Dyes analysis in the Armitage laboratoryWe
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.
What do we get:
main colorant in dye made from Indigo is indigotin. The positive
ion DART mass spectrum for a blue yarn dyed with natural indigo extract
(above, left) shows primarily indigotin as the protonated (M+H) ion. For a museum sample, we use even smaller samples for analysis, only about 2 mm long.
The spectrum on the right shows the DART mass spectrum 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. 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.