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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. 4 — Raw SERS spectra of caffeine (a), theobromine (b) and paraxanthine (c) (all at 0.01 mol/dm³) and a 50:40:10 ratio of caffeine/theobromine/paraxanthine mixture (d). A silver citrate colloid and a 1 mol/dm³ NaCl aggregating agent were used for SERS, and the collection time is 30 s