Journal of Criminal Law and Criminology

Volume 61 | Issue 3 Article 9

1971

Forensic Applications of the Scanning ElectronMicroscopeE. J. Korda

H. L. MacDonell

J. P. Williams

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Recommended CitationE. J. Korda, H. L. MacDonell, J. P. Williams, Forensic Applications of the Scanning Electron Microscope, 61 J. Crim. L. Criminology &Police Sci. 453 (1970)

Tim TounNAL o7 Canun LTw, Cmncwo, or Aw Powcz SrczCopyright 0 1970 by Northwestern University School of I w

Vol. 61, No. 3Printe in U.S.A.

FORENSIC APPLICATIONS OF THE SCANNING ELECTRON MICROSCOPE*

E. J. KORDA, H. L. MACDONELL AND J. P. WILLIAMS

E. J. Korda is a Research Scientist, Instrumental Analysis, Coming Glass Works, Coming, NewYork. He was an Associate Professor of Metallurgy at the Drexel Institute of Technology, Phila-delphia, and Director of their Electron Microscopy Laboratory.

H. L. MacDonell is a Research Chemist, Coming Glass Works and a Consulting Criminalist. Mr.MacDonell has previously served as a Forensic Scientist at the Rhode Island State Crime Laboratoryand has taught police science courses at the Corning Community College. He is a fellow of the Amer-ican Academy of Forensic Sciences and a member of the Canadian Society of Forensic Sciences. Hispublications have appeared in a number of professional journals.

J. P. Williams, Ph.D. is Manager of the Instrumental Analysis Research at the Research and'Devel-opment Laboratories, Coming Glass Works. He has worked in the area of chemical and instrumeatalanalysis at Coming since 1950.-EnrTop-

The physical sciences have long been utilizedby those concerned with law enforcement. Chemi-cal analysis, for example, has always been anessential portion of toxicology. In 1832 Coleypublished the book Poisons and Asphyxia con-taining the chemical analysis procedures of thatperiod. (1) Bertillon developed a system of anthro-pometric measurements in 1882 which permittedthe classification of prisoners. (2) Though thesystem was useful, it was not infallible and finallyyielded to the more accurate and practical systemof fingerprint identification. (3, 4) Likewise, withimprovements in firearms and ammunition therefollowed advances in firearms identification usingcharacteristics not possible with earlier cap andball smoothbore weapons.

During the past fifty years a few dedicatedpioneers in scientific crime investigation haveslowly incorporated more science into police sci-ence. As a result, a new discipline known ascriminalistics has evolved which is primarily con-cemed with physical evidence and the use of sci-ence in its examination. It is both logical and un-fortunate that considerable time is often requiredbefore developments in methodology and/orinstrumentation reach their ultimate utilization.Frequently, an instrument is available that maysolve a particular problem, and yet its applicationto the problem may go unnoticed for many years.The objective of this paper is to present some of

*Portions of this paper were presented by Dr.Williams during the Symposium on Forensic Chemistry,American Chemical Society, 156th National Meetingheld in Atlantic City, New Jersey, Wednesday 11,September 1968. Mr. MacDonell was Chairman of theSymposium.

the possible applications of scanning electronmicroscopy (SEM) to the field of criminalisticsand thus to suggest expanded use of this newmicroscope for forensic purposes.

The SEM has evolved from the work of a num-ber of scientists over the past 30 or more years. (5)The first commercial instrument, which becameavailable in 1965, was based primarily on theactivities of a group at the University of Cam-bridge under the direction of Oatley. (6) A sche-matic drawing of the SEM is shown in Figure 1. Abeam of electrons is generated by a tungsten

FIGURE 1Schematic Drawing of SEM.

E. J. KORDA, H. L. MACDONELL AND J. P. WILLIAMS

COMPARISON OF

TABLE 1OPTICAL, SCANNING, AND TRa ArsMIssIoN ELEcTRoN MIcRoscoPY

Comparison Factors Optical Microscopy Scanning Electron Microscopy Transmission ElectronMicroscopy

Magnification Range 1 to 1200X 50 to 30,000X 500 to 250, OOOXResolution Possible 2,000 A 150 to 200 A <5 to 50 ARelativ' Depth of Focus 1 300Type of Obseivation Direct Direct Mainly Replica Direct

DifficultSample Preparation Time Minutes tolhouis Minutes Hours to DaysOther Modes' IR, UV, Luminescent & Conduc- Electron Diffraction

tive

filame it'aitd demagnified by thiee -elctromagneticlenses to a: spot sizeof about 100 A.' The'accelerat-ing voltage for the electrons can be varied from 1to 20 kv. When the primary beam strikes thespecimen, the energy is converted to heat, light,X-rays, ack-scattered electron reflections, andsecondary ,electron emission. In the SEM emissivemode the secondary electrons are collected fromeach point and integrated into a total imageon acathode ray tube (CRT) which scans visually insynchronism with the primary beam. Magnifica-tion is achieved by the relationship of the area onthe specimen scanned by the primary beam andthe area pictured on the CRT.

