Identification of an unusual contaminant in wet deposition samples: 2-ethylhexanoic acid

The presence of contaminants and interference derived from the plastic material used in sampling device is a major Achilles' heel for analytical chemists. The use of field and laboratory blanks can limit but not eliminate such a source of errors because of the wide variability of the environmental conditions to which sampler devices are exposed in the field. Reproduction of these conditions in laboratory experiments is impossible, so that a search for the source of a particular interference occurring in experimental work could be a hard task. This paper reports on a peculiar interference observed during a monitoring campaign on wet deposition in four sub-alpine sampling sites (Morbegno, Longone, Castione and Tignale) located in Lombardy, Northern Italy. During June 1996, a period characterised by very scarce precipitation, sampled wet deposition showed a large anion deficiency in ionic balance (about 50%), which could not be explained by the usual chemistry of deposition. This phenomenon seemed to be correlated with the particular meteorological condition of that period, during which an anticyclonic area developed over Italy, with high temperatures (more than 30 degrees C) and high solar radiance. This climatic situation also promoted the formation of high levels of ozone in sub-alpine areas: for example, 40 ppbv of ozone, the highest level of the year, were measured in Sondrio tin the same valley and at the same altitude as Morbegno) in the first week of June 1996.(1) The aim of this communication is to identify the source of this anionic deficiency in order to understand whether It was caused by an anomalous atmospheric pollutant or if it was due to a contaminant derived from the sampler bucket.