In the field of Defence and Public Security, standoff detection is a useful tool to identify many threats, such as chemical warfare agents, toxic industrial compounds and other chemical substances under various physical states (gas, liquid and solid). Over the years, Defence Research and Development Canada (DRDC) - Valcartier Research Centre has built an extensive expertise in the field of hyperspectral standoff detection. Although the technology behind the detection instruments is well known, the interpretation of spectral signatures of materials can sometimes be difficult. In reflectance infrared spectroscopy of solids, the same molecule can yield different signatures depending on sample surface roughness and particle sizes as opposed to the transmission and absorption infrared spectroscopies for which the signatures are significantly less influenced by these parameters. In principle, molecules containing one ionic bond, such as NaCl, must not exhibit infrared peaks, since only covalent bonds are supposed to generate active bands in the thermal infrared region. However, experimental results show significant spectral features in reflectance infrared spectroscopy. In fact, in reflectance spectroscopy, the volume diffusion phenomenon can generate spectral features even if no fundamental vibration occurs in the molecule. An effort has been undertaken at DRDC to understand the phenomenology associated with these variations. This paper summarizes experimental results which emphasize the roles of surface roughness and particle size in the interpretation of reflectance infrared spectra of solid materials.