Using an Optical Probe as the Microdrop Holder in Headspace Single Drop Microextraction: Determination of Sulfite in Food Samples

A novel headspace single-drop microextraction method (HS-SDME) for determination of sulfite in the form of sulfur dioxide was developed. An optical probe was used as the droplet holder in the HS-SDME procedure, and the analytical signal (absorbance) was monitored online during the extraction process. The method is based on the conversion of sulfite to volatile sulfur dioxide by acidification of the analyzed solution. The liberated SO2 was absorbed by 25 μL of an aqueous mixed reagent solution placed on the optical probe tip and containing Fe­(III), 1,10-phenantroline, and an acetic buffer solution of pH 5.6. During the extraction process, Fe­(III) reduces to Fe­(II) and the Fe­(II) formed then reacts with 1,10-phenantroline to form a colored complex. Absorbance was measured at 510 nm. The calibration plot was linear in the range 0.032–0.320 mg L–1 of sulfite (as SO2), with a correlation coefficient of 0.9989. The limit of detection (LOD), calculated as three times the standard deviation of the blank test (n = 10), was found to be 8 μg L–1. The method was applied for analysis of real food samples, such as wine, jam, and juice.