Raman spectroscopy is a fast and easy analytical tool for quantifying the amount of methanol contamination present in alcoholic beverages. It is an ideal method for the discrimination of very similar molecules like ethanol (CH₃CH₂OH) and methanol (CH₃OH), as shown in Figure 1.

Raman spectroscopy is superior to comparative technologies such as infrared spectroscopy (e.g., FTIR) because of its:

• ability to measure through optically transparent containers
• insensitivity to interference from water
  

These two key properties enable accurate detection of methanol down to approximately 1% by volume in the field with no need to open the bottles being tested.

An in-house study measured commercially available coconut rum that was spiked with methanol in concentrations between 0.33% and 5.36%. The i-Raman® Plus, a sensitive high resolution laboratory system with a fiber-optic probe, was used to collect Raman spectra of the mixtures, shown in Figure 2. Table 1 lists the relevant equipment and instrument settings used for this application study.

The peak at around 1000 cm^-1 visibly increases with increasing concentration of methanol, becoming significant at approximately 1%.

This data was analyzed with Vision software, and a partial least squares (PLS) regression model was developed on normalized data. The two-factor model developed over the range from 920–1580 cm^-1 gave the calibration curve shown in Figure 3, which has a root mean square error of cross-validation (RMSECV) of 0.1069 (Table 2). The R² value of 0.9977 shown in Table 2 means that the Raman method used here can be used to confidently quantify the amount of methanol in a mixed alcohol sample.

These results verify that Raman can be used for rapid, quantitative screening of dangerous adulterants in alcoholic beverages that pose a public safety risk. This technique can be expanded to investigate adulteration in other media such as food, petroleum, and pharmaceutical drugs [4].