FBI Laboratory Chemistry Unit Paints and Polymers SOP Manual PPSU 203-0.doc Issue Date: 06/21/06 Revision: 0 Page 1 of 10
X-Ray Diffraction Analysis of Tapes 1 Introduction X-ray diffractometry (XRD) is an instrumental technique used for characterizing, comparing, and identifying solid materials. Diffraction is directly related to a compound's crystalline nature, in that a specimen will scatter an incident beam of X-rays only at specific angles, which is dependent upon the spacing between the crystal planes that exist within the compound. There are published diffraction patterns for tens of thousands of materials that are updated periodically. Comparison to these powder diffraction records can facilitate the identification of crystalline compound(s) present in a specimen.
2 Scope This procedure describes sample preparation and instrumental parameters for the XRD analysis of pressure sensitive tapes.
3 Principle When a sample with a crystalline morphology is placed in a beam of monochromatic X-ray radiation (of wavelength λ ), the X-rays will be diffracted away from the sample at an angle theta (Θ) as defined by the crystallographic pattern formed by the atomic structure of the compound. This phenomenon occurs in accordance with Bragg's Law: nλ = 2d sin Θ Therefore, every crystalline material yields a unique X-ray diffraction pattern. Based on these principles, X-ray diffractometry can be utilized to identify different tape components such as inorganic pigments, fillers, additives, and polymeric materials. Samples can be analyzed intact, where both the adhesive and backing are characterized simultaneously, or the backing may be separated and analyzed independently.
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FBI Laboratory Chemistry Unit Paints and Polymers SOP Manual PPSU 203-0.doc Issue Date: 06/21/06 Revision: 0 Page 2 of 10
4 Equipment/Materials/Reagents a.
PANalytical X=Pert PRO MPD X-ray Diffractometer system (or equivalent) with copper (Cu) anode X-ray tube
b.
Haskris Model R100 recirculating water chiller (or equivalent)
c.
Sample holder and single-crystal silicon wafer (supplied by instrument manufacturer)
d.
Corundum (silicon carbide) disc, (National Institute of Standards Technology SRM 1976 or equivalent)
e.
Silicon, pressed powder sample (supplied by instrument manufacturer)
f.
Stereo microscope (6X to 50X) with appropriate lighting
g.
Stainless steel ruler with metric (1 mm) and English (1/64") graduations
h.
Scalpel with blades
i.
Glass microscope slides
j.
Tweezers
k.
Disposable wipes
l.
Cotton-tipped applicators
m.
Acetone (Reagent grade)
n.
Chloroform (Reagent grade)
o.
Hexane (Reagent grade)
p.
Methanol (Reagent grade)
q.
Powder Diffraction File, Inorganic Volume, Sets 1-34 published by the International Centre for Diffraction Data, Newtown Square, Pennsylvania, 1989, or newer edition This controlled document. Do not photocopy. This is is anauncontrolled copy of a controlled document.
FBI Laboratory Chemistry Unit Paints and Polymers SOP Manual PPSU 203-0.doc Issue Date: 06/21/06 Revision: 0 Page 3 of 10
r.
High Score search software, Version 1.0d or newer (PANanalytical B.V.)
5 Standards and Controls 5.1 Standards Manufacturer-supplied and commercially available tapes, polymers, and adhesives are maintained in reference collections within this laboratory. When used in casework, these materials are validated and documented in accordance with the Chemistry Unit=s Procedure for Verification of Reference Standards and Procedure for Documentation of Specimens Used as Known Standards. 5.2 Performance Checks As a performance check, Corundum and Silicon (Si) standards are analyzed once per week by personnel in the Mineralogy Subgroup of the Trace Evidence Unit (TEU). These standards require no preparation and do not expire. Refer to the Quality Assurance section of the Trace Evidence Unit procedure X-Ray Powder Diffractometry Using X=Pert MPD for further information.
