1) n-Alkanes


Available n-alkanes are listed in a pdf file along with δ-values (http://mypage.iu.edu/~aschimme/files/list%20of%20n-alkanes.pdf).

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 (e.g., DesMarais and Hayes, 1976. Tube cracker for opening glass-sealed ampoules under vacuum. Analytical Chemistry 48, 1651-1652. http://dx.doi.org/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.

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.

Higher n-alkanes are available either pure as liquid or solid substances, or as dissolved mixtures of selected n-alkanes in hexane (mixtures are discussed separately below). Pure n-alkanes are flame-sealed in glass ampoules or capillaries, or crimp-sealed in glass vials. Some n-alkanes are stored as a drop of solidified wax at the end of a glass stick inside of a crimp-sealed glass vial. Hexatriacontane (i.e. C-36 n-alkane) is a lightweight powder that may be difficult to weigh. I recommend to gently melt the powder and pour the melt onto a sheet of cold and clean (e.g., pre-annealed) heavy-duty aluminum foil. The solidified wax can be peeled off easily, fragmented, and filled back into a glass vial.

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 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 http://mypage.iu.edu/~aschimme/files/list%20of%20n-alkanes.pdf. 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 Chemistry 73 (2), 200-207. http://dx.doi.org/10.1021/ac000488m. Sessions et al., 2001b, Correction of H3+ contributions in hydrogen isotope ratio monitoring mass spectrometry. Analytical Chemistry 73 (2), 192-199. http://dx.doi.org/10.1021/ac000489e).  In addition, the solution can be used to measure the H3+ factor under conditions closely matching those experienced by analytes.
How do I use n-alkane mixtures for calibration? Typically an n-alkane mixture is injected between samples. The n-alkanes in the mixtures vary in δ2H by over 200 per mil, and have been calibrated relative to both VSMOW and SLAP standards. The n-alkanes can therefore be used to normalize isotopic measurements to the accepted VSMOW-SLAP scale. The following example is based on experience and measurements using a customized Finnigan MAT 252, but similar observations are expected from Finnigan Delta +XL and newer mass spectrometers (Alex Sessions, pers. comm.).  For example, when using a C-16 to C-30 mixture (types A or B), C-18 and C-28 can serve as reference peaks with assigned δ2H values. The other δ2H values are then calculated by the computer relative to C-18 and C-28. The resulting individual raw δ2H values determined by GC-irm-MS can be plotted versus those determined by off-line combustion of pure compounds (as listed on the Certificate of Analysis).  Ideally, the points fit well along a straight line with a linear regression R2 larger than 0.999. At WHOI, the slope of the line may vary from 0.92 to 1.01, but it is typically stable to about ± 0.02 over the course of one day. The Y-intercept varies from about -2 to +2 per mil. Thus there is no evidence for substantial internal fractionation in the WHOI instrument, although there is some δ2H -dependent effect which causes the slope of the line to vary. For each day, a normalization served to calibrate the instrument with regard to a particular slope and intercept (Alex Sessions, pers. comm.).
How do I open a sealed glass capillary with a free sample of an n-alkane mixture? Carefully remove the label from the capillary.  Centrifuge the liquid in the unopened capillary to one end. Score the end of the capillary where the air resides. Clean the exterior of the capillary with solvent. Break off the tip of the capillary at the score mark with a clean tool. Use a clean syringe to retrieve the needed volume of the n-alkane mixture. You can store an opened capillary with n-alkane mixture by placing it upright into a glass vial that has some liquid hexane at the bottom. The headspace of the closed vial will become saturated with hexane and thus limits the loss of hexane from the opened capillary.
How do I store an n-alkane mixture?  Partial evaporation of the solvent hexane from an open glass ampoule containing an n-alkane mixture can be avoided by transferring the mixture into a different container that can be closed tightly for storage.  Some laboratories prefer glass containers fitted with Mininert® valves (e.g., http://www.restek.com/catalog/view/148) that allow the insertion of a syringe needle through a narrow valve opening for taking samples for injection. The vapor pressure of hexane can be reduced by storage of containers in a refrigerator.
