MASKING AS A MECHANISM FOR EVAPORATIVE LOSS OF TRACE ANALYTE, ESPECIALLY AFTER SOLID-PHASE EXTRACTION

Using a Pasteur pipette plugged with silanized glass wool and packed with C-18-silica particles, we attempted to remove K2CO3 from an aqueous acetonitrile solution. In spite of extensive washing of the column with water after the sample was applied, elution with acetonitrile followed by evaporation gave a visible, white residue. It was found that the residue was derived from both the sample and the packing, including particles from the latter. Substitution of a plastic column/polyethylene frit for the Pasteur pipette/glass wool gave a more consistent residue, apparently because this improved the retention of particles. Subsequent experiments were conducted in the plastic hardware. The amount of the residue was observed to vary as much as 19-fold when C-18-silica particles were tested from different manufacturers, and the residue could be reduced in amount as much as 9-fold when a column was prepared in the laboratory vs. the use of a comparable, pre-packed column. The water itself contributed some of the residue: even the ''purest'' water routinely available left a visible residue when 1.0 mi was appropriately evaporated (e.g. on Saran Wrap in a microwave oven). The recovery of an arbitrary trace analyte and internal standard (pentafluorobenzylated nucleobases at the low pg level) was 32% less when they were evaporated in acetonitrile that had been passed through an acetonitrile and water-washed cartridge containing C-18-Si vs. evaporation in untreated acetonitrile. Collectively these results reveal that an evaporation can risk some loss of an analyte from masking by even subtle solvent contaminants. This tends to counteract the common strategy of redissolving a trace analyte by using a good solvent for it.