NEW USES FOR ORNL'S ULTRASENSITIVE MASS SPECTROMETER

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Oak Ridge National Laboratory has an ultrasensitive instrument that can
reduce the cost of monitoring workers for radiation exposure, determine
concentrations of trace elements in tree cores to assess the effects of
acid rain on soil chemistry and tree growth, and even identify
counterfeit bolts that need to be replaced.

Developed for ORNL's Analytical Chemistry Division (ACD), the
inductively coupled plasma mass spectrometer (ICP-MS) has also been
helpful to other ORNL divisions in their research. "The new mass
spectrometer is creating wonderful new opportunities for ORNL and the
Department of Energy," says Joe Stewart, leader of the Special Projects
Group in the division.

One area in which the ICP-MS may someday demonstrate its value is in the
DOE-required monitoring of workers exposed to radioactive materials such
as uranium. The Oak Ridge Y-12 Plant uses alpha spectrometry to check
900 to 1000 employees for uranium exposure on a monthly basis by
measuring the alpha-particle radiation emitted by uranium in samples of
the employees' urine over a 16-hour period. The alpha-counter method
requires a 1-liter sample from each worker collected over a 24-hour
period each time the test is due. "Having to collect, store, and analyze
this much urine," Stewart says, "presented obvious technical and
aesthetic problems."

Cumbersome as the alpha-counting procedure is, the Y-12 effort has been
equal to or better than that of any other facility in the DOE system.
Recent tightening of the regulations, however, makes it necessary to
detect 0.1 picocurie, or 10-13 curies, of each isotope of uranium
(U-234, U-235, and U-238) per liter of urine.

Stewart believed that the ICP-MS could be used for the urine bioassays,
but he had to find a way to improve its sensitivity. The government
standard required measurements of concentrations as low as five parts
per trillion, or 5 x 10-12 grams per gram dry weight, for the U-234
isotope. This is a quantity about one million times smaller than can be
weighed on a sensitive laboratory balance. His group chose to
concentrate on U-234, the isotope that is the most radioactive and the
most difficult to detect because it is in the lowest concentration in
natural and enriched uranium. The researchers knew that if they could
detect U-234, they could detect any other uranium isotope.

ORIGIN OF THE METHOD

While at home one evening, Stewart read about some related work being
done at Argonne National Laboratory. Researchers there had developed a
uranium extraction method for measuring uranium isotope ratios to
determine the age of dinosaur bones found in tar pits in the West. Their
method combined the best features of liquid-liquid and solid-phase
extractions to obtain pure uranium samples and eliminate the troublesome
interferences of calcium, potassium, sodium, and iron ions.

The next day, Stewart called Philip Horwitz, the Argonne researcher who
had developed the method. Horwitz agreed that the method might be useful
in solving the U-234 problem and arranged for extraction columns and
instructions for their use to be sent to ORNL. Jeff Wade, leader of the
Low Level Radiochemical Analysis Group, and Perry Gouge, a technician in
the group, began to develop the extraction procedure. Shelby Morton, a
technician in the Chemical and Physical Analysis Group, began to test
the ICP-MS using effluents from the Argonne columns.

Wade and his colleagues sought to demonstrate the effectiveness of the
chromatography material in the Argonne extraction columns to separate
uranium from urine. In chromatography, each element or compound moves
through a column at a characteristic rate based on its strength of
adhesion to the chromatography material, thus making separations
possible.

The researchers first used aqueous samples spiked with known amounts of
uranium to test the column material. After repeated successful
separations with the spikes, the researchers donated urine samples for
testing. They succeeded in separating the uranium from a 100-milliliter
aliquot of urine and concentrate it into 1 milliliter, "quite an
accomplishment considering the complexity of urine," says Wade.

The next step was to find a way to sufficiently purify the separated
uranium from the residual organic material that remained in the
1-milliliter solution. Purification was needed, the researchers found,
because even trace amounts of organic compounds would interfere with the
analysis on the ICP-MS. The researchers discovered that, if the
solutions were evaporated in the presence of nitric acid and then placed
in a furnace overnight, all of the organic material was easily
destroyed. The residue that remained could then be dissolved in 1
milliliter of weak nitric acid for the ICP-MS analysis.

