Comparison of dual solvent-stir bars microextraction and U-shaped hollow fiber–liquid phase microextraction for the analysis of Sudan dyes in food samples by high-performance liquid chromatography–ultraviolet/mass spectrometry

Two sample preparation methods, dual solvent-stir bars microextraction (DSSBME) and U-shaped hollow fiber–liquid phase microextraction (U-shaped HF–LPME), are proposed and critically compared for high-performance liquid chromatography (HPLC)–ultraviolet (UV)/mass spectrometry (MS) analysis of Sudan dyes in this paper. In DSSBME, the organic solvent was confined to a pair of hollow fiber membrane fixed on a stir bar, which can stir by itself, while the hollow fiber in U-shaped HF–LPME was fixed by two microsyringes. The significant factors affecting the microextraction of Sudan dyes in both microextraction techniques have been examined and no obvious difference in the effect of extraction solvent, pH and salt concentration on the extraction efficiency of Sudan dyes was observed except extraction time and stirring speed. Both microextraction techniques were similar in terms of analytical performance from aqueous solutions (LODs ranged from 0.09 to 0.95 μg L−1 by HPLC–UV and 2.5–6.2 μg L−1 by HPLC–MS; the absolute LODs ranged from 0.9 to 11.25 pg by HPLC–UV and 5–21.2 pg by HPLC–MS), however, DSSBME was more stable (lower stirring speed required), less sample consuming and much shorter time required to reach extraction equilibrium; while U-shaped HF–LPME was easier to operate and no more special device required. The two microextraction techniques combined with HPLC–UV/MS were successfully applied to the analysis of real samples including strawberry sauce, capsicum oil, salted egg, and two kinds of chilli sauce. Although the LODs of HPLC–UV are lower than that of HPLC–MS by a factor of 10 in this work, the absolute LODs for both HPLC–UV and HPLC–MS are comparable. HPLC–UV cannot identify the suspicious peaks at the same retention time as that of Sudan II and III in salted egg, while HPLC–MS can give exact information of Sudan I–IV in real sample analysis and is more reliable. The sensitivity of HPLC–MS is enough for real sample analysis.