Comparison of ultrasound-assisted extraction and microwave-assisted digestion for determination of magnesium, manganese and zinc in plant samples by flame atomic absorption spectrometry

In this paper, a sample preparation method based on acid extraction of magnesium, manganese and zinc from plant tissue by means of high intensity probe ultrasonication is described. Acid extracts obtained upon sonication were directly nebulised into an air–acetylene flame for fast metal determination by atomic absorption spectrometry. Parameters influencing extraction such as sonication time, ultrasound amplitude, sample mass, particle size, extractant composition and volume were fully optimised. Optimum conditions for metal extraction were as follows: a 3-min sonication time, a 30% ultrasonic amplitude, a 0.1-g sample mass, a particle size less than 50 μm, a 0.3% m/v HCl concentration in the extractant solution and a 5-ml extractant volume. Six plant samples used in the human diet were analysed, the concentration range of the three metals approximately being in the range of 1500–3000 μg g−1 for Mg, 30–735 μg g−1 for Mn and 20–45 μg g−1 for Zn. Limits of detection corresponding to the ultrasound-assisted extraction method were 0.10, 1.26 and 0.65 μg g−1 for Mg, Mn and Zn, respectively. Between-batch precision, expressed as R.S.D., was about 0.5, 1.5 and 1% for Mg, Mn and Zn, respectively. Analytical results for the three metals by ultrasound-assisted extraction and microwave-assisted digestion showed a good agreement, thus indicating the possibility of using mild conditions for sample preparation instead of intensive treatments inherent with the digestion method. The advantages and drawbacks of ultrasound-assisted extraction in respect to the microwave-assisted digestion are discussed.