Density-Gradient Column
Definition
Glass column filled with stepwise layers of liquids of decreasing density (commonly bromoform 2.89 g/cm3 and bromobenzene 1.50 g/cm3 mixed in known ratios). Soil grains settle at the level matching their density, giving a fingerprint banding pattern unique to the source.
- Column setup
- Tall glass cylinder containing stepwise or continuously graded liquid layers
- Primary liquids
- Bromoform (2.89 g/cm³) and bromobenzene (1.50 g/cm³)
- Measurement principle
- Soil grains settle at their neutral buoyancy depth, creating a unique mineral-assemblage profile
Common questions
How does a density gradient column work?+
Soil grains and mineral fragments are placed in a tall glass column filled with special liquids arranged by density. Each grain floats or sinks until it reaches a level where the liquid density matches its own density. This creates a banded pattern showing the mineral composition of your soil sample.
What liquids are used in a density gradient column?+
Bromoform and bromobenzene are the standard liquids. Bromoform is denser (2.89 g/cm³) and sinks to the bottom, while bromobenzene is lighter (1.50 g/cm³) and floats on top. They can be layered in steps or mixed in ratios to create a smooth density gradient from top to bottom.
Why is the banding pattern useful in forensic soil examination?+
Each soil source has its own unique mineral composition and density profile. When soil grains settle in a density column, they create a distinctive stratified pattern that acts like a fingerprint for that location, helping forensic examiners compare soil samples from a suspect's clothing or vehicle to soil from a crime scene.
Related terms
- ASTM E2365
- The American Society for Testing and Materials standard for forensic comparison of glass by density; specifies temperature control, calibration, and precision requirements...
- Bromobenzene
- C6H5Br, density 1.50 g/cm3; the lighter component of the forensic density-gradient liquid mixture.
- Bromoform
- CHBr3, density 2.89 g/cm3; the denser component of the forensic density-gradient liquid mixture, used to create the bottom of the density range.
- Bromoform-Bromobenzene System
- The reference liquid mixture used in forensic density-gradient columns and sink-float tests; bromoform (2.890 g/cm³) and bromobenzene (1.495 g/cm³) mix in variable...
- D-Spacing
- The perpendicular distance between adjacent parallel planes of atoms in a crystal lattice, measured in angstroms or nanometres. Each set of planes...
- Density
- Mass per unit volume (ρ = m/V), expressed in g/cm³ or kg/m³; a physical property used to compare and discriminate glass, mineral,...
- DFSS
- Directorate of Forensic Science Services, India; issues the Forensic Laboratory Manual that governs glass and soil examination procedures, including the bromoform-based density...
- Diatom Frustule
- The siliceous cell wall of a diatom, composed of biogenic opal; species-specific in shape and ornament, and resistant to degradation in soil...
- ENFSI ENG3-2013
- The ENFSI Glass Expert Working Group guideline for glass comparison casework; specifies the examination sequence, within-source variation requirements, statistical comparison framework, and...
- Forensic Palynology
- The application of pollen and spore analysis to legal investigations. A forensic palynologist identifies the species composition of pollen assemblages from crime-scene...
- Heavy-Mineral Fraction
- The sub-fraction of soil mineral grains with density above 2.85 g/cm3, including pyroxene, hornblende, garnet, zircon, rutile, and tourmaline; particularly diagnostic of...
- Hydrostatic Weighing
- Density measurement exploiting Archimedes' principle; the apparent weight loss of a solid submerged in a liquid of known density equals the weight...
Explained in these topics
- Density Measurement: Forensic MethodsA tall glass cylinder filled with a liquid of continuously increasing density from top to bottom; solid fragments migrate to their neutral-buoyancy depth, prod...
- Dust and Soil: Nature, Types and Forensic ExaminationGlass column filled with stepwise layers of liquids of decreasing density (commonly bromoform 2.89 g/cm3 and bromobenzene 1.50 g/cm3 mixed in known ratios). So...
- Soil Examination: Density Gradient, Mineralogy and PalynologyA tall glass cylinder containing a continuously graded liquid mixture (bromoform and bromobenzene) in which soil mineral grains float at positions correspondin...