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The Precise World of Isotope Ratio Mass Spectrometry "Isotope ratio MS (IRMS) is probably the first form of analytical mass spectrometry," says Thomas Brenna of Cornell University's Division of Nutritional Sciences. First developed in the 1920s, basic features of IRMS instruments have slowly changed over the decades. "Now," Brenna says, "techniques are evolving at an accelerated rate." IRMS is used today by a small but diverse community of researchers, many of whom still focus almost exclusively on determining isotope ratios. "These researchers regard it as an application tool," observes Brenna. The IRMS community includes geochemists, ecologists, food scientists, nuclear engineers, and pharmacologists. In a broad sense, IRMS is used to track the natural cycle of the elements. Applications range from determining human energy use to dating rocks to testing for adulterated foods. The field is further subdivided because each element is best measured on one of three types of specialized MS instruments. Brenna represents the gas IRMS community, which focuses on isotopic ratios for light elements (C, H, N, 0, S); Donald Bogard, a space and planetary scientist at the Johnson Space Center, provided advice on noble gas instruments; and geochemist Richard Walker from the University of Maryland at College Park
Applications dominate this small and fragmented market
pic ratios, including glow discharge and ICPMS, optical emission spectroscopy, ion microprobe, and NMR But, as a rule, these techniques lack the high precisions of the instruments in Table 1. In an isotope ratio experiment, samples are ionized and deflected along a curved path by the magnetic mass analyzer according to their m/e value. Instruments are often run with the slits wide discussed isotopic ratios by thermal ioniza- open to let all the ions through. Ion currents from two or more beams are coltion MS (TIMS)) A sampling of commercial instruments lected simultaneously by different detectors. For quantitative measurements, it is from five manufacturers is listed in Table important that detectors collect and record 1; other systems, some marketed for specific applications, are mentioned in the text all of the ions for a particular mass. "Sensitivity is the key," says Brenna. Response below. Micromass is the group that just completed a buyout of VG Organic's mass linearity is another important factor in judging instruments stresses Chuck spectrometer line, formerly owned by Fisons. Analytical Precision, Ltd. has just Douthitt of Finnigan. started production. Instruments must handle widely varying isotopic compositions. For example, says Bogard, the 3He/4He ratio in the Some points in common Earth's atmosphere is 1.4 x 10"6, but in luIRMS is a classic analytical technique in which many tried and true instrumental fea- nar soil exposed to the solar wind the ra4 tures are used. All of the instruments llsted tio is as high as 4 x 10" . in Table 1 are magnetic sector mass specAll of the experts had favorable comtrometers with multiple Faraday detectors ments about their commercial IRMS inoptimized to detect isotopes of interest Ad- struments. "The cost of yearly operation is dition of electron multiplier detectors exrelatively inexpensive," says Walker. tends the detection system's dynamic range This comment is echoed by the other by measuring weaker ion currents. Other mass spectrometrists. "Magnetic fields methods are capable of determining isotochange quickly and settle down quickly," Analytical Chemistry News & Features, June 1, 1996 3 7 3 A
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Walker finds. This allows operators to switch or "peak hop" rapidly to different masses, which is an important feature on some instruments. The experts also noted improvements in vacuum systems, especially with the introduction of the newer turbomolecular pumps, and in detector design. Ensuring that detectors collect all the ions from the mass analyzer poses a technological challenge. As manufacturers
ets, says Walker. "If you see 142Nd [isotoadd more Faraday cups to measure isopic] anomalies, it suggests a major chemitopes simultaneously, the size of these cups is getting smaller, notes Walker. This cal fractionation event" To find these anomalies, a TIMS instrument must measure can lead to problems with ions escaping differences at a part in 40,000 to 50,000. from the detector or stray ions for other masses falling into the wrong detector, as All of the experts said that they found well as degradation of the cups. It is one current software systems somewhat inflexiarea in which manufacturers are making ble for their needs—the one discordant improvements, adds Walker. note in the evaluation. Manufacturers report work on upgrading instrumental softFor example, the ratio of 142Nd/144Nd may hold clues to the early history of plan- ware.
