Infrared spectroscopy can be used to detect these drugs in the liquid phase but no techniques have been developed to detect them in the much more common powder form. Now S N Thakur of the Banaras Hindu University in Varanasi and co-workers have applied laser photoacoustic spectroscopy to the problem, using samples of almost 100% purity supplied by the Central Forensic Science Laboratory in Calcutta.

Heroin, morphine and narcotine are large molecules containing 50, 40 and 53 atoms respectively. Like all molecules they preferentially absorb and emit radiation at certain wavelengths to give a distinctive molecular “fingerprint”. Thakur and co-workers recorded photoacoustic spectra using a carbon dioxide laser that could be tuned to wavelengths in the 9.6 and 10.6 micron regions of the spectrum.

The quality of the spectra were much higher than has been obtained with infrared spectroscopy in the past. In particular there were more peaks in the photoacoustic spectra, and the peaks were also sharper. Thakur and co-workers then went on to use ab initio quantum chemistry calculations to identify the particular molecular vibrations responsible for most of the peaks. The team claims that the technique could be used to detect trace amounts of all three drugs.