product review
Plasma Opens New Doors in Isotope Ratio MS The development of multiple-collector inductively coupled plasma systems allows previously difficult stable isotope studies of transition metals. Michael J. Felton
F
rom the discovery in 1912 that neon atoms had different masses, and therefore additional neutrons, the isotopes of elements have been used to achieve nuclear power, date fossils, deduce the chronology of Earth’s surface, determine illegal drug use in athletes, and detect adulteration of food (1). They may even be used to prove or disprove the existence of life on Mars. Isotope analysis is basically determining the ratio between two isotopes of the same element in a sample. This isotope ratio is of interest because it is changed by various natural and synthetic processes that fractionate isotopes or, in the case of radioactive isotopes, radioactive decay that forms lighter isotopes. Measuring isotope ratios has become a highly specialized field, which uses mass spectrometric techniques to achieve spectacular precision. “Basically, you can do isotope ratios on [any mass spectrometer]; the question is, what do you need as far as precision and usage?” says Chuck Douthitt of Thermo Finnigan. For example, an instrument must be able to measure relative errors of 20–50 ppm on a measurement of 0.011200 to accurately determine 13C/12C isotope ratios. Three mass spectrometric techniques are used for highly precise isotope ra-
tios: isotope ratio mass spectrometry (IRMS) for stable isotopes of lighter elements (light, stable isotopes); thermal ionization for radiogenic and some heavy stable isotopes; and the newest technique,
multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS) has opened up the analysis of heavy stable isotopes, which were all but inaccessible with previous methods.
M A R C H 1 , 2 0 0 3 / A N A LY T I C A L C H E M I S T R Y
119 A
product review
Table 1. Summary of representative MC-ICPMS products. Product
Neptune
Nu plasma
Nu plasma 1700
Company
Thermo Finnigan 355 River Oaks Pkwy. San Jose, CA 95134 408-965-6800
Nu Instruments, Ltd. Unit 74, Clywedog Rd. South Wrexham LL13 9XS North Wales, United Kingdom Tel: +44 (0)1978 661304
Nu Instruments, Ltd. Unit 74, Clywedog Rd. South Wrexham LL13 9XS North Wales, United Kingdom Tel: +44 (0)1978 661304
Website
www.thermo.com
www.nu-ins.com
www.nu-ins.com
Ion source
ICP
ICP
ICP
Mass analyzer
Electrostatic analyzer and magnetic sector
Electrostatic analyzer and magnetic sector
Electrostatic analyzer and magnetic sector
Detectors
Up to nine Faraday cups and eight secondary electron multipliers
12 Faraday cups and up to 3 ion-counting multipliers (fixed adjustments by ion optics)
10 fixed Faraday cups, an additional 6 movable Faraday cups for high-dispersion beams
Sample introduction
Nebulizer or laser ablation
Microcyclonic spray chamber with low-flow Microcyclonic spray chamber with low-flow nebulizer, desolvator system, or laser abla- nebulizer, desolvator system, or laser ablation system tion system
Sensitivity
>30 V/ppm
35 V/ppm at U (2.2 Gcps)
50 V/ppm at U (3.1 Gcps)
Mass resolution (10% valley)
R = 8000 (edge resolution)
500
1000–10000
Mass range
3–310
3–280
3–280
System software
Multicollector software package (Windows Windows 2000 software (Linux version from University of Oxford) 2000)
Additional features
Largest Faraday cups, zoom optics, modular Pseudo high-resolution, enhanced interface Enhanced interface pumping and highpumping, and high-abundance sensitivity detectors (swappable) abundance sensitivity options. options
Light, stable isotopes Stable isotope ratios of H, C, N, O, and S are used in a wide range of fields, including geology, environmental science, and medicine. For instance, plants tend to fractionate carbon isotopes during photosynthesis, allowing lighter 12C to evaporate while retaining heavier 13C and, thus, deviating from the atmospheric ratio of carbon isotopes (2). Oxygen isotope ratios in carbonate fossils have been used to determine past water temperatures, because heavier isotopes with lower vibrational energy, and thus slightly lower reactivity, will preferentially incorporate into compounds with higher vibrational frequencies at thermodynamic equilibrium. These same compounds will lose lighter oxygen isotopes because they are slightly more reactive and will break free. Therefore, the temperature of the water in which a carbonate fossil formed can be deduced from a ratio of 16O to 18O. 120 A
Systems used to determine the isotope ratios of these light elements first convert the samples to a set of common gases, such as CO2, N2, O2, H2, and SO2 (1). These gases are then fed into a dual-inlet mass spectrometer that typically uses electron impact ionization. The two inlets allow the feed to the mass spectrometer to be switched between a sample gas and a standard gas, which has a known isotope ratio. The offset in isotopic composition between the sample and standard is therefore determined independently of any mass fractionation by the instrument. Moreover, by using the same standard as a reference for all samples, the isotope fractionation between samples can be readily measured, according to Ariel Anbar of the University of Rochester. These dual-inlet mass spectrometers are limited to light elements that can be converted to common gas-phase molecules relatively easily.
A N A LY T I C A L C H E M I S T R Y / M A R C H 1 , 2 0 0 3
Windows 2000 software (Linux version from University of Oxford)
Thermal ionization Isotope ratios of some heavier elements can be determined using thermal ionization as a source for MS. Thermal ionization typically uses filaments of rhenium or tungsten to heat, vaporize, and ionize micro- or nanograms of sample that have been deposited as a salt or oxide on the filament. Because the filament is heated, lighter isotopes come off first, and the isotopes are fractionated to a much greater degree than in light, stable IRMS. Several improvements have been made to thermal ionization MS (TIMS) instruments in the past several decades. “The first thermal machines were single collectors,” says Douthitt. “Then multiple collectors were added, followed by multiple moveable collectors, then negative-ion [detection and] second stages for lower sensitivity. And the latest step has been multiple ion counting.” Experimental techniques are just as
product review
Table 1. Summary of representative MC-ICPMS products (continued). Product
IsoPlatform ICPMS
Isoprobe-ICPMS
Company
Micromass Waters Corp. 34 Maple St. Milford, MA 01757 800-252-4752
Micromass Waters Corp. 34 Maple St. Milford, MA 01757 800-252-4752
Website
www.micromass.co.uk
www.micromass.co.uk
Ion source
ICP
ICP
Mass analyzer
Hexapole and quadrupole
Hexapole and magnetic sector
Detectors
Dynolite photomultiplier and conversion Up to nine Faraday cups and one fixed dynode and seven movable ion counters
Sample introduction
Direct injection, concentric or ultrasonic nebulization, or laser ablation
Direct injection, concentric or ultrasonic nebulization, or laser ablation
Sensitivity
>2 108 cps/ppm U
>109 cps/ppm U
Mass resolution (10% valley)
