A remotely operated, field deployable tritium analysis system for surface and groundwater measurement Page: 6 of 11
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prior to introduction to the on-line purification system. The anions, cations, and organic impurities in
the sample are then removed by passing through a commercially available, water purification column.
Up to 32 sealed "tritium columns" are housed in a pneumatically actuated carousel. The tip of an
unused column is first broken off and the 50 mL aqueous sample is injected through a septum in the top
at a flow rate of ~ 5 mL per minute. The first 20 mL aliquot of eluent is automatically discarded as
being non-representative, and a subsequent 20 mL fraction is delivered to a sample reservoir equipped
with a capacitive level-sensing device. The remainder of the eluent is automatically diverted to waste.
Once the meniscus sensing apparatus is triggered, an output signal is sent to the tritium analyzer, which
then prompts a valve/pumping sequence to transfer the purified sample to the counting cell for liquid
scintillation counting. The autosampler is then flushed with deionized water and purged with air, once
the tritium analyzer begins its analysis sequence. Micro-bore tubing is used throughout the system to
minimize holdup and cross contamination. The autosampler may act as a stand-alone device, and is
enclosed in a rugged, field-portable case equipped with wheels for ready transport. It weighs
approximately 40 pounds. Figure 2 shows a schematic of the autosampler.
A Packard Radiomatic 525TR liquid scintillation counter was selected as the basic instrument
for low-level tritium measurements but was extensively modified by scientists at CAIS in order to meet
the required sensitivities. To accommodate measurement of the low levels of tritium in aqueous
environmental samples, a special quartz cell was developed to increase the aqueous volume to -5 mL
and permit measurements on a stop/flow basis; the 525TR was designed for continuous flow
measurements. The reduction of background was also critical in achieving the required sensitivity. This
was accomplished by redesign of the counting chamber and sample vial holder, increased lead shielding,
and the incorporation of Bi4Ge3O12 (bismuth germanate, BGO) windows in the counting chamber,
coupled with after-pulse counting electronics. This dual-scintillator technique has been shown to be
effective in reducing backgrounds in liquid scintillation counting systems (4).
The prototype FDTAS is enclosed in a field-portable steel housing containing additional lead
shielding. The redesigned counting chamber is made of high-purity electrolytic copper that is coated on
the interior with a white reflective paint to enhance photon collection by the photomultiplier tubes. The
chamber accommodates a specially designed -10 mL flow-through quartz sample vial of 8 mm
thickness and 40 mm diameter. A pair of 6 mm thick BGO windows are incorporated into the counting
chamber for background reduction and bracket both sides of the quartz sample vial. The counting
chamber, quartz vial, and BGO window configurations were optimized through a series of laboratory
tests and optical modeling software.
To accommodate computer-automated sequencing of remotely collected samples, a sample
collection and injection system was assembled using solenoid valves, metering pumps, and flow
controllers. All operations on the autosampler are conducted pneumatically using a combination
compressor/vacuum pump. The sequencing of valves, and monitoring of level indicators, flow meters,
and pressure switches in the autosampler are controlled by the PLC. In the tritium analyzer, the flow
system is controlled by a 24-channel relay accessory board and parallel I/O board in the local computer.
The software for spectral data acquisition and analysis is the standard Packard FLO-ONE code for the
525TR system. Both components (autosampler PLC and Packard 525TR tritium analyzer) are controlled
and monitored by custom software operating in WINDOWS 3.1 which permits the collection of samples
from multiple sites, computer automated sequencing of sample injection, cocktail mixing, sample
counting, sample outflow and system flushing and cleaning.. Two-way communications with the
autosampler and tritium analyzer is accomplished along a single telephone line using a 9600 baud
modem with multiplexed inputs from both systems.
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Cable, P.R.; Hofstetter, K.J.; Beals, D.M.; Jones, J.D.; Collins, S.L.; Noakes, J.E. et al. A remotely operated, field deployable tritium analysis system for surface and groundwater measurement, report, December 31, 1996; Aiken, South Carolina. (https://digital.library.unt.edu/ark:/67531/metadc676402/m1/6/: accessed May 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.