Analytical note
X-Ray fluorescence analysis of trace elements in fruit juice

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Abstract

X-Ray fluorescence spectrometry is applied to the determination of trace elements in fruit juice characterized by a high content of sugar and other soluble solid substances. Samples are prepared by evaporation, carbonization and pressing into discs. The synthesis of standards is described in detail. All element concentrations are directly estimated from linear calibration curves obtained without any matrix correction. The results of the analysis are in good agreement with those given by inductively coupled plasma-atomic emission spectrometry and atomic absorption spectrometry techniques.

Introduction

X-Ray fluorescence spectrometry (XRFS) thin sample technique has been applied to the determination of inorganic elements such as Ca, Mg, K and Na, etc., in drinks [1], [2], but it is not suitable for trace element analysis as the amount of sampling is not enough to obtain sufficient X-ray intensity. Chemical approaches and instrumental analytical techniques such as atomic absorption spectrometry (AAS) and inductively coupled plasma-atomic emission spectrometry (ICP-AES), suitable for the analysis of solution, are often utilized for trace element determination in drinks [3], [4], [5]. However, fruit juice containing significant amounts of soluble solid substances (such as sugars, proteins, amylums, additives, etc.) will cause sample injection difficulties for these instrumental techniques, due to their high viscosity and solids content. On the contrary, the high content of the soluble solid substances that can be carbonized easily to form a solid sample will facilitate the preparation of pellets for X-ray fluorescence spectrometry. This note reports on the sample preparation procedures and the measuring conditions for X-ray fluorescence spectrometric determination of trace elements in fruit juice.

Section snippets

Reagents, instrument and working conditions

Specpure reagents Na2SO4, Na6P6O8 (99.9%), Al2O3, KCl, CaCO3, MnO2, RbCl, SrCO3, ZrO2, amorphous carbon powder (made by Shanghai Chemical works, China) and MgO, SiO2, TiO2, Fe2O3, NiO, CuO, ZnO (made by Johnson Matthey Chemicals Ltd, England) were used for the preparation of a standard series. A Rigaku 3080E3 X-ray fluorescence spectrometer, equipped with an Rh target X-ray tube, was employed at a tube voltage of 50 kV and a tube current of 50 mA. The window width of the pulse height analyzer

Calibration and corrections of matrix and instrument drift

Correction of matrix effects is unnecessary because the matrix tends to be constant after the sample has been evaporated to dryness and carbonized. For the elements K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, Rb, Sr and Zr (called Group 1) the RhKα Compton scattering peak intensity is used as an internal standard to correct the instrument drift. The calibration equation is:W=BIiIRhKαc+C

Here, Wi and Ii are the weight fraction and the X-ray intensity of element i, respectively, and IRhKαc is the Compton peak

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