Introduction to Glass Examinations

Transcription

1 Module # 3 Introduction to Glass Examinations Fracture Match, Isotropy, Sn, Curvature, Bugs Fluorescence, Density, Refractive Index Calibration of the GRIM2/GRIM3 System 1

2 Problem A. The accurate association of a questioned glass fragment(s) to a source of glass. The questioned glass is analytically indistinguishable from the glass source that was submitted as a standard B. The assignment of a value of strength for that association. The frequency of occurrence for the RI is 4 % and the frequency of occurrence for the [Zr] is 8 %... or The glass evidence (strongly) supports the hypothesis of contact with the source of the broken glass rather than not 2

3 Glass Analysis Visual Inspection of Known/ Questioned for Fracture Matches Comparison of Glass: Physical Properties Optical Properties Chemical Properties Classification of Glass into End Use Category Discrimination between glass samples Interpretation and Value of Results 3

4 Glass Fracture Characteristics (Terminology, Significance, Limitations) Crime Scene Reconstruction (Direction of Force, Sequence of Shots, Laminated Glass Considerations) 4

5 Radial and Concentric cracks Elasticity permits bending until radial cracks form on the opposite side of the force Continued force places tension on the front surface (force side), forming the concentric cracks 5

6 Conchoidal Striations (fractures) Source: Forensic Examination of Glass and Paint. Caddy, B. Ed.; Taylor & Francis: London,

7 Direction of force determination The perpendicular edge always faces the surface on which the crack originated Source: Forensic Examination of Glass and Paint. Caddy, B. Ed.; Taylor & Francis: London,

8 Direction of force determination The perpendicular edge always faces the surface on which the crack originated Source: Forensic Examination of Glass and Paint. Caddy, B. Ed.; Taylor & Francis: London,

9 The 3R Rule Radial cracks form a Right angle on the Reverse side of the force Need to know: 1) Which side of the window is examined 2) Whether a radial or a concentric edge is examined Source: Forensic Examination of Glass and Paint. Caddy, B. Ed.; Taylor & Francis: London,

10 Conchoidal Striations Source: Forensic Examination of Glass and Paint. Caddy, B. Ed.; Taylor & Francis: London,

11 Exceptions to this rule Tempered glass because it dices and a frost in the middle prevents the observation Source: Forensic Examination of Glass and Paint. Caddy, B. Ed.; Taylor & Francis: London,

12 Other exceptions Very small windows held in a tightly held frame windows broken by heat or explosion Laminated glass is a special case: There are two glass sheets sandwiching a vinyl film. The sheet opposite the force exhibits the fracture marks. 12

13 Plastic Identification and Sn surface Plastic can be eliminated by testing for indentation by a needle point Fluorescence upon short wave (254nm) illumination of an original surface can detect the Sn contamination on one side of float glass. 13

14 Interference Objective An Interferometer can be used to detect the most minimal curvature on the glass surface Curvature indicates possible sources: windshield containers other non-flat glass source 14

15 Thickness Considerations Tempered glass is greater than 3.0 mm thick Vehicle side windows are typically mm thick Tolerances: 3.0 mm ( mm) 4.1 mm ( mm) Typical auto standard is +/ inches. 15

16 Bugs - Manufacturer Identification 16

17 Bugs -DOT numbers on vehicle glass 17

18 18

19 Density Measurements High correlation between D and RI observed Literature review Slater and Fong (1982) Stoney and Thorton (1985) Koons and Buscaglia (2001) 19

20 RI (n D ) and Density correlation RI (Nd) v Density (USA casework) Density RI (Nd) 20

21 Density Method Sink/Float Vary the mix of two liquids having different densities until the K fragment floats in the mixture Place Q in same solution and determine if it floats At that point, the density of the liquid and glass (K and Q) are equal 21

22 Density Determination Once fragments match, their densities are equal and the density of the liquid can be determined Plummet and balance method Digital densitometer method Do we really need to know the density? Is it better as a comparative exam? What is the variability of a source? How reliable are results for small fragments? 22

23 Density Liquids Methylene Iodide d ~ 3.32 g/cm 3 Bromoform d ~ 2.85 g/cm 3 Acetylene Tetrabromide d ~ 2.96 g/cm 3 Sodium Polytungstate d ~ 2.89 g/cm 3 Na 6 H 2 W 12 O 40 x H 2 O (water soluble) 23

