edited bv Marlborough School 250 S. Rassrnore Avenue Los Angeles. CA 90004
A Tool for Forensic Science Keith 0. Beny University of Puget Sound, Tacoma, WA 98416 In a crime laboratory, chemists must often determine whether two s a m ~ l e are s "identical" or have a common origin. Su(:hdeterminationsmiyhr avsist the progressof a criminal trial hv establishinr! a link between seeminrlv seDarate events or l&ations. 1dentity must be shown "biond'a reasonable doubtv-in the language of the law-by exhibiting a list of identical physical properties for the two samples. The optical properties of materials are of particular importance in the crime laboratory and several are routinely measured in the context of identification determinations. Only a relatively low probability of similar origin can be established from a single measurement, so values for a varietv of characteristic ~ . r o.~ e r t i eares normallv determined. he list may include refractive index, optical dispersion, and color in addition to a number of nono~tical~ r o ~ e r t i such es as melting and boiling points, h a ~ d n eand ~ , 'density (I).For further discussion of optical properties, the reader's attention is directed to the Charmot and Mason "Handbook" (2), widely considered to be the prime reference in this field. The nonoptical properties are commonly discussed in general chemistry courses and studies are likely to have measured several of them. Optical properties are less commonly discussed, but their prominence in forensic work suggests that their study could be a fruitful and interesting addition to the laboratory syllabus. The index of refraction or refractive index is a good choice because it is based on relatively easily understood parameters and uses unusual laboratory techniques. The ;et'rartive index is a measurement based on the variation in the \doritv of liaht in different substances. It causes a light ray to bend-slightly change direction-upon passing from one medium to another. You can demonstrate the phenomenon by immersing a straw in a glass of water and observing from the side: the straw appears to be bent a t the water line.
The refractive index is defined as the ratio of the speed of light in air to the speed of light in the substance, or speed of light in air n= speed of light in the object Since light travels faster in air than in any other medium (except a vacuum), the refractive index is always greater than unity. The refractive index may also be determined from measurements of the ray angles (Fig. l).In that case, the refractive index is given by n =-sin a sin b Refracllve Index of Liqulds The refractive index of a liquid can be measured in several ways. The Physical Science Study Committee has developed and published a method using ray tracing and the simple e a u i ~ m e ntwical t of the P.S.S.C. Phvsics curriculum (3). .. A fo'rensic labo;atory typically uses an Abbe refractometer, which is capable of five-sianificant-fiaure measurements under well-co&olled conditions. Operation of the refractometer is relatively simple. Figure 2 presents a schematic diagram of the Bausch and Lomb Abbe refractometer used in our laboratory (4). The liquid sample is spread between two prisms and a light beam is passed through the assembly. The beam is reflected from a plane mirror attached to a scale, passed through a colorcompensating and other compensating prisms, and through a lens with cross hairs to an observer. The field of view is divided into dark and light halves. The observer aligns the light-dark division over the cross hairs (Fig. 3) and reads the scale.
OBJECTIVE LENSES
--
REFRACTING PMM B --
FIELD IPRISMJ LAMP_,-
ABBE-3L OPTICAL DIAGRAM Figure 1. Definnion of refractive index.
Figure 2. Schematic diagram of Abbe Refractometer. Photo courtesy Milton Roy Company.
Volume 63 Number 8 August 1986
70 1
@=
$??-
Becke line
OBJECTIVE CONVERGING
LIGHT RAYS
VERTICAL FOR READING LINE SCALE
Figure 3. Field of view for refractometer (2).
Refractive index is temperatwe-dependent, so a good instrument will have provisions for circulating water of a known temperature through the prisms, and tabulated values include the temperature. Secondly, refractive index depends on the waveiength of the light source, so the most accurate determinations employ filters t o obtain relatively monochromatic light (4). Refractive lndex ofSollds A refractive index can be measured for a wide variety of solids including but not limited t o slivers of glass, hair, minerals, soil, sand grains, textiles and fibers, plastic materials, soils. dust and ash. and minerals and other crvstals. ~n'mostforensic'lahoratories, the refractive-index of small particles is determined by a microscopic method. A small sample is immersed in a liquid; if the refractive indices of the sample and liquid are the same, light will pass through each a t the same rate, and the sample will he nearly invisible. This method is applicable to single particles and amorphous materials, and is a nondestructive test. When a transparent object such as a small chip of glass is immersed in a liquid, the edges appear to have adark boundary. The effect is enhanced if the edge is particularly sharp. Just inside or outside the dark boundary one may observe a light-colored line or "halo", called the "Becke line" (5) (Fig. 4). The line is a result of the dis~ersionof lieht a t the sham edge of the sample and is used to estimaG the refractive index. In aeneral, the greater the difference in refractive index hetGeen the s&le and the liquid, the greater the intensity of the Becke line. here is one further phenomenon that makes the Becke line useful in estimating the refractive indexofthesolid.'I'he sample is observed through the microscope, and the working distance (distance between the object and the objective lens) is sliehtlv ,. . increased hv . sliehtlv .. . defocusine. T h e Becke line will appear to move toward the medium of higher refractive index. Thus. bv rhoosine a liauid of known ret'ractive index and determinLg the dGectiin of Becke line motion, it is oossihle to determine whether the refractive index of the sample is less or greater than that of the immersion liquid. T h e process can be repeated usine other immersion liquids u n t i i t h e refractive iddex of theusample has been lo&d within an acceptably small range. The refractive index of a liquid is more sensitive to changes in temperature than that of a solid by a factor of about 1000. This ohsewation suggests that the refractive index of a sample could be determined using a single immersion liquid and a temperature-regulated microscope stage. In fact, several methods use this idea (6). However, for the purposes of this demonstration experiment, the measurements are made a t room temperature. Procedure Refractive lndex of a Liquid Students are asked to measure refractive index for several liquids. The most significant problem with the determination is sample contamination. The prisms must he cleaned between samples using alcohol or acetone and a lens tissue, and the students must he particularly careful about exchanging droppersfrom sample bottles.