The energy range of the secondary electrons islow, in the order of 6 to 50 ev, and accordingly thesecondaries can be drawn in curved trajectoriesby a small positive bias to a collector-detectorsystem consisting~,f a focusing electrode, scintil-lator, light pine, and photomultiplier. The abilityto select and detect the secondary electrons in

TABLE 2SEM ExAMINATION OF PHYSiCALr "EVIDENCE

Object

Copper CladLead Bullet

Cartridge Case

Cartridge Case

Iron Wire

Fingernails

Characteristic

Rifling Grooves

Firing Pin Im--presion

Extractor Im-pression

Wire Cutter Im-pression

LongitudinalStiations

MaxiUseful Ma

OpticalMicros-

copy

500X

SOX'

bOox

300X

200X

gnification

SEM

1000X

300X

4000X

3000X

IOox

this manner, coupled with the fact that they arebeing generated at a very fine spot on'the surfaceof the specimen, makee the emissive mode of theSEM a superior means to examine surface topop-raphy directly with great resolution and -depth offocus. A comparison of the characteristi of opti-cal, scanning, and transmission electron micros-copy is summarized in table 1.

Specimen preparation for SEM observation isminimal. Electrical conductors, such as metals,can be studied directly. Insulators, which includeorganic materials, require a thin conducting coat-ing. A recommended coating is a 400 A layer ofvacuum-evaporated aluminum. Samples,. up toapproximately 2 by 1 by 1 6n, are examined on aspecimen stage capable of three-dimensionalmovement, as well as rotation and tilting, withinthe vacuum chamber of the SEM.

FORENSIC APPLICATIONS

Because of the unique capabilities of the SEM, anumber of objects frequently encountered asphysical evidence were examined in an attempt todefine advantages of this new microscopic device.Results of the SEMA analyses are summarized intable 2 and are described in some detail in thetext and micrographs which follow.. Two .45 ACP copper clad lead projectiles wererecovered after-firing and examined for striationsproduced by the barrel rifling. Optical micrographsof the bullets are shown in figure 2. The micro-graphs picture a bullet mounted on a SEM sped-men holder to indicate the approximate size ofsample suitable for convenient viewing. Figure 3illustrates the detail possible for the matching ofthe rifling grooves of the two bullets by means ofSEM analysis. The higher magnification used formatching of the rifling grooves in the SEM micro-graphs shown in figure 3 is approximately 300X.

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SCANNING ELECTRON, MICROSCOPE

FiGuRE 2Optical micrographs of .45 ACP bullet. Top: Bullets

on SEM specimen mounting. Bottom: Rifling grooveson the two bullets. Scale given as diameter of whitecircle in microns (,L).

FiGuRE 4Micrographs of firing pin impression in .38 SPL

cartridge cases. Top: Optical micrographs illustratingdepth of focus limitations. Bottom: SEU micrographsshowing superior depth of focus.

A,

4

F GURE 3SEM micrographs of .45 ACP bullets. Top: Matched

rifling grooves of two bullets pictured in Figure 2. Bot-tom: Matched rifling grooves of same bullets.

Note that the greater magnification does not helpthis comparison and, if anything, it is more con-fusing. It will be demonstrated in succeedingexamples that more detailed markings can befound on objects composed of relatively hardmetals such as iron and brass than on objectsmade of soft metals such as copper and lead.

There are a number of characteristic markingswhich can be found on cartridge cases; these in-clude firing Din impressions and imprints made by

/ &

I~ I

FGURE 5SEM micrographs of firing pin impression. Top:

Detail at center of pin impression in two cases. Bottom:Increased detail of pin impression in two cases.

the breechblock, extractor, and ejector of theweapon. Since detail in this type of marking tendsto be found at the bottom of impressions, the greatdepth of focus of the SEM provides a distinctadvantage over other forms of microscopic exami-nation. Figures 4 and 5 show optical and SEMmicrographs of firing pin impressions in two .38SPL cartridge cases. Similar distinguishing topog-raphy on the bottom surface of the two impres-

E. J. KORDA, H. L. MACDONELL AND I. P. WILLIAMS

FioGuR 6Micrographs of extractor markings on .45 ACP

cartridge cases. Top: Optical micrographs of extractormarkings. Bottom: SEM micrographs of portion ofextractor markings above.

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FiGuRE 9SEM micrographs of cut ends of 18-gauge iron wire.

Top: Low magnification micrograph picturing excellentdepth of focus of the two cut end surfaces. Bottom:Micrograph of the two cut surfaces illustrating theoverall matching character of both specimens.

