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A Rapid Screening Method for 241 Am in Urine using a DIPEX Based Extractive Scintillator and PERALS Spectroscopy. Robert L. Metzger Pierre Pouquette. Need for the Method.
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A Rapid Screening Method for 241Am in Urine using a DIPEX Based Extractive Scintillator and PERALS Spectroscopy Robert L. Metzger Pierre Pouquette
Need for the Method • In the Broken Arrow Drill in Tucson, Physicians were asked to evaluate hundreds of worried well in the second phase of the accident. • The attending physicians would not chelate patients without a positive nasal smear or positive bioassay result showing a significant body burden.
Need for the Method • The presence of significant contamination on the patient’s hair, clothing or vehicle was not sufficient for the physicians to initiate treatment. • Nasal smears are only useful for about four hours after the initial exposure. • A fast urine bioassay screening method is needed with a sensitivity of ~1 ALI.
Method Requirements • Evaluate screening urine bioassay samples for actinides in a clinicallyusable time frame. • A clinically usable time frame is one in which the attending physician receives the results back in time to contact the affected patient and initiate effective treatment.
Method Requirements • For actinides, treatment normally consists of chelation therapy. This therapy only is effective if administered before the radionuclides bind to bone. Normally the treatment must be started within a couple of days of the initial exposure. • For the worried well that appear in the second phase of an accident, one to two days have already been lost before they appear for evaluation.
Method Requirements • A simple screening method that can be performed either in the hospital, or in a mobile lab just outside. • Sensitivity of 1 ALI for most actinides. • Suitable for batch processing • Fast. • 241Am chosen for this work due to prevalence in industry.
One ALI – Defines LLD • For an airborne exposure to one ALI of Solubility Class M 241Am with an AMAD of 5 μm, 1.5 x 10-3 of the intake would be expected to appear in urine one day after the exposure. • Assuming 1350 mL/day excretion, and an ALI of 6 x 10-3 μCi, an LLD of 8 pCi/L is required for the screening test. • No direct counting method can achieve this sensitivity in a short time frame.
DIPEX • DIPEX has an extraordinary k’ for Am, Pu, Th, & U. • Unfortunately, it is almost impossible to strip. • If, however, we make an extractive scintillator using DIPEX, there is no need to strip the isotopes.
Fe Interferencein DIPEX • Fe(III) is a strong interference, but can be managed by reducing it to Fe(II) with ascorbic acid.
Complexing Anions • DIPEX tolerates phosphates and sulfates up to 1 M in concentration without significantly reducing actinide retention.
DIPEX Extractive Scintillator • DIPEX, in liquid form, was purchased from Eichrom and mixed at 5 g/L in Ultima-Gold F liquid scintillation cocktail from Perkin Elmer. • Ultima-Gold F does not contain a detergent, so it is suitable for use as a solvent extraction cocktail. • Ultima-Gold αβ contains a slower fluor and is better suited to methods requiring αβ pulse shape discrimination, but is not sold without a detergent.
Approach • This screening method involves the direct extraction of 241Am from raw urine by solvent extraction with minimal preparation of the sample. • Ascorbic acid is added to reduce Fe interference and the high k’ of DIPEX is used to overcome interferences from complexing anions in the sample.
PERALS • Photon Electron Rejecting Alpha Liquid Scintillation involves the use of extractive scintillators and pulse shape discrimination to separate α and β pulses. • 99.6% counting efficiency for alphas. • Near zero background. • Small and easily transportable.
PERALS Time Spectrum • A user-controlled discriminator is set to ensure a good separation of the α and β pulses for each sample. • It is necessary to monitor each sample to ensure the discriminator is properly set for these samples.
Method • Take 50 mL of raw urine and decolor it by passing it through an amberchrome cartridge (Eichrom Industries). This is an optional step. • Make sample 0.1 M in HCL and add 20 mg of ascorbic acid. • Transfer to a teflon separatory funnel and add 10 drops of antifoam B. • Agitate for 5 minutes and allow to separate for 5 minutes. • Drain and discard aqueous.
Method • Centrifuge supernate containing the extractive scintillator and other organics from the raw urine sample. The extractive scintillator should separate from the other organics after two minutes of centrifuging at high speed on a common tabletop centrifuge. • Extract, sparge with dry argon to remove oxygen, and count for 10 minutes.
Performace • Nine spiked urine samples from office staff were directly extracted using the developed method. • The recovery of the 241Am from the samples averaged 85.5% with a standard deviation of 5%. • The limited resolution of the PERALS spectrometer prohibits the use of a tracer for 241Am. 236Pu may be used as a tracer for Pu samples.
LLD • The lower limit of detection must be determined as the upper limit of radioactivity that could be present in the sample and still yield a zero count in the 241Am window (see Hanford Procedures Manual). • With the observed 85% recovery of the Am, a 50% recovery of the volume of extractive scintillator, and a 50 mL urine sample, the LLD is 6.3 pCi/L for a 10 minute count.
Time • From receipt of the sample to the completion of the count, the total time required was less than two hours per sample. • The method can be run as a batch process, particularly if the extractions are performed in disposable centrifuge tubes. • With the exception of the PERALS spectrometers and the reagents, the remainder of the laboratory equipment needed is commonly found in hospital laboratories.
One ALI • The measured LLD is below the expected activity of 241Am to be found in urine one day after an inhalation intake of one ALI. • The method is usable as a screening tool to identify exposed individuals that should be treated after an accident involving 241Am and is fast enough to report results in a clinically usable time frame.
In the Hospital • With the exception of the PERALS spectrometers and reagents, which are portable, the method only utilizes lab equipment commonly found in hospital laboratories. • Analyses could be performed on-site, thereby reducing the time required to deliver results to the attending physicians.
Plutonium • The method will also work with Pu, but does not have the sensitivity to identify exposures at one ALI. • For high fired PuO2 (Solubility Class S), the fraction to urine one day after an exposure is only 2.1 x 10-6. • Requires an LLD of ~0.04 pCi/L urine.
Radiation Protection Assumptions • The use of radiation protection assumptions buried in bioassay and dose assessment methodologies in a clinical setting can produce massive errors due to the bias associated with these conservative assumptions. • The bias can both overestimate and underestimate the dose, depending on the setting.
Attending Physicians • The average attending physician does not understand committed dose, effective dose, LNT, weighting factors, etc. • They do not understand the difference between risk and assumed risk. • Great care must be taken in reporting intake, dose, and risk assessments to them. • They do not want “worst case” analysis as they are trying to determine a course of treatment for their patient where the treatments also entail some risk.
Example: Pu Intake Assessment • In the Broken Arrow drill, physicians were reluctant to treat patients in Phase II absent hard bioassay data. • Intake assessment based on a spot urinalysis at 24 hours (such as those derived from a method described in this work), can be problematic if conservative assumptions are used. • Default solubility classes are conservative.
Plutonium • Plutonium has a default solubility class of Type S (slow). • Most actinides have solubility profiles that are a mixture of the different classes. Even high fired PuO2 has a couple of percent Class F or M. • The fraction of a 100% Class S Pu intake that appears in urine 24 hours after an exposure is 2.1 x 10-6 • With 2% Class F and 98% Class S, the Fu at 24 hours is 4.3 x 10-5.
Plutonium • Use of the conservative default solubility class results in a 20 fold error in the intake assessment. • When operating in a clinical environment, care must be taken to remove assumptions that will bias intake and dose assessments.
