ENVIRONMENTAL SCIENCE & TECHNOLOGY
ES&T
The premiere research publication in the environmental field. Environmental science continues to be one of the fastest growing fields. And ES&T has grown right along with it! ES&T continues to give you the practical, hard facts you need on this science . . . covering research, techniques, feasibility, products and services. Essential reading for environmental scientists both in the business and academic world . . , ES&T has increased its emphasis on peerreviewed research dealing with water, air, and waste chemistry in addition to adding critical reviews of important environmental science issues—all relevant to understanding the management of our natural environment. Also included are discussions on environmental analyses, governmental regulations, current environmental lab activities, and much more! For rate information, and to subscribe, call toll free:
{800) 424-6747 476 A · ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
the absence of effects such as energy transfer. In PDCD, the differential absorption of left and right circularly polarized light is detected by the relative intensity differences of the fluorescence light. Thus, the FDCD spectrum is related to the CD spectrum since the intensity of fluorescence is proportional to the intensity of absorbed light at low absorbance. The major advantages gained by FDCD are greater sensitivity and selectivity. Selectivity is gained because not every chromophore in a chiral mixture will fluoresce. This technique allows the potential of isolating a single chiral fluorophore in a mixture of chiral chromophores for determination of the structure at that particular location. A theoretical discussion of the FDCD technique has already been published (41). Early publications cited some problems in the theoretical and experimental techniques (42-44), but these were later resolved (45). The first application of FDCD demonstrated the validity of the technique and showed the method's usefulness as a bioanalytical tool (46). I n s t r u m e n t a t i o n . Conventional spectrometers can be used to measure luminescence polarization by adding linear polarizers in the excitation and emission beams of light. Several commercial instruments are available with polarization attachments. Polarization spectra must be corrected to account for a wavelength-dependent polarization response inherent in the detection system (monochromator, optics, detector). There are several methods to calculate the instrumental correction constant, the G factor. Techniques for determining the G factor and further details on the theory of luminescence polarization and depolarization are given elsewhere (12). Many early experiments in the field of FDCD were done on conventional CD instruments that were modified by placing a photomultiplier tube at a right angle to the excitation beam. Recently a new multidimensional instrument has been described that acquires ellipticity as a function of multiple excitation and emission wavelengths through the use of an intensified diode array combined with a polychromatic dispersion system (47). A block diagram of this instrument is given in Figure 6. The instrument acquires complete fluorescence spectra at an excitation wavelength by illuminating the sample with left circularly polarized light and then with right circularly polarized light. The FDCD in terms of ellipticity spectra is acquired as a difference in these two measurements. The data are temporarily stored in the memory of a microprocessor-based interface that also controls the excita-
