Simultaneous determination of preservatives in soft drinks, yogurts and sauces by a novel solid-phase extraction element and thermal desorption-gas chromatography.

A novel solid-phase extraction element combined with thermal desorption-gas chromatography (TD-GC) using a pyrolysis-GC system was applied for the determination of the five preservatives - benzoic acid, sorbic acid, and methyl, ethyl and propyl esters of p-hydroxybenzoic acid - in soft drinks, yogurts and sauces. The extraction element is made of a titanium (Ti) tubing (30 mm x 1.2 mm i.d.x1.6 mm o.d.) coated with a chemically bonded layer of polydimethylsiloxane (PDMS) to a thickness of 500 microm. A sample pH of 2.0, sample volume of 5 ml, extraction time of 45 min with addition of salt (15% m/v NaCl) and TD temperature from 40 to 280 degrees C at a heating rate of 40 degrees Cmin(-1) and maintained at 280 degrees C for 9 min, were selected as optimum extraction and desorption conditions. The linearity of the method was satisfactory over a concentration range of 2-1000 mg L(-1) for benzoic acid and sorbic acid, 0.2-300 mg L(-1) for methyl ester of p-hydroxybenzoic acid, and 0.02-300 mg L(-1) for ethyl and propyl esters of p-hydroxybenzoic acid, and the correlation coefficients were higher than 0.997. The limit of detection ranged from 0.002 to 0.2 mg L(-1). The potential of the proposed method was assessed by applying it to the determination of preservatives in real samples. The recoveries (92-106%) and precisions (R.S.D.s of 0.9-4.6% (n=5)) of the method were examined by analyzing soft drink, yogurt and sauce samples spiked at levels of between 20 and 100 mg L(-1). This simple, accurate and highly sensitive method may be used in the determination of the preservatives in various water-based food samples.