5.3. DART parameters and method development

Helium (He) gas was used (at 2.5 L/min) to promote the most efficient ionization mechanism employed by DART; the ionization of water in the air and subsequent ionization of the sample. Temperature trials for this polyester fiber (single thread, approximately 30 filament fibers) were run between 200–500°C to determine the optimum ionization temperature while limiting sample degradation/destruction. The trials were run in triplicate with an extended, 30 s exposure. However, the final analysis requires significantly less time for ionization and detection of dye molecules. It was found that an ionization temperature of 200°C or below had a negligible effect on the thread, while 300–350°C showed some signs of distortion with extended exposure. 400°C and above instantaneously melted the fibers placed in the stream. Balancing ion abundance and physical disruption of the substrate, 300°C was chosen for this method. Narrowing of DART‐to‐interface spacing was also required, with the highest clear response occurring at a spacing of approximately 1–1.5 cm; a larger spacing decreased the resolution and abundance of ionized dye molecules, so ultimately, a 1 cm spacing was settled on for analysis.

Multiple introduction methods were observed to verify the versatility of DART‐MS for disperse dye analysis in multiple matrices. Solutions were analyzed first to confirm the dye molecules themselves would ionize and be detectable in the DART gas stream before introducing them into a more solid medium. Solutions (dye dissolved in acetone as well as in 80:20 ACN/water) were introduced via a glass probe (the closed end of a glass melting point tube) from both fiber extractions and spiked solutions. Single threads and thin solid and perforated strips of fabric were introduced both by using tweezers and by mounting each on IonSense QuickStrip wire mesh cards, with care taken not to obstruct the flow of gas. All methods were comparable where ionization is concerned, though introduction via mesh card on a sliding rail provides the most reproducible measurement, standardization, and removes error or differences in introduction angle, the distance between source and MS inlet, etc. The mesh cards also benefit the method by preventing loose fibers from dislodging and entering the mass spectrometer.