A new, simple and fast automatic spectrophotometric flow injection (FI) method to determine glucosamine in dietary supplements has been developed and compared against a batch method, both based on the same reaction: “Schiffs base” formation as a result of the reaction between ninhydrin and glucosamine. Temperature and flow-rate were the main parameters to optimize the FI method because the reaction kinetic is slow. With the optimized flow-rate and temperature (0.8 ml min-1 flow-rate and 75 °C), the complex was measured in less than three minutes. By the batch method, 30 min of reaction, 20 min for a cooling step and 92.5 °C to measure the complex were needed. The sensitivity of the FI method was much lower than that of the batch method but the linear range was longer: for the FI method was from 0.3-% to 1-%; and for the batch method from 0.01-% to 0.04-%. The FI method allowed a throughput of 24 h-1 with a precision of 1.0-% for repeatability and of 3.5-% for reproducibility. The batch method had a repeatability of 2.1-% and a reproducibility of 4.1- %. The proposed method allowed an on-line determination with a very little human intervention and can be used in routine analysis laboratory. With this method the analysis time decreased 20 times with respect to the batch method demonstrating that the flow methods can be used in slow kinetic reactions improving also the precision.
Keywords: Dietary supplements, Flow Injection, Glucosamine, Ruhemmann complex, Spectrophotometry, Schiff's base, Reproducibility, Repeatability, Precision, Kinetic reactions
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