Trace Evidence Examination Questions and Answers

Answer: C

Such coatings are called lacquers.

A lacquer is a solution of a film-former, such as nitrocellulose or an acrylic resin, in a volatile solvent. It dries quickly by simple evaporation of the solvent, with no chemical change, and the dried film can be redissolved by the same solvent.

A varnish dries by oxidation and polymerisation of drying oils and resins. The vehicle is the liquid part of a paint that carries the pigment, and resins are the film-forming materials themselves, not a class of coatings defined by how they dry.

Answer: B

When a liquid is heated, its refractive index decreases.

Heating makes a liquid expand, so its density falls and there are fewer molecules per unit volume to slow the light. The refractive index therefore falls, by about 0.0004 to 0.0005 per degree Celsius for typical organic liquids.

This property is the basis of the glass refractive index measurement used in forensic glass comparison (the hot-stage method): a glass fragment is immersed in silicone oil, the oil is heated until its refractive index falls to match the glass, and the fragment's edges vanish at the match point.

Answer: B

Paint analysis does not include a mineralogical profile.

Forensic paint examination compares a questioned chip with a known sample by its sequence of layers (colour, number and thickness, seen under the microscope), by the organic binder, often identified by pyrolysis gas chromatography or infrared spectroscopy, and by its pigments and fillers through trace elemental composition (for example by SEM-EDX or XRF).

A mineralogical profile, the identification of the mineral grains present, is a method of soil and geological examination, not part of the standard scheme for paint.

Answer: C

Soil grains are separated by specific gravity using bromoform.

Bromoform (tribromomethane) is a dense liquid, with a specific gravity of about 2.89. When soil is added to it, or to a density-gradient column made by mixing bromoform with a lighter liquid such as ethanol, light minerals such as quartz and feldspar float while heavy minerals such as magnetite and garnet sink, so the grains separate by specific gravity. Comparing these profiles helps match soil samples.

Ethanol is used only as the lighter liquid in the mixture, and sodium hydroxide solution and nitrobenzene are not heavy liquids used for this.

Answer: C

Mohair is obtained from Angora goat hair.

Mohair is the long, lustrous, silky fibre shorn from the Angora goat, which originated in the Ankara (Angora) region of Turkey. Under the microscope its scales are flat and barely visible, which helps distinguish it from wool.

Camel hair comes from the Bactrian camel, vicuna from the vicuna, a wild relative of the llama, and cashmere, not mohair, comes from the Asiatic (Kashmir) goat. Angora wool, confusingly, comes from the Angora rabbit, not the goat.

Answer: A

Casein is an example of a regenerated protein fibre.

Regenerated fibres are made by dissolving a natural polymer and spinning it back into filaments. Casein, the main protein of milk, is dissolved and extruded to make a wool-like protein fibre, sold historically as Lanital and Aralac; other regenerated protein fibres were made from soya and groundnut protein.

It is not a synthetic polymer, which is built from small petrochemical molecules (nylon, polyester), not a mineral fibre such as asbestos or glass, and not a vegetable fibre such as cotton or jute, which is cellulose.

Answer: D

R-ratio, percent extraction and dye ratio relate to the relative age of ink.

These are parameters of ink-dating methods developed by Antonio Cantu and Richard Brunelle. As ballpoint ink ages on paper it becomes harder to extract with solvents. Percent extraction measures how much of the ink dissolves in a weak solvent compared with a strong one, and the R-ratio compares extraction under two conditions. The dye ratio, measured by TLC with densitometry, compares the relative amounts of dye components, which change as some dyes degrade. Together they indicate whether one ink entry is older or newer than another.

The options about pigment, vehicle and layering belong to paint analysis, which uses different measures.

Answer: D

The correct matching is a-iii, b-iv, c-ii, d-i.

• Silica (fused quartz) glass is made from molten quartz (iii).
• Light-sensitive (photochromic) eyeglasses contain silver halide crystals that darken in ultraviolet light (iv).
• Decolorized glass is paired with barium sulphate (ii) in the key.
• Colored glass uses metal oxides such as chromium oxide, which gives green (i).

All four codes agree on (iii) for silica glass. Only the last code pairs photochromic glass with silver halide, which is the most distinctive pair, and it then gives chromium oxide to coloured glass, so it is the answer.

Answer: B

Club formation takes place during the catagen phase.

Hair grows in cycles. In anagen, the active growth phase lasting years on the scalp, the root is pigmented and attached to the papilla. In catagen, a short transitional phase of a few weeks, growth stops, the follicle shrinks and the root becomes keratinised into a club shape, detaching from the papilla. In telogen, the resting phase, the fully formed club hair stays in the follicle until it is shed.

So the club is formed in catagen and present in telogen. Root morphology matters forensically: a naturally shed telogen club hair has little nuclear DNA, while a forcibly pulled anagen hair with sheath tissue is better for DNA typing.

Answer: B

Spandex and Lycra are urethane (polyurethane) fibres.

Spandex, called elastane outside North America, is a synthetic fibre made of at least 85 per cent segmented polyurethane, which gives it exceptional elasticity: it can stretch to several times its length and recover. Lycra is DuPont's (now Invista's) trade name for spandex.

Acrylic fibres are based on acrylonitrile, olefin fibres on polypropylene or polyethylene, and polyester fibres on polyethylene terephthalate. In fibre examination, these classes are distinguished by microscopy, solubility, melting behaviour and infrared spectroscopy.