Table 1. Summary of representative products
INA = information not available
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cf = continuous flow
Analytical Chemistry News & Features, June 1, 1996
Gas IRMS
Gas IRMS instruments are optimized to detect variations in the stable isotopes of C, H, N, 0, and S, generally as the gases C02) 0 2 , N2, H2, and S0 2 . "[Instruments] are optimized for best precision on a few masses," says Brenna. Thus, an IRMS with a three Faraday cup detector system set for C0 2 would collect masses 44,45, and 46. It's been estimated that as much as 70% of the analyses run are for isotopic ratios of C0 2 (2).
Traditionally, gas IRMS used a dual viscous-flow inlet system, developed in the 1950s, to introduce sample and reference gases for ratio measurements. The inlet was designed to circumvent the isotopic fractionation of samples prior to ionization in the source. Beginning in the late 1970s, researchers (2) developed a different inlet for introducing samples in a He stream. This new inlet system, called isotope ratio monitoring or continuous flow (cf), allows signifi-
cantly smaller samples to be measured than the dual inlet, as little as ng or picomole quantities. It has also opened up IRMS to new hyphenated techniques, including those using GC and an elemental analyzer (EA). Brenna predicts that eventually the cf inlet will completely supplant sample introduction by dual inlet for all but the most exacting measurements. In EA-IRMS, the sample undergoes flash combustion prior to introduction into the MS. This technique has been used to
Analytical Chemistry News & Features, June 1, 1996 375 A
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analyze isotopic ratios for C, N, H, and S in such matrices as soil and leaves, as well as in pure materials. As one example, EAIRMS is used to identify honey adulterated with corn syrup based on differences in their carbon isotope ratios. Capillary GC-IRMS is used to analyze atmospheric constituents such as C0 2 and N20 at ppm concentrations or, with preconcentration, at ppb levels. A broader range of compounds can be analyzed by adding a combustion unit after the GC, which converts chromatographically separated analytes into gases detectable by IRMS. "[GC-C-IRMS] allows compound specific analysis," says Brenna. "The technique is fairly routine now, but requires good baseline GC separation." Currently, most GC-C work is aimed at carbon and nitrogen isotope ratios, but other elements, including hydrogen, are amenable to the technique. The GC effluent can also be divided so that part of the sample undergoes combustion and analysis by IRMS while a second portion is diverted to another MS for structural analysis, points out Douthitt. With isotopically enriched samples, GC-C-IRMS can measure as little as femtomole quantities. GC-C-IRMS has emerged as a powerful technique and is the basis of new methods to detect several difficult natural analytes. For example, the technique is being used to identify athletes who misuse synthetic testosterone to boost their performance. Analysis of the 13C/12C ratio for testosterone, two of its natural precursors, and a metabolite in urine samples reveals variations in the isotopic ratios when synthetic testosterone is present (3). Combustion techniques need plenty of reagent oxygen, which therefore rules out isotopic analysis for that element. Pyrolysis-GC-IRMS, GC-pyrolysis-IRMS, and EApyrolysis-IRMS are now being developed to address this problem in organic compounds. Other techniques under development include laser ablation to volatilize materials like carbonates and microcombustion for dissolved organic carbon. A commercial LC combustion interface for carbon compounds may also be a future possibility, says Brenna. "Handling samples is a field by itself," he adds. In addition to the instruments listed in Table 1, a host of gas IRMS instruments are on the market Several are dedicated to specific cf techniques. Finnigan markets the delta plus and the MAT 252, modular IRMS systems to accomodate all EA and GC techniques. Dedicated GC-C-IRMS instruments built around 376 A