Because the British reprocess nuclear fuel, they are also very
interested in new methods of monitoring workers for uranium exposure. VG
Elemental, the British manufacturer of ORNL's ICP-MS, worked closely
with ORNL and Argonne to perfect the uranium extraction and detection
processes. The instrument manufacturer built and tested the
electrothermal vaporization (ETV) attachment, which ORNL's mass
spectroscopy team now runs with the ICP-MS. Energy Systems purchased the
ETV, the first at a Martin Marietta facility.

The new extraction method combined with the ETV attachment for the
ICP-MS has drastically reduced the requirements for sample volume and
analysis time. "Now we can work with only 100 milliliters, instead of a
liter, of urine and still meet the new government standard. We've
reduced the counting time from 15 hours to 20 minutes and shortened the
overall process from three days to two," Stewart says. "That wasn't
supposed to be possible."

Alpha counting may still be used for confirmation of any samples that
test positive by the ETV method, but Stewart says that "doing the
initial screening with the ICP-MS could save taxpayers a lot of money
and help Martin Marietta and DOE take care of their people at the same
time." ORNL researchers are now involved in the process of getting their
new U-234 detection method approved for use at the Y-12 Plant by DOE and
Martin Marietta.

ENVIRONMENTAL APPLICATIONS

ORNL's Environmental Sciences Division (ESD) has also found uses for the
ICP-MS. In one of the first demonstrations of the instrument, scientists
analyzed the annual rings of tree cores to determine each tree's
exposure to toxic materials at different stages of its growth.

Exposure to toxins may be a factor in forest decline, which has been
linked to increases in acid rain and other forms of industrial
pollution. Trees in Europe have been dying for decades.

"In this country severe decline in the growth of red spruce trees has
been observed in New England and at high elevations in the Smokies,"
says Sandy McLaughlin of ESD. "We need to know why and what we can do
about it.

"The ICP-MS provides an excellent capability for measuring changes in
wood chemistry that are linked to the mechanisms by which acid rain
affects tree growth. Evaluating the trends over time in concentrations
of elements, such as aluminum and calcium, in the wood formed annually
in each tree core provides an indication of the extent to which acid
deposition has changed soil chemistry and associated tree growth."

McLaughlin and the late Ernie Bondietti had asked Wade and Morton to use
the ICP-MS to measure trace amounts of aluminum in growth rings in cores
extracted from tree trunks in a nondestructive procedure. The ICP-MS's
predecessor, an ICP optical instrument, had measured lead concentrations
in the tree rings that correspond with increases and decreases in the
use of leaded gasoline. By comparison, the ICP-MS can measure a greater
number of elements at lower detection limits in the same rings.

Using the laser ablation attachment recently purchased by ORNL,
researchers can vaporize trace amounts of elements and identify them
with the ICP-MS. The initial "quick and dirty" trial of this automated
process was done using a pencil as a surrogate core. The data were
crude, but agreed well enough with previous results that ORNL
researchers decided to continue the trials. They are currently testing
the laser on actual core samples and learning to measure precisely the
concentrations of the identified elements.

ORNL researchers have done some initial tests on fish scales, which also
have growth rings, looking for signs of mercury exposure. Future
projects include studies of oyster shells and tooth enamel.

OTHER POTENTIAL BENEFITS

Ceramics and steels are also candidates for study using the ICP-MS laser
ablation method. These substances are hard to prepare for analysis by
conventional methods. If digested--the standard method of sample
preparation--the samples are easily contaminated and tend to plate out
on the sides of their containers. Laser ablation overcomes these
problems by eliminating the digestion step and vaporizing the samples
for direct analysis by the mass spectrometer.

The ICP-MS may also help make structures safer for people. Counterfeit
bolts, sold in place of the required stronger bolts by organizations
looking for an easy profit, threaten the safety of people using tank
turrets, helicopter blades, nuclear power reactors, bridges, and
buildings. With the new laser technology, scientists can detect
counterfeit bolts in less than a minute. With one laser blast, they can
tell if the bolt is coated with the required cadmium or with a cheaper
substitute such as zinc. With a second blast, they can determine if the
composition of the bolt itself meets federal standards.

