Methylphosphonic acid as a 31 P-NMR standard for the quantitative determination of phosphorus in carbonated beverages† Marek K~ ollo, Marina Kudrja ˇ sova, Maria Kulp and Riina Aav * Methylphosphonic acid (MPA) is introduced as an internal standard for the determination of the phosphorus content of carbonated beverages (cola drinks) by quantitative 31 P-NMR analysis. Phosphorus in cola beverages was analyzed as phosphate in ten commercially available drinks. Phosphate concentration in the studied samples was in the range of 3.5 to 6.1 mM. The preparation of the samples included: degassing of commercially available beverages, addition of MPA as an internal standard, D 2 O for signal locking and a solution of NaOH to ensure a basic environment in the sample. The calibration graphs of the MPA standard and analyte showed a linear response in the concentration range of interest. The measured values of the phosphoric acid standard solutions lay within the confidence limits of the MPA, showing that these compounds give a similar response in NMR analysis. The measured concentrations of phosphorus were in good agreement with the values detected by the traditional spectrophotometric method. The presented 31 P-NMR method using MPA as an internal standard is reliable and simple for the determination of the phosphorus content of cola type beverages. Introduction Phosphorus is an essential element and makes up about 1% of the adult body. 85% of body phosphorus is in the bones and the remaining is distributed throughout the so tissues. In blood and extracellular uid, an inorganic phosphate compartment is located, into which phosphate is inserted upon absorption from the diet. The recommended daily allowance of phosphorus for adults is 700 mg. 1 Phosphoric acid and its phosphates are used as food addi- tives for acidity regulation and chelation, and as antioxidants or as enhancers for other antioxidants. Phosphates can be found in carbonated beverages, chocolate, processed meat, cheeses, dressings, fats, oils and bakery products. There are several methods used to quantify phosphate concentration in food. The Association of Official Analytical Chemists (AOAC) recommends a colorimetric method, applying antimony phosphomolybdate. 2 International standard ISO 6878 species spectrophotometric determination of phosphorus using ammonium molybdate in water. 3 The determination of phosphorus in beverages has been described several times by colorimetric analysis using molyb- date/vanadate, 4 molybdate 5,6 color reagents and pH titration. 4,7 Among other methods, ion chromatography, 8 ow injection methods with IR 9 and colorimetric detection applying thy- molphthalein 10 have been reported. Nuclear magnetic resonance spectroscopy is a good method to gain simultaneous qualitative and quantitative data in the analysis of food products. 11 The most widely applicable analysis method is proton NMR, which was recently well reviewed by Mannina et al. 12 The carbon NMR is probably the most attractive method for qualitative analysis of mixtures, however due to low natural abundance of the magnetically active 13 C isotope, simultaneous quantitative information is difficult to collect. 13–18 Quantitative 31 P-NMR analysis accuracy has been shown to be comparable to chromatographic methods. 19 31 P-NMR has also been proven to be a good method for food analysis and has been covered in a review by Spyros et al. 20 Phosphates have been analyzed by 31 P-NMR in milk, 21,22 meat 23,24 and olive oil. 25,26 So drinks have been analyzed only by 1 H-NMR 27,28 but, somewhat surprisingly, we were unable to nd any literature data on the determination of phosphates in beverages by 31 P-NMR. While 1 H-NMR has been shown to be a suitable quality control method for so drinks, 27 an additional 31 P-NMR measurement from the samples under study would clearly enrich the available data. Also methylphosphonic acid has not been evaluated as a stan- dard for the quantitative 31 P-NMR analysis. In this paper, we describe a new method for the quantitative determination of phosphate in so drinks by 31 P-NMR, applying methylphosphonic acid (MPA) as an internal standard. Department of Chemistry, Faculty of Science, Tallinn University of Technology, Akadeemia tee 15, Tallinn, 12618, Estonia. E-mail: riina@chemnet.ee † Electronic supplementary information (ESI) available: Experimental and calculation details. See DOI: 10.1039/c3ay40743h Cite this: Anal. Methods, 2013, 5, 4005 Received 3rd May 2013 Accepted 27th May 2013 DOI: 10.1039/c3ay40743h www.rsc.org/methods This journal is ª The Royal Society of Chemistry 2013 Anal. Methods, 2013, 5, 4005–4009 | 4005 Analytical Methods PAPER