702
Journal of Chemical Education
SIDE VIEW Figure 4. Dispersion of light and the Becke line.
FIELD OF VIEW
Typlcal Values tor Refractive lndex ol Llqulds (9) Liouid Methanol Ethyl amate l~opropanol lsobutanol c-Pentand Chloroform Glvcerin
RI
LlouM
RI
1.33 1.36 1.38 1.39 1.41 1.44 1.47
Castor 011 Chlorobenrene Bromabenrene Bromoform lodobenzene a-bomonaphthalene Methvlene iodide
1.48 1.52 1.56 1.59 1.62 1.66 1.74
A wordof caution about scratching- the -prisms may save you from an expensive problem later. The tablelists values far refractive index of severalliquids.Values more accurate than the 0.60.8% imolied in the table reouire accurate calibration, controlled temperature and wavelength, and very pure samples. Refractive lndex Of A Solid--Immersion Method Place a small fragment of glass on a clean microscope slide. Select a liquid of known refractive index, and place a drop or two on the glass fragment. Place the slide on the microscope stage, and observe the glass fragment, focusing on a sharp edge, if possible. Observe the Becke line. It will appear as a Light-colored halo near the edge of the glass. Now, while observing the glass through the eyepiece, increase the working distance by rotating the focus knob aliphtlv. --.-~--~, The Heckc line ail1 appear to move toward the medium of highest refractive index. For instance, if it appears ta move"uut" or intothe liquid, then the liquid hnr a greater refractive index than the glass. Remove the slide. Use a tissue paper to soak up and remove the liquid. Then choose another liquid with a refractive index closer to that of the glass as indicated by the first measurement. Repeat the process using as many liquids as is necessary todetermine the refractive index of the sample within limits set by the instructor. Note that by using this method it is not possible to determinean exact value unless one of the immersion liquids has the same refractive index as the sample. The object is to find two liquids such that the refractive index for liquid 1 is less than that of the sample which is less than the refractive index of liquid 2. The table gives the refractive index for some common liquids (79). This data could be used for calibration of an instrument or in conjunction with determination of refractive index of an unknown solid.
Literature Cited (1) Walton, G. "A Laboratory Manual for IntroduMry Forensic Science"; Glsnaa: Ercino. CA. 1979 pp 23-26. (2) Charnot, E. M.; Mason, C. W. "Haodbook of Ch.mical Mimcopy", 3rd d.; Widey: New York, 1958;vol 1. (3) Hsber-Sehaim, U;Dodg8.J. H.;Walter, J. A. "Laboratory Guide PSSC Physics", 5th ed.; He& Lexington, MA, 1981:p 61. ( I ) Pavia. D. L.: Lampman. G. M.; K z , G. 5.. Jr. "lntrduetion to Orpanic Laborstory Techniques":Ssunders: Philadelphia, 1916:pp 613445. (51 Safentein, R.:Jarnes,R.;Meloan, C. E. "Laboratory Manual forhtotmduMry Climinalisfies"; Prentice-HalL Englewood Cliffliff,NJ, 1980; pp 2733. (6) Miller, E. T. "Forensic Glass Compsrisona". In "Forensic Science Handbook"; S d a atein. R.. Ed.: Prentice-Hall: Endewmd Cliffs..NJ.. 1982: no 139-183. (71 saferstkin,'~,eisi. I" rei6, p 29. " (81 Kirk, P. L. '"Density md Refractive 1ndex":Thom:Springfield. 1951. (9) Shrine,, R. L.:Fusoo. R. C.; Curtin, D. Y. "The Systematic Identdkation of Orgaoie Cornpounda"; W i l W NeluYork.
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