This paper (open access) reports the development of a hand-held device that can detect methanol addition in alcoholic spirits by scanning directly through the unopened glass bottle. Such a device has obvious benefit to enforcement inspectors at ports and retail outlets. The paper also describes the operating principle of the device, including all the modifications made by the authors and why they were needed, in clear language understandable to non-specialists.
For an overview of Raman spectroscopy see FAN’s method explainers.
The authors of this paper describe the three main challenges to overcome in order to make a practical Raman Spectroscopy scanner which can read through glass bottles; 1) the spectroscopic signal from the container masking the sample signal; 2) the intrinsic fluorescence signal of the sample that can overwhelm the weaker Raman peaks; and 3) the opacity and colour of the glass attenuating the signal both entering and exiting the container.
They use of a combination of approaches to circumvent these challenges. They use an axicon lens to generate a conical excitation beam, which effectively circumvents the bottle signal. They also use a relatively long-wavelength excitation combined with wavelength modulation (Wavelength Modulated Raman Sprectroscopy, WMRS) to minimise and then offset any natural fluorescence from components in the drink.
To quantify, they compared the signals attributable to methanol with those from ethanol as an internal standard. They used the nominal %ABV of ethanol for this calculation, on the assumption that adulterated spirits would have a lower than declared ethanol content and therefore they would over-estimate the methanol content (i.e. erring on the side of caution, for a screening test).
They report the successful detection of methanol adulteration at well below the 2% level that causes acute serious health concerns. The method has been validated on one real spirit sample but has yet to be tested for robustness over a range of samples.
Image from the publication
