Effect of Test Portion Mass on Vitamin A Testing in Animal Feed Materials

BACKGROUND: Vitamin A test results have historically been notorious for poor repeatability and reproducibility. This problem has been discussed at length in Association of American Feed Control Officials Laboratory Methods and Services Committee meetings. OBJECTIVE: The objective of this work was to assess the effect of test portion mass on the repeatability of vitamin A test results. METHODS: The study was conducted in two parts. In Part I, fundamental sampling error (FSE) was determined experimentally through replicated (n = 16) vitamin A testing of three animal feed materials. The testing followed rigorous test portion selection for 10 g and 100 g test portions. In Part II, FSE calculations were made (1) using theoretical equations based on vitamin A as a liberated analyte and (2) on representing the particles in feed materials. Particle size characterization of vitamin A ingredients was estimated by microscopy and further evaluated by particle size analysis. RESULTS: RSDs, % for vitamin A determinations ranged from 10.5–24.7, and 2.26–10.7 for 10 g and 100 g test portions, respectively. FSE calculated for Ingredient A ranged from 18.3–101% and 5.79–32.0% for 10 g and 100 g test portions, respectively, and for Ingredient B, ranged from 10.2–56.2% and 3.21–17.8% for 10 g and 100 g test portions, respectively. CONCLUSION: Test portion mass has a substantial impact on FSE and is an important factor in controlling the random error in vitamin A testing. FSE equations are useful to approximate minimum test portion mass. HIGHLIGHTS: Vitamin A method development should use theoretical predictions and experimental verification to guide test portion mass. Strategies to deal with the larger test portion masses will be key to validating new methods.