Unlike the basic dyes, the disperse dye standards contained many unidentified compounds in addition to dyes. This made monitoring the dye m/z ion difficult, because in many cases, the dye was not distinguishable from the background. This is important as the mass spectrometer voltages are optimized for the dye molecule in order to maximize detector sensitivity. These dyestuffs contain additional compounds, such as auxiliaries, which are used to facilitate dyeing polyester fibers. After the dye is affixed to the fiber, these other chemicals are removed from the final product, leaving only the dye bound to the fiber. Thus, "real-life" cases involving disperse dyes may not necessarily involve the detection or classification of these other compounds, which, in the dyestuff itself, are masking the presence of the dye molecule. Therefore, it was deemed necessary to separate the dye molecule from the neat dyestuff before injection on the LC/MS. Organic solvents were selected as possible purifying solvents, because unlike the auxiliaries, the dye molecules have polar functionalities and would be left in the aqueous phase. Several organic solvents were investigated, including toluene, hexane, and chloroform. Toluene resulted in the highest concentration of dye in the aqueous phase, thus resulting in more "purified" dye standards. The solvent–solvent extraction gave adequate results and facilitated the optimization of both the HPLC and the mass spectrometer. A chromatogram of the purified disperse dyes can be seen in Fig. 2. FIG. 2—Chromatogram of separated disperse dyes.
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