<I'FIGTRE 7

SEM micrographs of match extractor markings on two cartridge cases.

sions is clearly demonstrated in the SEM micro-graphs at 300 X magnification. Extractoridentations on two .45 ACP cartridge cases arepictured in figures 6 and 7. Groove matching athigh magnification is portrayed in the SEM micro-graphs. Similar detail could be found but withmore difficulty in the case of breechblock andejector markings. Detail was not easy to resolvein the latter two instances because of toolinggrooves on the cartridge cases.

The surface of the cut ends of wire carry impres-sions of the shape and contour of the cutting edgeof pliers or wire cutters. A comparison of the in-formation obtainable from optical micrographs of

two 18-gauge iron wire ends cut with the samewire cutters as contrasted to SEM micrographs isrecorded in figures 8, 9, and 10. The optical micro-graphs pictured in figure 8 indicate that the lightmicroscope has definite limitations for examiningsuch irregular surfaces. The optical micrographsare of poor quality compared to the SEM micro-graphs in figures 9 and 10 which show excellentresolution and detail from magnifications of 100 to3000 x.

For a number of years one of us (MacDonell)has collected fingernails to determine whether thepattern of longitudinal growth striations remainsunchanged over a long period of time. A similar

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SCANNING ELECTRON MICROSCOPE

FIGURE 8Optical micrographs of cut ends of 18-gauge iron wire. Top: Micrographs of two wire cuttings. Bottom: Micro-

graphs of the cut surfaces showing maximum useful resolution of optical microscope examination.

_ : < : ;

FIGURE 10SEM micrographs of detail in surface topography of

iron wire cuttings. Top: Detail of loop in center ofmicrographs pictured at bottom of Figure 9. Bottom:High magnification matching of wire cutting groovesshown at center above. The total cross section illus-trated (top to bottom) is less than Xo0 th of the diam-eter of a human hair.

F GURE 11Micrographs of fingernail sections collected one year

apart. Top: Optical micrograph of Al coated specimensusing reflected light. Bottom: SEM micrographs of twofingernail sections showing matching striations.

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ii +!:<i !: =ii >:{, i-+i i II ::::1:777 iiii .... ::i17:" i' + . :F$'O::

E. J. KORDA, H. L. MACDONELL AND J. P. WILLIAMS

FIGuRE 12Composite SEM micrograph of fingernail sections collected over time interval of five years.

investigation was reported in 1965 by Thomas andBaert (7) who used a light microscope to evaluateand match striations. While some detail is presenton the top surface of fingernails, the area is usuallyabraded and polished to such a degree that it is oflittle value for identification. The underside, how-ever, contains much more detail as can be seen infigures 11 and 12. The aluminum conductive coat-ing deposited on the fingernail specimens for SEMexamination also improved the clarity of the re-flected light optical micrographs. Both the opticaland the SEM micrographs in Figure 11 presentmatched longitudinal striations of sections of twofingernails collected about one year apart. Figure12 pictures a composite SEM micrograph ofstriations of three fingernail clippings collected attime intervals of about one and one-half years be-tween each specimen. Note that the agreement ofstriations is more apparent when the viewer holdsthe illustration so they run in a vertical ratherthan a horizontal configuration. This is true forany striation comparison.

CONCLUSION

The SEM can be utilized to advantage for thedirect examination of surface topography onphysical evidence of forensic interest. The great

depth of focus and high resolutions of the emissivemode of the SEM can reveal important micro-structural detail not readily obtained by othermicroscopic means. While not employed in thisstudy, the luminescent and conductive modes ofthe SEM could also prove valuable in criminalisticinvestigations.

ACKNOWLEDGMENTS

Mr. Raymond A. Fritz and Mrs. Georgia A.Norris provided valuable assistance in obtainingthe micrographs pictured in this report.

REFERENCES

1. COLEY, HENRY, POISONS AND ASPHYXIA, publisherunknown, New York, New York (1832).

2. BERTILLON, ALPHONSE, Identification Anthropo-metrique, ANN. DE DEMOGRAPHIE, (1882).

3. FAULDS, HENRY, Skin Furrows of the Hand, 22NATURE, 605, (28 Oct. 1880).

4. GALTON, FRANCIS, FINGE.RPRINTS, MacMillan,London, (1892).

5. THORNTON, P. R., SCANNING ELECTRON MICROSCOPY,Chapman and Hall Ltd., London, (1968).

6. OATLEY, C. W., NxoN, W. C., AND PEASE, R. F. W.,Advances in Electronics and Electron Physics, 21,181, (1965).

7. THoms, F., AND BAERT, H., A New Means ofIdentification of the Human Being: The Longi-tudinal Striation of the Nails, 5 MEDICINE,SCIENCE AND THE LAW, 39, (1965).

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