24 Non-Toxic Float liquid Poly Gee Brand; Sodium Polytungstate density = 2.89 g/ml non-toxic adjustable density (with addition of water) reusable (evaporate water) ~ $ / lb sold as solid or liquid from: Geoliquids Inc., 1618 Barclay Blvd. Buffalo Grove, Illinois (708)

25 Density Discrimination of Samples with Similar RIs SAMPLE SOURCE TEMPER FLOAT nd nc nf DENSITY W-8 RESIDENCE N N W-38 RESIDENCE N N Data from S. Ryland, FDLE 25

26 Characterization of Glass Fragments Refractive Index Determination (GRIM and GRIM2, Foster and Freeman) Elemental Composition by sensitive method ( XRF, ICP-AES, ICP-MS) Application of Statistical Techniques (Classification Schemes, Descriptive Statistics (Graphical), Match Testing, Informing Power Statements) 26

27 Snell s Law Light Angle of Refraction I Angle of Incidence Glass R RI = sin sin I R V V Air Glass The ratio of the wave s velocity in a vacuum (~air) to the wave s velocity in a transparent medium. 27

28 Becké Line Method (1892) When the objective of the microscope is raised (focus up), a bright line moves into the direction of the material of higher R.I. Once the line disappears or doesn t move, the R.I. of the oil can be measured by a refractometer. The Becké line is best observed with contrast microscopy. 28

29 Emmons Temperature Variation Method (1930) Emmons observed the variation of R.I with temperature ( temp, R.I.) for oils but not (much) for glass. He used a circulating water bath to heat the oil with an immersed glass in it and watched for the Becké line to disappear. (He then measured the oil R.I. with a refractometer at that temperature) 29

30 Emmons Double Variation Method Used with a Mettler Hot-Stage for better temperature control and involved the variation of both the wavelength of the light coming through the sample and the temperature. (Description of method in handout, Theory description in Saferstein s Handbook) 30

31 Refractive Index RI of glass is known to be affected by both wavelength and temperature n match temperature wavelength n 25 D 31

32 Dispersion (V) The relationship of the indices at different wavelengths. V = (n D -1) (n F -n C ) where: n D is measured at 589 nm (Sodium D line) n F is measured at 486 nm (blue) and n C is measured at 656 nm (red) 32

33 Value of Dispersion dispersion data does not provide added discrimination... large error inherent in dispersion data dominates the error associated with the k value Cassista et al, Canadian Society of Forensic Science Journal, Vol. 27, No. 3,

34 Refractive index oil immersion method Immerse the selected fragment in a previously calibrated silicon oil and mount on a slide for viewing under phase contrast microscopy. Change the temperature of the oil in a controlled fashion to reach the endpoint where the RI of the oil is equal to the RI of the glass (viewed as maximum contrast). Use the calibration curve for the oil to interpolate the RI of the glass. 34

35 Phase Contrast Microscopy 35

36 Calculating the Mean Match Temperature Heating Cycle Cooling Cycle Mean Match Temperature Image Contrast Temperature 36

37 Oil Immersion at the Match Temperature 37

38 Calculating the Mean Match Temperature Heating Cycle Cooling Cycle Mean Match Temperature Image Contrast Temperature 38

39 Oil Calibration RI Glass Oil Match Temp Temperature 39

40 Oil Calibration 1.54 A Oil RI 1.52 Glass B Oil Match Temp 1.49 C Oil Temperature 40

41 Locke Silicone Immersion Oils Oil A n 25 D =1.540 n D range Oil B n 25 D =1.520 n D range Oil C n 25 D =1.481 n D range (Locke Scientific or Foster and Freeman) 41

42 Oil Operating Temperatures A Oil RI C C B Oil RI C C C Oil RI C C 42

43 Oil Calibration Curve for GRIM2 Locke B Oil M ean M atch Temperature vs Refractive Index 1.53 B 1, M M T = B 3, M M T = Refractive Ind B2, MMT= Locke B Glass Standards Correlation Coefficient= RI= MT(b) + a, where a= and b= B 6, M M T = B11, MMT= B 7, M M T = B12, MMT= B10, MMT= M ean M atch Temperature ja, 3/93 43

44 RI determination by GRIM2 Good Precision: SD s of over 5 hr. period (using optical reference glass) and over 5 days. Fast analysis routine (~ 5-10 min. / reading) Semi-automated, reduced operator bias Improved data handling, reduces transcription errors, facilitates data manipulation Published by ASTM E-30 as a standard method of analysis. 44

