David A. Hickman Metropolitan Police Forensic Science Lab 109 Lambeth Road London SE 1 7LP, U.K.
Linking to the Scene Criminals Robbery turned to murder when a young man walked up to the counter of a bank in a quiet residential district of west London, leveled a sawed-off shotgun at a female cashier sitting behind the glass "security" screen, and demanded money. She handed over f 2500, but seconds later the robber fired his gun at point-blank range. A lV4-in.-diameter hole was blasted through the screen, which thus provided little security for the 20-year-old woman sitting behind it. The shot hit her squarely in the neck and chest, and moments later she was dead. The murderer walked calmly out of the bank and disappeared. After an interval of two weeks he was recognized by the police and chased, but again he escaped. He was arrested two months later while trying to cash a stolen check. At first he denied responsibility for the bank robbery and murder but later admitted them, although he maintained that the shooting was accidental. He told the police that he had thrown the gun into the River Thames at Hampton Court and police frogmen were able to recover it. The gun was examined at the Metropolitan Police Forensic Science Laboratory (MPFSL) in London. There was a fired cartridge in the right-hand barrel and an unfired cartridge, as well as at
least a thousand fragments of glass in the left-hand barrel. Glass as evidential material does not always arrive in a forensic laboratory under such dramatic circumstances, but it is encountered frequently in the crimes investigated by forensic scientists. Murders, assaults, and automobile accidents may on occasion provide such evidence, but burglaries form the majority of cases involving broken glass. In the London Metropolitan Police area during 1982 there were 78 000 reported instances of forcible entry to houses and apartments, 50 000 forcible entries to nonresidential buildings, and 104 000 cases of theft from vehicles. Although missing the "glamor" of major crime, burglaries and robberies are offenses of concern to the general public in the U.K., and as such must be investigated thoroughly in a forensic laboratory. Until recently forensic scientists regarded glass as having a limited evidential value, but it is now possible, using advances in analytical instrumentation, to obtain extremely useful information from the examination of glass particles no larger than the head of a pin. The Composition of Glass Glass is one of the oldest of all manufactured materials. A simple fusion of sand, soda, and lime—all of which
844 A · ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
are opaque—produces a transparent solid when cooled. This supercooled liquid is rigid and fundamentally fragile, although it can be made relatively strong by varying its composition. Silicon is the glass-forming element in the majority of glasses, and the addition of stabilizers such as CaO, MgO, BaO, and especially AI2O3 provides chemical resistance. Fluxes, generally oxides of the alkali metals, improve the melting properties of glass but impart low chemical resistance. Chemical decolor ization is sometimes necessary if iron, an impurity in the raw materials, has imparted an unwanted colpr to the glass. Although there are more than 700 glass compositions in commercial use today, the type of glass encountered most often by forensic scientists is flat glass, the generic term for the glass used in windows and doors. Until the late 1950s flat glass was produced entirely by the sheet and plate processes (J). In 1959 the U.K. firm of Pilkington introduced the float glass process in which molten glass is "floated" over a bath of molten tin to produce a distortion-free sheet. The production of float glass has since been licensed to 31 manufacturers in 21 countries, and the manufacture of sheet glass has been virtually eliminated in the developed world. Glass as Forensic Evidence At speeds of 500 frames per second, films of breaking glass show particles of glass flying backwards from all parts of the window where cracks appear—not just from where the first puncture was made. This "backward fragmentation" (Figure 1) results in the breaker of a window being subjected to a shower of minute glass parti-
0003-2700/84/0351-844 A$01.50/0 Published 1984 by the American Chemical Society
The Analytical Approach Edited by Jeanette G. Grasselli
of the Crime with Glass Analysis
Figure 1. The "backward fragmentation" effect Adapted from high-speed film taken by Photographic Branch, New Scotland Yard
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984 · 845 A
υ
cφ
CT
ω
1.5150
1.5120
1.5180
1.5210 Rl
1.5240
1.5270
1.5300
