C1-C5 Hydrocarbons in Gas Samples
Method brief: The default method for C1-C5 involves direct injection of a gas volume into GC/IRMS, in the split mode (wellhead gas) or in splitless mode with cryofocusing. With cryofocusing, it is possible to perform carbon isotope analysis of samples with concentrations as low as 1 ppmv, such as soil gas.
Isotope ratios: KEI can determine both carbon, and hydrogen isotope ratios.
Examples of target compounds:
C1-C5 hydrocarbons (methane, ethane, propane, i-butane, n-butane, i-pentane, n-pentane).
Sample requirements:
We can work with samples provided in 12 mL exetainers or in 20 mL autosampler vials with thick Bellco stoppers. The samples should be provided at near-atmospheric pressure, to minimize the risk of failure of the closure and to assure there is enough material for analysis (if the actual samples constitute only a small portion of the container volume, the effective concentration of the sample becomes proportionally diluted). We recommend the use of pre-evacuated vials with Bellco stoppers rather than exetainers for field sampling due to their robust mechanical strength. We can also potentially accept other containers, including Isotubes, however, this should be verified in advance by contacting KEI to discuss the details. We specifically do not accept Isotubes for analysis of ppmv-level concentrations. Gas samples require determination of gas composition prior to CSIA. If such data are not available, we can process the samples, but it will increase the cost of analysis and the oeral processing time.
H N O CO₂ in Gas Samples
Method brief: The default method for permanent gases and CO₂ involves direct injection of a gas volume into GC/IRMS in the split mode. We can apply cryofocusing to CO₂ only. Our recent achievable reporting limits are conservative and based on existing QAQC data produced for specific range of concentrations, not representing best achievable values after additional optimization. Consult the lab for feasibility of analyzing lower target concentrations.
H₂ 100 ppmv, N₂ 250,000 ppmv, O₂ 10,000 ppmv, CO₂ 10 ppmv
Isotope ratios: KEI can determine the following isotope ratios: H (H₂), N (N₂), O (O₂, CO₂) and C (CO₂).
Examples of target compounds:
See the gas species itemized in the paragraph above. KEI uses well-calibrated reference materials for all these compounds.
Sample requirements:
See above, in the context of C1-C5 analysis.
n-Alkanes in Petroleum, Rock Extracts, etc.
Method brief: Samples are analyzed by split/splitless injection of low-viscosity hydrocarbon liquid (viscous samples are diluted in a solvent such as hexane) into GC/IRMS.
Isotope ratios: KEI can determine C and H isotope ratios.
Examples of target compounds:
n-alkanes up to C31-C36 (varies, depending on the level of baseline noise contributed by the sample matrix).
C4-C10 compounds in light condensate samples.
Sample requirements:
We can analyze whole oils/extracts or saturate fractions of the extracts. Sample preparation (extraction and fractionation) must be performed by the client. We recommend assessing a chromatogram (full scan GC/MS or GC-FID) of the sample prior to CSIA to determine if the sample is suitable for CSIA to confirm the sample has well-resolved peaks with an adequate signal-to-noise ratio. We can receive samples without GC data, but they will instead require screening by KEI. Whole oil, condensate, and similar organic liquids should be provided in 2-4 mL vials with PTFE-lined closures. Note that volatile hydrocarbons undergo isotope fractionation during volatilization: do not ship small aliquots of volatile liquid in larger vials (headspace should be at 10-30 % of the vial volume). Most affected compounds are those in C4-C6 range. The fractionation problem becomes increasingly negligible with an increased molecular mass of the target compound. We prefer extracts and extract fraction samples to contain at least 10 mg of the sample material (this is separate from solvent added to the vials, if any). Smaller samples can be typically successfully analyzed, but we recommend consulting us prior to shipping the samples.