the delta plus and MAT 252 are also available. Finnigan also sells the MAT 271— a magnetic sector IRMS for ultrahighprecision gas analyses such as calibration gas mixtures or nuclear fuel reprocessing. Finnigans' MAT 281 is used for uranium hexafluoride analysis. Finally, Finnigan sells the THQ or thermionic quadrupole MS system, a low-cost alternative to magnetic sector systems. The THQ offers negative ion mode and accomodates the stable isotope dilution method. Micromass offers EA, GC, GC-C, and laser ablation inlets under its Isochrom brand. These are built around the manufacturer's Optima instrument. Micromass also markets Prism, a larger geometry (50-cm dispersion radius) stable IRMS with a four-Faraday bucket, adjustable collector system. With its larger radius, the Prism can determine isotopic ratios for sulfur as SF6. Prism comes with many of the same options as Optima. Europa Scientific offers a number of systems, all built around the basic design of its
Thermal ionization
Geochemists have become surprisingly adept at correlating isotopic ratios with a variety of phenomena. Strontium, for example, can be used to date marine carbonates because the element enters seawater via the weathering of the continents, and thus its isotopic composition varies overtime.Radioactive 234U is used to date corals, but because corals live only at certain ocean depths, the uranium dating also indicates past sea levels. TIMS instruments are highly sensitive detectors designed to tackle this wide variety of isotopes. "A large number have been sold to geology departments," says Walker. "No facility is complete without one." TIMS is also heavily used by the nuclear industry to analyze fuel rods. In TIMS the chemically purified elements to be analyzed are placed on a filament for thermal ionization. Both Micromass's Sector 54 and Finnigan's MAT 262 provide multifilament turrets for running samples sequentially. Each filament is used for one sample only. Filament metals, such as Ta or Re, are sold as ribbons and spot welded to posts. Walker estimates that he spends $2000 to $4000 each year for filaments but can recycle the precious metals. A TIMS instrument is only half of what researchers such as Walker need to run their samples. This community also requires a classical analytical laboratory to prepare samples. Materials are digested in acids and run through several ion20-20 IRMS. ORCHID ii its sedicated GC- exchange columns to purify and separate C-IRMS which, according to Europa's Si- analytes from the sample matrix. Walker says that he may begin with as much as mon Prosser, contains ion optics specifically for cf operation. ANCA is a specific line 20 g of rock to obtain as little as 20 pg of Os for isotopic analysis. New elementfor use with raw samples in environmental and nutritional research. Hydra is a cf sys- specific resins are an aid in these procedures, although the extra costs may limit tem especially for measuring hydrogen in their use. "The real way to do [this rea He stream, a complex problem because 1 2 search] is to have good clean chemistry " H H is easily swamped by 3He and the tailing of the 4He peak. To handle this probAs expected, the more refractory elelem, the 20-20 was modified to provide im- ments such as Os are the most difficult to proved resolution of the m/z 2, 3, and 4 analyze. Walker, in fact, relies on Re and peaks (4) says Prosser Os isotopic ratios to date iron meteorites, A novel application of IRMS is spurred which are otherwise difficult to date. For by studies that suggest a contributing fac- Walker, a major advance, introduced tor in ulcers is an infection by Helicobac- around 10 years ago, was negative-ion ter pylorii Thii sacterium produces large TIMS to determine these difficult-to-ionize amounts of urease in the gut. Ingestion of elements as the negative ion metal oxide. For an element like osmium, says Walker, a 13C urea tracer leads to an increase in 13 C02 in exhaled breath. Micromass (Iso- negative ionization rates reach as high as 1% with the metal oxide. "Parr of the evoluchrom-uG), Europa Scientific (ABCA tion of TIMS in earth sciences is learning [automated breath carbon analyzer]), to run elements as different species." NegFinnigan (BreathMAT), and Analytical Precision, Ltd. (2003 Breath) market dedi- ative ion is a standard feature of TIMS systems cated, automated 13C breath analyzers.
