mn Rb Measure of precision for Pb 5 Sr Zr Ba La Ce Concentration, ppm Hf Pb ExCal ID LA Methods [Pb] is ~ 2 ppm in 1831 glass

22 Comparison of Discrimination Power of EC and LA-ICP-MS Subset CFS Headlamps Containers Number of samples Number of comparison pairs Undistinguishable pairs by RI Undistinguishable pairs by EC-ICP-MS + RI (p<0.05) Undistinguishable pairs by LA-ICP-MS + RI (p<0.05) * % 100 % 100 % T. Trejos, S. Montero, J.R. Almirall,, J. Anal Bioanal. Chem., 2003.

23 Repeatability Ba137 Pb208 Sr88 Co59 Mn55 Ti49 0 CFS 355 CFS 116 Repeatability studies of five different glass fragments of CFS 355 and CFS 116

24 Definition of sampling strategies Homogeneity Fragment size study Spot size study Trejos T, Almirall JR. Sampling strategies for the analysis of glass fragments by LA-ICPMS. Part I. Microhomogeneity study of glass and its application to the interpretation of forensic evidence,, Tatanta, 2005: 67, Trejos T, Almirall JR.Sampling strategies for the analysis of glass fragments by LA-ICPMS Part II. Sample size and shape considerations, Tatanta, 2005:67,

25 Homogeneity at micro-scale (0.5 1mm) 0.78 mm 0.65 mm 50 µm

26 Homogeneity study of glass Is the elemental composition of a glass homogenous within the piece of interest?? Glass window Windshield Containers Tempered glass

27 Architectural Glass B (10 different fragments) A (10 replicates on the same fragment) 1 2

28 t-test results for architectural 1 <x> MNRB 5.0 P value = For normal distribution <x> 2s 95% p> 0.05 <x> 3s 99% p> Count Count SAMPLE All ratios were indistinguishable for this set

29 Windshields Side A Plastic film Side B 28 samples from each windshield: 14 samples A (7 replicates and 7 dif. Fragments) 14 samples B (7 replicates and 7 dif. Fragments)

30 Elemental Ratio profile for windshield elemental ratios U/Th PbTh Hf Zr CeLa BaLa SnBa SrZr MnRb CoSr TiMn AlCa AlK Li/B 1: replicates from same fragment 2: different fragments from windshield

31 Homogeneity of containers

32 Sampling of containers A) Within a single bottle: First Approach: 10 different fragments 10 measurements in a single fragment Second approach Comparison of 3 replicates of each fragment B) Between the four pack 3 replicates of 3 fragments from each bottle C) Outside - Inside

33 Homogeneity single bottle Ratio p > 0.01 p >0.01 single wine bottle single beer bottle AlK = BLi = = MgAl TiMn = = SrZr = ZrSn = = UTh = = CeLa = = PbHf = MnRb = = BaLa = = FeMn RbSr = =

34 Homogeneity within single bottles Natural heterogeneity in the elemental composition within the bottle Manufacturing process more prone to contamination 7 8 Variation due to heterogeneity in the sample is bigger than instrumental variation Use MSE of fragments within a single bottle as match criteria!! In casework: guarantee representative sampling from known samples!!

35 Comparison between wine bottles Least Squares Means MNRB W1 W2 W3 w4 SAMPLE The 4 bottles from the same pack were undistinguished using Mean Squared Error (MSE)

36 Comparison of elemental profile: different sources Fe/Mn Pb/Hf TiMn MnRb AlK B/Li ZrSn Mg/Al SrZr Rb/Sr CeLa U/Th B1 B2 B3 B4 B5 B6 W1 W2 W3 W4

37 Tempered Glass Glass thickness compression tension Ordinary flat glass that has been exposed to a tempering process: Additional strength Safety breakage patterns compression Differences in refractive index within a single fragment

38 Tempered Glass Side A Side B Are there variations on elemental composition due to tempering process? Samples from vehiclewindows: CFS 143, 165, 167 and 594. No significant difference in he elemental composition!!

39 Float glass MUST take into account that Sn content is significantly different in the float side vs the other sides

40 Recomendations Conduct a good sampling is essential to arrive to accurate conclusions Take as many samples as practical Characterize the known source in order to account for natural heterogeneities particularly for containers Sample glass from different areas Windshields: be aware of differences between elemental composition of outer and inner pane sides Analyze each glass fragment at least in triplicate

41 Fragment size study Does fragment size really matter? Glass fragments collected from the crime scene and from suspect(s) and victim(s) are random in size Recovered fragments 0.1-1mm in length Known fragments > 3mm Bulk digestion analysis (6mg crushed glass) LA : craters of ~50µm (250ng) SRM 610 and SRM different sizes (0.1-6mm)

42 0.78 mm 0.65 mm

43 Mean values and standard deviation of Sr/Zr ration for 7 glass fragments (SRM 612) 2.4 true ratio Sr/Zr ratio fragment size ANOVA results showed no significant difference between the fragments (p>0.05)

44 Quality control Perform tuning of the instrument every day and make sure specifications are within the range Sensitivity Oxides Double charge U/Th Standards as calibrators Standards as control checks Document quality control charts Precision Accuracy

45 Laser type affect results Practical considerations Flat beam profile better than gaussian profiles 193 nm > better results than > 213 > 266nm Femtosecond lasers better results than nanosecond lasers Particle size The smaller the size of particles produced by the LA, the better results Gas environment For the majority of LA systems, a He environment improved the sensitivity and precision Size of the spot Small as possible (less sample consumption) At the end, try to minimize causes of fractionation and improve sensitivity! Internal standard Minimize matrix effects and improve data Trejos T, Almirall JR. Effect of Fractionation on the Forensic Elemental Analysis of Glass Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Anal Chem, 2004:76,

46 LA-ICP-MS: Summary LA allows rapid direct sampling of solid materials For some of these matrices, such as glass, LA have been deeply studied and have demonstrated to be the technique of choice to conduct trace elemental analysis The application of LA to forensic evidence will be mainly limited to the natural heterogeneity of the matrix, variation within different sources and availability of standards and/or quantification strategies Proper sampling and statistical analysis are fundamental to arrive to accurate conclusion

47 Current Status of LA-ICP-MS Technique has been strongly evaluated for glass Publications for peer review (>150 papers on analytical applications) Inter-laboratory studies (NITECRIME group and NIJ funded working groups) Continuous improvement in LA instruments It is in use in forensic labs