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 w ith C-18-silica particles, we attempted to remove K2CO3 from an aqueou s acetonitrile solution. In spite of extensive washing of the column w ith water after the sample was applied, elution with acetonitrile foll owed by evaporation gave a visible, white residue. It was found that t he residue was derived from both the sample and the packing, including particles from the latter. Substitution of a plastic column/polyethyl ene frit for the Pasteur pipette/glass wool gave a more consistent res idue, apparently because this improved the retention of particles. Sub sequent experiments were conducted in the plastic hardware. The amount of the residue was observed to vary as much as 19-fold when C-18-sili ca particles were tested from different manufacturers, and the residue could be reduced in amount as much as 9-fold when a column was prepar ed in the laboratory vs. the use of a comparable, pre-packed column. T he water itself contributed some of the residue: even the ''purest'' w ater routinely available left a visible residue when 1.0 mi was approp riately evaporated (e.g. on Saran Wrap in a microwave oven). The recov ery of an arbitrary trace analyte and internal standard (pentafluorobe nzylated nucleobases at the low pg level) was 32% less when they were evaporated in acetonitrile that had been passed through an acetonitril e and water-washed cartridge containing C-18-Si vs. evaporation in unt reated acetonitrile. Collectively these results reveal that an evapora tion can risk some loss of an analyte from masking by even subtle solv ent contaminants. This tends to counteract the common strategy of redi ssolving a trace analyte by using a good solvent for it.