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Pigment Analysis in the Forensic Examination of Paints. I. Pigment Analysis by X-Ray Powder Diffraction
1.F.S.S. ORIGINAL PAPERS
Pigment Analysis in the Forensic Examination of' Paints. I. Pigment Analysis by X-Ray Powder Diffraction C. J. CURRY, D. F. RENDLE and A. ROGERS The Metropolitan Police Forensic Science Laboratory, 109 Lambeth Road. London, United Kingdom SEI 7LP
Abstract A brief description of pigment analysis by X-ray powder diffraction is presented. Diffraction data obtained by DebyeScherrer powder photography on some seventy-one pigments have been collected. The d-spacings of the three most intense diffraction lines of each pigment are listed in a form to allow identification using the Hanawalt search-and-match technique. Journal of the Forensic Science Society 1982; 22: 17.3-177 Received 6 March 1981
Introduction X-ray powder diffraction (XRD) has long been used for the analysis of pigments and dyestuffs (Susich, 1950) and specifically for the analysis of pigments in paints (Scott, 1969). I n this laboratory X R D has been used routinely in the analysis of paints for a number of years and a relatively small number of inorganic materials (e.g. titanium dioxide, lead chromate, the iron oxides and hydrated iron oxides) have been found to produce various colourations in paints. As a result, identification of these pigments and extenders by X R D has become a relatively simple matter. However, replacement of inorganic pigments by organic pigments in modern paints has posed certain problems for the X-ray analyst: (a) Organic pigments are generally poorer scatterers of X-rays than their inorganic counterparts, yielding fewer diffraction data. This makes positive identification difficult. (b) Some organic pigments, because of their intense colouration, tend to be present in much lower concentrations than inorganic pigments in paints. This results again in weak diffraction patterns which give rise to uncertainties and ambiguities in identification. (c) Unit cells of organic pigments are generally much larger than those of inorganic pigments which means that the majority of their diffraction lines will be confined to low 8 angles. I n Debye-Scherrer powder photography (the technique used) this angular region is unfortunately the one of highest background level and this may mask some of the weaker diffraction lines. (d) Organic pigments may be present in paints in non-crystalline (amorphous) forms and as such they cannot be characterised by X-ray diffraction. (e) A large number of organic pigments may be derived from one parent compound, e.g. the phthalocyanines, where chemical substitutions or metal insertions produce dramatic colour changes yet sometimes only subtle changes in diffraction pattern.
Powder diffraction patterns of a number of organic dyes and pigments are included in the Powder Diffraction File (PDF, 1979) e.g. Toluidine Red, C,,H,,N303 File No. 25-1879, but this number falls far short of what could be described as a comprehensive collection of data. Consequently individuals or organizations interested in pigment analysis by X R D have had to amass their own diffraction data, a task the size of which has depended upon just how comprehensive a data collection was desired. One useful source of reference to X R D data on synthetic dyes and pigments has been compiled by Whitaker (1977). The object of this present study was to generate and tabulate additional pigment X-ray powder diffraction data for use in the identificationof pigments in vehicle paints which occur as casework specimens in this laboratory. This report presents the results of X-ray powder diffraction studies on a total of seventy-one pigments known to be used in vehicle and/or household paints and kindly supplied by various manufacturers.
