Vacuum Metal Deposition (VMD)
Definition
A technique in which gold and zinc are sequentially evaporated in a vacuum chamber; the gold deposits uniformly while zinc nucleates differentially on the background versus print residue, revealing ridge detail on polymer surfaces.
- Metals used
- Gold, then zinc
- Process
- Sequential vacuum evaporation
- Mechanism
- Zinc nucleates differentially on background versus print residue
- Best substrate
- Polymer and other non-porous surfaces
Common questions
Why is gold applied before zinc rather than zinc alone?+
The initial gold layer deposits as an extremely thin, uniform film across the whole surface, which creates the differential nucleation sites that the subsequent zinc layer needs to selectively build up over the fingerprint ridges versus the background.
Why is VMD reserved for difficult surfaces rather than used routinely?+
It requires a dedicated vacuum chamber and trained operator, is slower and costlier than powder or cyanoacrylate fuming, so labs typically apply it after simpler methods fail or when the exhibit is a polymer bag or other surface known to respond poorly to standard techniques.
Related terms
- Alpha-Amino Acids
- Molecules bearing an amine group on the carbon adjacent to the carboxyl group; the primary molecular target of ninhydrin, DFO, and indanedione...
- DFO (1,8-Diazafluoren-9-One)
- A fluorescent ninhydrin analogue used for latent fingerprint development on porous surfaces; excite at 450-470 nm (with secondary absorption extending into the...
- HFE (Hydrofluoroether)
- A class of solvents used as carriers for ninhydrin, DFO, and indanedione formulations; lower toxicity and flammability than older acetone or ethanol...
- Indanedione-Zinc
- A fluorescent amino acid reagent (indane-1,3-dione complexed with zinc chloride) that produces an orange-yellow fluorescent product; reported to be more sensitive than...
- Ninhydrin
- A chemical reagent (2,2-dihydroxyindane-1,3-dione) that reacts with alpha-amino acids in latent print residue to produce a purple-blue compound (Ruhemann's Purple); the primary...
- Physical Developer (PD)
- A multi-bath silver-deposition wet-chemistry method that deposits silver metal onto fatty latent print residue on porous surfaces; the only technique that recovers...
- Ruhemann's Purple
- The purple-blue condensation product formed when ninhydrin reacts with alpha-amino acids; the visible development indicator used in latent fingerprint examination on paper...
- Sequential Processing
- The application of chemical development methods in a fixed, evidence-preserving order that maximises cumulative recovery while preventing earlier treatments from destroying the...
- Treatment Log
- The contemporaneous written record of every chemical method applied to an item of evidence, including reagent batch, examiner identity, date/time, and result;...