Experimental evidence of matrix effects in total-reflection X-ray fluorescence analysis: Coke case

Total-reflection X-ray Fluorescence (TXRF) analysis is an established method used in instrumental analytical chemistry. Benefits and drawbacks of the method are well known. They have been described in the past and presented in many papers in the last decades. One of the most impressive features, not common to other X-ray fluorescence methods, is the absence of matrix effects and therefore the use of a simple and reliable quantification. Nevertheless, in some cases, TXRF suffers from matrix effects if the sample amount exceeds the condition of a transparent film-like sample or simply if the system is overloaded. In this work, we present the results of TXRF analysis of the different components of beverages summarized under the generic name 'Coke'. Some of them contain up to 10% of sugars and others only mg-amounts of sweeteners. Different sample pre-treatments were applied in order to get reliable results of trace element contents in Coke. A significant deviation of the concentration of elements detected by their characteristic X-ray radiation between 1 and 5 keV (e.g. for P, S, K and Ca) was observed depending on the sample amounts and sample preparation. This behaviour could only be explained by the presence of matrix effects. Transmission curves depending on the residual film-like sample after drying the beverages were calculated and compared with measurements. The obtained deviations for the different sample preparations and different amounts presented to the spectrometer are in good agreement with the theory. TXRF results obtained after microwave digestion for traditional and light Coke were validated by ICP-MS analyses.