J. Forens. Sci. Soc. (1975), 15, 189
Method for Fixing Latent Fingerprints Developed with Iodine F. TROWELL Chemistry Division, Atomic Weapons Research Establishment, Aldermaston, Reading, Rerkshire, England Methods .for jixing and transferrring latent jingerprints revealed by exposure to iodine vapour are described. A solution of #,p'-tetramethyldiaminodiphenylmethane ("tetrabase") i n 1,1,2 trichlorotriJEuoroethane (Freon 113) is used to react with the iodine to produce a green-blue colour which is permanent for at least G monthr. The solvent is chosen for its low to.rici@, non-jiammability and rapid rate of evaporation. Where the use of solvent is to be avoided, the application to the iodine print oJa pad of uncured silicone rubber putty into which dry tetrabase has been incorporated, converts the print to a permanent blue colour.
Introduction Latent finger and palm prints may be revealed by a number of techniques (Kirk, 1960) which include: dusting with a suitable powder; converting the chloride in the print to silver chloride followed by reduction of the silver chloride to metallic silver; reacting the urea in the print with a solution of p-dimethylamino-cinnamaldehyde (Morris, Goode and Godsell, 1973); exposing the latent print to iodine vapour; reacting the amino-acids in the print with a solution of ninhydrin. I n important cases iodine may be used as the first reagent in a sequence of three; iodine treatment followed by ninhydrin and silver nitrate respectively. If a print is revealed by iodine, it can be fixed by the technique described below. O n the other hand, if no print is revealed by iodine, one would proceed to the other two reagents. Prints older than 24 hours on absorbent surfaces may not be revealed by exposure to iodine vapour, but prints deposited on non-absorbent surfaces such as glass, metal, some plastics and paints, may yield iodine prints after several days or even weeks. Dipping paper into a solution of iodine in an inert solvent is not a satisfactory way of developing an iodine print. If the paper contains any material such as starch, which reacts with iodine, the whole paper is discoloured. This does not happen if the same paper is exposed to iodine vapour. Latent prints developed with iodine vapour fade quickly and may be fixed for a longer period by spraying with a solution of starch in water. Care is needed not to wet the sprayed surface with too much water which may cause the developed print to become diffuse or, where documents are being examined, may cause inks to run. The following describes a simple system (Trowell, 1973) for fixing iodine prints without using aqueous systems. Choice of Reactant and Solvent Iodine reacts with a number of organic reagents giving coloured products (Wclcher, 1945-9). One such is p,pftetramethyldiaminodipheny1methane ("tetrabase"). This also reacts with various other oxidizing agents to give yellow, green or blue colours. Iodine dissolves in organic solvents to produce sometimes brown and sometimes violet solutions. I t has been pointed out (Feigl, 1949) that in some reactions, brown solutions possess greater reactivity than violet solutions. Brown solutions are usually produced by solvents whose molecules contain oxygen or nitrogen, or sulphur atoms present in ether-like linkage. In such cases the -
-
iodine is co-ordinated on these atoms with the formation of brown solvates. When a dilute violet-coloured solution of iodine in an organic solvent is mixed with a solution of tetrabase in the same solvent, there is no noticeable change. If, however, a drop of the reaction mixture is placed on filter paper and the solvent allowed to evaporate, a deep blue residue is left. The same colour may be produced by grinding together solid tetrabase and a small crystal of iodine. The redox reaction between an oxidizing agent such as iodine and tetrabase, can be represented as in Figure 1 (Feigl, 1954).
colourless
b l u e quinonoidal f o r m
Figure 1. Oxidation of Tetrabase to coloured form
The simplest way of bringing the tetrabase into contact with the iodine print is by means of a solvent solution of the reagent. The solvent chosen should meet the following specification : (1) a sufficient amount of tetrabase can be dissolved in the solvent; (2) the solvent itself should not cause the print to become diffuse; (3) it should not damage inks; (4) it should evaporate quickly; (5) it should be of low toxicity and non-flammable. A solvent which meets these requirements is 1,1,2 trichlorotrifluoroethane (Freon 1 13), boiling point 47°C. The threshold limit value for the toxicity of Freon 113 in air is 1000 parts per million as opposed to lOppm for carbon tetrachloride and 25ppm for chloroform (Sax, 1968).