Materials and Methods Diffractometry and Guinier powder photography yield data which are very accurate but both methods, in particular diffractometry, require large specimens. Casework paint flake specimens are usually very small and are best examined using Debye-Scherrer powder photography. Since diffraction patterns from these specimens are the ones with which the pigment data are to be compared, Debye-Scherrer powder photography was also used for recording the pigment diffraction patterns. Powdered pigments were loaded into thin-walled 0.3mm internal diameter glass capillary tubes (PANTAK (EMI) Ltd., Windsor, Berks). Diffraction mode: 114.6mm dia. Debye-Scherrer powder cameras Film: Kodak K O D I R E X or NO-SCREEN Radiation: Iron-filtered Cobalt K a from fine-focus X-ray tubes powered at 35 kV 34 mA Exposure : 2 hrs Spacings of diffraction lines on film (d-spacings) were measured using a Huber film-measuring device 622 (Gordon (Churt) Ltd., Farnham, Surrey) and their intensities on a Joyce-Loebl Mark I I I C double-beam recording microdensitometer. The intensities were normalised and in the tables of results they are recorded as subscripts to the d-spacings. The maximum intensity 10 is denoted by x. Errors in the measurement of intensities and d-spacings are 0-1A (d > 10A) or 0.01A (d < 10A). respectively f 1 and either Results and Discussion The number of diffraction lines visible on film varies considerably from pigment to pigment. Accordingly the number of d-spacings and intensities recorded for each pigment varies but in each case as many spacings and intensities as possible have been measured and recorded on file cards in the format used in the PDF (1979). The diffraction data so recorded can then be used to identify pigments from their diffraction patterns using a search-andmatch technique known as the Hanawalt method (PDF, 1979). Very briefly, observed and standard data are compared for the three most intense diffraction maxima and when a "match" has been obtaincd between observed and standard data in terms of d-spacings and intensities, the remainder of diffraction maxima are compared to confirm the identity. For the sake of brevity, only the spacings of the three most intense diffraction lines of each piqment have been recorded in Table 1. A complete listing of the spacings is available from the authors upon request. The spacings in Table 1 are arranged to suit a Hanawalt-type search. The important aspect of this Table is the linking of C I pigment numbers with dye type and diffraction data.
TABLE 1 DIFFRACTION DATA (d-SPACINGS AND INTENSITIES), CI PIGMENT NUMBERS AND COLOURANT CLASS FOR A SELECTION O F POWDERED PIGMENTS. DIFFRACTED INTENSITIES ARE DENOTED BY T H E SUBSCRIPT ( x = 10). d - Spacings 19.5, 19.1, 18.8, 18.6, 18.4, 17.2, 16.8, 16.0, 14.9, 11.3, 11.2, 10.2, 9.30, 3.46, 3.37, 3.37, 3.37. 3.35, 3.34, 3.34, 3.34, 3.33, 3.31, 3-31, 3.29, 13.5, 13.4,
With Intensities 8.16, 3.46, 3.40, 4.09, 3.72, 3.42, 3.45, 4.92, 3.42, 3.25, 3.44, 6.28, 3.29. 3.23, 3.27, 6.32, 3.36, 6.1 1, 5.10, 3.29, 3.34, 4.63, 3.25, 20.7, 3.32, 3.23, 12.2, 4.87, 6.88, 14.7, 10.6, 5.63, 5.92, 7.41, 7.27, 6.50, 6-88, 10.2, 3.51, 4.84, 2.78, 3.50, 1 1.4, 6.67, 16.4, 4.95, 3.23, 3.56, 8.00, 3.40, 24.4, 3.45, 3.51 . 8.76,
CI Pigment Number Pigment Red 48 : 4 Pigment Red 52 Pigment Red 48 : 2 Pigment Red 48 : 3 Pigment Red 57 Pigment Red 149 Pigment Red 223 Pigment Red 122 Pigment Red 5 Pigment Red 58 Pigment Red 10 Pigment Red 144 Pigment Red 166 Pigment Red 170 Pigment Red 209 Pigment Red 11 Pigment Red 112 Pigment Red 114
Class of Colourant Monoazo Monoazo Monoazo Monoazo Monoazo Anthraquinone Monoazo Indigoid Monoazo Monoazo Monoazo Disazo Azo Monoazo Quinacridone Monoazo Monoazo Monoazo
Pigment Red 168 Pigment Red 168 Pigment Red 12 Pigment Red 146 Pigment Red 88 Pigment Red 3 Pigment Yellow 128 Pigment Yellow 83 ~iEmentYellow 129
Anthraquinone Anthraquinone Monoazo Monoazo Thioindigoid Monoazo Disazo Disazo Azo-Methine
Pigment Yellow 1 Pigment Yellow 12
Monoazo Disazo
Pigment Yellow 17 Pigment Yellow 110 ~ i i m e n Yellow t 154 Pigment Yellow 109 Pigment Yellow 156 Pigment Yellow 74 Pigment Yellow 13 Pigment Yellow 14 Pigment Yellow 3 Pigment Yellow 24 Pigment Yellow 151 Pigment Yellow 73 Pigment Violet 23 Pigment Violet 19 Pigment Violet 19 Pigment Violet 37 Pigment Violet 15 Pigment Blue 15 : 2 Pigment Blue 16 Pigment Blue 15 : 4 Pigment Blue 15 : 3 Pigment Blue 64 Pigment Blue 60 Pigment Blue 29 Pigment Brown 25 Pigment Brown 23 Pigment Brown 32 Pigment Green 10 Pigment Green 7 Pigment Green 8
