A rapid ion chromatography column-switching method for online sample pretreatment and determination of l-carnitine, choline and mineral ions in milk and powdered infant formula

Abstract

A simple and rapid column-switching ion chromatography with non-suppressed conductivity detection method was developed for simultaneous determination of l-carnitine, choline and mineral ions (free sodium, potassium, magnesium and calcium ions) in milk and powdered infant formula samples. On-line sample clean-up was achieved on the pretreatment column Dionex IonPac NG1 (50 mm × 4 mm, i.d.), with pretreatment elution (3 mmol L-1 MSA) and washing solution (100% acetonitrile) at a flow rate of 1.0 mL min-1. After column switching, the separation and determination of analytes were performed using guard column Dionex IonPac SGS (50 mm × 4 mm, i.d.) and analytical column Dionex IonPac SCS (250 mm × 4 mm, i.d.) with an isocratic mobile phase consisting of 3 mmol L-1 MSA and 10% (v/v) acetonitrile, at flow-rate of 1 mL min-1, followed by non-suppressed conductivity detection. The organic matrices clean-up performance of the proposed method was studied using an UV detector working at 200-400 nm. Under the optimal conditions, detection limits (S/N = 3) ranging from 0.05-0.30 mg L-1, good linearities in the range of 1-100 mg L-1 with a correlation coefficient over 0.9973, and satisfactory recoveries in the range between 94% and 105% with relative standard deviation less than 2.1% for real samples were attainted. The proposed method was successfully applied to rapid, simultaneous, and labour-saving detection of l-carnitine, choline, and mineral ions in milk and powdered infant formula samples.