Application Research Of Vacuum Metal Deposition Developing Latent Fingerprints On Metals

Vacuum metal deposition（VMD）is a fingerprint display technology with extremely high sensitivity and wide application range,especially in the display of potential fingerprints in old and humid conditions.At present,most reports are based on VMD technology to reveal the potential fingerprints on the surface of plastic bags and textiles,but there are few reports on the potential fingerprints on metal objects.Metal brass bullet casings,stainless steel door handles and knives are often the key material evidence at the scene of a criminal case.The appearance of potential fingerprints on such objects plays a crucial role in the identification of suspects.Therefore,further exploring the ability of VMD to the potential fingerprints on the surface of metal objects and expanding the scope of objects applicable to VMD have become important issues to be solved in the field of forensic science.In this paper,four common metal objects are used as the test materials,and the optimal dosage of silver,silver-zinc and traditional gold-zinc VMD on the surface of various metal objects is determined by comparing the effect of different metal evaporation VMDs.The results showed that the optimal dosage of gold-zinc VMD on the surface of four common metals was 4 mg,and the optimal dosage of silver VMD was 12 mg.The dosage is 12 mg,and on the aluminum foil object,8 mg of silver-zinc VMD works best.On this basis,by comparing the display effect and sensitivity of the three kinds of VMDs on various metal surfaces,and exploring the changes of the three kinds of VMDs with the remaining time,this paper obtained the best VMD schemes for the four kinds of metal object surfaces.The results show that silver VMD is the least affected by the remaining time and has the highest sensitivity on the four common metals.It can be seen that silver VMD can be used as an effective supplement to traditional gold-zinc VMD.On this basis,this paper conducts a comparative and combined study of VMD and other fingerprint visualization methods.On the one hand,through the comparison and combination of VMD and CAF（Cyanoacrylate Fuming）,this paper determines the enhancement scheme of potential fingerprints on four metal surfaces under "normal temperature and normal humidity" normal legacy conditions.The results show that on brass objects,silver VMD has the best display effect;on copper and stainless steel objects,silver VMD+CAF is the best display solution;and on aluminum foil objects,the best display effect is obtained by using CAF + silver VMD.On the other hand,by comparing the effects of VMD and SPR（small particle reagent）on the potential fingerprints on the metal surface immersed in four types of water,and exploring the influence of the retention time and immersion time on the display effect,the four types of immersed in water were determined.The best visualization solution for metal surfaces.The results show that for brass and red copper objects soaked in water,silver VMD has the best display effect;for stainless steel objects soaked in water,silver-zinc VMD is the best solution;for aluminum foil objects soaked in water,SPR is the best solution.is the best display solution.Finally,the accuracy and feasibility of the above experimental conclusions are verified by applying the best visualization scheme obtained from the previous experiments to brass bullet casings,stainless steel door handles and knives.Using silver VMD alone on the 14-day-old brass cartridge case still showed recognizable fingerprints with a score of 1.At the same time,on the stainless steel doorknobs and knives that were left for 30 days,the silver VMD+CAF solution still showed About 10% of identifiable fingerprints can be obtained.Even if some fingerprints do not meet the identification standards,it also provides the possibility for subsequent physical and chemical tests.