Environmentalists Toast New Liquid Scintillation Cocktails

Many biochemical studies routinely performed by researchers, such as protein assays and DNA studies, require the detection of extremely minute quantities of material. However, such small quantities are usually undetectable by the Lowry assay, UV spectroscopy, and other conventional methods of chemical analysis. This problem was solved with the development in the 1940s of chemical tests that use radioisotopes to label the material in question before it is assayed. This technology has allowed scientists to detect substances at extremely low concentrations with extraordinary sensitivity. Radioisotopes are alternative, unstable forms of common elements that carry extra particles (neutrons) in their nucleus. As a result of spontaneous nuclear decay, radioisotopes emit nuclear radiation. There are several kinds of emitted radiation, including alpha-particles, beta-particles, and gamma-rays. The isotopes most commonly used in radioisotopic labeling are those that emit á-particles, such as 3H, 35S, and 32P. Energy from the emitted á-particles can be easily detected, making these particles a sensitive label for many biochemical assays. To detect substances labeled with a radioisotope, researchers often use the process of liquid scintillation. A sample labeled with a radioisotope is dissolved or suspended in a solution called a scintillation cocktail, an organic solvent that contains one or more fluorescent chemicals, such as PPO (2,5-diphenyloxazole) or POPOP 1, 4- bis- 2- (5-phenylox-azol)-benzene). The á-particles emitted from the sample enter the solvent. As the solvent absorbs the á-particles, the solvent molecules become excited, jumping to a higher energy level. When the excited molecules return to the ground state, they release energy in the form of a photon of very short wavelength light. The fluorescent substance, the fluor, in the cocktail acts as a wavelength shifter by absorbing the short wavelength photons and re-emitting them at a longer wavelength. Thus, the energy of emitted alpha-particles is converted to a flash of light. A photomultiplier tube in a liquid scintillation spectrometer detects this flash of light and converts it into a voltage. The spectrometer then translates the voltage into a numerical representation of the amount of radioactivity in the sample. This information is given as counts per minute, or cpm. The efficiency of the liquid scintillation process depends on the properties of the scintillation cocktail. Most liquid scintillation cocktails use aromatic ring compounds, such as dioxane, toluene, and xylene, for the solvent because they efficiently absorb the energy of the emitted á-particles. However, these substances are hazardous and toxic, and therefore present a safety problem to workers in the laboratory. Solvent-based cocktails generally have a low flash point, earning them an NFPA (National Fire Protection Association) rating as a flammable liquid, one that readily ignites and burns with intensity. In addition, these types of solvents readily vaporize, and some can penetrate the walls of the plastic vials used for scintillation counting, thus releasing hazardous vapors into the laboratory. The toxic vapors can cause respiratory tract irritation and even central nervous system damage. Because of the inhalation hazard associated with these solvents, they must be used and stored in a fume hood. ICN Biomedicals Inc. P.O. Box 19536 Irvine, Calif. 92713 (800) 854-0530 Products (prices are for 1 gallon): Ecolume - $83 Ecolite(+) - $83 Universol - $77 Cytoscint - $75 BetaMax - $45 National Diagnostics 1013-1017 Kennedy Blvd. Manville, N.J. 08835-2031 (800) 526-3867 Products (prices are for 4 liters): Ecoscint H - $58 Ecoscint A - $58 Ecoscint O - $28 Research Products International Corp. 410 N. Business Center Dr. Mount Prospect, Ill. 60056-2190 (800) 323-9814 Products (prices are for 4 liters): Bio-Safe II - $55 Bio-Safe NA - $30 Moreover, solvent-based liquid scintillation cocktails pose an environmental hazard as well as an occupational one. The U.S. Environmental Protection Agency (EPA) classifies toluene, xylene, and dioxane, the three most common solvents, as toxic waste. According to current EPA guidelines, established in the late 1980s, these solvents can no longer be disposed of in landfills, but instead must be sent to EPA-certified facilities for incineration. In the meantime, vials containing the cocktail must be stored in a fume hood in the laboratory, a practice that takes up valuable space and is potentially dangerous. Disposal of these toxic and hazardous chemicals represents a major expense for the researcher. With the recent increased emphasis on environmental awareness, researchers have developed new environmentally safe liquid scintillation cocktails to take the place of solvent-based solutions in the lab. These products, marketed by a growing number of companies, are reportedly completely biodegradable, converting to carbon dioxide and water following disposal. All of the components are nonhazardous and nontoxic, thus eliminating the occupational risks associated with conventional scintillators. Unlike solvent-based cocktails, which are rated as flammable by NFPA, the environmentally safe cocktails are rated as combustible, meaning that they will not readily ignite unless the temperature exceeds 100øF. The evaporation rate is also much lower, which virtually eliminates the possibility of toxic vapor buildup in the laboratory. Owing to the recent deregulation of radioactive waste disposal, these cocktails can be disposed of down the drain as normal liquid waste when they contain only low levels of some types of radioactive material (subject to local, state, and federal regulations). National Diagnostics, headquartered in Manville, N.J., makes three biodegradable scintillators, Ecoscint H, A, and O, which can be used with aqueous or nonaqueous samples. These products are not flammable and are considered environmentally safe in accordance with EPA regulations. The Ecoscint solutions cannot permeate plastic, as do conventional cocktails, and do not readily vaporize, which means that they can be used on a bench top without the need of a fume hood. The company claims that the Ecoscint solutions are comparable in efficiency to solvent-based cocktails, providing researchers with high counting efficiency without the associated health hazards. The Mount Prospect, Ill.-based Research Products International Corp. has developed the Bio-safe line of environmentally safe liquid scintillation cocktails, Bio-safe NA and Bio-safe II. Bio-safe NA is formulated especially for use with nonaqueous samples and filters, while Bio-safe II is a complete counting cocktail. These products boast a very high flash point (in excess of 300øF). They are completely biodegradable and even come packaged in environmentally safe amber glass bottles that can be recycled. These formulations provide an efficient transfer of energy from the radioactive sample to the fluors, resulting in high counting efficiencies for many different types of samples. ICN Biomedicals Inc., based in Irvine, Calif., markets a full line of ready-to-use biodegradable cocktails that can be used in place of conventional cocktails in any application. For scientists interested in switching from a solvent-based cocktail to an environmentally safe formulation, ICN provides guidelines on making the proper choice without loss of counting efficiency during the conversion process. Samples of the company's products are available at no cost to the researcher for evaluation and comparison with solvent-based cocktails. Manufacturers claim that environmentally safe liquid scintillation cocktails are comparable in efficiency and overall cost to the more conventional solvent-based cocktails. Switching to biodegradable liquid scintillation cocktails greatly reduces the personal safety hazards associated with use of aromatic solvents and reduces toxic waste. Moreover, since disposal of toxic and hazardous waste has become a major environmental issue in recent years, interest in environmentally safe products is expected to continue to grow. Holly Ahern teaches immunology and cell and molecular biology at the State University of New York, Albany.

Environmentalists Toast New Liquid Scintillation Cocktails

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