Geochemistry and Geology
Natural variation in the relative abundances of elements and isotopes of various elements can be used as a tool to explain the mechanisms behind geological systems on Earth and in the cosmos. These variations provide insights into a variety of processes, like the formation and evolution of the solar system, the birth and early history of the Earth, the evolution of the Earth surface, the evolution of the ocean chemistry and Earth climate, weathering processes, and many more.
Thermo Fisher Scientific offers the complete range of instruments required for high-precision elemental and isotope ratio analysis in a wide variety of geological samples.
For stable isotope ratio analysis, we offer:
- Thermo Scientific Delta V Isotope Ratio Mass Spectrometer (IRMS)
- Thermo Scientific 253 Plus IRMS
- Thermo Scientific Delta Ray IRIS systems
These systems are available with unique sample preparation systems, including:
- Thermo Scientific EA IsoLink IRMS
- Thermo Scientific ConFlo IV universal continuous flow interface
- Thermo Scientific GasBench II universal on-line gas preparation and introduction system
- Thermo Scientific GC IsoLink II IRMS system
- Thermo Scientific LC IsoLink IRMS system
- Thermo Scientific Kiel IV Carbonate Device
For the analysis of radiogenic and stable metal isotopes (“non-traditional stable isotopes”), the instruments of choice are:
For noble gas isotope analysis:
- Thermo Scientific Helix SFT split flight tube noble gas mass spectrometer for simultaneous 3He- 4He isotope measurements
- Thermo Scientific Argus VI static vacuum noble gas mass spectrometer optimized for Ar-Ar dating
- Thermo Scientific Helix MC Plus multicollector noble gas mass spectrometer for high-resolution noble gas isotope ratio analysis.
Geochemists have access to various analytical techniques for elemental analysis:
- Atomic Absorption Spectroscopy (AAS)
- Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)
- Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
- X-Ray Fluorescence (XRF)
Thermo Fisher Scientific offers the whole portfolio of instruments for these analyses.
Mineral structure and composition
Electron microscopy and molecular spectroscopy are established analytical techniques for the analysis of geological samples, not only providing identification of various mineral components but also offering detailed structural and chemical information. Thermo Fisher Scientific offers a wide range of microscopes and spectroscopy products:
Trace elemental analysis of geological materials
Trace element geochemistry, like the entire field of geochemistry, has many applications within the earth sciences, such as understanding the origin and evolution of our planet, understanding rock formation, undertaking provenance studies, constructing geodynamic models, understanding mineralizing processes, predicting volcanic eruptions, studying magma dynamics, determining paleo-ocean conditions, studying soil formation processes, and many more.
X-Ray Fluorescence (XRF) is ideally suited for determining elements in various materials, such as metals, slags and glasses, but also soil, aqueous solutions and air. Elements from boron to uranium can be analyzed with high accuracy, precision and reliability. XRF is a non-destructive analytical technique to determine the chemistry of a sample by measuring the fluorescent (or secondary) X-ray emitted from a sample when it is excited by a primary X-ray source. Each of the elements present in a sample produces a set of characteristic fluorescent X-rays ("a fingerprint") that is unique for that element, making XRF spectroscopy an excellent technology for qualitative and quantitative analysis of material composition.
Another powerful technique is based on inductively coupled plasma (ICP). ICP is a plasma, usually argon, that is partially ionized. It is a very powerful ionization technique and can be combined with various mass spectrometers. ICP-MS is widely used in geochemical applications for performing accurate and reliable quantification of inorganic trace elements in ppm to ppq concentration levels. Combined with laser ablation, ICP enables scientists to do in situ analysis. Additionally, coupling ICP with gas chromatography enables speciation studies of elements, such as S, Hg, Pb and Mg.
Mapping mineralogy & textures at the micron-scale
For more than 150 years – since the advent of light microscopes and petrographic thin sections – geoscientists have been examining and describing rocks. Today, the goal is still to unravel geologic history, including environmental conditions and climate, by making detailed observations of a rock's mineralogy and microstructure.
To this end, geoscientists work on quantification of modal mineralogy (major, minor, trace), grain size estimations of key components, associations between minerals and grains, and various microstructural features that may be present such as fractures, veins, and different types and styles of alteration and weathering.
Electron microscopy provides images containing a wealth of information about the texture and mineral composition of a rock. These images are used by petrologists to augment conventional optical petrographic observations, as well as provide guides for further investigations of specific areas of a sample. Automated data collection and sample holders that can accommodate multiple thin sections and polished blocks, enable highly efficient mineral analysis, with the ability to process several hundred points per second and resultant images typically containing millions of pixels.
Raman spectroscopy is also an established analytical technique for the analysis of geological samples. It not only provides a fast and efficient way of identifying specific materials, but also considerable information on molecular structure and chemical environments. The easy-to-use Thermo Scientific DXR2xi Raman imaging microscope and Thermo Scientific OMNICxi software provide a powerful, research-grade platform to explore geological history.
Radiogenic isotope ratio analysis of geological materials
In addition to using radiogenic isotopes for absolute age dating, geoscientists can use them to trace the origin of a component. They make use of the fact that isotope ratios can act as fingerprints that constrain the origin of the studied object. Variations in the abundance of isotopes such as 143Nd and 87Sr can be used to unravel the origin of a material. A material can be anything from ancient human teeth to an oceanic water mass to rocks and fluids.
