SEM Analysis of Paints, Tapes, and Polymers

Transcription

1 SEM Analysis of Paints, Tapes, and Polymers FBI Laboratory Page 1 of 13 1 Introduction Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) is an instrumental technique used to define and compare the structure and elemental composition of paints and coatings, tape components, and adhesives and polymers. The characterization and comparison of structure and elemental composition is useful for determining whether items may have been formulated by the same production process. 2 Scope This document is an outline of SEM methods applied to the following materials: paint (e.g., automotive paints and architectural paints), tape (e.g., duct tape and electrical tape), and polymers (e.g., plastics and adhesives). The particular methods employed for each examination will depend upon sample suitability (e.g., sample size), analytical requirements, and equipment availability. The term SEM refers to the entire analytical system, including the SEM and an Energy Dispersive X-ray Spectrometer (EDS). The methods of analysis that are more commonly used are presented; however, other methods are available that may be used at the operator=s discretion. 3 Principle The SEM is a high power microscope that can image and analyze minute amounts of material in a non-destructive way. An electron source in the SEM serves as an excitation source by focusing and scanning a beam of electrons on the sample, resulting in electrons being dislodged from within the sample. Multiple signals are produced, including backscattered electrons, secondary electrons, and characteristic X-rays. Through detection of the backscattered and secondary electron signals, SEM can be used to image layer structure and topography. In EDS, the characteristic X-rays from within the sample are detected and identified by the EDS spectrometer for multi-elemental analysis. Therefore, SEM/EDS can be used to characterize the elements present from the pigments and additives used in paints, tapes, and polymeric materials.

2 Page 2 of 13 4 Equipment/Materials/Reagents a. Scanning Electron Microscope (JEOL JSM6300) with Backscattered Electron (BE) detector (Model Tetra, Oxford Instruments) b. Energy Dispersive X-ray Analyzer ISIS L300 (Oxford Instruments) c. Spectral Library Identification and Classification Explorer (SLICE) (EDAX) d. Stereomicroscope (6X to 100X) with appropriate lighting e. Compound light microscope f. Digital camera adapted for stereomicroscope or light microscope g. Glass microscope slides h. Cover slips i. Compressed gas duster j. Acetone (Reagent grade) k. Ethanol (Reagent grade) l. Liquid nitrogen m. Distilled water n. Cotton tipped applicators o. Manganese (Mn) pure element reference material (Alfa Aesar) p. Adhesive-backed and double-sided adhesive tape q. Graphite paint (Ernest F. Fullam, Inc.) r. Embedding molds (Ted Pella, Inc.)

3 Page 3 of 13 s. Epo-kwick epoxy resin and hardener (Buehler) t. Oven with temperature capability to cure epoxy resin, approximately 100 F -150 F (40 C -65 C) u. Microtome (Leica Ultracut UCT) v. Carbon coater (Cressington 208carbon or equivalent) w. Vacuum evaporator (Denton Vacuum DV-502A or equivalent) x. Carbon rods (SPI Supplies) y. Aluminum wire (Ernest F. Fullam, Inc.) z. Ultrasonic cleaner (Fisher Scientific) aa. bb. cc. dd. ee. ff. Scalpel with blades Pyrolytic carbon planchets (Ernest F. Fullam, Inc.) Durotak (National Starch) X-ray energy slide rule (Oxford Instruments) Wood applicator sticks Square plastic petri dishes 5 Standards and Controls 5.1 Standards Manufacturer-supplied and commercially available paints, tapes, and polymers are maintained in reference collections within this laboratory. When used in casework, these materials are validated and documented in accordance with the =s Procedure for Verification of Reference Standards and Procedure for Documentation of Specimens Used as Known Standards.