Developing Latent Prints with Iodine Vapour Method Prepare a glass tube about 2cm diameter and about lOcm long bearing at one end a socket fitted with a glass stopper, and the other end drawn down to a tube about 5mm diameter. Place a small wad of cotton wool in the constriction and fill the tube to within Icm of the top with a lightly ground mixture of equal parts of iodine crystals and common salt. Insert a loose plug of cotton wool above the filling. Attach a hand blow-bulb to the narrow glass tube and hold the open end of the tube about 2cm above the surface to be examined. Blow iodine over the surface by squeezing the blow-bulbseveral times. When not in use, stopper the iodine tube and push on a closed rubber cap at the open narrow end. Fixation of Iodine Prints by a Reagent Solution Method Prepare a 0.2:/, w/v solution of tetrabase in Freon 113. Expose the latent print to iodine vapour either in a sealed container or by blowing iodine vapour on to the surface suspected of containing the latent print (Figure 2(a)). Small objects bearing iodine prints may then be dipped into the solution of tetrabase, withdrawn immediately and the solvent allowed to evaporate, when the brown iodine print changes to a green-blue colour (Figure 2(b)) which is permanent for at least some months. (Prints produced on paper by this technique have been kept for 8 months to date.) Larger objects (Figures 3(a) and 3(b)) may be sprayed with the reagent solution. A
(a) (h) Figure 2. (a) Latcnt print on paper developed with iodine vapour. (b) Same print fixed by dipping in reagent solution.
La1 (b) Figure 3. (a) Latent print on painted wall developed with iodine vapour. (b) Same print fixed by spraying with reagent solution.
convenient method is to apply the reagent directly from a n aerosol spray canister filled with the reagent solution mixed with a Freon of lower boiling point to create the necessary pressure. Prints developed with iodine can be transferred by placing a piece of suitable material such as paper, impregnated with the tetrabase solution and dried, in contact with the iodine print and holding it in place for 15-20 seconds. A blue negative reproduction of the print is obtained on the paper (Figure 4). I t is possible to re-expose the print to iodine and obtain a second satisfactory reproduction. I t should be noted that iodine prints produced on papers which contain starch will give a blue print when dipped in Freon 1 13 alone and dried. Surfaces such as painted walls which become stained by the blue reaction product of iodine and tetrabase may be cleaned by lightly rubbing with a wet ;ag and domestic scouring powder.
Solventless Fixation of Iodine Prints There may be occasions when it is undesirable to dip or spray with solvent
Figure 4. Latent print on glass dcvrloped with iodirle arid transferred to paper imptegnatecl with tettabaae.
solutions, since either leaves a residue of unreacted tetrabase over the area sprayed or dipped. If left exposed to air and sunlight, the residual reagent becomes brown. A method by which this can be avoided is as follows. Method Make a dough-like mixture of 0.2g tetrabase powder in about log of uncured silicone rubber (Silastoner 91 59, Heavy Grade, Midland Silicones), by kneading
Figure 5.
Cake of silicone rubberltetrabase mixture pressed 192
011 to
sheet of polythene.
the powder into the rubber. Place a sheet of polythene on a flat surface and press a piece of the mixture to a flat cake about 2.5cm diameter and 3mm thickness (Figure 5). Develop the iodine print in the usual way, and apply the flat piece of rubber to the print, with the face which was next to the polythene in contact with the print. Press the rubber down, starting a t one edge, working across the middle to the opposite edge to eliminate air bubbles Figure 6 ( a ) ) . Leave for about 30 seconds and remove the rubber. The print will be fixed (Figures 6(b) and 7), without leaving a considerable amount of unreacted reagent on the surface. The silicone rubber mix can be kneaded again and re-used many times.
