visualisation of latent prints using Ninhydrin-Katherine

Question 1

ninhydrin

Answer 1

reacts with amino acids in sweat to give a purple colour (ruhemann’s purple)

amino acids have a high affinity for cellulose and do not migrate with age

prints can be developed after a long period of time

crystalline solid, soluble in water and polar solvents, turns red in sunlight and on heating

Question 2

formulation of ninhydrin reagent

Answer 2

originally made up with acetone or ether as solvent. conc acetic acid added to improve sensitivity

1974-first non-flammable formulation

does not dissolve ink but gives coloured background on banknotes and rag based writing papers

banned in 1977 due to ozone layer

Question 3

processing conditions

Answer 3

heating accelerates the development of fingerprints but temps in excess of 100 degrees cause unwanted background reactions and damage to the paper

relative humidity of 65% needed as water catalyses the reaction but too much may cause water droplet formation and diffuse fingerprint

exposure time is 3 mins

marks developed using ninhydrin fade need to be images asap.

Question 4

name a ninhydrin analogue

Answer 4

DFO

Question 5

main uses of DFO

Answer 5

latent and bloodmarks on semi-porous and porous surfaces

reacts with amino acids present in fingerprints

fingerprint developed with DFO is less visible in white light than after treatment with ninhydrin but with green light it fluoresces strongly

fluorescence is viewed and photographed through orange or red filter

Question 6

comparison between ninhydrin and DFO

Answer 6

ninhydrin produces purple, DFO pale pink

Ruhemmans purple fluoresces by complexing with metal salts but this additional process is still not as sensitive as DFO. Metal complexation with DFO does not further improve the fluorescence

DFO requires heat for the reaction to proceed while ninhydrin will react at room temp provided moisture avaliable

Question 7

1,2-indanedione

Answer 7

most sensitive AA reagent

produces weak inital print (pale pink) but has strong fluorescent detail when viewed under green light

fluorescence stronger after ZnCl2 post treatment

Question 8

disadvantages of 1,2-indanedione

Answer 8

relatively short life as a working solution and should be made up only when needed

less likely to develop prints on low quality paper than DFO

Question 9

5-methylthioninhydrin

Answer 9

combines strong purple colour of ninhydrin and fluorescence of DFO and 1,2-indandione