4 Page 4 of Performance Check A manganese (Mn) pure element reference material, embedded and polished, is analyzed each day that casework is performed in order to measure the performance of the EDS system. Refer to the Performance Monitoring (QA/QC) Protocol for the instrument for complete details. 6 Calibration Not applicable. 7 Sampling As determined by macroscopic and/or microscopic examinations, for physically consistent specimens originating from the same scene, a sampling or portion of one sample may be chosen as representative of the others for analysis by this technique. 8 Procedure 8.1 Sample Preparation 1. The choice of a specific method for sample preparation will depend on the size and condition of the specimen, as well as the analytical request. It may be necessary to use multiple preparation methods in order to determine all sample characteristics. For the accurate comparison of elemental composition and structure, samples must be prepared in as similar a manner as possible. Refer to the SOP for General SEM Methods for complete details of the steps discussed below. 2. Samples are examined with a stereomicroscope. If extraneous (contaminant) materials are present, they can often be removed by a method such as shaving or Adabbing@ with adhesive tape. Extraneous surface material may also be removed with a cotton-tipped applicator moistened with water. For adhesives, adhering extraneous material may be removed by picking it out with a fine-tipped scalpel. When extraneous materials cannot be removed, their location should be noted during light microscopy or backscatter electron SEM observations. An attempt to avoid areas with extraneous material can then be made during

5 Page 5 of 13 subsequent SEM analysis. 3. A paint smear is composed of commingled particles and fragments. Initially a light microscopic search is made for the largest fragments present. Particles that are approximately 50 µm can often be selected individually and attached to a mount for analysis. It is also possible to lift a collection of deposited particles with a sticky material, such as tape adhesive, which can be attached to an SEM mount. The particles can then be analyzed individually. 4. Small samples or shavings may be attached directly to an SEM mount, such as a pyrolytic carbon planchet, using double-sided tape or a thin adhesive layer. In the case of tape, a backing can be adhered with its own adhesive. Adhesives may be spread into a thin layer of uniform thickness directly onto a pyrolytic planchet. In order to facilitate sampling, adhesives may be cooled with liquid nitrogen, causing them to temporarily lose their tackiness. 5. Multi-layered samples, such as automotive paints and some tape backings, are often embedded in order to provide mechanical support for the examination and analysis of the layers. This is accomplished by placing the sample (and a sample identification label) in a mold, positioned to reveal the structures of interest when subsequently cross-sectioned or polished. Small samples are attached to the bottom of the mold with a thin adhesive layer. Liquid epoxy is added to the mold slowly so as to prevent air bubble entrapment, and the amount of epoxy added should fill the mold entirely. The epoxy will cure at room temperature or alternatively, it can be placed in an oven and allowed to cure. 6. Once embedded, samples are cross-sectioned by hand with a scalpel, by microtomy, or by polishing, in order to produce a flat block or section of the sample. Microtomy, is the most commonly used cross-sectioning technique. It is accomplished by first trimming the block face to an area approximately 2 x 3 mm and then clamping the block in a block holder. The block holder is attached to the microtome arm and appropriate adjustments are made (e.g., knife angle, cutting speed, and cutting thickness). The block face is then trimmed with the knife with rough cuts followed by fine cuts. If sections are desired, they can be removed from the knife face; if a faced-block is desired, the block is removed from the holder and processed for analysis. 7. It may be necessary to apply a conductive layer such as carbon to the sample surface to eliminate sample charging. Carbon rods are placed in the electrodes of the vacuum evaporator, the sample is placed on the base plate, and the evaporator is cycled to high

6 Page 6 of 13 vacuum. Current is induced through the carbon rods in order to evaporate the carbon onto the sample. The chamber is then pressurized, and the sample is removed. 8. If fabrication markings are of interest, the specimen is photographed under a light microscope at a magnification suitable to demonstrate surface features. Application of a thin metallic film may be used to enhance surface features. This is achieved by placing an additional piece of sample onto a glass microscope slide and coating with aluminum (Al) by vacuum evaporation. The procedure is analogous to that of carbon-coating, except that Al wire, rather than carbon rods, is used by balling it up and placing it in a tungsten basket installed in an electrode set in the vacuum evaporator. 9. When analyzing multiple samples, a map is constructed identifying sample location on or within the sample holder. This may be in the form of a sketch, a photomicrograph, or a captured video image. 8.2 Analytical Procedures 1. Structural imaging: a. Light microscopy can demonstrate layer structure as well as some structural detail within each layer of a multi-layered sample. Either a thin cross-section or the prepared block can be observed. b. Reflected light microscopy, often following aluminum coating, is useful for characterizing the surface or cross-section of a tape backing. c. A backscatter electron image, which is based on average atomic number of the matrix, is useful for defining layers and structures and for defining distribution of particulate components. 2. Collection of a Abulk@ EDS spectrum permits determination and comparison of average elemental composition of a material. A bulk spectrum is collected after the raster area of the SEM is selected to yield the largest sample area possible. If it is not possible to select one large area, several small areas are analyzed, and the data from each are summed. 3. Collection of a Aparticle@ EDS spectrum permits determination and comparison of individual particle compositions (e.g., aluminum flakes, decorative flakes, CaCO 3, BaSO 4,

