|Isotope Ratios||% Composition||Sample Type|
|Elemental Analyzer (EA)|
Thermal Conversion Elemental Analyzer(TC/EA)
Solid samples and fresh water
Solid (Carbonates only)
Total Inorganic Carbon/Total Organic Carbon Analyzer (TIC/TOC)
Water (fresh water only)
|Gas Chromatograph (GC)|
13C/12C 2H/1H 15N/14N
|Mixtures of organic compounds (solutions and gases)|
Elemental Analyzer (EA)
The Carlo Erba NA1500 Series II EA is currently used for the determination of 13C/12C and 15N/14N, and for elemental composition (%C, %N). Samples (solid inorganics and organics) are weighed into tin capsules and introduced into the EA via an autosampler. They are flash combusted with oxygen in a 1020-1050 °C furnace. Helium carrier gas moves the resultant gases through oxidation and reduction reactors and a water trap. The gases (CO2, N2) are then separated on a GC column before entering the IRMS via a continuous-flow interface.
The types of samples analyzed with this equipment include minerals, soils and sediments, and many types of biological specimens (fish tissue, collagen, plant material, etc.).
The ThermoElectron GasBenchII is used for the determination of 13C/12C and 18O/16O in carbonates. Samples are weighed into glass vials and capped. The vials are placed in a heated block (50 °C) and flushed with helium prior to injection with phosphoric acid, which generates CO2 and water. The headspace of each vial is sampled using helium as carrier gas. The resultant gases pass through Nafion™ tubing for removal of water and are separated on a GC column prior to entering the IRMS.
Total Inorganic Carbon/Total Organic Carbon (TIC/TOC) Analyzer
The OI Analytical Aurora 1030W TOC Analyser is used for the analysis of freshwater samples for inorganic and/or organic carbon content by wet chemistry. First, phosphoric acid is added to the sample, which converts inorganic carbon to CO2 (TIC). Then, a sodium persulfate solution is added to convert organic compounds into CO2 (TOC). The CO2 gas evolved from each reaction may be analyzed independently. It is carried by helium through a Nafion™ dryer, halide scrubber and water trap before entering an infrared detector (for %C). It then passes through a reduction furnace and an additional water trap, and finally through a GC column before entering the IRMS for isotope ratio analysis (13C/12C).
Thermal Conversion Elemental Analyzer (TC/EA)
With the Thermo Scientific TC/EA, samples are pyrolized at 1400 – 1450 °C in an oxygen-free atmosphere over glassy carbon in a continuous flow process. Oxygen and hydrogen in the sample are quantitatively converted to CO and H2, respectively, which are separated on a GC column before being introduced into the IRMS. This allows for determination of18O/16O, 2H/1H, and %O and %H. The TC/EA has a zero-blank autosampler for solid samples (weighed into silver cups), and a PAL autosampler for water analysis.
We are currently developing methods for the analysis of Ag3PO4 (precipitated from PO43- in bones of archeological significance) for 18O/16O, and for the analysis of water samples for 18O/16O and 2H/1H.
Gas Chromatograph (GC)
The Agilent 6890N GC in conjunction with the DeltaVPlus II allows for compound specific isotope analysis. The sample (solution or gas) is injected and separated in its components on a GC column. When the individual components elute from the GC column, they can either be oxidized (for 13C/12C and 15N/14N determination), or pyrolized (for 2H/1H determination).
Oxidation to CO2 and H2O (and NOx if the compound contains N) takes place in a reactor at 940 °C. After passage through a reduction reactor, where NOx is converted to N2, and through Nafion™ tubing for removal of H2O, the CO2 and N2 are directed into the IRMS for 13C/12C or 15N/14N determination of the individual compounds (for 15N/14N determination, a cold trap is needed to remove CO2).
This set-up is currently used mainly for the analysis of 13C/12C of fatty acids from soil and fish, CO, CO2 and C1-C4 hydrocarbons in gas mixtures, and amino acids in soil, fish muscle and fish otoliths. For amino acids, 15N/14N determination is also possible. Note that both fatty acids and amino acids need to be derivatized in order to be amenable to GC analysis.
For pyrolysis, the separated components are directed through a reactor at 1400°C, in the absence of oxygen. Under these conditions, all bound hydrogen is converted to H2, and compound specific 2H/1H determination is possible. This is currently mainly used for the analysis of CH4, where hydrogen isotope ratios, in combination with carbon isotope ratios, are characteristic for its origin (bacterial or thermogenic).