NEW USES FOR ORNL'S ULTRASENSITIVE MASS SPECTROMETER This article also appears in the Oak Ridge National Laboratory Review (Vol. 26, No. 1), a quarterly research and development magazine. If you'd like more information about the research discussed in the article or about the Review, or if you have any helpful comments, drop us a line. Thanks for reading the Review. 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 6000øC. 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) ------------------------------------------------------------------------ Please send us your comments. Date Posted: 1/11/94 (ktb)