Figure 2. Refractive index distributions for flat glasses (histogram) and container glasses (curve)
cles from the surface nearer t o him. Good forensic evidence can result if glass found on a suspect proves to be indistinguishable from a broken win dow fragment from t h e scene of t h e crime. T h e items submitted to the forensic
Β
C
laboratory in a typical case of burglary will be a control sample from a broken window and articles of clothing from t h e suspect. Glass fragments may be recovered from the clothing and their n u m b e r and distribution are impor t a n t . A piece of glass embedded in the sole of a shoe will have a low eviden tial value whereas the presence of m a n y small fragments of glass on a shirt or sweater can be highly signifi cant. Usually the forensic scientist will be asked if t h e suspect sample matches t h e control (discrimination) and be re quested t o identify t h e type of glass (classification). Although t h e class of t h e control sample will be known (e.g., window, bottle, headlamp), classifica tion of suspect samples is i m p o r t a n t to refute t h e possible claim t h a t glass found on a suspect's clothing is of a different type; for example, from a broken bottle and not from a broken window. The Forensic Examination of Glass
A
200
240
280 nm
320
360
Figure 3. Fluorescence excitation spectra for (a) nonfloat glass or the nonfloat side of float glass, (b) the float side of a specimen of float glass, and (c) the float side of a different speci men of float glass Emission monitored at 400 nm, excitation slit 25 nm, emission slit 10 nm, on a Perkin-Elmer SPF2A spectrofluorometer
846 A ·
Refractive index (RI) and density m e a s u r e m e n t s of glass have been used routinely in forensic science laborato ries for m a n y years and are still used extensively. RI determination, a very sensitive technique, is an excellent discrimination test. W i t h suitable equipment, differences of a t least 0.0001 RI units can be measured. At this level of sensitivity, it is possible to detect the small variations in RI t h a t occur across a two- by one-foot p a n e of float glass. T h e surface of t h e float glass t h a t was in contact with molten tin exhibits a RI different from the " b u l k " glass, allowing the classifica tion of recovered glass.
A N A L Y T I C A L CHEMISTRY, VOL. 5 6 , NO. 7, JUNE
1984
T h e m e a s u r e m e n t of R I alone is of limited use for classification because the RI distributions of flat glasses and container glasses overlap (Figure 2). T h u s , several alternative analytical approaches have been investigated in forensic science laboratories. Recent advances in microscopic techniques enable forensic scientists to establish whether m i n u t e glass frag m e n t s carry a portion of the original surface of t h e p a r e n t object. This can indicate the type of glass. Float glass is absolutely flat, wine glasses are slightly curved and milk bottles (still used extensively in t h e U.K.) and pat terned windows have microscopic de fects from the mould into which t h e molten glass was blown or from t h e rollers used to create the p a t t e r n . Many glasses, when excited by ul traviolet radiation, exhibit fluores cence in the visible region. T h e fluo rescence results from the presence in the glass of a variety of heavy metal atoms, including tin, a n d can be ob served visually or by using a spectro fluorometer (2). Fluorescence excita tion spectra can differentiate between float a n d nonfloat window glasses a n d potentially they can discriminate be tween different samples of float glass (Figure 3). T h e determination of a n u m b e r of elements in glass samples has proved to be very successful for b o t h classifi cation and discrimination. T o be suit able for forensic purposes, however, t h e analytical technique employed m u s t be capable of the rapid determi nation of several elements (some of which may be present in the glass a t p p m levels) in very small fragments of glass. Spark source mass spectrometry
(b)
(a) Si
Ca
Mo (s)
Έ Ο
Ο
I
Ca Na Mg
Ζ
1
Si Ar
Κ
Fe
Cu S
κ
Fe
Ca
Rb Sr
(s) Mo
Mn
Al
Ti/Ba
Energy/eV •
Energy/eV
Figure 4. (a) SEM and (b) XRF spectra for a fragment of modern flat glass (s) denotes scatter peak
has been employed to analyze mgsized fragments for u p to 16 elements (3), b u t this technique suffers from a n u m b e r of operational drawbacks, which have severely limited its accep tance in forensic laboratories. In con trast, scanning electron microscopes (SEMs) have now been installed in most forensic science laboratories in t h e U.K., and microprobe analysis using S E M s is employed for analyzing glass (4). T h e major components (Na, Si, Ca, Mg, and K) in t h e glass are de tected a n d samples smaller t h a n 50 μg can be examined on a routine basis. X-ray fluorescence (XRF) spectrome try (4,5) can also be used to detect major elements in glass samples, and
