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HankronThe use of UV light and a IR viewer can be used to quickly weed out fake mustards, and chromatography can be used to determine how long the mustard has been on the ticket. However, a sample of the mustard must be removed and destroyed for the later test-- how much depending if the chemist is about the make a sandwhich.
Off topic, there was a Texas county coroner who kept his sandwhiches and condiments in the same refrigerator that the bodies were stored. He also once lost the head to one of the bodies, had no idea where it went.