Coupling ion chromatography and inductively coupled plasma-mass spectrometry with ultrasonic nebulization for sulfur speciation in dry foods

This paper describes an ion chromatography (IC) inductively coupled plasma mass spectrometry (ICP-MS) method, coupled with ultrasonic nebulization, for the speciation analysis of sulfur-namely sulfite (SO32-), sulfate (SO42-), and thiosulfate (S2O32-). These ionic sulfur compounds were separated well within 4.5 min through IC when using a Hamilton PRP-X100 column as the stationary phase and a mobile phase comprising 60 mmol L-1 NH4NO3 and 0.1% (v/v) formaldehyde solution (pH 7.0). The analyses were performed using a dynamic reaction cell (DRC) ICP-MS system. The sulfur-selective chromatogram was recorded at m/z 48, representing the (SO+)-S-32-O-16 ion, formed through reactions with O-2 in the reaction cell. This approach of detecting S-32(+) as its oxide ion (SO+)-S-32-O-16 at m/z 48 avoided the effect of polyatomic isobaric interference towards the signal of S-32 + at m/z 32 caused by the ions (OO+)-O-16-O-16 and (NO+)-N-14-O-18. The limits of detection of the various species were in the range from 3.1 to 3.7 ng S mL(-1) (6.2 to 7.4 mg kg(-1)). Accuracy of the method was verified by comparing the sums of the concentrations of the individual sulfur compounds, as measured using this new procedure, with the total concentrations of sulfur in a standard reference material (NIST 1573a Tomato Leaves) and in several samples of dry foods purchased locally. After ultrasonication for 90 min, the sulfur species had leached quantitatively from the leaves and the dry foods into the mobile phase solution. The extraction efficiency was > 95%. The spike recoveries of the various studied compounds ranged from 96 to 102%.