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Dye analysis
Dye analysis
We use mass spectrometry to identify dye compounds in fibers without using chromatography. Since 2010, we have been using Direct Analysis in Real Time (DART) ionization to directly identify natural dyes in textiles of historical and archaeological interest. DART requires no sample preparation: clumps of fibers or a section of yarn are held in the gap between the ionization source and the mass spectrometer. There, the dye molecules are desorbed and either gain (in positive ion mode) or lose (in negative ion mode) a hydrogen atom, forming a charged species called an ion. The ions then pass into the time-of-flight mass spectrometer, where they can be identified based on their molecular mass. To see this process in action, here is a short video from JEOL by Dr. Robert B. Cody, co-inventor of the DART ion source.
The main colorant in dye made from Indigo is indigotin. The positive ion DART mass spectrum for a blue yarn dyed with natural indigo extract shows primarily indigotin as the protonated (M+H) ion.
One of the limitations of DART-MS is that the molecules are identified only by their molecular mass; another compound called indirubin, which adds a purplish color to the blue, is an isomer of indigotin (it has the same molecular mass) and can also be present in Indigo-dyed fibers. So the peak observed at 263.078 could be either indigotin, indirubin, or a mixture of the two.
We use samples about 1-2 mm long, and analyze the fibers by introducing them into the space between the ion source and the mass spectrometer Orifice 1.
This DART mass spectrum was collected from a tiny fragment of yarn removed from the ball shown, one of several that are part of a weaver's basket in the collections of the Carlos Museum at Emory University.
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