Instrumental Analysis, ChFE 5503
Metals in a Juice-Type Drink
(Rev. 1/27/04)
Scope: This method describes the determination of Ca, Mg, Fe, K, Na, Sn and Zn in fruit juices and similar drinks. The method may also be applicable to other elements. Dry ashing or wet oxidation can be used to prepare the sample, however these methods are time consuming (sometimes requiring up to 2 days). The juice sample can be hydrolized by a strong acid, which allows the preparation of many samples at one time. The sample is acidified, filtered, washed, and made up to volume for analysis by atomic absorption.
TYPICAL ANALYTICAL PROCEDURE
Sample Preparation: To 20 ml of juice in a 100 ml volumetric flask, add 10 ml of concentrated HCl (37%) then make up to volume with deionized water. Note: always add acid to water and beware of excess heat even using that method. If solid particles are visible in the juice, filter the sample ( using approximately 50 ml) prior to making up to 100 ml. Include all of the wash water in the final solution.
Analysis: Dilution may be necessary for elements present in high concentrations. Make dilutions so that the final solution contains 4-5% (v/v) HCl. The solution must also contain 0.5% La if calcium is being determined. Prepare standards containing the same acid (and La)
concentrations as the samples. Potassium standards should be made from KCl. Dissolve 1.907 g of potassium chloride in deionized water and dilute with deionized water to 1.00 liter in a volumetric flask. Better results for potassium may be realized if 0.1 % or more cesium or La is added to the samples and standards.
OTHER APPLICATIONS
Wine: The method above can also be used to determine metals in wines. Sodium and potassium can be determined by diluting the wine sample (the P.E. manual does not say how much dilution is required) and analyzing using aqueous samples as described above. Cu and Zn may be determined by direct aspiration versus standards containing equal amounts of alcohol. (Note: a guess would be to dilute the wine 1:10 so that the alcohol is about 1 %)
Spirits: Copper and iron (plus some other elements) can be determined in spirits, gin, whiskey, rum and similar beverages in the manner above. The samples are aspirated directly but some modifications in the flame may be needed (e.g. lean, blue flame). Standards are made up in alcohol to match the contents of the particular sample.
Beer: A method similar to the above can also be used to determine Na, K, Ca, Mg, Pb, Ni, Cu, Fe, and Zn in beer. Most elements are determined directly but if in high concentration, the sample can be diluted. Pb, Ni, and Fe may be very low which requires a solvent extraction procedure to concentrate them. All beers must be decarbonated by shaking or rapid transfer from beaker to beaker several times. Add 1-2 drops of octyl alcohol to bottled beer to control foam if necessary. Standards should have alcohol as mentioned in the wine and spirits analysis.
Reference:AAnalytical Methods for Atomic Absorption Spectroscopy,@ Perkin Elmer Instruments (2000).