LABORATORY EXAMINATION

You may find it advantageous to delay submission to the laboratory of some items, such a pieces of broken headlights found at the scene of a fleeing-the-scene vehicle accident. Temporary retention of such pieces permits visual matching of remaining pieces found on a suspected vehicle, to be later confirmed if possible, by laboratory analysis.

Conversely, early submission for laboratory analysis may facilitate the investigation, as in the case of an accident where no suspect vehicle is promptly located. The laboratory may, if sufficient glass is available, identify the vehicle as to make, model, and year of manufacture, or eliminate from consideration glass from a suspect vehicle, thus narrowing the ^{investigative field.} Since glass is not destroyed or appreciably altered in laboratory examination (except in spectrographic examination of small fragments), the evidence pieces are available for later comparison with suspect pieces.

The material contained in the following paragraphs on laboratory methods of analysis is not intended to make you an expert in this field, but only to give you general working knowledge of various methods of analysis that qualified laboratory technicians can provide to assist you in your investigation.

Glass Fragments

Various glass fragments such as broken headlight lenses, broken bottles, and similar glass materials may furnish important investigative leads when examined by properly trained technicians to determine their composition and possible identity or nonidentity with other fragments. A scientific examination of several particles of glass may disclose identical physical and chemical characteristics, indicating that all particles came from the same piece of glass. It can be determined whether minute particles having the physical appearance of glass are actually particles of glass.

Glass Fractures

Laboratory examination of fractured glass found at the scene of an incident may indicate:

1. Type of glass

2. Manufacturer of the glass

3. Glass did or did not come from a certain place or thing

4. Direction of blow

5. Direction and angle of impact of bullet

6. Sequence of holes

Fluorescence

While it may not produce results as positive as those by other examinations, examination of glass fragments under ultraviolet light may be useful in determining that two pieces of glass could or could not, have originated from the same object.

The fluorescence examination is based on the fact that mineral constituents impart a distinctive type and degree of fluorescence to the glass originating from one molten batch. While the results of ultraviolet comparisons of two or more pieces may not be positive in establishing similarity, they may be positive as to dissimilarity and thus useful in the elimination process.

Since fluorescence examination requires that the glass be absolutely clean to preclude fluorescent reaction from contaminants, wash the glass in acetone or a similar solvent, and in water. Therefore, caution should be taken that this examination is not performed until after other examinations for fingerprints, surface debris, and so forth have been completed.

Spectrographic Analysis

The constituents of glass, especially the minor or trace constituents such as contaminants and minor elements added for specific purposes, lend themselves readily to spectrographic analysis, which identifies and measures the quantities of these constituents. The presence or absence, and the quantities, of these minor constituents are of more importance in the spectrographic analysis than are the basic constituents that are ordinarily present in such comparatively great quantities that differences in the spectral lines of various fragments are often difficult to detect. The spectrograph, for this reason, is of greatest value in demonstrating major differences between two samples, indicating their nonidentity, rather than in indicating acceptably proven identity.

Spectrographic analysis should not be routinely requested in all cases, since it is not always necessary or useful to the investigation, and since small fragments are destroyed by the analysis. Other examinations should be completed first, and spectrographic analysis requested only if considered necessary.

Refractive Index

Refraction refers to the change of direction of a ray of light in passing through a medium whose density is not uniform, as through a piece of glass that has been ground to specific requirements.

The refractive index can be measured either microscopically or with a refractometer. The latter method, however, requires that the two sides of the sample be parallel.

Another method known as flotation, in which a chemical solution is used with the refractometer, can be used when the specimen does not have parallel sides.

Density

This analysis is based on the relative densities of two or more pieces of glass. Comparison of the densities of a known and an unknown piece of glass may aid in determining if they are similar or dissimilar.

Transmittal to Laboratory

It is your responsibility, under supervision of the evidence custodian, to prepare and ship (or carry) the evidence to a laboratory. The evidence should be packed to prevent breakage, friction, shifting, or contact with other items, which would result in destruction, loss, or contamination of the evidence.

All available pieces and fragments pertaining to one incident should be submitted at the same time.

To facilitate the work at the laboratory, each piece of evidence should be identified clearly on individually _{wrapped items.} The evidence number shown on individually wrapped items and on the laboratory request may not necessarily be the same as the exhibit number to be listed in the report of investigation.