A method based on thermogravimetry/differential scanning calorimetry for the forensic differentiation of latex gloves

Abstract

Latex gloves are quite frequently employed by felons, when committing crimes, if they want to avoid shedding fingerprints or biological traces. Less forensic aware crime committers, though, may discard the gloves at the crime scene or nearby. If found and properly analyzed, these items may therefore provide useful information to investigators. An analytical procedure of such items is proposed, using thermogravimetry and differential scanning calorimetry. By these very straightforward and inexpensive techniques, without sample preparation, nearly all the considered samples could be discriminated. 99.5% of the possible pairs of latex gloves, indistinguishable by visual examination, were differentiated. The rather large degree of variability existing in latex gloves, even though they are mass produced, was shown, confirming that these items can be useful in comparisons between items related to the crime scene and those seized in the suspect's premises.

Introduction

Latex gloves are commonly used by felons who wish to avoid shedding fingerprints on the crime scene. Although it is currently possible, albeit not in every case, to detect DNA traces or latent prints in the inside of the gloves, the forensic awareness of some crime committers is limited and gloves themselves can be found discarded at the crime scene or nearby. In some instances it has been proved useful to compare the gloves found at the crime scene with those seized at the suspect's premises. The literature on the forensic analysis of latex items is extremely limited. The only examples that could be found involve the examination of condoms, but they are focused mainly on the detection of lubricants and spermicides [1], [2], [3], [4].

Latex, a natural rubber, is a highly regular cis-1,4-polyisoprene produced by more than 400 different species of plants. The majority of the most common techniques for the characterization of polymers do not yield interesting information for the forensic differentiation of similar latex products. This material is amorphous, so X-ray diffraction will not be useful [5]. The analysis of the inorganic content [6], [7] could be in principle a viable technique, but it does not allow to single out the additives from the contaminants. Infrared spectroscopy is not very informative because all latex gloves yield extremely similar spectra, except in the case of chlorinated, powderless gloves that show some additional and potentially useful signals [8]. Physical and mechanical characterization cannot be applied in the majority of real cases because it requires samples of relatively large size and of standardized shape. Most of the times, there is not enough material to perform the replicate measurements necessary to obtain a reliable result.

Thermal analysis is a family of techniques that study the behavior of materials as a function of temperature. Differential scanning calorimetry (DSC) and thermogravimetry (TGA) are the most popular members of this family. DSC detects the variations in heat flow in or out of the sample when subject to a temperature program. It is the ideal technique to measure features such as the glass transition temperature or to study phase transitions like melting or crystallization [9], [10]. The glass transition temperature of natural rubber is located at extremely low temperatures, about −70 °C, and it is not easily measurable with accuracy by common equipment. The amorphous nature of latex makes otherwise featureless the DSC trace in the temperature scale investigated by normal stand alone DSC instruments (<300 °C).

TGA detects the variations in sample mass with temperature as a consequence of degradation, oxidation or other reactions. The usefulness of this technique for forensic purposes and in material analysis has already been suggested [11], [12]. Thermal degradation of polymer materials depends on their formulation and slight variations in additivation can bring about measurable changes in their behavior at high temperatures [13].

The purpose of this work is to investigate the value and reliability of a differentiation procedure of similar latex gloves by TGA. With this objective, 28 items were analyzed and the discriminative power of each characteristic feature of the thermogram was computed.