Disazo Isoindolinone Monoazo Isoindolinone Azo Monoazo Disazo Disazo Monoazo Anthraquinone Monoazo Monoazo Dioxazine Quinacridone Quinacridone Dioxazine Inorganic Phthalocyanine Phthalocyanine Phthalocyanine Phthalocyanine Anthraquinone Anthraquinone Inorganic Monoazo Disazo Monoazo Monoazo Phthalocyanine Nitroso continued
TABLE 1-continued d
- Spacings
With Intensities
CI Pigment Number Pigment Green 36 Pigment Green 7 pigment Orange 43
Class of Colourant Phthalocyanine Phthalocvanine ~nthra~6inone
Pigment Orange 52 Pigment Orange 5
Pyranthrone Monoazo
Pigment Orange 36
Monoazo
The Colour Index (CI), mentioned in the following paper (Castle, 1982), is a universally recognised system and the CI number identifies the pigment. Some pigments supplied to us by manufacturers were labelled with only the manufacturer's trade name and in most cases it was possible to deduce the CI pigment number via the Colour Index. Absence of a CI number therefore, implies a pigment whose trade name could not be associated with a CI pigment number. Some pigments appear in Table 1 with the same CI number-this can arise as follows: (a) different metal ions have been substituted in the pigment molecule and as a result slight differences occur in diffraction pattern e.g. Pigment Red 48:2, 48:3 and 48:4. (b) the pigments have the same chemical formula but their crystalline phases are different. This means that their diffraction patterns will be quite different. The usefulness of the diffraction data in Table 1 is illustrated by the following example: red paint from a case opener was submitted for analysis with the request that it be compared with red smears of paint found at the scene of a break-in. Analysis by XRD revealed similar diffraction patterns from control and suspect specimens. Three inorganic pigments were identified-lead chromate molybdate, haematite and rutile. Additional d-spacings, thought to be from a n organic pigment, were compared with d-spacings listed for red pigments. According to the list, Pigment Red 48 and Pigment Red 52 were possibilities. The manufacturer of the paint supplied to the makers of the case opener confirmed that the paint did in fact contain three inorganic and two organic components, one of the latter being mentioned above and another which was present in too small a n amount to be detected by XRD. No attempt has been made to estimate detection limits of organic pigments in paints by X R D here, since accurate estimates are difficult. This is because of uncertainties in the initial ratios of resins/pigments/extenders and in the weight losses which occur on drying. I n any case it is unwise to make generalisations since the X-ray scattering power of one pigment may be far greater than that of another, and the presence of extenders of varying scattering power will increase or decrease the detection limits of the ~ i ~ m e n tAs s . far as ~ a i n t flake analysis is concerned, the minimum size recommended is 0.5 x 0.5mm, corresponding to an approximate weight of 40 micrograms. 1
"
Conclusions Diffraction data acquired from pure pigments can be used to identify pigments in paints providing their concentration in the paint is sufficiently high for detection by XRD. I n cases where X R D cannot provide a positive identification, confirmation is sought in chemical tests (Castle, 1982) and transmitted light microscopy (Hamer, 1982).
Acknowledgements The authors are indebted to two summer students, Susan Wilson and Eileen Stocks, for their contributions to this work. They also wish to thank various pigment manufacturers for generously providing the samples used in this study. References CASTLE,D. A., 1982, J. Forens. Sci. Soc., 22, 179. HAMER,P. S., 1982,J. Forens. Sci. Soc., 22, 187. PDF, 1979, Powder Diffraction File, Joint Committeeon Powder Dzflaction Standards, International Centre for Diffraction Data, 1601 Park Lane, Swarthmore, PA 19081, U.S.A. SCOTT,R. W., 1969,J. Paint Technol., 41, 422. SUSICH,G., 1950, Anal. Chem., 22, 425. WHITAKER, A., 1977, The Analytical Chemistry of Dyes, Ed. K. Venkataraman, p. 269, John Wiley & Sons Inc., New York.
