Answer to Question #7531 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

The following question was answered by an expert in the appropriate field:

How can I concentrate a volume of oil to be analyzed in a proportional counter?

Whether or not you can concentrate the oil to the extent desired depends on a number of factors. These include the original volume of oil necessary to yield sufficient activity to be analyzed with the proportional counter, the volatility of the oil, the volatility of the radionuclide(s) of interest in the oil, and your ability to carry out the required volume-reduction procedure. You do not specify what the approximate activity concentration of specific radionuclides are so it is difficult to know what amount of oil you will require and whether alternative counting approaches may be desirable. I will briefly discuss some of these alternatives before considering the volume-reduction possibilities.

The use of a proportional counter normally applies to solid samples, although it is possible to count small amounts of liquids directly if the radionuclides are beta emitters and activity concentrations are sufficiently high. Such measurements usually require a manual counter since automatic sample changers may not be appropriate for handling liquid samples; also a thin-window detector is preferable to a windowless counter to avoid potential contamination problems. Liquid scintillation counters that have been used for measurements of radioactivity in some oil samples (for example, tritium in vacuum pump oil) provide a possible alternative, although the activity must be high enough that a few milliliters of oil will provide enough activity to be measured. Another possibility for assessing gamma-emitting radionuclides is to count a large-volume sample on a laboratory system using either a NaI(Tl) scintillation detector or a germanium detector with a Marinelli type beaker; such a container has an inverted well that fits around the detector; a large Marinelli can hold up to about four liters. If these alternatives are not acceptable, and volume reduction is required to obtain necessary sensitivity, there are a number of considerations that apply.

Oils come in a wide variety of physical/chemical characteristics. Some are very lightweight (relatively low molecular weight) with relatively high volatility at elevated temperatures and can be effectively evaporated in a moving airstream, especially when heated; some care must be taken to avoid heating the oil beyond the flashpoint and causing it to combust. Many oils, however, are quite heavy (viscous with relatively high molecular weight) and are not amenable to practical evaporation techniques. Many such hydrocarbon-based oils are combustible at elevated temperatures and it may be possible to do a controlled dry combustion. Usually this involves igniting the oil and allowing it to burn in a partially closed container with controlled oxygen ingress so as to avoid loss of liquid in the form of small droplets and associated loss of the radionuclide. Complete combustion of the liquid leaves a residue, which would contain the nonvolatile radionuclides. High-temperature treatment may be necessary to convert any residual organic tar to a carbonaceous ash suitable for transfer to a planchette for counting in the proportional counter. Naturally, such combustion procedures are not suitable if radionuclides of concern are volatile at the combustion temperatures (e.g., iodine isotopes or tritium). Combustion may be impractical on a laboratory scale if you are trying to reduce large volumes. You must abide by proper safety requirements if combustion is attempted, as some of the combustion products in the form of incompletely combusted hydrocarbons and other combustion products may be toxic. Proper ventilation must be used, and you must abide by applicable release limits that might be dictated by your facility and local and other governmental control groups.

As an alternative to dry combustion (also sometimes referred to as dry ashing), another possible route to volume reduction is a wet ashing procedure. The latter is sometimes used for treating organic samples that contain certain moderately volatile radionuclides (such as isotopes of cesium) and involves oxidizing the organic matter using strong oxidizing acids; these procedures usually require the use of relatively large volumes of nitric acid at elevated temperatures to destroy most of the organic components. Additional treatments with smaller volumes of higher boiling sulfuric acid and possibly other more extreme oxidizing acids may be necessary. I have applied wet ashing procedures to many organic environmental samples, but I have never used it for oil samples and cannot predict whether you might encounter any unusual problems. At best, such procedures are messy and can be quite dangerous, especially if one must resort to the use of very strong oxidizing acids such as perchloric acid, which is prone to producing explosive reactions with organic materials and requires special hoods free of organic contaminants in the ductwork. If you have never carried out either dry or wet ashing procedures, I would strongly recommend that you consult someone in your area who has used such techniques to obtain some more specific guidance and instruction before attempting these.

Another possible volume-reduction technique is liquid extraction, in which the oil is shaken with an aqueous solution. The nature of the aqueous solution (e.g., salt content, pH, etc.) may be varied depending on the radionuclides of interest and the physical/chemical state in which they are present. Some such extractions must be done rather gently to avoid forming oil-water emulsions that are difficult to separate. Sometimes radionuclides present in oil as metallic ions form micelle-type structures with components in the oil (the metallic ion is at the center and multiple "fingers" with hydrophobic organic molecules extend outward from the center into the oil). These are often very difficult to break up to free the ions for extraction. Sometimes extraction with strong hydrochloric acid can be effective in breaking up these structures. Again, if you have not done similar liquid-liquid extractions in the past, you should seek some help from a qualified chemist before proceeding. If extraction into an aqueous solution is successful, the aqueous solution may then be reduced in volume by evaporation to obtain a residue that can be counted in a proportional counter.

Clearly, a lot of details that are important in carrying out any of the above procedures are not dealt with here, and it is critical that, if you decide to pursue any of these approaches, you do additional research and contact qualified individuals in your area who can give you more direction. Good luck.

George Chabot, PhD, CHP

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