Figure 6. (a) Pad of silicone rubber/tetrabasc applird to iodinc print on bottle.
Figure 6. (b)
Print on bottle fixed by the application of silicone rubberltetrabase.
As far as is known, tetrabase is of low toxicity and according to the Merck Index (Merck Incorporated) is non-carcinogenic when fed to rats. Conclusions Latent prints revealed by exposure to iodine vapour may be fixed by: dipping the object bearing the iodine print into a solution of tetrabase in Freon 113, spraying the object with the same solution, or applying to the iodine print a pad consisting of tetrabase powder dispersed in a silicone rubber putty. Prints developed with iodine may be reproduced as a negative by the
Figure 7.
Iodine print on paper fixed hy the application of a pad of silicone ruhherltetrabase.
application to the print of tetrabase impregnated paper. Prints fixed by these techniques are stable for at least several months. Acknowledgement The author thanks the Director (AWRE) for permission to publish this paper. References FEIGL,F., 1949, Chemistry of Spec@, Selective and Sensitive Reactions, pp. 62 and 373, Academic Press Inc. FEIGL,F., 1954, Spot Tests, Vol. 11, p. 322, Elsevier Publishing Co. KIRK, P. L., 1960, Crime Investigation, Interscience Publishers. Merck Index of Chemicals and Drugs, 8th edn. Merck Incorporated, Rahway, New Jersey, USA. MORRIS, J. R., GOODE, G. C., and GODSELL, J. W., 1973, Police Research Bulletin, No. 21, Her Majesty's Stationery Office. SAX,N. L., 1968, Dangerous Properties of Industrial Materials, 3rd edn. Reingold Publishing Corporation. TROWELL, F., 1973, U.K. Patent Application Nos. 33488174 and 38864174. WELCHER, F. J., 1945-9, Organic Ana(ytica1 Reagents, Vols. 1-4, Van Nostrand Co.
Method for Fixing Latent Fingerprints Developed with Iodine F. TROWELL Chemistry Division, Atomic Weapons Research Establishment, Aldermaston, Reading, Rerkshire, England Methods .for jixing and transferrring latent jingerprints revealed by exposure to iodine vapour are described. A solution of #,p'-tetramethyldiaminodiphenylmethane ("tetrabase") i n 1,1,2 trichlorotriJEuoroethane (Freon 113) is used to react with the iodine to produce a green-blue colour which is permanent for at least G monthr. The solvent is chosen for its low to.rici@, non-jiammability and rapid rate of evaporation. Where the use of solvent is to be avoided, the application to the iodine print oJa pad of uncured silicone rubber putty into which dry tetrabase has been incorporated, converts the print to a permanent blue colour.
