Directly coupled sample treatment-high-performance liquid chromatography for on-line automatic determination of liposoluble vitamins in milk.
We developed an on-line system for the determination of liposoluble vitamins A, D3 and E in milk, both liquid and in powder form, using an automated sample treatment system coupled to chromatographic determination. For this, C18 cartridges were used because of the strong capacity of this material for the extraction and preconcentration of such vitamins, its ease in handling and the possibility of coupling it with automatic analysis systems. Alkaline hydrolysis of the samples was performed in an on-line system comprising two confluent channels through which the sample solution and alcoholic sodium hydroxide plus ascorbic acid flowed for a given period of time. A third channel merged with the other two to neutralize the solution before it arrived at the C18 cartridge. The latter, inserted into a loop with a six-port injection valve, retained the soluble vitamins. The vitamins were eluted with a stream of methanol and the eluate was automatically injected into the chromatographic system. Variables affecting the on-line system were optimized: sample size, flow-rate, preconcentration, washing and elution times, etc. The recoveries for powdered and liquid milk for the three vitamins assayed ranged between 80 and 105% (n = 10). Additionally, a day-to-day precision (10 days) of the method of 4.5% was obtained for the different vitamins.