Volume 23, Issue 1 (January 1978)
Forensic Applications of Sapphire Cell-Infrared Spectroscopy: Companion to the Diamond Cell in Explosive and Leg Wire Identification
The application of the diamond cell sample holder to the infrared examination of trace evidence has been reported by several investigators. Materials examined by this technique have included paints [1–4], blasting cap leg wire insulation , and explosives [6,7]. With the diamond cell, the usable infrared spectral region is approximately 1800 to 200 wave numbers (cm-l). While this region allows the characterization of many materials, the analyst may desire examination of the range 4000 to 200 cm-1. With a sapphire cell, the region 4000 to 1600 cm-1, which is partially opaque to the diamond, can be used to obtain complete coverage of the desired working range . A sapphire window sample holder compatible with the diamond cell holder mount is commercially available and is appreciably less expensive than the diamond cell. A blank spectrum obtained with the sapphire cell is shown in Fig. 1. In practice, a sample can be transferred manually from the diamond cell to the sapphire cell to obtain the total infrared spectrum. All spectra were obtained on a Perkin-Elmer Model 621 Grating Infrared Spectrophotometer equipped with a 4 × all-reflecting beam condenser and sapphire cell sample holder. An unattenuated signal of 4 to 6% was attenuated in the reference beam to 75 to 80% transmittance. Instrument operating settings and parameters were slits, 1100; gain, 8; attenuator speed, 1000; scan time, 10 min; suppression, 0; source current, 0.8A; and slits at 2 × programmed. When the sapphire cell is used excessive pressure should be avoided since the softer window material is more easily fractured than the diamond.