Freeze-drying brings about errors in polychlorinated biphenyl recovery calculations

The influence of the freeze-drying process on the recovery of 12 chlorobiph enyl standards spiked in a wet candidate reference material (mussel) was in vestigated. Losses of spiked amounts tended to reduce with increasing numbe r of chlorine atoms in the molecule. The aqueous phase in wet materials in combination with small volumes of organic solvents introduced by spiking ma y serve as an intermediate in the loss processes observed during freeze-dry ing, facilitating differential losses of spiked surrogate standards as well as analytes originally present in the material. Overall losses will be det ermined both by the amount of polychlorinated biphenyls stored in the aqueo us phase (solubility) and by evaporation from the aqueous and tissue phases (vapour pressure) in a kinetic and complex way. Hence, recovery figures fo r surrogate standards thus obtained may lead to an overestimation of the ac tual efficiency of the analytical procedure for biologically bound polychlo rinated biphenyls. (C) 2000 Elsevier Science B.V. All rights reserved.