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. 2015 Sep 7;407(27):8253–8261. doi: 10.1007/s00216-015-9004-8

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© The Author(s) 2015

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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Fig. 2 — Plots of SERS peak area for the C=O deformation band at 693 cm⁻¹ against pH of the SERS association for caffeine and paraxanthine, whilst the peak at 1303 cm⁻¹ (ν(C–N) + ρ(CH₃)) is plotted for theobromine. Points are the average of five repeat measurements, and the % relative standard deviations were between 0.8 and 2.7 %