Pulsed, visible and near-infrared laser light is coupled into an optical fiber, which is wound into a loop using a fiber splice connector. The light pulses traveling through the fiber-loop are detected using a photomultiplier detector. It is found that once the light is coupled into the fiber it experiences very little loss and the light pulses do a large number of round trips before their intensity is below the detection threshold. Measurements of the loss-per-pass and of the ring-down time allow for characterization of the different loss mechanisms of the light pulses in the fiber and splice connector. This method resembles “cavity ring-down absorption spectroscopy” and is well suited to characterize low-loss processes in fiber optic transmission independent from power fluctuations of the light source. It is demonstrated that by measuring the ring-down times one can accurately determine the absolute transmission of an optical fiber and of the fiber connector. In addition it is demonstrated that the technique is useful as an absorption spectroscopic technique of very small sample volumes. A solution of an organic dye was placed between the fiber ends instead of the usual index matching fluid, and an absorption spectrum of 7×10−15 mol of the dye 1,1′-diethyl-4,4′-dicarbocyanine iodide in 7×10−12 L of dimethylsulfoxide was recorded.

Interpretation of toxicity test results may be hampered when doubt exists about the actual exposure concentration. Processes that are responsible for differences between the nominal and the actual concentration in aqueous test systems may include sorption, precipitation, volatilization, chemical and biological degradation, and uptake into biological or test tissue. In this study, the use of a poly(dimethylsiloxane) (PDMS) film containing the test compound is introduced as a versatile technique for partition controlled delivery of hydrophobic compounds to aqueous toxicity tests. Two methods developed produced preloaded films, having toxicant added to the PDMS prepolymer solution before film deposition and curing, and postloaded films, which are created by the addition of toxicant in a solvent to an already-polymerized PDMS film. Preloaded films were generally more easily prepared, may better accommodate larger molecules, and have a higher capacity than postloaded films. Postloaded films provided film-solution partition coefficients with higher precision and allowed for the use of films from stock and thus for a more portable technique. Chemical analysis showed that equilibrium between films and the aqueous solution was established within 7-10 min and was maintained for a suite of aromatic compounds (log Kow ranging from 2.8 to 6.1). The reliability of the film technique was demonstrated by application to the Microtox bacterial toxicity tests of solutions of polycyclic aromatic hydrocarbons (PAHs).

The most potent carcinogenic aromatic hydrocarbon known to date-dibenzo [a,Z]pyrene (DB[o,Z]P)~has been directly determined in crude extracts of several soil and sediment samples by applying Shpol'skii spectroscopy, a low-temperature high-resolution molecular fluorescence technique. Unambiguous identification and quantitation of DB[o,Z]P was possible on the basis of vibrationally resolved fluorescence spectra obtained under site/energyselective tunable laser excitation and cryogenic sampling conditions. Owing to a relatively long-lived fluorescence ACKNOWLEDGMENT This research was conducted within the long-term fellowship program financed by the European Environmental Research Organization (EERO). Joop Harmsen and Rick Wieggers (The Winand Staring Centre, Wageningen, The Netherlands) aregreatly acknowledged for supplying the extracts. We also thank Steven Kok, Gerard Hoomweg, and Ronald van de Nesse for excellent technical support during LESS measurements. Special thanks to Freek Ariese for drawing our attention to the DB[a,Z]P analysis and valuable comments.

Spiro-BODIPYs with a diaryl chelate unit have been found to form J-aggregates in methanol-water solvent mixture and brightly emissive in the solid state. The diaryl chelate unit has a significant impact on J-aggregates and fluorescence of BODIPYs. Crystal structural analysis reveals that the spiro-structures facilitate J-stacking in the solid state.