Discrimination between authentic and false tax stamps from liquor bottles using laser-induced breakdown spectroscopy and chemometrics

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This work describes the preliminary application of a compact and low-cost laser-induced breakdown spectroscopy (LIBS) instrument for falsification detection of tax stamps used in alcoholic beverages. The new instrument was based on a diode-pumped passively Q-switched Nd:YLF microchip laser and a mini-spectrometer containing a Czerny–Turner polichromator coupled to a non-intensified, non-gated, and non-cooled 2048pixel linear sensor array (200 to 850nm spectral range). Twenty-three tax stamp samples were analyzed by firing laser pulses within two different regions of each sample: a hologram and a blank paper region. For each acquired spectrum, the emitted radiation was integrated for 3000ms under the continuous application of laser pulses at 100Hz (integration of 300 plasmas). Principal component analysis (PCA) or hierarchical cluster analysis (HCA) of all emission spectra from the hologram or blank paper region revealed two well-defined groups of authentic and false samples. Moreover, for the hologram data, three subgroups of false samples were found. Additionally, partial least squares discriminant analysis (PLS-DA) was successfully applied for the detection of the false tax stamps using all emission spectra from hologram or blank paper region. The discrimination between the samples was mostly ascribed to different levels of calcium concentration in the samples.