Pigment Analysis in the Forensic Examination of' Paints. I. Pigment Analysis by X-Ray Powder Diffraction C. J. CURRY, D. F. RENDLE and A. ROGERS The Metropolitan Police Forensic Science Laboratory, 109 Lambeth Road. London, United Kingdom SEI 7LP
Abstract A brief description of pigment analysis by X-ray powder diffraction is presented. Diffraction data obtained by DebyeScherrer powder photography on some seventy-one pigments have been collected. The d-spacings of the three most intense diffraction lines of each pigment are listed in a form to allow identification using the Hanawalt search-and-match technique. Journal of the Forensic Science Society 1982; 22: 17.3-177 Received 6 March 1981
Introduction X-ray powder diffraction (XRD) has long been used for the analysis of pigments and dyestuffs (Susich, 1950) and specifically for the analysis of pigments in paints (Scott, 1969). I n this laboratory X R D has been used routinely in the analysis of paints for a number of years and a relatively small number of inorganic materials (e.g. titanium dioxide, lead chromate, the iron oxides and hydrated iron oxides) have been found to produce various colourations in paints. As a result, identification of these pigments and extenders by X R D has become a relatively simple matter. However, replacement of inorganic pigments by organic pigments in modern paints has posed certain problems for the X-ray analyst: (a) Organic pigments are generally poorer scatterers of X-rays than their inorganic counterparts, yielding fewer diffraction data. This makes positive identification difficult. (b) Some organic pigments, because of their intense colouration, tend to be present in much lower concentrations than inorganic pigments in paints. This results again in weak diffraction patterns which give rise to uncertainties and ambiguities in identification. (c) Unit cells of organic pigments are generally much larger than those of inorganic pigments which means that the majority of their diffraction lines will be confined to low 8 angles. I n Debye-Scherrer powder photography (the technique used) this angular region is unfortunately the one of highest background level and this may mask some of the weaker diffraction lines. (d) Organic pigments may be present in paints in non-crystalline (amorphous) forms and as such they cannot be characterised by X-ray diffraction. (e) A large number of organic pigments may be derived from one parent compound, e.g. the phthalocyanines, where chemical substitutions or metal insertions produce dramatic colour changes yet sometimes only subtle changes in diffraction pattern.
Powder diffraction patterns of a number of organic dyes and pigments are included in the Powder Diffraction File (PDF, 1979) e.g. Toluidine Red, C,,H,,N303 File No. 25-1879, but this number falls far short of what could be described as a comprehensive collection of data. Consequently individuals or organizations interested in pigment analysis by X R D have had to amass their own diffraction data, a task the size of which has depended upon just how comprehensive a data collection was desired. One useful source of reference to X R D data on synthetic dyes and pigments has been compiled by Whitaker (1977). The object of this present study was to generate and tabulate additional pigment X-ray powder diffraction data for use in the identificationof pigments in vehicle paints which occur as casework specimens in this laboratory. This report presents the results of X-ray powder diffraction studies on a total of seventy-one pigments known to be used in vehicle and/or household paints and kindly supplied by various manufacturers.
