Traditional analytical techniques for sodium determination are complex and time consuming. In this study, sodium content in food samples was determined by potentiometric multiple standard addition technique with a sodium ion-selective electrode. The results were compared with results obtained with classical analytical methods for sodium content determination (AAS, IC and ICP-MS). The comparison indicates that sodium content in food samples can be determined directly, quickly and accurately by potentiometric multiple standard addition. This analytical technique represents an alternative to sophisticated analytical techniques for routine analysis and quality control in food production.
Sodium is an essential ingredient in processed foods, but too much can be detrimental to health. Accurate and efficient sodium determination is therefore necessary during food production and quality control processes. It has to be reliable, accurate, precise, straightforward, and fast. Some of the commonly used methods for direct sodium analysis are atomic absorption spectroscopy (AAS), ion chromatography (IC) or inductively coupled plasma mass spectroscopy (ICP-MS). However, not only do these methods involve tedious sample preparation and time-consuming system calibration procedures, they also require significant capital investment and in-depth user knowledge. In contrast, the potentiometric multiple standard addition technique stands out as a simple, fast, reliable and economic method to accurately determine the sodium ion content in food products.
Principle of the Multiple Standard Addition Method
The sodium determination is performed with ion-selective sensors. As the membrane potential cannot be observed directly, the membrane potential of the ion-selective electrode (ISE) half-cell is measured against a reference sensor. First, the electrode potential (Ex) of the sample solution is measured in mV. Then, a small amount of sodium standard solution (∆Vs), of known concentration, is added to the sample solution several times in succession. The addition of the sodium standard increases the sodium concentration (cs) in the sample. The potential (Es) of the sample mixture is measured after every addition. The differences in potential (∆E) resulting from the known volumes (∆Vs) of added standard are used to determine the sample concentration directly with an iterative evaluation algorithm, which is based on the Nernst equation (Fig. 1). The limit of detection for sodium ions is 0.1 mg/L (or 4.7•10-6 mol/L) sodium.
In potentiometric multiple standard addition, sodium ions have to be freely available for detection by the sodium ISE. In particular, they have to be released completely from the sample matrix, and dissolved in aqueous solutions to allow for potentiometric measurements. The key factor in sample preparation for multiple standard addition is the contact area between the solvent (water) and the sample, to allow for the extraction of as many sodium ions as possible. In solid samples this is achieved by mechanical sample preparation procedures such as grinding or mixing. After homogenisation of the sample, the content determination is performed by dissolving a sample aliquot with deionised water.
Sample preparation for AAS, IC and ICP-MS is more complex. The sample preparation procedure applied in sodium content analysis by AAS is ashing. Digestion sample preparation techniques are chosen for IC and ICP-MS measurements (Bader et al., 2012; Cross, 2015). In both cases, the sample is decomposed into its elements (atomization) by means of combustion, strong heating, as well as the use of strong acids such as sulphuric and nitric acid, to completely release the elements into solution (Nielsen, 2010; Cross, 2015). These sample preparation procedures generally require additional devices and hazardous chemicals, whereas the sample preparation for potentiometric multiple standard addition analysis is straightforward without the need of time-consuming and tedious steps such as filtration or digestion.
Outline of the comparative study
Aim of this study is to compare the potentiometric multiple standard addition method with the commonly used analytical techniques for sodium determination, such as AAS, IC and ICP-MS.
Food samples were selected to cover a wide range of sodium content and food matrix variety. The sodium content analyses by potentiometric multiple standard addition technique were carried out at the Mettler-Toledo Analytical Headquarters. The remaining tests (AAS, IC and ICP-MS) were carried out at the laboratory of Swiss Quality Testing Services (SQTS, www.sqts.ch). Analyses for all techniques were performed in triplicate.
Comparison of the Results
The results demonstrate both the high precision and the excellent recovery of the multiple standard addition method compared to AAS, IC and ICP-MS techniques (see table 1 and figure 2).
As seen in figure 2, the results produced with multiple standard addition technique show good correlation with the AAS method. Recovery rate of the sodium determination of AAS compared to multiple standard addition is high, with 102.0% for the cacao sample and 98.5% for the milk sample.
Cacao and milk have very low sodium content: 200 mg/L to 400 mg/L respectively, which corresponds to 0.02 g/100 g to 0.04 g/100 g. As is evident in figure 2, the measured sodium content of the cacao and the milk samples was lower than the declared sodium content value on the respective food product labels. This finding applies to all four techniques. Regulations define wider declaration tolerances for samples with very low sodium content since they are more prone to variations and deviations (EU, 2012).
The salad dressing and cornflakes samples display only a small deviation between all techniques; the multiple standard addition technique lies well within the range of the conventional techniques.
The results obtained with multiple standard addition analysis in cheese also yielded a result comparable to the AAS and ICP-MS results. Finally, compared with the standard addition technique, the recovery rate of the sodium determination in meat by AAS and IC is 96.5% and 99.4%, respectively.
The relative standard deviations (srel) of the multiple standard addition technique are comparable to those obtained with AAS, IC, and ICP-MS. As shown in table 1, the multiple standard addition srel is small; it ranges from 0.5% to 1.2% for the cornflakes and salad dressing samples, respectively. As expected, higher relative standard deviations were observed across all techniques especially in samples with low sodium content, such as cacao powder and milk.
Accuracy and precision are the main goals in chemical analysis. Moreover, QC needs fast and straightforward analytical procedures to fulfil the demanding requirements of routine checks in food production in a reasonable timeframe.The outcome of the comparative study reveals the potentiometric multiple standard addition method to be a straightforward, accurate and reliable analytical technique. It compares very well with the established techniques, AAS, IC, and ICP-MS. In addition, the sample preparation for potentiometric multiple standard addition is straightforward and fast; in many cases it consists of homogenisation with a high-speed mixer, followed by a suitable dilution step. Moreover, unlike conventional analytical techniques, no external calibration is needed. The measurement procedure is fast and does not require in-depth operator knowledge.
Julia Werder has a degree in Food Science. For 3 years, she has worked for Mettler-Toledo as an applications specialist for titration. Prior to that, she held a position as head of quality control at a manufacturer of food and pharmaceutical products.
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