Scientists are currently working on a third method of testing whether
the bolt has undergone the required heat treatment that makes the metal
ductile instead of brittle. Once these three tests are consolidated into
one efficient detection technique, the safety of many devices can be
drastically improved by replacing the identified counterfeit bolts with
the required ones. The Department of Defense has not yet provided
funding for this project. If funding is provided by the U.S. military,
however, ORNL could test the use of the ICP-MS for detecting counterfeit
bolts.

DEVELOPMENT OF THE INSTRUMENT

The ICP-MS was developed in 1982, when British researchers replaced the
ICP's optical spectrometer with a mass spectrometer. The MS is much more
sensitive for many elements. Unlike the ICP, it can also detect
nonmetallic elements and distinguish between different isotopes of the
same element. Before the introduction of the ICP-MS, even tests for
elements such as mercury, arsenic, selenium, cadmium, and lead, which
could be detected by the ICP, often had to be run using a graphite
furnace to meet the required low detection limits. Graphite furnace
analysis is expensive and time consuming. Samples can be analyzed for
only one element at a time. Stewart knew that, if ORNL had an instrument
that could test for most or all of the desired elements at one time and
at the required low detection limits, the Laboratory could quickly
recoup its investment in the instrument.

Working together, Stewart, David Smith, and Warner Christie wrote the
specifications for the instrument they wanted VG Elemental to build for
them. "They were tough specs," Stewart says. "We wanted the new ICP-MS
to do things no instrument had done before, and we wanted an instrument
that would not become obsolete in a short time. We wanted to be able to
upgrade the instrument without expensive modifications. We wanted
attachments that might be purchased later, such as the ETV and the laser
ablator, to be perfectly compatible with the existing unit."

The instrument maker accepted the challenge, even sending an engineer to
ORNL from England to work closely with the ORNL MS section. The new
instrument arrived on December 17, 1989, and Stewart says it was "a
beautiful Christmas present." The ICP-MS, which was up and running by
the end of December, was accepted by ORNL in the second week of February
1990.

ORIGINS OF SPECTROMETRY

Spectrometry has a long history. Since the early part of the 19th
century, scientists have been able to identify elements in a material by
looking at its spectral lines--lines of colored light emitted when a
material is heated. For example, yellow lines indicate the presence of
sodium and green lines the presence of copper.

Over a century later, in the 1970s, British scientists invented an ICP
optical spectrometer that could identify and measure concentrations of
up to 40 elements at a time at detection limits 100 times more sensitive
than ever before. The key to this technology is a radiofrequency power
supply similar to one that might power a local FM radio station. It
converts an aqueous sample, such as groundwater containing trace amounts
of cadmium or lead, into a plasma--a hot mass of positively charged ions
and electrons that may reach a temperature of 6000讈. The solution to be
analyzed is instantly vaporized. As a result, it gives off light that is
diffracted by a grating into the different colors characteristic of the
elements present in the sample.

ORNL bought its first ICP optical spectrometer in 1982. At that time,
the instrument met all DOE standards for detection limits of trace
elements. About 200,000 analyses of materials, such as groundwater and
soil samples, were needed each year, and the ICP optical spectrometer
met the demand. However, the federal government lowered its required
detection limits, requiring ACD to find a new, more sensitive
instrument.

In 1985 the division began to consider the inductively coupled plasma
mass spectrometer, another product of British ingenuity. Instead of
relying on the wavelength emission of light to determine which elements
are present, the researchers using this technology measured elemental
concentrations using the ions created by the flame torch, which were
sent into an ordinary mass spectrometer. In the mass spectrometer, the
ions, each with a characteristic mass and charge, travel through
magnetic fields and are deflected according to their masses. In this
way, isotopes of the same element as well as different elements can be
identified and quantified.

The laser ablation attachment has one additional advantage: it can
analyze solid samples directly, without digesting them first. Sample
digestion usually involves heating the raw sample with a strong acid to
bring the desired analytes into solution. The digestion process is time
and labor intensive and a frequent source of sample contamination.

Future uses of the ICP-MS at ORNL have yet to be imagined. The
Laboratory is in the process of acquiring a second instrument for
special research projects. "We're now entering phase two," says Stewart,
who thinks the ICP-MS is just beginning to show its research potential.
With its help, ORNL should continue as a world leader in mass
spectrometry.

Marilyn Morgan

(keywords: mass spectrometry, radiation monitoring, radiation exposure)

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Date Posted: 1/11/94 (ktb)