Materials and Methods Diffractometry and Guinier powder photography yield data which are very accurate but both methods, in particular diffractometry, require large specimens. Casework paint flake specimens are usually very small and are best examined using Debye-Scherrer powder photography. Since diffraction patterns from these specimens are the ones with which the pigment data are to be compared, Debye-Scherrer powder photography was also used for recording the pigment diffraction patterns. Powdered pigments were loaded into thin-walled 0.3mm internal diameter glass capillary tubes (PANTAK (EMI) Ltd., Windsor, Berks). Diffraction mode: 114.6mm dia. Debye-Scherrer powder cameras Film: Kodak K O D I R E X or NO-SCREEN Radiation: Iron-filtered Cobalt K a from fine-focus X-ray tubes powered at 35 kV 34 mA Exposure : 2 hrs Spacings of diffraction lines on film (d-spacings) were measured using a Huber film-measuring device 622 (Gordon (Churt) Ltd., Farnham, Surrey) and their intensities on a Joyce-Loebl Mark I I I C double-beam recording microdensitometer. The intensities were normalised and in the tables of results they are recorded as subscripts to the d-spacings. The maximum intensity 10 is denoted by x. Errors in the measurement of intensities and d-spacings are 0-1A (d > 10A) or 0.01A (d < 10A). respectively f 1 and either Results and Discussion The number of diffraction lines visible on film varies considerably from pigment to pigment. Accordingly the number of d-spacings and intensities recorded for each pigment varies but in each case as many spacings and intensities as possible have been measured and recorded on file cards in the format used in the PDF (1979). The diffraction data so recorded can then be used to identify pigments from their diffraction patterns using a search-andmatch technique known as the Hanawalt method (PDF, 1979). Very briefly, observed and standard data are compared for the three most intense diffraction maxima and when a "match" has been obtaincd between observed and standard data in terms of d-spacings and intensities, the remainder of diffraction maxima are compared to confirm the identity. For the sake of brevity, only the spacings of the three most intense diffraction lines of each piqment have been recorded in Table 1. A complete listing of the spacings is available from the authors upon request. The spacings in Table 1 are arranged to suit a Hanawalt-type search. The important aspect of this Table is the linking of C I pigment numbers with dye type and diffraction data.
TABLE 1 DIFFRACTION DATA (d-SPACINGS AND INTENSITIES), CI PIGMENT NUMBERS AND COLOURANT CLASS FOR A SELECTION O F POWDERED PIGMENTS. DIFFRACTED INTENSITIES ARE DENOTED BY T H E SUBSCRIPT ( x = 10). d - Spacings 19.5, 19.1, 18.8, 18.6, 18.4, 17.2, 16.8, 16.0, 14.9, 11.3, 11.2, 10.2, 9.30, 3.46, 3.37, 3.37, 3.37. 3.35, 3.34, 3.34, 3.34, 3.33, 3.31, 3-31, 3.29, 13.5, 13.4,
With Intensities 8.16, 3.46, 3.40, 4.09, 3.72, 3.42, 3.45, 4.92, 3.42, 3.25, 3.44, 6.28, 3.29. 3.23, 3.27, 6.32, 3.36, 6.1 1, 5.10, 3.29, 3.34, 4.63, 3.25, 20.7, 3.32, 3.23, 12.2, 4.87, 6.88, 14.7, 10.6, 5.63, 5.92, 7.41, 7.27, 6.50, 6-88, 10.2, 3.51, 4.84, 2.78, 3.50, 1 1.4, 6.67, 16.4, 4.95, 3.23, 3.56, 8.00, 3.40, 24.4, 3.45, 3.51 . 8.76,
CI Pigment Number Pigment Red 48 : 4 Pigment Red 52 Pigment Red 48 : 2 Pigment Red 48 : 3 Pigment Red 57 Pigment Red 149 Pigment Red 223 Pigment Red 122 Pigment Red 5 Pigment Red 58 Pigment Red 10 Pigment Red 144 Pigment Red 166 Pigment Red 170 Pigment Red 209 Pigment Red 11 Pigment Red 112 Pigment Red 114
Class of Colourant Monoazo Monoazo Monoazo Monoazo Monoazo Anthraquinone Monoazo Indigoid Monoazo Monoazo Monoazo Disazo Azo Monoazo Quinacridone Monoazo Monoazo Monoazo
Pigment Red 168 Pigment Red 168 Pigment Red 12 Pigment Red 146 Pigment Red 88 Pigment Red 3 Pigment Yellow 128 Pigment Yellow 83 ~iEmentYellow 129
Anthraquinone Anthraquinone Monoazo Monoazo Thioindigoid Monoazo Disazo Disazo Azo-Methine
Pigment Yellow 1 Pigment Yellow 12
Monoazo Disazo
Pigment Yellow 17 Pigment Yellow 110 ~ i i m e n Yellow t 154 Pigment Yellow 109 Pigment Yellow 156 Pigment Yellow 74 Pigment Yellow 13 Pigment Yellow 14 Pigment Yellow 3 Pigment Yellow 24 Pigment Yellow 151 Pigment Yellow 73 Pigment Violet 23 Pigment Violet 19 Pigment Violet 19 Pigment Violet 37 Pigment Violet 15 Pigment Blue 15 : 2 Pigment Blue 16 Pigment Blue 15 : 4 Pigment Blue 15 : 3 Pigment Blue 64 Pigment Blue 60 Pigment Blue 29 Pigment Brown 25 Pigment Brown 23 Pigment Brown 32 Pigment Green 10 Pigment Green 7 Pigment Green 8
