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Application of Cathodoluminescence in Paint Analysis Scanning Microscopy Volume 6 Number 3 Article 4 6-30-1992 Application of Cathodoluminescence in Paint Analysis W. Stoecklein Bundeskriminalamt R. Göbel Bundeskriminalamt Follow this and additional works at: https://digitalcommons.usu.edu/microscopy Part of the Biology Commons Recommended Citation Stoecklein, W. and Göbel, R. (1992) "Application of Cathodoluminescence in Paint Analysis," Scanning Microscopy: Vol. 6 : No. 3 , Article 4. Available at: https://digitalcommons.usu.edu/microscopy/vol6/iss3/4 This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Scanning Microscopy by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. Scanning Microscopy, Vol. 6, No. 2, 1992 (Pages 669-678) 0891- 7035/92$5. 00 +. 00 Scanning Microscopy International, Chicago (AMF O'Hare), IL 60666 USA APPLICATION OF CATHODOLUMINESCENCE IN PAINT ANALYSIS W. Stoecklein* and R. Gobel Bundeskriminalamt, Forensic Science Institute D-6200 Wiesbaden, Federal Republic of Germany (Received for publication May 3, 19921, and in revised form June 30, 1992) Abstract Introduction When solving cases of burglary or investigating Paints and their components often play a decisive ship collisions, the forensic scientist frequently has to role as evidence in solving crimes. The forensic scien­ examine several layers of paint of the same color, often tist analyzing such material must not only determine the white. As a rule, the usual microscopic and spectro­ origin of unknown paints (identification and classifica­ scopic methods [fluorescence microscopy, FT-IR (Four­ tion) but in addition compare traces collected from ob­ ier Transform Infrared Spectroscopy), pyrolysis, GC/MS jects in the suspect's possession or found in his environ­ (Gas Chromatography - Mass Spectrometry), etc.] are ment with those traces secured at the scene of the crime. not sufficient to prove that the paint traces found on the In most cases, these comparative analyses are scene, which are often only available in the form of conducted to answer the question whether the incriminat­ fragments, originated from the same source as the refer­ ing material and the comparative sample are identical or ence material. It is possible to achieve convincing proof whether significant differences exist between the two . of this using either an optical cathodoluminescence­ If the traces prove to be analytically and morphological­ microscope or a cathodoluminescence-scanning electron ly identical, the forensic scientist is also required to microscope, both of which can be coupled to a visible furnish a statement on the evidential value of his results (VIS)-spectrometer . for court interpretation purposes. This can only be achieved through a comprehen­ sive analysis of the paint's composition, provided suffi­ cient data on the frequency of the paint or its compo­ nents of use or occurrence is available . Paint compari­ sons are especially difficult if the trace specimen con­ sists of several coats of paint of similar color - especial­ ly white and cream colors. These flakes often occur in connection with burglaries (e.g., paint flakes from doors and windows) and ship collisions. Flakes with up to 35 layers of a similar color are not unusual. The individual layers can be between 20 µm and 200 µm thick and of­ ten, thicknesses within small sections of the same coat Key Words: Scanning electron microscopy, cathodo­ can vary by more than 100 % . When such flakes are ex­ luminescence (CL), paint, pigments, forensic science, amined with the help of optical microscopic methods optical CL microscopy. (polarization, interference contrast, fluorescence, bright­ field and dark-field illumination), it is almost impossible to determine the number and thickness of the individual layers. In addition, it is often the case that layers dis­ integrate due to chipping and poor adhesion, so that only • Address for correspondence: fragments are found. Assigning these fragments to com­ Wilfried Stoecklein, parative specimens with intact layer structures is often Kriminaltechnisches Institut, impossible under these circumstances and using the Bundeskriminalamt, methods mentioned above. Thaerstr. 