A liquid chromatographic (LC) method was developed for fast and simple measurement of retinyl palmitate (vitamin A) in fortified milk. Retinyl acetate internal standard was added to a test portion of milk followed by extraction into hexane. The hexane extract was analyzed by LC using a normal-phase silica gel column equilibrated with mobile phase (conditioned hexane–isopropanol, 99.85 + 0.15, v/v) about 1 h before injections. The retinyl palmitate concentration was calculated by using a relative response factor determined with calibration standards. In the collaborative study, 11 laboratories analyzed 13 pairs of fluid milk materials in blind duplicate. Twelve of the materials were composed of skim milk (<0.5% fat), 1% fat milk, 2% fat milk, and 1% fat chocolate milk. Each material was fortified at 3 concentrations of retinyl palmitate of approximately 581 μg/L (1000 IU/qt), 1163 μg/L (2000 IU/qt), and 2236 μg/L (4000 IU/qt). The 13th material, unfortified skim milk, served as a matrix blank. Repeatability standard deviations (RSDr) without outliers ranged from 1.5 to 5.7% and reproducibility standard deviations (RSDR) without outliers ranged from 5.0 to 22.7%. cis-Isomers co-eluted with the predominant trans-retinyl palmitate isomer and were included in the results reported by all the collaborative laboratories. Endogenous long-chain esters from milk fat were also measured with the retinyl palmitate additive. The Study Director recommends that this method for determination of retinyl palmitate in fluid milk by LC be adopted First Action.
Micronutrients are widely used in feed products and are very important to the health and productivity of farm and domestic animals. It is imperative to have analytical capabilities for rapid and accurate results. By using a simple nitric acid digestion of the feed coupled with inductively coupled plasmamass spectrometry (ICP/MS), rapid and accurate results can be obtained. ICP/MS has many advantages over classical wet chemical methods, atomic absorption methods, and other ICP methods: speed of analysis, reduced standards preparation, reduced waste stream, and qualitative scan capabilities. One gram portions of feed products were placed into 250 mL volumetric flasks and digested with 30 mL concentrated high-purity nitric acid for about 20 min. They were then brought to volume with ultra-high-purity water, shaken, and filtered. Ten milliliter portions were taken, and 0.5 mL internal standards were added. Standards were made up in concentrations to agree with the expected range of the desired element. Analysis of check samples from the Association of American Feed Control Officials shows that this method is comparable with other methods of choice
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