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 specimen may be chosen as representative of the others for analysis by this technique.
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8 Procedure 1.
Sample preparation will depend on sample type, size, and condition. If the sample is contaminated or too limited in size for a meaningful analysis, document as such in the case notes.
2.
All tapes are generally analyzed in “as-received condition.” For example, duct tape is analyzed intact (adhesive layer/scrim fabric/polymeric film backing). Additionally, the film backing may be analyzed separately.
3.
a.
Prior to analysis, clean the tape backings using a cotton swab or disposable wipe and as necessary, an appropriate solvent (e.g., methanol or acetone).
b.
To analyze an intact specimen, obtain a sample area large enough to fill the sample holder. Using the sample holder as a template, cut a sample of the specimen with a scalpel blade. If the specimen is a partial roll of tape, the roll should be unwound approximately three to four inches before sampling in order to eliminate stretching or distorting the film.
c.
The same sample preparation should be used for the analysis of the tape backing. The adhesive (and scrim fabric, if present) is removed using a suitable solvent (e.g., hexane or chloroform for rubber-based adhesives, acetone for acrylic-based adhesives).
d.
Specimens must be placed on the sample holder in a manner that results in a flat, uniform surface for X-ray beam interaction. Specimens containing adhesive may be affixed directly onto the silicon wafer of the XRD sample holder. To ensure a flat sample when only the film backing is analyzed, the sample may be held in place with a small amount of adhesive applied to the sample holder outside the sampling area where the X-ray beam impinges. The sampling area is approximately 13 mm by 15 mm (determined by the beam mask size, the fixed divergence slit width, and the initial o2Θ).
Ensure that the water chiller is turned on and is operational. Observe the instrument for any error messages. If an error is indicated or the chiller is not functioning properly, see the appropriate Operator’s Guide for troubleshooting assistance.
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4.
Ensure that the quality assurance procedures have been conducted. Refer to the instrument logbook for this information. If a QA/QC test has not been performed, consult the Trace Evidence Unit (TEU) procedure, X-Ray Powder Diffractometry Using X’Pert MPD, or a member of TEU’s Mineralogy Subgroup for specific instructions on this procedure.
5.
Analyze the sample(s) by XRD using the instrument parameters in section 9.
6.
If phase identification in the resulting diffraction pattern is necessary, use the Powder Diffraction File (available in either text or software formats), or analyze an appropriate standard under the same operating conditions as the unknown sample.
7.
Upon completion of the analysis, remove all samples from the sample chamber and return the instrument to the standby conditions.
9 Instrumental Conditions The following instrumental conditions apply for all standards and samples analyzed by XRD: Generator Settings: Standby: Analysis: Anode material: Filter type:
40 kV 10 mA 45 kV 40 mA Copper (Cu) Nickel (Ni)
Scan range: Step size: Scan type:
8 – 80 ° 2Θ 0.0170 ° 2Θ Continuous
Incident Beam Path: Soller slits: Fixed divergence slit: Fixed anti-scatter slit: Fixed beam mask:
0.04 rad. ¼° ½° 10 mm
10 Decision Criteria a.
If significant differences are observed between the diffraction patterns of two (or more) samples being compared, then it may be concluded that the specimens are not consistent with one another.
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b.
If no significant differences are observed between the diffraction patterns of two (or more) samples being compared, then it may be concluded that they are consistent with one another.
c.
If XRD is being used to classify a material, the diffraction pattern should compare favorably to a corresponding reference powder diffraction file, within a reasonable degree of scientific certainty.
d.
Proper sample preparation is critical for XRD analysis. The following are some common sample preparation problems encountered in XRD analysis: i.
The displacement of the surface of the specimen away from the diffraction axis is the primary source of error in the measurement of diffraction peak positions. The sample must be spread flat on the specimen holder without extending above or below the instrument's diffraction axis.
ii.
