Technical note: latent fingermarks, colloidal gold and multimetal deposition (MMD): Optimisation of the method

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Abstract

Operational details and optimisation of the colloidal gold or multimetal deposition technique (MMD) for the detection of latent fingermarks on non porous and porous surfaces demonstrate the power of the method. Control of particulate size, pH, reagent, handling are shown to be essential. A difficult case example illustrates the potential of MMD.

Introduction

The use of colloidal gold into the domain of forensic sciences was introduced by Saunders [1], as a method for the detection of latent fingermarks. He called the technique multi metal deposition (MMD). The treatment comprises two stages: the first is the immersion of the object to be treated in a solution containing colloidal gold as the active component; and the second is the enhancement of the detected fingermarks by the use of a physical developer. This technique is applicable for the detection of prints on both porous and non-porous supports.

Several forensic science laboratories have tested this technique [2], [3], and concluded that colloidal gold is a particularly effective reagent. It is their opinion that the development of latent fingermarks by MMD on smooth surfaces is of equal, if not superior, efficiency when compared to the well known methods of cyanoacrylate fuming and vacuum metal deposition. However, despite these positive conclusions the colloidal gold technique has been tested, evaluated and subsequently abandoned by a majority of forensic laboratories throughout the world. No obvious reasons have been given for such an outcome but the authors have gathered that one main reason is the lack of controlled activity, i.e. inconsistency of results combined to the operational difficulty of working according to tight laboratory conditions (cleanliness, etc.).

Following optimisation, MMD was used on the basis of biochemical enhancement techniques intended for a multiplication of the signal given by weak fingermarks [4]. The process of multiplication should by-pass whatever improvements that could be obtained by changing single reagent structures, such as observed with the intensive research activity around ninhydrin analogues. Structural modifications only offer marginal improvements, whereas a multiplication of the signal should allow a quantum leap in the improvement.

It is not intended here to discuss and explain the scientific basis of the process described by Schnetz [4], but the operational optimisation of the technique for fingermark detection.

Section snippets

MMD: methods and materials

According to the optimisation proposed by Allman et al. [3], using data presented by Saunders [1], the parameters were set as follows: during the first stage of the detection procedure, the object to be treated was immersed in an acidified colloidal gold solution (adjusted to pH 3 by the addition of citric acid). The quantity of sodium citrate that is added enabled the formation of gold particles with an average diameter of 30 nm as suggested by Frens [5] and Goodman et al. [6]. Five ml of Tween

Secondary treatment: physical developer

When considering amplification by a physical developer, there are four potential sources of silver ions: silver lactate, silver acetate, silver bromide (from photographic emulsion) and silver nitrate [10], [11], [12], [13]. After testing several reducing agents, hydroquinone emerged as the best reagent and the formulation with silver acetate as the most sensitive. The silver lactate is also sensitive but has the disadvantage of being strongly photosensitive. Treatment in dark rooms is

Reagents

EXTRAN, MAO 01 Alcalin, MERCK No 75553-aminopropyltriethoxysilane 98%, SIGMA No A-3648

Acetone 100%

Active solution: 4 ml of 3-aminopropyltriethoxysilane in 200 ml of acetone (100%)

Procedure

  • Immerse glassware overnight in 10% solution of Extran.

  • Rinse well with hot water, followed by a second rinse with cold water.

  • Dry in oven at 100°C.

  • Remove from oven and let cool to ambient temperature.

  • Immerse in active solution for 5 seconds.

  • Rinse twice with 100% acetone.

  • Rinse twice with distilled water.

  • Dry

New formulation of MMD

The following photographs show the final results of the detection of latent fingermarks on porous (Fig. 8) and non-porous (Fig. 9) surfaces. The left half of the fingermark was developed using the method proposed by Saunders [1], and the right half using the method proposed above.

The same results are obtained regardless of the age of the latent fingermark. On first observation, the right half of the prints appear sharper and, for the second example (Fig. 9), there is less background

Discussion and conclusion

The tests undertaken using the new MMD formulation highlight three improvements: an increased reactivity due to the homogeneity of the solution and the controlled pH; an improved resolution, due to the smaller gold particle size; and a greater amplification selectivity (due to an improved targeting of the fingermarks developed with colloidal gold by the silver species), resulting in decreased background interference.

Despite an apparent complexity, the preparation of this MMD formulation is

References (13)

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  • D.S. Allman, S.J. Maggs, C.A. Pounds 1992. The use of colloidal gold/multi-metal deposition for the detection of latent...
  • B. Schnetz 1999. La révélation des empreintes digitales par l’or colloı̈dal: l’amplification du signal par des...
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