will sometimes detect t h e minor and trace level components. Spectra from b o t h S E M and X R F analyses of t h e same sample of modern flat glass are compared in Figure 4. For t h e S E M spectrum, 32K counts for Si are dis played as 8K. For t h e X R F spectrum, Ar is from t h e air, Cu from t h e X R F chamber, and Mo from t h e X-ray t u b e . During t h e past 12 years, various analytical techniques for glass analysis based on atomic spectroscopy have been employed at t h e M P F S L . Emis sion spectrographic (6) a n d atomic ab sorption spectrometric (7,8) methods have now been replaced by an induc tively coupled argon p l a s m a - a t o m i c emission spectrometric (ICP-AES)
Figure 5. Size of typical glass fragments occurring in forensic casework, compared to head of a pin The large fragment is ~ 5 0 0 μg, the smaller ones, tens of micrograms
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A N A L Y T I C A L CHEMISTRY, V O L . 5 6 , NO. 7, JUNE 1 9 8 4
procedure (9, 10). Sample sizes of 200-500 μg are preferred for this anal ysis, although on occasion glass frag ments as small as 50 Mg (Figure 5) have been analyzed successfully. Six elements are determined in t h e I C P A E S procedure used routinely for casework glass samples and these ele m e n t s are shown in t h e chart recorder trace of Figure 6. Information from the Elemental Analysis of Glass Multielement analyses of a selection of colorless flat, container, and table ware glasses within t h e R I range of 1.5177 t o 1.5183 (11) have shown t h a t Mg, Li, Co, Sr, Fe, and As are good classifying elements (Figure 7). A combination of these six best elements performs some 85% of the separation produced by 22 elements. A similar multielement study (12) has identified Ba, R b , Sr, Fe, Κ, Μ η , and Li as good discriminating elements. T h e classifying elements can some times be related to manufacturing processes; for example, since 1930 Pilkington has closely monitored t h e level of Mg added to flat glass to con trol t h e flow properties of t h e molten glass. In contrast, container glass often has relatively high concentra tions of As, if this element has been used as a batch-refining agent. Many of t h e good discriminating elements are trace impurities t h a t enter t h e glass from t h e raw materials; two Pilk ington plants in t h e U.K. have been characterized by t h e different levels of Sr and R b in their products (13). In casework a simple discrimination test is used to compare control and suspect glass analyses: t h e ranges (mean ± 2 s t a n d a r d deviations) of each of t h e elements determined are com pared for the two samples. If t h e ranges overlap for every element, t h e
B a r i u m 455.4
:] Aluminum 396.15 Magnesium 285.2
Magnesium 278.0
Chromium 267.7
Iron 259.95
Manganese 256.7
Figure 6. Emission spectrum for six elements in a 4 0 0 - μ 9 sample of flat glass Wavelengths are in nm and the duplicate peaks for each analytical line result from forward and backward scans across the selected wavelength region
t h e glass security screen a n d t h e m a n y glass fragments recovered from the gun were examined in the laboratory. T h e screen comprised three glass sheets separated by two plastic sheets. T h e m e a s u r e m e n t of RI and elemental analysis showed t h a t the three glass layers of t h e screen were indistin guishable from each other and also were indistinguishable from all the re covered fragments t h a t were tested. F u r t h e r samples of glass found in the car used on the day of the robbery and
samples are reported as indistinguish able, whereas if the ranges for one or more elements are separate, t h e n t h e samples are reported as distinguish able. T h e standard deviations used for this test are those calculated from more than 200 analyses of a typical flat glass carried out during a 10month period. U s e of Glass Analysis in Forensic C a s e w o r k
In the murder of the bank cashier