45 Proficiency Test Results (R.I.) 1993 (23 of 71 labs used GRIM or GRIM2) 1994 (31 of 88 labs used GRIM or GRIM2) 1995 (39 of 95 labs used GRIM or GRIM2) (7 labs reported elemental analysis) (7 labs reported use of statistical analysis) 1996 (46 of 106 labs used GRIM or GRIM2) (40 labs reported elemental analysis) 45

46 Two Problems Classification: The ability to use some measured characteristics of a questioned object to place it into a product use class. (Need to be accurate over long term, characteristic compositions for each class) Discrimination: The ability to distinguish between two or more objects within the same product use class. (Need to be precise, small variability within sample and wide range across all samples) 46

47 Analytical Requirements for Classification Classification requires good accuracy, but not necessarily good precision. The best elements for classification are those added by the manufacturer to effect a desired end-use property, ex. B in headlamps, Ba in TV screens or Ce in decolorized glass. Interpretation of results requires access to a high-quality database. 47

48 Analytical Requirements for Discrimination Discrimination requires excellent precision, but does not generally require good accuracy. The best variables for discrimination are those not tightly controlled by the manufacturer, but for optimum discrimination, the maximum number of variables should be determined. All measured variables must be indistinguishable for a common source to be indicated. 48

49 Factors Affecting Classification Capability Analytical accuracy over long time period Consistency of analytical method over time Within-class heterogeneity (not within-sample) Distinctiveness of variables (not number) Completeness and accuracy of database Contamination control 49

50 Factors Affecting Discrimination Capability Short term analytical precision Heterogeneities of both sample and source Range of measured values across a class Number and independence of measured parameters Contamination removal 50

51 Annealing Schedule Start temperature Ramp rate 1 Level 1 Dwell time 1 Ramp rate 2 Level 2 Dwell time 2 Ramp rate 3 Level 3 25 C 600 C/h 555 C 2 h - 4 C/h 500 C 10 min - 4 C/min 25 C W. Stoecklein, BKA 51

52 Container RI homogeneity Sample RI SD a b

53 Characterization of glass sources It is useful to determine 1) variation within a single source and 2) variation within all sources 1) Within source studies (literature) (containers, float sheet, vehicle windows, headlamps) 2) Reference Databases (FBI - RI and ICP-AES from casework, FIU - RI and ICP-MS from surveys) 53

54 Frequency Distribution (USA vs UK) % % UK USA % % % % USA UK % % Frequency Refractive Index

55 Frequency Distribution (Dade vs USA) % % 8.000% 6.000% 4.000% Dade Dade USA 2.000% 0.000% Refractive Index 55 Frequency

56 Headlamp Glass Within and Between Distribution Range of R.I. within one lamp (10 readings) Lens Reflector units units Range of R.I. between all lamps measured (73) Lens Reflector Overall units units units Minimum: Maximum: See: Ojena and DeForest, JFS,

57 ?? Questions to Ponder?? Does the measurement of dispersion increase the discrimination capability over nd alone? Does the measurement of density increase the discrimination capability over nd alone? Does the measurement of change in nd with annealing increase the discrimination capability over nd alone? Are these all measures of the same thing? 57

58 Why was research initiated into the use of elemental analysis for glass in the 1980s? First, there was interest in classifying specimens as to source to rule out or confirm alibis. Later, there was concern that manufacturers had improved their quality control and refractive index was not as discriminating as it had been historically - greater discrimination power was needed. 58

59 Classification of Sheet and Container Glasses using %Mg 80 No. of Samples Containers Sheets Magnesium (%) 59

60 Refractive Indices of Auto Side Windows Number of Samples 0 Refractive Index, nd 60

61 Discrimination Potential of Elemental Composition Analysis Coleman and Goode (1973) Used NAA looking at 25 elements Were able to distinguish all but two pairs from 539 different glass samples Al, As, Ba, Ca, Hf, Mn, Na, Rb, Sb, Sc, and Sr provided the most discrimination 61

62 Discrimination Potential of Elemental Composition Analysis 38 of 40 windows were distinguished by EDX Andrasko et al, JFS, of 28 windows were distinguished by SSMS Haney, JFS, of 81 windows were distinguished by EDX Reeve et al, JFS, of 81 vehicle windows were distinguished by ICP Koons et al, J Analytical Atomic Spectrometry, 1991 Headlamps, Containers, Non-Vehicle and Vehicle 62