7 Page 7 of 13 and clay). Particle analysis is performed when bulk analysis alone is insufficient to perform the adequate structural and compositional discrimination. It can be used to associate elements in a specific particle or structure which have previously been identified by a bulk analysis and to determine the presence of trace elements, should those elements be located in specific particles or structures. A particle analysis is performed by directing the electron beam of the SEM directly onto the structure of interest, either by increasing the magnification or placing the scan generator in spot mode. 4. Once an X-ray spectrum is collected, a qualitative analysis is performed in order to determine the elements present. 5. Similarity of elemental composition may be determined by direct spectral comparison using SLICE. The compositional similarity of questioned materials to reference materials may also be performed using SLICE. 9 Instrumental Conditions The following operating conditions are meant as general guidelines for starting conditions. Actual requirements may vary as determined by specific analytical needs. Beam voltage: Beam current: Live counting time: Pulse processor time constant: 25 kv adjusted to yield 25 to 35% deadtime 200 seconds mid-range Generally, changes in the suggested instrumental conditions, listed above, are required under the following circumstances: a. The beam voltage is increased when higher energy X-ray line excitation is required. b. The beam voltage is decreased when greater spatial resolution is required. c. The pulse processor time constant is lengthened when greater spectral resolution is required. d. The pulse processor time constant is shortened when a greater count rate is required (e.g., for trace element analysis or construction of elemental distribution maps).

8 Page 8 of 13 e. The EDS detector-to-sample distance is reduced to increase X-ray collection efficiency. f. The spectral energy display scale is expanded when sufficient detail is not evident. g. The beam current is increased when the X-ray count rate is too low. The beam current may be increased by decreasing the condenser lens current or increasing the final aperture size. h. The beam current is decreased when the X-ray count rate is too high. The beam current may be decreased by increasing the condenser lens current or decreasing the final aperture size. 10 Decision Criteria Comparisons of elemental composition can be made by the direct spectral comparison of the materials analyzed under similar analytical conditions. Depending upon their correspondence, the spectra are characterized as similar, generally similar, or different. a. Spectra are considered similar if they only exhibit channel-to-channel intensity variations. b. Spectra are considered generally similar if they exhibit only a few slight differences. These differences may be qualitative or quantitative. Permissible qualitative differences in materials considered to be generally similar may consist of the absence of trace amounts of common elements. Permissible quantitative differences may consist of slight ratio differences in the peaks heights of major or minor elements. c. Spectra are considered different if they exhibit distinctive, significant differences. These differences may be qualitative or quantitative. Significant qualitative differences may consist of the absence of trace amounts of rarely occurring elements or the absence of a significant peak from a more commonly occurring element. Quantitative differences may consist of significant ratio differences in the peak heights of major and minor elements. 11 Calculations Not applicable. 12 Uncertainty of Measurement

9 Page 9 of 13 Not applicable. 13 Limitations The methods described in this guide may have some limitations, including the inability to detect elements in trace concentrations, the need for a conductive coating of the sample, and the discoloration of materials by irradiation. a. The information available from a specimen may diminish as its size is reduced and its condition degrades. As specimen size is reduced or the material becomes degraded, it may no longer be representative of the original material. b. A disadvantage of embedment is the inability to remove a sample from most embedding materials after analysis. c. Although the natural X-ray line width is approximately 2 ev, EDS resolution is generally no better than approximately 140 ev. As a result, overlap of peaks in the EDS spectrum of materials containing several elements may occur. The following are some commonly occurring overlaps encountered in EDS: TiKα/VKα, VKβ/CrKα, CrKβ/MnKα, MnKβ/FeKα, FeKβ/CoKα, PbMα/SKα/MoLα, BaLα/TiKα, KKβ/CaKα, ZnLα/NaKα, PKα/ZrLα, and AlKα/BrLα. d. Any individual particle or fragment from an inhomogeneous material may not be compositionally representative of the bulk and therefore would not be expected to produce spectra similar to the bulk material. 14 Precautionary Statements If spectral differences are detected, it is likely that the materials that produced them are not similar in composition; however, several alternative explanations are possible: a. Dissimilar sample geometry. b. Spectral collection from non-representative areas of an inhomogeneous sample.

