Isotope Geochemistry Services
(Available forms for requesting isotopic services)
Radiocarbon (14C) Dating
Liquid Scintillation Counting (LSC) Method is currently employed in our radiocarbon dating laboratory for 14C analysis. The LSC method allows dating of samples ranging from 150 to 50,000 years old with a precision better than 2.5%. Our laboratory is equipped with a benzene synthesis system, and three low-level scintillation counters with a maximum analytical capacity of 350 samples per year. We are capable of processing a variety of samples including: charcoal, wood, bone, paleosol, coral, shell, dissolved inorganic carbon (DIC) in groundwater and landfill leachates, and landfill produced gases such as methane (CH4) and carbon dioxide (CO2). In addition to LSC dating, we also process samples for our customers who request Accelerator Mass Spectrometry, or AMS, dating.
Radiometric Tritium Analysis
Tritium (3H) is a radioactive isotope of hydrogen (1H). It is produced in the atmosphere through interaction between cosmic radiation and the air. Tritium is picked up by water vapor in the atmosphere and transferred to the earth surface through precipitation. Tritium has a short half-life of 12.43 years and is, therefore, a good tracer for monitoring recent groundwater movement and recharge in shallow aquifers.
Our lab uses Liquid Scintillation Counting Method (LSC) to determine tritium content in samples. We are capable of analyzing both liquid (ex. groundwater, leachate) and gas (methane) samples. Samples with high tritium content can be counted directly. Samples with low tritium content will go through an "enrichment" process before counting, which allows tritiums to be concentrated in the water. Three low-level scintillation counters are currently used in our lab, including two Packard Model 2000CA/LL and one Wallace Quantulus 1200-200. The precision for tritium analysis is 0.25 TU or 5%, whichever is greater.
Stable Isotope Analysis
Hydrogen and Oxygen in Water
Hydrogen and Oxygen are the two major elements in water. Ratios of different isotopes of hydrogen and oxygen (for example, 18O /16O and D/H) vary with different processes involved in the water cycle. Thus these ratios are often used to provide information about water samples such as latitude or elevation of precipitation, mixing of groundwater, and recharge sources of an aquifer. Oxygen isotopic ratios can also be used in examining the extent of water-rock interaction.
The ISGS Isotope Geochemistry Laboratory employs water-zinc reaction for analyzing hydrogen/deuterium ratios in water samples. Samples are filtered (<0.45 ôm) before analysis. Two microliters of water are sufficient for analyzing H/D ratios. Sample reacts with Zn at 500°C to form hydrogen gas. Water-CO2 equilibration is used for analyzing oxygen isotopic ratios in water. One milliliter of water is used for each oxygen isotopic analysis. For handling purpose and replicate analyses, samples are usually requested for δD and δ18O analyses of water.
A Finnigan Mat Delta E isotope ratio mass spectrometer is used for isotopic analysis of light gases such as hydrogen, oxygen, CO2 and nitrogen. Our precision is ±0.1 per mil for oxygen isotopic analysis, and ±1 per mil for hydrogen/deuterium analysis.
Carbon can be found almost everywhere on earth in different forms such as carbonate minerals, organic carbon, and dissolved carbonate species. The two most abundant carbon isotopes in nature are 12C and 13C. The 13C/12C ratio is useful for tracing the origin of different carbon species. Carbon isotopic analysis is available at the Isotope Geochemistry Laboratory for six different types of material, including:
- solid organic samples;
- carbonate samples;
- dissolved inorganic carbon in groundwater;
- dissolved inorganic carbon in leachate samples;
- methane samples;
- soil CO2 samples.
Samples are converted into CO2 through different processes for their isotopic analysis. Our precision for carbon ±0.1 per mil.
Nitrogen is the most abundant gas in the atmosphere. Through the nitrogen cycle, different forms of dissolved nitrogen species (nitrate, ammonium and nitrite) are found in water. Nitrate contamination in water system is a serious environmental problem all over the world because of intensive land usage and the employment of nitrogen-fertilizer in agriculture. The Isotope Geochemistry Laboratory is able to analyze the 15N/14N ratio of dissolved nitrate, ammonium, and the 18O/16O ratio of dissolved nitrate for source determination.
An anion-exchange column is used for extracting nitrate from water samples. Nitrate is then converted into AgNO3 salt. Combustion methods are employed to convert AgNO3 into nitrogen gas and carbon dioxide gas for nitrogen and oxygen isotopic analysis. A diffusion method is utilized for nitrogen isotopic analysis of dissolved ammonium and nitrate. Our standard deviation for replicate sample preparation is generally better than Ø 0.5 per mil.
The isotope geochemistry laboratory has used sulfur isotope analyses in coal research, groundwater research, and environmental applications. In coal, sulfur isotopes can be used to help determine the origin of the different forms of sulfur or to help follow the behavior of the different forms of sulfur (ie. organic, pryritic, sulfatic) during various treatment to coal such as pyrolysis or chemical extractions. In groundwater, sulfur isotopes are useful for determining the source of dissolved sulfate and/or determining the geochemical processes occurring which might affect the sulfate concentration in the groundwater.
Currently, our services only apply to groundwater on a case-by-case basis. For groundwater, the samples should be filtered (<0.45 µm). Dissolved iron is removed from the sample. The sample is acidified; the dissolved sulfate is precipitated as barium sulfate, and then prepared for isotopic analysis. Our prepared samples are now sent to outside labs for isotopic analysis along with several standards for which the isotopic compositions are known.
Gas Chromatography Analysis
The Isotope Geochemistry Section performs GC analysis on soil gas, natural gas, landfill gases and dissolved gases from groundwater samples. The instrument used in the lab is a Varian Model 3800, gas chromatography system equipped with both thermal conductivity detector (TCD) and flame ionization detector (FID) capable of analyzing fixed gases such as nitrogen, oxygen, carbon dioxide, carbon monoxide, and hydrocarbon gases from C1 up to C6.
Updated 09/14/2009 CAB