As variations in isotope abundances are very small, highly sensitive and accurate mass spectrometers are required for reliable results. Traditionally, radiogenic isotope ratio analysis is performed by thermal ionization mass spectrometry, a very powerful and accurate technique. However, over recent decades, ICP-based instrumentation has also been developed for this purpose. The advantage of an ICP ionization source that it can ionize almost all elements because the argon plasma has a very high ionization efficiency.
Analysis of non-traditional stable isotopes
Non-traditional stable isotopes, i.e., the stable isotopes, excluding H, C, N, O and S, are a growing field of research. The development of multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) at the end of the 1990s enabled non-traditional stable isotope geochemistry to flourish. Light stable isotopes, such as B, Li, Mg, Fe, Ca, Cr, as well as heavy stable isotopes, such as Tl, U, are now routinely measured at a precision that is high enough to resolve natural variations.
Non-traditional stable isotopes have several distinctive geochemical features compared to traditional stable isotopes. Many of these elements are trace elements, redox-sensitive, biologically active and range from highly volatile to refractory (e.g., Ca and Ti). These features, together with the fact that many of them have high atomic numbers with more than two stable isotopes, make the different elements susceptible to different fractionation mechanisms. As such, these non-traditional stable isotopes are unique tracers of different cosmochemical, geological and biological processes.
Carbon, nitrogen, oxygen and sulfur stable isotopes
In addition to using carbon and oxygen isotopes for paleoclimate research, geoscientists use stable isotopes for a wide variety of applications. Understanding isotopic fingerprints and the sources of fractionation that lead to stable isotope ratio variations can help address a diverse array of questions ranging from ecology and hydrology to geochemistry.
Stable isotope ratio analysis is usually performed by gas isotope ratio mass spectrometry (gas IRMS), where samples are introduced to the mass spectrometer as pure gases. This is achieved through combustion, gas chromatography or chemical trapping. The detected isotope ratios are compared to those of a measured standard, enabling accurate determination of the isotope ratio of the sample. Generally, samples are combusted or pyrolyzed and the desired gas species (e.g., H2, N2, CO2 or SO2) is purified by means of traps, filters, catalysts and/or chromatography.
The most common types of IRMS instruments are continuous flow and dual inlet. In dual inlet IRMS, purified gas obtained from a sample is rapidly alternated with a standard gas of known isotopic composition. This is done by means of a system of valves, so that a number of comparison measurements can be made of both gases. In continuous flow IRMS, sample preparation occurs immediately before introduction to the IRMS, and the purified gas produced from the sample is measured just once. The standard gas may be measured before and after the sample or after a series of sample measurements.
Another technique to measure carbon and oxygen isotope ratios is isotope ratio infrared spectrometry (IRIS). Instead of using a magnetic sector instrument, the isotopes are analyzed using laser-based spectroscopy, taking advantage of the different absorption spectra of the different isotopologues of CO2 in the gaseous phase.
Speciation studies of sulfur are of particular interest to petrochemical researchers. In order to analyze isotopes of individual sulfur species within oil samples, geoscientists use gas chromatography coupled with multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Sensitivity and high resolution are key to obtaining high-precision, accurate data.
New amplifier technology
One of the major challenges in geosciences is the analysis of small ion beams. Analysis of ion beams is of particular interest for studies that focus on isotope composition of scarce materials (e.g., dust in ice cores, inclusions in diamonds, components of extraterrestrial material), or materials that have ultra-low concentration of the isotopes of interest (Hf in depleted peridotite, Re and Os in silicate rocks), but also in studies that aim to resolve isotopic variability on a small spatial scale (growth zones in minerals, teeth or hairs). Ultimately, the analytical precision of such studies is limited by the detection system of the mass spectrometer.
To address this challenge, Thermo Fisher Scientific developed 1013 Ω amplifiers, part of a revolutionary new resistor design that enables fast response times and extremely low signal to noise.
When analyzing small ion beams using multicollection mass spectrometry, precision is fundamentally limited by:
- The error introduced by counting statistics
- The electrical Johnson-Nyquist noise of the resistor used in the multiplier’s feedback loop.
From the Johnson-Nyquist noise equation, it follows that if the resistor value (and thus the output V) is increased by a factor of 100, as is the case when a 1013 Ω amplifier is used, noise will increase by only the square root of 100. As a result, when compared to a default 1011 Ω amplifier, the signal-to-noise ratio is expected to improve by a factor of 10.
The new amplifiers open the door to more applications that were not previously explored as a result of the limited detection technology. With a factor of 4-5 better precision compared to the 1011 Ω current amplifiers, scientists can now analyze sample sizes smaller than ever before. These samples include those with large isotope variations, such as melt inclusions, extraterrestrial material, dust inclusions in ice cores and nuclear safeguards. In these samples, variations can be large, but material is usually very scarce. The large variations make precision in the tens to a couple hundred ppm a requirement.
The new 1013 Ω amplifier technology has potential for use in applications that were previously conducted using a single ion counting peak jumping routine. In this method, all ion beams are measured sequentially using a single collector ion counting detector, i.e., a Daly or a secondary electron multiplier. Such analyses do not take advantage of multicollection and require reasonable ion beam stability. Sequential measurement also raises concerns about sample utilization. Precision and accuracy are also affected by at least two major ion counter characteristics:
- Linearity effects related to the dead time of the ion counter
- Mass-dependent detection efficiency of the ion counter, inducing an instrumental mass bias effect.
Both effects need proper calibration and monitoring to avoid systematic errors. For instance, the accurate measurement of a 1 MHz signal down to 0.01% requires an accurate dead time correction to <100 ps, which in itself is a challenge. Proper calibration of detector-induced mass bias effects to the required precision of 0.01% is also difficult.