GC-C--RMS is a fairly routine method that allows compound specific analysisi
Analytical Chemistry News & Features, June 1, 1996
The other big advance, says Walker, has been improvements in the mass abundance sensitivity on TIMS instruments. For example, the 234U signal appears as a small peak off a larger peak. To improve abundance sensitivity, Finnigan and Micromass offer second-stage filters as an option. Finnigan's second stage filter is described as a retardation potential quadruplle that offers < 10 ppb abundance sensitivity; Micromass offers the wide aperture-retarding potential filter with an abundance sensitivity of < 10 ppb. Micromass also fits its instrument with a Daly detector, which replaces electron multipliers for ion counting. "We've never replaced one in the field," says Steven Fulkerson of Micromass. He says they also provide a very linear response for detecting minor isotopes and weak ion currents. Noble gas According to Bogard, the noble gas community is the smallest of the three IRMS groups. Traditionally, it was a community characterized by home-built or modified instruments and largely used by cosmologists like Bogard. In the last decade, says Bogard, the numbers of noble gas MS instruments being sold to terrestrial geologist have risen, and most new purchasers have relied on commercial instrumentation. Although he warns, "a lot of this work is still not cookbook." The growing interest among terrestrial researchers in noble gas IRMS was sparked by lunar sample studies that showed the value of Ar dating and the recognition that the He and Ne isotope ratios from the Earth's interior weren't the same as those from the atmosphere. In addition, noble gas analysis has found applications in the search for oil and gas, and in the nuclear industry where fuels are labeled with trace amounts of Xe by the Department of Energy says Fulkerson. (Kr and Xe are the two of the rarest elements on Earth adds Bogard.) In noble gas MS analysis, researchers need to "protect the vacuum as much as possible," says Bogard. These instruments require a very good vacuum in order to analyze samples with typically very low concentrations of noble gases. For example, Xe in many meteorites is found at concentration levels of about 10"16 mol/g, notes Bogard. Static vacuum systems do not use dynamic vacuum pumping during analysis. As a result, attainable background is an important factor in judging commercial instrument performance, says David Olsen
of Mass Analyzer Products. For example, background species such as HC1+ or CJ can interfere with 36Ar detection. Therefore, instruments are pumped down for some time, prior to analysis, and often baked out at temperatures as high as 300° C. In a typical experiment, a solid sample is melted under ultrahigh vacuum to unlock gases, says Bogard. After chemically active gases such as 0 2 and N 2 are removed with hot metals, the remaining pure noble gases are admitted to the ion source. Samples can be as small as 0.01 mg, and a researcher will typically collect one gas sample and measure the isotopic spectrum several times to insure accuracy. Varying the magnetic field to measure different isotopes ("peak jumping") is also a common practice. In addition to the 5400, Micromass also markets the smaller 3600 (36 cm) 1.1 L static vacuum MS system, which is often dedicated to Ar dating. Mass Analyzer Products also markets the 216, which is a dedicated Ar detection system with a 15-cm radius, 1.2 L-static volume, and retractable Faraday and electron multiplier.
Laser applications are among the new frontiers for noble gas IRMS systems, according to Bogard. Lasers are being used as an in situ probe to raster across samples. The laser can then be used to release noble gas samples from a particular phase in materials with mixed geological phases. This technique allows researchers to investigate smaller samples. There have also been experiments with resonance ionization of targeted elements by use of lasers. Finally experiments have measured noble gas isotopes by time-of-flight MS but says Bogard the magnetic sector instruments still have the best mass resolution Alan Newman References (1) Brand, W. A./. Mass Spectrom. 1996,96, 225-35. (2) Matthews, D. E.; and Hayes, J. M. Anal. Chem. 1978,50,1465-73. (3) Becchi, M.; Aguilera, R.; Farizon, Y.; Flament, M.-M.; Casablanca, H.; and James, P. Rapid Commun. Mass Spectrom. 1994, 8,304-08. (4) Prosser, S. J.; Scrimgeour, C. M. Anal. Chem. 1995,67; 1992-98.
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