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I recently travelled to the USA from Asia. When I arrived in Detroit they marked my E-cigarette with a clear colorless liquid using a small paint brush used for water colors.

What chemical(s) do they use as markers?

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As a related question what is the purpose of the marking? – Phil May 27 '14 at 23:59
@Phil: "marking" is to elicit a chemical reaction with compounds of explosives that most likely makes the clear liquid change color. – Jonas May 28 '14 at 8:52
@Jonas, thanks. I wasn't aware they did that with liquids too. Whenever I've been checked it's been with the cloth in in the wand thing – Phil May 28 '14 at 16:00
This question appears to be off-topic because it is about science/chemicals - not a travel question. – Mark Mayo May 29 '14 at 13:51
@MarkMayo But the fact that something similar may be experienced by travellers makes it relevant right? – Vedant Chandra May 31 '14 at 15:48

They are known as taggants. I found an article about them on Wikipedia.

Hope it helps.

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