Disazo Isoindolinone Monoazo Isoindolinone Azo Monoazo Disazo Disazo Monoazo Anthraquinone Monoazo Monoazo Dioxazine Quinacridone Quinacridone Dioxazine Inorganic Phthalocyanine Phthalocyanine Phthalocyanine Phthalocyanine Anthraquinone Anthraquinone Inorganic Monoazo Disazo Monoazo Monoazo Phthalocyanine Nitroso continued
TABLE 1-continued d
- Spacings
With Intensities
CI Pigment Number Pigment Green 36 Pigment Green 7 pigment Orange 43
Class of Colourant Phthalocyanine Phthalocvanine ~nthra~6inone
Pigment Orange 52 Pigment Orange 5
Pyranthrone Monoazo
Pigment Orange 36
Monoazo
The Colour Index (CI), mentioned in the following paper (Castle, 1982), is a universally recognised system and the CI number identifies the pigment. Some pigments supplied to us by manufacturers were labelled with only the manufacturer's trade name and in most cases it was possible to deduce the CI pigment number via the Colour Index. Absence of a CI number therefore, implies a pigment whose trade name could not be associated with a CI pigment number. Some pigments appear in Table 1 with the same CI number-this can arise as follows: (a) different metal ions have been substituted in the pigment molecule and as a result slight differences occur in diffraction pattern e.g. Pigment Red 48:2, 48:3 and 48:4. (b) the pigments have the same chemical formula but their crystalline phases are different. This means that their diffraction patterns will be quite different. The usefulness of the diffraction data in Table 1 is illustrated by the following example: red paint from a case opener was submitted for analysis with the request that it be compared with red smears of paint found at the scene of a break-in. Analysis by XRD revealed similar diffraction patterns from control and suspect specimens. Three inorganic pigments were identified-lead chromate molybdate, haematite and rutile. Additional d-spacings, thought to be from a n organic pigment, were compared with d-spacings listed for red pigments. According to the list, Pigment Red 48 and Pigment Red 52 were possibilities. The manufacturer of the paint supplied to the makers of the case opener confirmed that the paint did in fact contain three inorganic and two organic components, one of the latter being mentioned above and another which was present in too small a n amount to be detected by XRD. No attempt has been made to estimate detection limits of organic pigments in paints by X R D here, since accurate estimates are difficult. This is because of uncertainties in the initial ratios of resins/pigments/extenders and in the weight losses which occur on drying. I n any case it is unwise to make generalisations since the X-ray scattering power of one pigment may be far greater than that of another, and the presence of extenders of varying scattering power will increase or decrease the detection limits of the ~ i ~ m e n tAs s . far as ~ a i n t flake analysis is concerned, the minimum size recommended is 0.5 x 0.5mm, corresponding to an approximate weight of 40 micrograms. 1
"
Conclusions Diffraction data acquired from pure pigments can be used to identify pigments in paints providing their concentration in the paint is sufficiently high for detection by XRD. I n cases where X R D cannot provide a positive identification, confirmation is sought in chemical tests (Castle, 1982) and transmitted light microscopy (Hamer, 1982).
Acknowledgements The authors are indebted to two summer students, Susan Wilson and Eileen Stocks, for their contributions to this work. They also wish to thank various pigment manufacturers for generously providing the samples used in this study. References CASTLE,D. A., 1982, J. Forens. Sci. Soc., 22, 179. HAMER,P. S., 1982,J. Forens. Sci. Soc., 22, 187. PDF, 1979, Powder Diffraction File, Joint Committeeon Powder Dzflaction Standards, International Centre for Diffraction Data, 1601 Park Lane, Swarthmore, PA 19081, U.S.A. SCOTT,R. W., 1969,J. Paint Technol., 41, 422. SUSICH,G., 1950, Anal. Chem., 22, 425. WHITAKER, A., 1977, The Analytical Chemistry of Dyes, Ed. K. Venkataraman, p. 269, John Wiley & Sons Inc., New York.