Introduction Latent finger and palm prints may be revealed by a number of techniques (Kirk, 1960) which include: dusting with a suitable powder; converting the chloride in the print to silver chloride followed by reduction of the silver chloride to metallic silver; reacting the urea in the print with a solution of p-dimethylamino-cinnamaldehyde (Morris, Goode and Godsell, 1973); exposing the latent print to iodine vapour; reacting the amino-acids in the print with a solution of ninhydrin. I n important cases iodine may be used as the first reagent in a sequence of three; iodine treatment followed by ninhydrin and silver nitrate respectively. If a print is revealed by iodine, it can be fixed by the technique described below. O n the other hand, if no print is revealed by iodine, one would proceed to the other two reagents. Prints older than 24 hours on absorbent surfaces may not be revealed by exposure to iodine vapour, but prints deposited on non-absorbent surfaces such as glass, metal, some plastics and paints, may yield iodine prints after several days or even weeks. Dipping paper into a solution of iodine in an inert solvent is not a satisfactory way of developing an iodine print. If the paper contains any material such as starch, which reacts with iodine, the whole paper is discoloured. This does not happen if the same paper is exposed to iodine vapour. Latent prints developed with iodine vapour fade quickly and may be fixed for a longer period by spraying with a solution of starch in water. Care is needed not to wet the sprayed surface with too much water which may cause the developed print to become diffuse or, where documents are being examined, may cause inks to run. The following describes a simple system (Trowell, 1973) for fixing iodine prints without using aqueous systems. Choice of Reactant and Solvent Iodine reacts with a number of organic reagents giving coloured products (Wclcher, 1945-9). One such is p,pftetramethyldiaminodipheny1methane ("tetrabase"). This also reacts with various other oxidizing agents to give yellow, green or blue colours. Iodine dissolves in organic solvents to produce sometimes brown and sometimes violet solutions. I t has been pointed out (Feigl, 1949) that in some reactions, brown solutions possess greater reactivity than violet solutions. Brown solutions are usually produced by solvents whose molecules contain oxygen or nitrogen, or sulphur atoms present in ether-like linkage. In such cases the -
-
iodine is co-ordinated on these atoms with the formation of brown solvates. When a dilute violet-coloured solution of iodine in an organic solvent is mixed with a solution of tetrabase in the same solvent, there is no noticeable change. If, however, a drop of the reaction mixture is placed on filter paper and the solvent allowed to evaporate, a deep blue residue is left. The same colour may be produced by grinding together solid tetrabase and a small crystal of iodine. The redox reaction between an oxidizing agent such as iodine and tetrabase, can be represented as in Figure 1 (Feigl, 1954).
colourless
b l u e quinonoidal f o r m
Figure 1. Oxidation of Tetrabase to coloured form
The simplest way of bringing the tetrabase into contact with the iodine print is by means of a solvent solution of the reagent. The solvent chosen should meet the following specification : (1) a sufficient amount of tetrabase can be dissolved in the solvent; (2) the solvent itself should not cause the print to become diffuse; (3) it should not damage inks; (4) it should evaporate quickly; (5) it should be of low toxicity and non-flammable. A solvent which meets these requirements is 1,1,2 trichlorotrifluoroethane (Freon 1 13), boiling point 47°C. The threshold limit value for the toxicity of Freon 113 in air is 1000 parts per million as opposed to lOppm for carbon tetrachloride and 25ppm for chloroform (Sax, 1968).
Developing Latent Prints with Iodine Vapour Method Prepare a glass tube about 2cm diameter and about lOcm long bearing at one end a socket fitted with a glass stopper, and the other end drawn down to a tube about 5mm diameter. Place a small wad of cotton wool in the constriction and fill the tube to within Icm of the top with a lightly ground mixture of equal parts of iodine crystals and common salt. Insert a loose plug of cotton wool above the filling. Attach a hand blow-bulb to the narrow glass tube and hold the open end of the tube about 2cm above the surface to be examined. Blow iodine over the surface by squeezing the blow-bulbseveral times. When not in use, stopper the iodine tube and push on a closed rubber cap at the open narrow end. Fixation of Iodine Prints by a Reagent Solution Method Prepare a 0.2:/, w/v solution of tetrabase in Freon 113. Expose the latent print to iodine vapour either in a sealed container or by blowing iodine vapour on to the surface suspected of containing the latent print (Figure 2(a)). Small objects bearing iodine prints may then be dipped into the solution of tetrabase, withdrawn immediately and the solvent allowed to evaporate, when the brown iodine print changes to a green-blue colour (Figure 2(b)) which is permanent for at least some months. (Prints produced on paper by this technique have been kept for 8 months to date.) Larger objects (Figures 3(a) and 3(b)) may be sprayed with the reagent solution. A
(a) (h) Figure 2. (a) Latcnt print on paper developed with iodine vapour. (b) Same print fixed by dipping in reagent solution.