10 c. X-ray fluorescence contributions from elements in adjacent structures. d. X-ray contribution from extraneous material. e. Contribution of carbon from a carbon SEM mount. FBI Laboratory Page 10 of Safety General precautions common to electron microscopy laboratories include minimization of contact with cryogens, venting or filtering of roughing pump discharge, and avoidance of direct exposure to beryllium metal. Use universal precautions when handling potentially biohazardous materials. Take standard precautions for the handling of chemicals and sharp cutting instruments. Refer to the FBI Laboratory Safety Manual and appropriate MSDSs for additional required practices and precautions. 16 References A Guide to Scanning Microscope Observation, JEOL Documentation of Specimens Used as Known Standards; FBI Laboratory, Quality Assurance Manual FBI Laboratory Safety Manual Garratt-Reed, A.J. and Bell, D.C. Energy Dispersive X-ray Analysis in the Electron Microscope, 2003, BIOS Scientific Publishers Limited, Oxford. General SEM Methods; FBI Laboratory, General Chemistry Subunit SOP Manual Goldstein, J.I., Scanning Electron Microscopy and X-ray Microanalysis, 2 nd ed., 1992, Plenum Press, New York Invitation to the SEM World, JEOL

11 Page 11 of 13 Jenkins, Jr., T.L., Elemental Examination of Silver Duct Tape Using Energy Dispersive X-ray Spectrometry. Proceedings of the International Symposium on the Analysis and Identification of Polymers, FBI Academy, Quantico, VA, July 31- August 2, 1984, Kee, T.G., The Characterization of PVC Adhesive Tape. Proceedings of the International Symposium on the Analysis and Identification of Polymers, FBI Academy, Quantico, VA, July 31- August 2, 1984, Keto, R.O., Forensic Characterization of Black Polyvinyl Chloride Electrical Tape. Proceedings of the International Symposium on the Analysis and Identification of Polymers, FBI Academy, Quantico, VA, July 31- August 2, 1984, Lee, R.E., Scanning Electron Microscopy and X-Ray Microanalysis, 1993, Englewood Cliffs, NJ Performance Monitoring Protocol (QA/QC) for the JEOL JSM-6300 Scanning Electron Microscope (SEM); FBI Laboratory, - Instrument Operation Subunit SOP Manual Performance Monitoring (QA/QC) Protocol for the Oxford ISIS Energy Dispersive X-ray Spectrometer (EDS); FBI Laboratory, - Instrument Operation Subunit SOP Manual Postek, M.T., Howard, K.S., Johnson, A.H., and McMichael K.L., Scanning Electron Microscopy, 1980, Ladd Research Industries, Inc. Scientific Working Group on Materials Analysis (SWGMAT). Forensic Paint Analysis and Comparison Guidelines. Forensic Science Communications, 1999; 1(2) Scientific Working Group on Materials Analysis (SWGMAT). Standard Guide for Using Scanning Electron Microscopy/X-ray Analysis Spectrometry in Forensic Paint Examinations. Forensic Science Communications, 2002; 4(4) Smith, J. The Forensic Value of Duct Tape Comparisons. Midwestern Association of Forensic Scientists, Inc. Newsletter, January 1998; 27(1): Vaughan, D., Ed., Energy Dispersive X-ray Microanalysis: An Introduction, 1989, Kevex Instruments Inc., San Carlos, CA

12 Page 12 of 13 Verification of Reference Standards; FBI Laboratory, Quality Assurance Manual Ward, D.C. A Small Sample Mounting Technique for Scanning Electron Microscopy and X-ray Analysis. Forensic Science Communications, 1999; 1(2)

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