DØsirØ Laza, 1 Ph.D.; Bart Nys, 1 Ph.D.; Jan De Kinder, 1 Ph.D.; AndrØe Kirsch-De Mesmaeker, 1 Ph.D; and CØcile Moucheron, 2 Ph.D. Development of a Quantitative LC-MS ⁄ MS Method for the Analysis of Common Propellant Powder Stabilizers in Gunshot Residue* ABSTRACT: In traditional scanning electron microscopy ⁄ energy dispersive X-ray analysis of gunshot residue (GSR), one has to cope more and more frequently with limitations of this technique due to the use of lead-free ammunition or ammunition lacking heavy metals. New methods for the analysis of the organic components of common propellant powder stabilizers were developed based on liquid chromatography coupled to tandem mass spectrometry (LC-MS ⁄ MS). A multiple reactions monitoring scanning method was created for the screening of akardite II, ethylcentralite, di- phenylamine, methylcentralite, N-nitrosodiphenylamine, 2-nitrodiphenylamine, and 4-nitrodiphenylamine, present in standards mixtures. Five out of seven of these target compounds can be selectively identified and distinguished from the two others with a high accuracy. Samples from the hands of a shooter were collected by swabbing and underwent solid phase extraction prior to analysis. Detection limits ranging from 5 to 115 lg injected were achieved. Results from several firing trials show that the LC-MS ⁄ MS method is suitable for the detection of stabilizers in samples collected fol- lowing the firing of 9 mm Para ammunitions. KEYWORDS: forensic science, organic gunshot residue, primer ingredients, propellant powder stabilizers, LC-MS ⁄ MS, SEM ⁄ EDX, solid phase extraction Current GSR Analysis When a firearm is discharged, the bullet is expelled together with a cloud of vapors and particles, which are deposited on any object in the area surrounding the firearm. This mixture contains metallic components of the primer as well as the remainder of the unburned and partially burned propellant powder. Sometimes known as ‘‘cartridge discharge residues,’’ or ‘‘firearms discharge residues,’’ these products of the firearm discharge are commonly called ‘‘gunshot residues’’ (GSRs). Gunshot residue analysis is routinely carried out in forensic labor- atories in different ways according to the chosen objective. It is used to estimate firing distances, to identify bullet holes in clothing and objects, and to determine if a person was involved in a firing inci- dent or was in the neighborhood during the firing of a weapon. Neutron activation analysis, atomic absorption spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), and scan- ning electron microscopy (SEM) detect the presence of the in- organic elements originating from the primer. Combined with energy dispersive X-ray probe (EDX), this last method (SEM ⁄ EDX) is now by far the most widespread because it offers the advantage of providing images as well as the chemical compo- sition by the analysis of X-ray spectra of individual GSR particles. In this respect, particles originating from a firearm discharge exhibit characteristic shapes and compositions, which are distinguishable from environmental interferences. Their composition depends on the primer ingredients and consists traditionally of Lead (Pb), Barium (Ba), and Antimony (Sb). More and more ‘‘lead-free’’ or ‘‘nontoxic’’ ammunitions are used, and it is consequently difficult to avoid false negatives when searching for GSR. A solution to overcome this problem may be sought in the detection of organic components of the propellant powders. Composition of Propellant Powders—Stabilizers as Indicator of GSR Smokeless propellant powders for small firearms are of two types: single base powders consist of nitrocellulose (NC); double base powders contain NC together with nitroglycerine (NG). Besides those two energetic ingredients, propellant powder compo- sition includes several additives such as gelatinizing agents, flash suppressors, plasticizers, or stabilizers. NC and NG are nitric esters, which are respectively obtained by the nitration of cellulose and glycerine (1). Nitric esters undergo a thermal decomposition initi- ated by the splitting of ester functional groups and the production of various gaseous molecules (2) of which nitrogen monoxide (NO) and dioxide (NO 2 ) react as strong oxidizers. This spontaneous decomposition is followed by the production of nitrous and nitric acids, which can accelerate the process of decomposition (3). To inhibit this autocatalytic effect, stabilizers are added to propellant powder formulations. Stabilizers exert their effect by binding the nitrogen oxides (4) and preventing them from reacting with NC or NG. Diphenylamine (DPA) is the most common stabilizer used in single base powders, while ethylcentralite (EC) is added in double base powders, acting as stabilizer as well as gelatinizing agent for NG. Usually, propellant powders contain DPA together with EC. In addition, EC is frequently substituted by methylcentralite (MC) in Chinese ammunitions (5). 1 National Institute of Criminalistics and Criminology (NICC), Ballistics Section, ChaussØe de Vilvorde 98, 1120 Brussels, Belgium. 2 Faculty of Sciences, Section Organic Chemistry and Photochemistry, UniversitØ Libre de Bruxelles, CP 160 ⁄ 08, Brussels, Belgium. *Financial support provided by the Belgian Federal Office for Scientific, Technical and Cultural Affairs. Received 11 Feb. 2006; and in revised form 30 July 2006 and 24 Feb. 2007; accepted 4 Mar. 2007; published 12 June 2007. J Forensic Sci, July 2007, Vol. 52, No. 4 doi: 10.1111/j.1556-4029.2007.00490.x Available online at: www.blackwell-synergy.com Ó 2007 Belgian government 842 Journal compilation Ó 2007 American Academy of Forensic Sciences