11, There is only a limited number of pigments and D-6200 Wiesbaden, Germany extenders that can be used in white, grey, and cream­ Phone No.: 0611-552609 colored paints (Table 1). Therefore, many white paints FAX No.: 0611-553875 produced by different manufacturers do not differ in pig- 669 W. Stoecklein and R. Gobel ment composition. This fact must not be neglected in Table 1. CL colors of white pigments and extenders. paint examination, especially as analysis of binders by BL: blue, GR: green , RE: red , YE: yellow, OR: or a nge, FT-IR (Fourier transform infrared spectroscopy) micros­ BR: brown , WH: white . copy is often not feasible for multilayered systems be­ WHITE PIGMENTS cause the thickness of individual layers is not sufficient. BL GR YE OR RE BR WH NO CL Nevertheless, further differentiation of paint pig ­ AND EXTENDERS /l- ZnS ment layers exhibiting identical phases is possible since SPHALERITE ® individual pigment phases may differ, depending on the ZnS production process used and subsequent treatment, for WURTZITE ® instance, in grain size distribution, crystal habit, and ZnO ® especially in the kind, and number, of their foreign ZINC WHITE ® ® atoms and other lattice imperfections. The analysis of CoCOs ® ® ® these latter characteristics also provides a basic approach CALCITE CoMg(COa}, to the problem of establishing identity in comparative DOLOMITE ® examinations. The classic methods of pigment analysis TiO, (X-ray diffraction, polarization microscopy) are not ca­ RUTILE ® pable of proving the presence of lattice imperfections. li02 ® Cathodoluminescence methods, however, have that ANATASE capability. Boso, ® Cathodoluminescence (CL) is the emission of ra­ PbCOs diation in the region of visible light and neighboring CERRUSITE ® wavelengths following excitation by electrons where Pb (COa),(OH). these, originating from a cathode and accelerated in an LEAD WHITE ® electric field, strike upon an insulator or semiconductor. SiO2 ® This paper describes CL-analysis methods which assist QUARTZ ® in solving complex paint analysis problems. CHINA CLAY ® PbSO4 Experimental Procedure ANGLESITE ® CoSO,·¥ Equipment and measurement procedures GYPSUM ® Cathodoluminescence analysis of paints can be performed in either a scanning electron microscope List of Abbreviations (SEM) or in a relatively simple specimen chamber de­ vised for optical microscopy. BSE: Backscattered Electrons Optical cathodoluminescence microscopy EDX: Energy Dispersive X-ray Spectrometry FT-IR: Fourier Transform Infrared Spectroscopy Vacuum specimen chambers with observation win­ GC-MS : Gas Chromatography - Mass Spectrometry dows and cold-cathode electron guns have been de­ PM: Photomultiplier scribed by a number of authors (Sippel, 1965 ; Geake et PMT: Photomultiplier Tube al., 1970; Bartz, 1973; Dudley, 1975 ; Barker and SE: Secondary Electrons Wood, 1987 ; Marshall, 1988) . We use a contrasting VIS : Visible unit according to Bartz (1973) produced by the firm WDX : Wavelength Dispersive X-ray Spectrometry Leica (Gobel and Patzelt , 1976). Specimens with a max­ imum diameter of 35 mm and up to 18 mm thick can be section is directly exposed to the electron beam . The placed in the chamber (Fig . 1). The chamber is evacu­ organic matrix of the uncovered area is pyrolytically de­ ated with a rotary pump . 10-15 kV are applied across composed within a few minutes . The interaction of the the discharge tube. The working pressure (approximate­ electron beam with the paint produces decomposition and ly 5.5 x 10-2 mbar) needed in the cham·ber in order to discoloration and the differences in decomposition of the obtain the optimal electron beam current density , and various paint layers can be utilized for material contrast thus the maximum luminescence intensity, is controlled and visualization . If the paint specimen is exposed to by means of a needle valve. It is advisable to use a pro­ the electron beam for only a short time (less than one tective gas (nitrogen or helium) as the specimens consist­ minute), which is more than sufficient for a lumines­ ing of organic matter oxidize within a very short time. cence analysis in the microscope, the depth of electron The beam currents and beam current densities utilized 2 beam induced damage remains below 1 µm. Light pol­ (0.3-0.8 mA = approximately 5 x 102 A/m ) pose a high ishing with diamond paste removes all damage. In order thermal strain on the paint specimen and the embedding to obtain qualitative and quantitative results using CL agents. In order to avoid thermal damage, the polished spectra, the Bartz cell is coupled to a microspectrophoto­ surfaces are covered with aluminum foil (0.03 mm thick) meter (Fig . 1). The Zeiss microspectrophotometer so that only a small area (5-10 mm2) on the flake cross- UMSP 80 used for CL measurements is equipped with an 670 Cathodoluminescence in paint analysis 3 6 4 5 12 2 Figure 1 . ~chematic diagram of a CL detection system FRONT for the v1s1ble range . The basic components are a curr ently available stage (Leica contrasting unit ; TOP VIEW Germany) for optical CL microscopes and a microspec­ trophotometer (Zeiss UMSP 80; Germany ). I : Photo ­ multiplier tube (PMT)-Detector ; 2: Filter ; 3: UV-VIS­ Figure 2. Schematic diagram of a SEM system
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