Any sample preparation technique should minimize introducing stress and orientation effects (preferred orientation) while maximizing a random distribution of crystallites with a uniform particle size (1-50 µm).
11 Calculations Not applicable.
12 Uncertainty of Measurement Not applicable.
13 Limitations a.
On average, the lower limit of detection of a component in a mixture is approximately 1%. This limit will vary depending on composition, degree of crystallinity, and crystallite size.
b.
Limited sample size may preclude analysis by this technique. This controlled document. Do not photocopy. This is is anauncontrolled copy of a controlled document.
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c.
Contamination of the adhesive may not be readily visible under macroscopic or microscopic examination, yet can bias a comparative analysis with a new and unused sample.
d.
The standard procedure may not provide accurate or reproducible results when one or more of the following circumstances occur: i.
The specimen is not homogeneous.
ii.
Crystallites are not randomly oriented; preferred orientation or stress exists within the specimen.
iii.
The specimen is amorphous.
iv.
The specimen contains variable or mixed, hydrated forms where each form yields a distinct diffraction pattern.
14 Precautionary Statements a.
If two (or more) components in a sample yield a similar diffraction pattern, the peaks may be overlayed; therefore, comparing the composite pattern with known powder diffraction patterns of individual compounds may be difficult.
b.
The elasticity inherent in some polymers may cause the flattened backing sample to curl up if insufficient mounting media is used to hold the sample flat and stationary.
15 Safety Take standard precautions for the handling of chemicals and sharps. Use universal precautions when handling potentially biohazardous materials. Refer to the most current revision of the FBI Laboratory Safety Manual and appropriate MSDS for further details. Personal radiation monitors (dosimeters) are administered by the Health and Safety group to monitor exposure to ionizing radiation. Operators should familiarize themselves with the PANalytical X=Pert PRO MPD safety manual before using the instrument.
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16 References Buhrke, V.E., et al., editors. A Practical Guide for the Preparation of Specimens for X-Ray Fluorescence and X-Ray Diffraction Analysis, Wiley-VCH, New York, 1998. Cullity, B.D. Elements of X-Ray Diffraction, 2d ed., Addison-Wesley, Reading, MA, 1978. Documentation of Specimens Used as Known Standards; FBI Laboratory, Chemistry Unit Quality Assurance Manual FBI Laboratory Safety Manual Jenkins, R., and de Vries, J.L. An Introduction to X-ray Powder Diffractometry. N.V. Philips Gloeilampenfabrieken, Eindhoven, Holland, pp.1-40. Johnston, J. Pressure Sensitive Adhesive Tapes: A Guide to Their Function, Design, Manufacture, and Use; Pressure Sensitive Tape Council, Northbrook, IL, 2000. Sata, D., ed. Handbook of Pressure Sensitive Adhesive Technology, 2d ed., NY: Van Nostrand Reinhold, 1989. Skeist, I., ed. Handbook of Adhesives, 3d ed., NY: Van Nostrand Reinhold, 1990. Smith, J. The Forensic Value of Duct Tape Comparisons. Midwestern Association of Forensic Scientists, Inc. News Letter, 1998: 27(1); 28-33. Snodgrass, H. Duct Tape Analysis as Trace Evidence. Proceedings of the International Symposium on the Forensic Aspects of Trace Evidence, FBI Academy, Quantico, VA, June 24-28, 1991, 69-73. Turley, J.W. X-Ray Diffraction Patterns of Polymers. International Centre for Diffraction Data, Newtown Square, PA, 1965. Verification of Reference Standards; FBI Laboratory, Chemistry Unit Quality Assurance Manual X=Pert PRO System - User=s Guide, 3d ed., Koninklijke Philips Electronics, The Netherlands, 2002. This controlled document. Do not photocopy. This is is anauncontrolled copy of a controlled document.
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X-Ray Powder Diffractometry Using X’Pert MPD; FBI Laboratory – Trace Evidence Unit SOP Manual
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