Drop Coating Deposition Raman (DCDR) for Microcystin-LR Identification and Quantitation

A drop coating deposition Raman (DCDR) method was developed for the analysis of 2-200 ng samples of microcystin-LR (MC-LR), a ubiquitous and deadly hepatotoxin secreted by cyanobacteria. Solid phase extraction (SPE) of the toxin from a water sample enabled identification of MC-LR at 5 mu g/L to 100 mg/L concentrations, and the collected results suggest lower detection limits can be readily attained following DCDR substrate modification. The DCDR process was applied to aqueous sample volumes of 0.5-20 mu L that generated sample deposits from which MC-LR Raman spectra could be obtained within seconds. Larger volume samples were not required to improve spectral resolution. Volumes of 2 mu L were ideal, producing "coffee-ring" MC-LR deposits that displayed distinct MC-LR Raman signals with high signal-to-noise within 1 s for a 200 ng sample and 300 s for a 2 ng sample. A linear correlation between Raman signal intensity and concentration was observed for 2-100 ng MC-LR samples after signal normalization. Reproducible MC-LR Raman spectra were collected from both fresh and aged samples. The presence of dissolved organic matter (DOM) did not preclude MC-LR identification in DCDR deposits of 3 mu g of DOM mixed with 0.7 mu g of MC-LR. Application of DCDR to environmental samples will require sample purification such as SPE before analysis, including critical cartridge wash and toxin rinsing steps. Raman based methods may one day facilitate simpler and faster sample throughput than traditional MC-LR detection methods.