La1 (b) Figure 3. (a) Latent print on painted wall developed with iodine vapour. (b) Same print fixed by spraying with reagent solution.
convenient method is to apply the reagent directly from a n aerosol spray canister filled with the reagent solution mixed with a Freon of lower boiling point to create the necessary pressure. Prints developed with iodine can be transferred by placing a piece of suitable material such as paper, impregnated with the tetrabase solution and dried, in contact with the iodine print and holding it in place for 15-20 seconds. A blue negative reproduction of the print is obtained on the paper (Figure 4). I t is possible to re-expose the print to iodine and obtain a second satisfactory reproduction. I t should be noted that iodine prints produced on papers which contain starch will give a blue print when dipped in Freon 1 13 alone and dried. Surfaces such as painted walls which become stained by the blue reaction product of iodine and tetrabase may be cleaned by lightly rubbing with a wet ;ag and domestic scouring powder.
Solventless Fixation of Iodine Prints There may be occasions when it is undesirable to dip or spray with solvent
Figure 4. Latent print on glass dcvrloped with iodirle arid transferred to paper imptegnatecl with tettabaae.
solutions, since either leaves a residue of unreacted tetrabase over the area sprayed or dipped. If left exposed to air and sunlight, the residual reagent becomes brown. A method by which this can be avoided is as follows. Method Make a dough-like mixture of 0.2g tetrabase powder in about log of uncured silicone rubber (Silastoner 91 59, Heavy Grade, Midland Silicones), by kneading
Figure 5.
Cake of silicone rubberltetrabase mixture pressed 192
011 to
sheet of polythene.
the powder into the rubber. Place a sheet of polythene on a flat surface and press a piece of the mixture to a flat cake about 2.5cm diameter and 3mm thickness (Figure 5). Develop the iodine print in the usual way, and apply the flat piece of rubber to the print, with the face which was next to the polythene in contact with the print. Press the rubber down, starting a t one edge, working across the middle to the opposite edge to eliminate air bubbles Figure 6 ( a ) ) . Leave for about 30 seconds and remove the rubber. The print will be fixed (Figures 6(b) and 7), without leaving a considerable amount of unreacted reagent on the surface. The silicone rubber mix can be kneaded again and re-used many times.
Figure 6. (a) Pad of silicone rubber/tetrabasc applird to iodinc print on bottle.
Figure 6. (b)
Print on bottle fixed by the application of silicone rubberltetrabase.
As far as is known, tetrabase is of low toxicity and according to the Merck Index (Merck Incorporated) is non-carcinogenic when fed to rats. Conclusions Latent prints revealed by exposure to iodine vapour may be fixed by: dipping the object bearing the iodine print into a solution of tetrabase in Freon 113, spraying the object with the same solution, or applying to the iodine print a pad consisting of tetrabase powder dispersed in a silicone rubber putty. Prints developed with iodine may be reproduced as a negative by the
Figure 7.
Iodine print on paper fixed hy the application of a pad of silicone ruhherltetrabase.
application to the print of tetrabase impregnated paper. Prints fixed by these techniques are stable for at least several months. Acknowledgement The author thanks the Director (AWRE) for permission to publish this paper. References FEIGL,F., 1949, Chemistry of Spec@, Selective and Sensitive Reactions, pp. 62 and 373, Academic Press Inc. FEIGL,F., 1954, Spot Tests, Vol. 11, p. 322, Elsevier Publishing Co. KIRK, P. L., 1960, Crime Investigation, Interscience Publishers. Merck Index of Chemicals and Drugs, 8th edn. Merck Incorporated, Rahway, New Jersey, USA. MORRIS, J. R., GOODE, G. C., and GODSELL, J. W., 1973, Police Research Bulletin, No. 21, Her Majesty's Stationery Office. SAX,N. L., 1968, Dangerous Properties of Industrial Materials, 3rd edn. Reingold Publishing Corporation. TROWELL, F., 1973, U.K. Patent Application Nos. 33488174 and 38864174. WELCHER, F. J., 1945-9, Organic Ana(ytica1 Reagents, Vols. 1-4, Van Nostrand Co.
