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Screening and classifying hazardous waste samples with fluorescence spectroscopy.

Timely and accurate analysis of hazardous materials is fundamental to environmental protection. In addition to the obvious necessity of identifying a suspected contaminant, a detailed analytical profile can help trace the source of a dangerous material and contribute to selecting the best containment and cleanup strategy.

A substantial body of research has clearly established the efficiency and economy of fluorescence spectroscopy as a technique for characterizing many hazardous substances. One recent effort in this area involved the use of a Spex Fluorolog-2 spectrofluorometer for detecting, classifying and quantifying samples containing petroleum oils or chemicals like polychlorinated biphenyls (PCBS) and aromatics.

The project was conceived and carried out by DeLyle Eastwood and Russell Lidberg, affiliated with the US Army Corps of Engineers. Their aim was to enlarge the utility of spectroscopic pattern recognition for environmental projects conducted by the Department of Defense under the Defense Environmental Restoration Account (DERA). The two researchers successfully applied fluorescence, low-temperature luminescence and, for limited comparative purposes, Fourier transform infrared (FT-IR) spectroscopy to the analysis of hazardous waste samples from DERA sites in Alaska and Kansas. Their work was also intended to augment the library of spectral references available for analysis of petroleum oils and fluorescent hazardous chemicals.

Eastwood and Lidberg used standard emission methods as well as excitation and synchronous scanning techniques at both room and liquid-nitrogen temperatures. With appropriate reference standards and emission methodology, samples can be quantified over a range from 100 parts per million to a few parts per billion, even where extraction is from difficult matrices such as river sediments. The experimental results compiled by Eastwood and Lidberg emphasize how spectrofluorometry can easily accomplish what is much harder to achieve via conventional approaches using gas chromatography and mass spectrometry.

Computerized search routines for classifying spectra on the basis of feature sets and similarity measures were developed and tested. For fluorescence, pattern recognition factors encompassed spectral area, peak positions and angular distance between spectra.

The Spex Fluorolog-2 research spectrofluorometer system used to acquire all fluorescence data included a singleg-rating excitation monochromator and a double-grating emission monochromator. The excitation source was a 150 watt xenon lamp. Photomultiplier tubes were employed for both emission and reference detectors.

Twenty-nine reference oil samples were obtained from the Environmental Protection Agency, Oak Ridge National Laboratory, and the US Coast Guard Research and Development Centre. These references were chosen to be representative of the principal types of petroleum oils - light fuels, heavy fuels and crudes. Standard solutions were prepared from the reference oils at a concentration of 20 [micro-m]/ g. Reference solutions for PCB analysis were prepared by dilution to concentrations of 10-30 [micro-m]/g.

Real-world samples obtained for analysis were divided into three groups. These were contaminated soil samples, neat' samples that appeared to be pure oils, and liquid samples that were not pure oils, many of which contained a water phase.

The 29 reference oils were characterized by their emission spectra as well as by data obtained from synchronous scanning. Eastwood and Lidberg have tabulated all of this information, categorizing the results of their experiments according to spectral features like maximum peak wavelengths, wavelengths for distinctive secondary peaks and shoulders, and the relative areas under corrected spectral curves.

Figure 1 shows actual emission spectra for a typical No. 2 fuel oil and a typical No. 6 fuel oil. The differences are very apparent, permitting easy classification. However, Figure 2 illustrates spectra for two No. 6 oils with slight differentiation. Furthermore, one of the spectra in Figure 3 is that of a Prudhoe Bay crude oil with spectra features analogous to those of the No. 6 fuels in Figure 2. Such close correspondence in emission spectra is very common and can lead to classification errors.

When emission data is not sufficient for conclusive sample identification, synchronous scanning can provide significantly more spectral structure with which to work. This technique entails simultaneous scanning of the excitation and emission monochromators with a constant offset between them. The recorded intensity is proportional to the product of the observed excitation and emission intensities. Accordingly, a significant difference can now be discerned between the maximum peak wavelengths of the No. 6 fuel oils in Figure 4 and the Prudhoe Bay Crude in Figure 5. Whereas the maximum peak positions were identical for the samples' emission spectra, synchronous scanning increases the difference in position by some 50 nanometers.

Figures 6, 7 and 8 provide real-world examples comparing emission spectra of unknown oil samples and library references. Figure 6 shows that an analytical samples extracted from soil will yield recognizable and useful spectra. Figure 7 reveals a close correspondence between a known Prudhoe Bay Crude and an unknown real-world sample received in isopropyl alcohol. Figure 8 is interesting because it compares a weathered real-world sample with a known sample of JP-4 jet fuel. While fluorescence analysis of samples weathered in a thin film or water is less common because of potential data distortion, research indicates that the correct analytical approach will reveal distinguishable spectra even after periods of weathering ranging from two days to several weeks.

Regarding hazardous chemicals, a 1979 Coast Guard report lists approximately 90 substances which can be readily identified by their room temperature fluorescence spectra. Eastwood and Lidberg indicate that this list could be extended to approximately 250 hazardous substances by including low-temperature luminescence/phosphorescence spectra.

Figure 9 shows a spectral comparison of PCB samples acquired at 77K, where the emission intensity is greater for phosphorescence than fluorescence. Low-temperature analysis is considerably more sensitive, allowing quantification in the parts-per-billion range. For field screening at the parts-per-million level, room temperature fluorescence appears satisfactory, since fluorescence quenching by the internal heavy atom effect is not complete.

In the future, Eastwood and Lidberg intend to use their Spex Fluorolog-2 system to expand the library of spectral references for petroleum oils and hazardous chemicals. The exceptional sensitivity and versatility of their Fluorolog-2 will also be applied to developing a more comprehensive system of classifying spectra, one that would encompass sub-classes based on spectral characterization rather than just API categories or physical properties. Information about the geographical origin of oils could also be integrated into such a framework.

Additionally, preliminary PCB measurements need to be expanded and refined to ensure better detection, identification and quantification for these ubiquitous chemicals, especially in the field. The same holds true for other important classes of fluorescing hazardous chemicals, among them organochlorine pesticides, dioxins and polynuclear aromatics (PNAs).
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Author:Eastwood, DeLyle; Lidberg, Russell
Publication:Canadian Chemical News
Date:Nov 1, 1990
Previous Article:A recent history of academic analytical chemistry.
Next Article:1990 IAPWS annual meeting.

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