Coupling liquid chromatography and inductively coupled plasma-mass spectrometry with ultrasonic nebulization for chromium speciation in rice

Abstract

In this study a procedure based on liquid chromatography (LC) and dynamic reaction cell inductively coupled plasma mass spectrometry (DRC ICP-MS), coupled with ultrasonic nebulization, for chromium speciation in rice was reported. Chromium (Cr) containing species, including Cr(III) and Cr(VI), were well separated by reversed phase HPLC with a C18 column as the stationary phase and a solution containing 0.5 mmol L⁻¹ tetrabutyl ammonium phosphate (TBAP), 0.1 mmol L⁻¹ EDTA and 2% (v/v) methanol (pH 6.9) as the mobile phase using isocratic elution. The chromatographic separation was complete in 4.5 min. Ammonia gas was employed as the reactive gas in the DRC to minimize spectral interference due to ⁴⁰Ar¹²C⁺ and ³⁵Cl¹⁶OH⁺ and ⁴⁰Ar¹³C⁺ and ³⁵Cl¹⁸O⁺ ions on the signal of ⁵²Cr⁺ and ⁵³Cr⁺, respectively. The detection limits were in the range 0.011–0.012 ng Cr mL⁻¹. To verify the accuracy, the developed procedure was applied on a certified reference material (NIST SRM 1573a Tomato Leaves) wherein the sum of the concentrations of individual species agreed with the total certified concentration of Cr. Species were extracted using microwave heating with 1% (v/v) HF and 2 mmol L⁻¹ EDTA. After ensuring the accuracy, this procedure has been applied to determine various Cr compounds present in rice and rice cereal. Supernatants were injected directly into the LC-ICP-MS system after dilution. The spike recovery was in the range of 95–105% and precision between sample replicates was better than 5% for all determinations. The LC-ICP-MS results showed satisfactory agreement with the total Cr concentrations obtained by ICP-MS analysis. Both Cr(III) and Cr(VI) were present in the samples analyzed. An unknown Cr compound was found in all the rice samples analyzed.