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 clinically
usable 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
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 Interference
in 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.
PERALS Time Spectra
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.