Standard addition flow method for potentiometric measurements at low concentration levels: Application to the determination of fluoride in food samples

• Multiple standard addition method through automatic flow system.
• Determination of the analyte at levels below the LOQ of the potentiometric detector.
• Determination of F− in sea salts, coffee beverages and water samples.
• Limit of quantification of the proposed manifold of 5×10−6 mol L−1 for F−.
• Novel approach for automatic routine analysis of low levels of F− in food samples.

A standard addition method was implemented by using a flow manifold able to perform automatically multiple standard additions and in-line sample treatment. This analytical strategy was based on the in-line mixing of sample and standard addition solutions, using a merging zone approach. The flow system aimed to exploit the standard addition method to quantify the target analyte particularly in cases where the analyte concentration in the matrix is below the lower limit of linear response of the detector. The feasibility of the proposed flow configuration was assessed through the potentiometric determination of fluoride in sea salts of different origins and different types of coffee infusions. The limit of quantification of the proposed manifold was 5×10−6 mol L−1, 10-fold lower than the lower limit of linear response of the potentiometric detector used. A determination rate of 8 samples h−1 was achieved considering an experimental procedure based on three standard additions per sample. The main advantage of the proposed strategy is the simple approach to perform multiple standard additions, which can be implemented with other ion selective electrodes, especially in cases when the primary ion is below the lower limit of linear response of the detector.