Determination of Soft Drink Ingredients

Soft drink formulas often include preservatives, artificial sweeteners, flavours or caffeine in their list of ingredients. Using the Acclaim OA column, as many as eight common additives may be determined in a single run. Many of these additives are hydrophilic organic acids for which this column was designed. Notably, benzoate and sorbate, which do not resolve on C18 columns at low pH, are fully separated.

Materials and Methods

All equipment was supplied by Dionex Corp., Sunnyvale, California, USA, including the HPLC system and Acclaim OA 4 × 150 mm column.

Mobile phase A: 14.2 g/L Na₂SO₄ (Sigma) and 0.550 mL/L methanesulphonic acid (Fluka); nominal pH is 2.65.

Mobile phase B: Methanol (Fisher).

The flow-rate was 0.80 mL/min, and the column temperature was set to 30 °C. The detector was monitored at 210 nm, with alternate wavelengths of 230 and 262 nm.

Samples were prepared by filtration through a 0.45 µm nylon membrane filter, and diluted 5× with mobile phase A. 10 µL was injected.

The standard was prepared in mobile phase A from individual stock solutions of 1000 µg/mL.

Discussion and Conclusion

The range of hydrophobicities for these additives precludes isocratic methods. At 210 nm, a plain gradient ramp would result in a sloping baseline that makes quantification difficult, therefore, a three-step solvent program was chosen. All the peaks elute in flat portions of the baseline.

Figure 1

Ordinary C18 reversed-phase columns are not suitable for this application for two reasons. First, the separation of erythorbic and citric acids requires a purely aqueous buffer, and conventional C18 columns tend to suffer from dewetting under these conditions. Second, sorbate and benzoate require the different selectivity of a polar-embedded stationary phase to separate them at the low pH needed for the other acids. The Acclaim OA is an aqueous-compatible, polar-embedded column that is designed for the resolution of hydrophilic organic acids, so it is well suited to this application.

Other flavour ingredients and colourings do not interfere. This method does not fully resolve erythorbic acid from ascorbic acid, nor aspartame from saccharin. Fortunately, these combinations are not usually seen in commercial soft drink formulas. Therefore, one may make an alternative standard mixture to accommodate a particular formula, without altering the conditions of the method.

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