The decrease in sulfite recoveries seems to be caused by a slow reaction of the sample matrix with the carbonate buffer. Appar-ently, this effect does not occur when sodium hydroxide solution is present. So we concluded, that sodium hydroxide solution is best suited for sample treatment, leading to higher sulfite recoveries than carbonate buffer. However, sodium hydroxide solution was used in combination with carbonate buffer,
in order to ensure the samepHvalue,andthereforematchingretentiontimesfordifferent samples.