A three-layer feed-forward neural network was used for the prediction of the polarity/polarizability parameter π 2 H . A simulated annealing algorithm was used to minimize the error at the neural network output. Descriptors related to the structure of the compounds were calculated as the input vector. The Kohonen neural network was used to split the data set into training and testing sets. The results obtained from the neural network were compared with the MLRA results.

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An additivity model of the apolar solute-solvent parameter log(L16) was verified using sets of 939 nonaromatic and 1075 aromatic compounds. Unbiased distributions of errors and of the contribution significance level were statistically tested. An analysis of the CH2 group contribution in 34 homologous series indicates that the differences among the homologous series are statistically insignificant and related to interactional contributions rather than to the nature of the CH2 group.

Sir, -In a recent article in this journal Seveik [I] described "the application of MDSS (multidimensional switching system) to the separation of naphthalenelbenzo(b)thiophene in an environmental sample ofcoal tar".Theauthorstill used two packedcolumnsof different polarities in his switching device. We would like to make the following comments on this paper:1) The idea of regulating the flow of solutes from one GC-column to another by specific external pressure regulations is not a new one, but was introduced by Deans [2] in 1968. The great advantage of this principle is the avoidance of contact between the solutes and any heated metal surfaces within valves situated in the GC oven. This idea was realized as two-dimensional packed columnlglass capillary gas chromatography [3,4] and glass capillarylglass capillary chromatography [5,6] in our laboratory between 1972and 1975.2) In environmental analysis of polynuclear aromatic hydrocarbons(PNAs),the higherboiling,4-6ringedcompoundsare ofparticular interest because of the carcinogenic properties of many PNA-species in this range. This fact is recognized in international

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