m u r d e r also were indistinguishable. T h e glass evidence in this case thus linked the gun and car to the scene of crime. F u r t h e r important forensic evi dence resulted from the analysis of the lead shot, b u t t h a t is another story (14). H i t - a n d - r u n car accidents without personal injury are relatively trivial crimes compared to murder, b u t one incident did furnish some scientifical ly interesting glass evidence. Some pieces of glass were found under the front b u m p e r of a car suspected of de molishing a traffic bollard. T w o dis tinct RIs—1.5102 and 1.5108—were found, and these would be very unusu al RIs for container or flat glasses. Analysis of the fragments confirmed the presence of two types of glass, dis tinguishable by the concentrations of M n , Fe, Al, a n d Ba. Comparison of t h e analyses with the reference collection showed t h a t both compositions were typical of light bulbs or fluorescent lighting tubes. Traces of calcium fluo ride orthophosphate were identified by X-ray diffraction; since this is a compound used in fluorescent tubes, both samples could be classified un ambiguously. T h e traffic bollard was constructed of plastic and had con tained two fluorescent tubes for illu mination. Control samples taken from these were indistinguishable, in both chemical composition and RI, from the recovered samples. T h u s there was strong evidence t h a t the car in ques tion had actually demolished t h e bol lard. Another scientifically interesting
Mg
0.0
0.5
1.0
1.5
2.0
2.5
Sr
3.0
20
40
60
80
%
100 120 ppm
140
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5
6 7 ppm
8
9
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% Co
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1.5 ppm
2.0
As
0
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2000
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ppm
Figure 7. Elements for classification Mean ± 1 σ ranges for elemental concentrations in groups of flat (red), container (blue) and tableware (black) glasses of RI 1.5177-1.5183
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984 · 8 5 1 A
In Denver, C O —
In Denver, C O . . .
The Colorado Section of ACS Presents . . .
An introduction to statistical and quality control methods
An introduction to concepts and techniques for characterizing water chemistry
Environmental Chemistry of Water August 8-10, 1984 A three-day short course on the applica tion of principles and concepts of physical, inorganic, and organic chemistry t o the description and characterization of natural waters. The text "Aquatic Chemistry" by W . Stumm and J. J. Morgan, and "Organic Carbon", a chapter from Organic Geochemistry of Natural Waters, by Ε. Μ. Thurman will be used f o r source material and are included in the course. The course consists of: a review of selected topics from the physical chemistry of aqueous solutions, an over view of the hydrogeochemical cycle, the distribution of organic carbon in natural waters, the nature and origin of organic substances in natural waters, the chemistry of organic reactions in natural waters, contaminants in the hydrosphere, and an optional field trip to an alpine field site. The course is intended for professional research and production staff needing a working knowledge of basic chemical prin ciples regulating the properties and com position of water. Emphasis will be placed on techniques for identifying and characterizing contemporary water quali ty problems, such as hazardous waste disposal, ground water contamination, and acid rain. Examples, illustrations, and class problems will be selected f r o m studies published in the recent literature. The third day offers an optional field trip t o an alpine field site located at 10,000 feet in the Rocky Mountains west of Denver.
Faculty Dr. Michael Thurman, Research Hydrologist, Organic Research G r o u p , U.S. Geological Survey, is the author of a recent book, "Organic Geochemistry of Natural W a t e r s " . Dr. Michael Reddy, Hydrologist, Water Resources Division, U.S. Geological Survey, is the author of over thirty research articles and book chapters dealing with aquatic chemistry. Fee ACS, SAS Members, $395; Nonmembers, $445. ($20 less without the field trip).
Statistical Quality Control August 8-10, 1984 A three-day short course on statistical quality c o n t r o l methods, based on an ASTM manual entitled " A S T M Manual on Presentation o f Data and C o n t r o l Chart Analysis." The course consists o f four parts: the presentation of data (includes descriptive statistics, frequen cy distributions, and functions o f a f r e quency distribution), presentation of limits o f uncertainty o f an observed average (confidence limits), c o n t r o l chart methods o f analysis (including general principles, types of c o n t r o l charts, and examples o f c o n t r o l charts), and o t h e r statistical methods ( A N O V A , regression and correlation, and statistical computer packages). This course is intended f o r scientists w h o are beginnners in the application o f statistical quality c o n t r o l . The student will learn h o w t o use statistical and quali t y c o n t r o l methods and the appropriate formulas w i t h o u t an emphasis on p r o o f s . The student should bring a hand calculator and be prepared t o w o r k o u t examples.
