Evaluation of an isotope dilution liquid chromatography tandem mass spectrometry method for pharmaceuticals in fish

An isotope dilution liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was successfully developed and applied for analysis of 15 pharmaceuticals and 2 pharmaceutically active metabolites in fish tissues. This method relied on electrospray ionization (ESI), for which the influence of sample matrix on analyte ionization efficiencies remains a persistent challenge to environmental analysis. Statistically derived method detection limits (MDLs) for most analytes ranged from 1 to 10 ng/g, independent of sample matrix, and were as low as 0.04 ng/g for the most sensitive compounds in fillet tissue. MDLs for fish fillets were determined for both 10 μL and 100 μL injection volumes; however, results showed that detection limits did not scale linearly with injection volume. Direct comparison of spike recoveries from fish liver demonstrated that isotope dilution was superior to matrix-matched calibration in compensating for matrix interference. Spike recoveries for the isotope dilution approach generally ranged from 91 to 112%, independent of tissue (i.e., fillet or liver). The developed method was applied to examine target analytes in brown trout (Salmo trutta), collected upstream and downstream from a municipal effluent discharge to East Canyon Creek, Park City, UT, USA. Though no pharmaceuticals were detected in fish samples from the upstream location, 3 and 10 compounds (out of 17 target analytes) were detected in fish fillet and liver samples, respectively, from the downstream sampling site. Pharmaceuticals in fish fillets were observed at concentrations ranging from 0.14 to 12 ng/g, while levels were markedly higher in liver tissues (range: 0.27-600 ng/g).