n-Alkanes

Information on n-Alkanes

Available n-alkanes are listed in a pdf file along with δ-values.

Download the list of n-alkanes

Various pure methanes and higher hydrocarbon gases are sealed at about ambient pressure in 9 mm o.d. Pyrex® glass tubes with an internal volume of at least 10 cm3 per tube. Each tube has a slender glass appendix at one end that can be easily snapped off for opening.

How do I open sealed glass tubes containing methane?

A tube cracker can be used to open the ampoule into a vacuum system. DesMarais and Hayes, 1976. Tube cracker for opening glass-sealed ampoules under vacuum. Analytical Chemistry 48, 1651-1652. https://pubs.acs.org/doi/abs/10.1021/ac50005a062.

Alternatively, methane can be transferred under water by scoring and breaking the tube and bubbling the methane (aided by an inverted funnel) into a water-filled inverted glass vial. Helium can be added to completely fill the vial under water before closing the vial with a septum and a crimp-seal. A gas-tight syringe can subsample the methane through the septum. The syringe should have a stopcock at its end to close off the syringe barrel before pulling the syringe needle out of the septum, in order to avoid air contamination. This simple technique renders gas at 100 % humidity. If needed, water can be frozen out by cooling the lower part of the methane-containing glass vial in dry ice before piercing the septum (kept at room temperature) with the syringe needle for subsampling of gas. Only a limited amount of liquid water should be in the vial during freezing, or the expanding ice may break the glass.

Using flexible bags

Flexible bags (e.g., Tedlar®) can be used to dilute a pure gas into various pure gases (He, N2, or "zero air") to ppm or part-per-thousand levels. Gas can first be moved from the glass tube to a septum-capped vial such as an Exetainer or Serum glass bottle using the underwater technique, followed by the use of a gastight syringe to inject a small portion (0.05 to 1 mL) into a 1-L Tedlar® bag. Thorough mixing can be accomplished by first filling the Tedlar® bag partially with the diluent, then injecting the methane (or another pure gas), and finally filling the remainder of the bag with the diluent expecting the turbulence to mix the two gases well. The solubility of methane, ethane and propane in water is small. We can store these gases in septum-capped glass vials for at least a month with no discernible isotopic changes by leaving a small volume of water in the Exetainer and storing the bottle upside-down to let water fully cover the septum. This technique offers sufficient time to repeatedly sample the gas, make dilutions and check the reproducibility of techniques.

How do I open a sealed glass capillary?

Sealed glass capillaries contain between 5 and 50 mg of n-alkane. Remove the label with a razor blade. Carefully score the capillary at several positions with a scoring knife to weaken the glass. Wipe the outside of the capillary with a clean solvent, dry off the solvent, weigh the capillary, and place the capillary into a clean glass flask. Use a clean glass or metal tool to push against the capillary and fragment it. Add clean solvent and gently heat the flask to thoroughly dissolve the contents of the capillary fragments into your solvent. Separate and recover the glass fragments from the solution, dry, then weigh the dryfragments in order to calculate the amount of n-alkane in solution. Adjust the solvent volume as needed to arrive at the required concentration.

Mixtures of n-alkanes

Mixtures of n-alkanes are dissolved in hexane and sealed as 0.5 milliliter aliquots under argon in glass ampules.  Exemplary chromatograms are available for mixtures of type A, type B, and type C. Currently available mixtures of types A, B and C are characterized compositionally and isotopically in the rightmost columns of this document. All mixtures of types A, B and C are using the same solvent hexane and often share some of the same pure dissolved n-alkane components. However, as older batches of pure n-alkanes become exhausted, newer mixtures contain replacement batches of individual, dissolved n-alkanes with different δ-values.

Mixtures of type A (i.e. 15 dissolved n-alkanes C-16 to C-30 in equal concentrations; see chromatogram of type A) and type C (i.e. 5 dissolved n-alkanes C-17, 19, 21, 23, 25 in equal concentrations; see chromatogram of type C) contain ca. 100 μmol of hydrogen H2 per compound per milliliter of solution. This is equivalent to approximately 1.4 mg of each n-alkane per milliliter of solution. These solutions are suitable for establishing the precision and accuracy of a GC-irm-MS instrument.

Mixtures of type B contain 15 dissolved n-alkanes (C-16 to C-30) with increasing concentrations along three pentads (see chromatogram of type B) covering a five-fold range of concentrations from 20 nmol H2 to 100 nmol H2 per compound per milliliter of solution. This mixture is designed specifically to test the accuracy of H3+ corrections in hydrogen-isotope-IRMS (Sessions et al., 2001a, Determination of the H3 factor in hydrogen isotope ratio monitoring mass spectrometry. Analytical Chemistry73 (2), 200-207. https://pubs.acs.org/doi/10.1021/ac000488m. Sessions et al., 2001b, Correction of H3+ contributions in hydrogen isotope ratio monitoring mass spectrometry. Analytical Chemistry73 (2), 192-199. https://pubs.acs.org/doi/10.1021/ac000489e).  In addition, the solution can be used to measure the H3+ factor under conditions closely matching those experienced by analytes.