GENERAL PAPER Development of metrological models for internal standard single-point and multi-point calibrations for the assessment of tear gas weapons compliance with legislation Daniel M. Silveira • Pedro A. S. Salgueiro • M. Filomena G. F. C. Camo ˜es • Ricardo J. N. Bettencourt da Silva Received: 29 August 2011 / Accepted: 17 December 2011 / Published online: 17 January 2012 Ó Springer-Verlag 2012 Abstract Tear gases are the most widely used non-lethal weapons, both by security forces and by the general public. The pepper spray, whose active agent is capsaicin, is the only self-defence aerosol allowed in Portugal, where capsaicin concentration must be below 5 g/100 mL. The cost-effective evaluation of the compliance of self-defence weapons with legislation involves the use of two measurement procedures with increasing quantitative capability. Samples are first assessed by preliminary measurement procedure based on single-point GC–MS calibration. Whenever the measure- ment uncertainty from this assessment makes evaluation inconclusive, the evaluation of sample compliance using multi-point GC–MS calibration is performed. Metrological models including sound criteria for the evaluation of sample compliance with legislation were developed for both mea- surement procedures. Such models include the evaluation of the impact of instrumental performance, calibration model, sample dilution and standards preparation on measurement uncertainty. The relative expanded uncertainty, in the stud- ied range (capsaicin 3–7 g/100 mL), of measurements supported in single-point calibrations ranged from 10 to 22% and the ones supported on multi-point calibrations from 8 to 12% depending on capsaicin concentration and daily GC–MS repeatability. Measurements are fit for the intended use since they present a relative expanded uncertainty smaller than a target value of 30, or 15%, for measurements supported in single- or multi-point calibrations, respectively. Keywords Internal standard  Single-point calibration  Multi-point calibration  Uncertainty  Capsaicin  Tear gas weapons Introduction The most common way civilians ensure their own security is by using self-defence weapons, SDW, based on riot control agents, RCA. The most widely used of such weapons is the pepper spray that contains Oleoresin Capsicum, OC, as ‘‘active agent’’. The pungency of the OC is due to the pre- sence of capsaicinoids. This family of compounds includes the capsaicin, the major capsaicinoid, and dihydrocapsaicin, the second major capsaicinoid, all together responsible for 90–95% of the OC pungency. Other compounds are used as the main active agent of SDW like 1-chloroacetophenone, known as ‘‘gas CN,’’ or o-chlorobenzylidene malononitrile, known as ‘‘gas CS.’’ Not all RCA are legal in many countries. For instance, in Portugal, the only self-defence aerosols allowed by legislation are the ones which contain capsaicin in a concentration lower than 5 g/100 mL as the active agent. Because of these legal restrictions, forensic chemists need to be able to perform reliable capsaicin quantifications that must be supported on the use of valid measurement proce- dures in quality-controlled test conditions. Owing to the fact that much of the OC components are volatile and thermostable [1], the aerosols can be analysed by gas chromatography techniques, GC, namely GC–MS, GC hyphenated with mass spectrometry. Presented at the Eurachem/CITAC Workshop on Measurement Uncertainty, June 2011, Lisbon, Portugal. D. M. Silveira  P. A. S. Salgueiro (&) Laborato ´rio de Polı ´cia Cientı ´fica, PJ, 1169-007 Lisboa, Portugal e-mail: passalgueiro@gmail.com D. M. Silveira  M. F. G. F. C. Camo ˜es  R. J. N. Bettencourt da Silva CCMM, Departamento de Quı ´mica e Bioquı ´mica, FCUL, Campo Grande, 1749-016 Lisboa, Portugal 123 Accred Qual Assur (2012) 17:115–127 DOI 10.1007/s00769-011-0861-3