Faculty D r . Robert Crovelli, Mathematical Statistician, Resource Appraisal G r o u p , U.S. Geological Survey, is Project Chief o f Probabilistic and Statistical Methodology f o r Petroleum Resource Appraisal. He has authored the b o o k "Principles of Statistics and Probabili t y , " and has t w e n t y years of college teaching experience. Fee ACS, SAS Members, $395; Nonmembers, $445.
Registration Registration Contact Dr. Tom Zamis, Colorado School of Mines, Chemistry and Geochemistry Department, Golden, C O 80401 (303) 273-3639; or Mr. Alan Kopelove, Hach Company, P.O. Box 389, Loveland, C O 80539 (303) 669-3050. Registration deadline is July 27, 1984.
Contact D r . T o m Zamis, Colorado School of Mines, Chemistry and Geochemistry Department, Golden, C O 80401 (303) 273-3639; o r Mr. Alan Kopelove, Hach Company, P.O. Box 389, Loveland, C O 80539 (303) 669-3050. Registration deadline is July 27, 1984.
852 A · ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
case involving glass evidence arose from a deliberate and costly fire in a school. A window had been broken by the arsonist in gaining entry to the school and a glass tank containing goldfish had been smashed prior to the fire being started. Control samples of paint and glass from the broken window and glass from the goldfish tank were examined. The goldfish tank glass had an unusual RI, encoun tered in only 7 out of 4500 glass sam ples (albeit of all types of glass) previ ously examined. A suspect was appre hended and his clothing was submit ted to the laboratory where it was searched for traces of paint and glass. Some of the fragments of glass found on the suspect's jeans matched the goldfish tank glass in chemical compo sition and RI, whereas other frag ments could not be distinguished from the window glass. This evidence link ing the suspect to the scene of crime was enhanced by the finding that frag ments of paint on his jeans were indis tinguishable, on a variety of tests, from the control paint taken from the window. To complete the forensic evi dence, a fish scale on the jeans was identified by a forensic biologist as originating from the same family of fish as goldfish. Acknowledgment
Among the many people who have been involved with glass analysis at the MPFSL during the past 10 years, particular mention should be made of Eye Blacklock (emission spectrography), Tim Catterick and Jim Russell (ICP-AES), and Brian Wheals, who has given constant encouragement to this project. References
(1) Pilkington, L.A.B. Proc. R. Soc. Land. 1969, A314,1-25. (2) Lloyd, J.B.F. J. Forensic Sci. 1981,26, 325-42. (3) Locke, J.; Boase, D.; Smalldon, K. W. J. Forensic Sci. Soc. 1978,18,123-31. (4) Keeley, R. H.; Christofides, S. Scan ning Electron Microsc. 1979,1, 459-64. (5) Dudley, R. J.; Howden, C. R.; Taylor, T. J.; Smalldon, K. W. X-Ray Spectrom. 1980,9,119. (6) Blacklock, E. C; Rogers, Α.; Wall, C; Wheals, Β. Β. Forensic Sci. 1976, 7, 121-30. (7) Hughes, J. C; Catterick, T.; Southeard, G. Forensic Sci. 1976,8,217-27. (8) Catterick, T.; Wall, C. D. Talanta 1978,25, 573-77. (9) Catterick, T.; Hickman, D. A. Analyst 1979,104, 516-24. (10) Catterick, T.; Hickman, D. A. Foren sic Sci. Int. 1981,17, 253-263. (11) Hickman, D. Α.; Harbottle, G.; Sayre, E. V. Forensic Sci. Int., 1983,23,189212. (12) Hickman, D. A. Forensic Sci. Int., 1983,23, 213-23. (13) German, B.; Scaplehorn, A. W. J. Fo rensic Sci. Soc. 1972,12, 367-74. (14) Hickman, D. A. Proc. Anal. Din. Chem. Soc. 1979,16, 186-88.