What are the contaminant peaks in the n-alkane mixtures?  All mixtures of types A, B and C are using hexane from the same original solvent bottle (Burdick and Jackson) that has developed small concentrations of contaminants over the years.  A Burdick and Jackson solvent guide contains a chromatogram detailing those peaks as the result of auto-oxidation of hydrocarbons (i.e. hexane).  The peaks correspond to a class of compounds called hydroperoxides.  Passing a sample of the hexane through a pipette packed with freshly baked glass wool and silica gel filter can remove the peaks. However, the position of the peaks does not interfere with the n-alkane peaks in mixtures of types A, B and C, and therefore a cleanup of the original solvent is deemed unnecessary.
Why is there no (M)SDS for the n-alkane mixtures?  It is unpractical to issue a single (M)SDS for the solvent and the 5 to 15 individual dissolved n-alkanes. The solvent hexane constitutes more than 99 wt. % of each mixture. From a practical point of view, the (M)SDS of hexane is the single most relevant (M)SDS. The dissolved n-alkane components are present in negligible concentrations and represent non-hazardous oily or waxy, inert compounds.
What can be the reason for unexpectedly small peak size?  Normally, n-alkane peak heights should be in the 2 to 6 V range (m/z = 2). For low-molecular n-alkanes which are not fronting, 100 ng of injected on-column should yield ~1 V (= 10 nA) peak height for mass 2. The following checks are recommended if peaks appear to be too small:  1. Check for leaks with a He sniffer through a GC temperature sequence while using the mass spectrometer to measure argon to see if leaks are opening up as the GC oven heats (sometimes a ferrule is tight at one temperature and opens up at higher or lower temperature).  2. Confirm the concentration of n-alkanes in your n-alkane mixture, either by running the mixture on a different GC (perhaps with a FID), or make your own comparative concentration standard using your own n-alkanes and solvent. Higher molecular weight n-alkanes may not completely dissolve in hexane if the temperature is too cold. 3. Sometimes problems occur in an open split when the split ratio is affected by the movable capillary slipping up or down. 4. The temperature of the TC unit might be lower than the controller is indicating, and thus the conversion of hydrocarbons to H2 may be limited. As a test methane can be injected and mass 16 be monitored.
Which GC program and column should I use for aliphatics?  Hardware and GC-programs vary widely among laboratories. However, some users have provided examples: GC-program for n-alkane mixtures:  Injection 50 °C in split mode, split flow 50 milliliter/min, oven 100 °C, ramp 20 °C/min to 180 °C, ramp 5 °C/min to 315 °C, hold 16 min.  Some laboratories inject in splitless mode and inject 2 or more microliters of sample. In that case, the n-alkane mixtures can be diluted 10-fold. GC-columns for liquid injections: 0.32 mm ID, film thickness of 0.25 to 1.0 μm, length 15 to 60 m. Most commonly 30 m, 0.32 mm ID, 0.25 um film thickness.  It is recommended to use a low column bleed GC-MS column with cross-linked stationary phases designed for MS work.  A DB-1 type column with a 1-μm film thickness has a high capacity for alkanes and can reduce peak fronting as a consequence of overloading. Many laboratories prefer the versatility of a slightly more polar DB-5 type column.
Why is the δ2H value of first eluted n-alkane not measured correctly? Several groups reported that the δ2H value of the first eluted compound (C-16 n-alkane) in mixtures of types A, B and C seems to be systematically shifted and should be ignored. The problem is likely attributed to conditioning and/or memory effects and can be mitigated if a sacrificial C-14 or C-15 n-alkane is co-injected to serve as the initial "throw-away peak" to condition the system (Nikolai Pedentchouk, pers. comm.). Inter-peak memory effects in compound-specific hydrogen stable isotope analysis by GC-TC-IRMS have been documented (Wang and Sessions, 2008, Memory effects in compound-specific D/H analysis by gas chromatography/pyrolysis/isotope-ratio mass spectrometry. Analytical Chemistry 80 (23), 9162-9170. http://dx.doi.org/10.1021/ac801170v). Systematic δ2H changes with peak size were observed as the peak size (mass 2 area) for an artificially enriched analyte (δ2H = +650 ‰, mass 2 at ~5000 mV) increased by a factor of 10. The measured δ2H values systematically increased by ~130 ‰ and trended toward a constant value at still higher concentrations. The identified culprit is a “background hydrogen” component with a δ2H value in the range of -250 to -200 ‰ that mixes with the analyte hydrogen. As a result, the measured δ2H value for analytes with δ2H > -200 ‰ is smaller than the real value due to the involvement of a more 2H-depleted background hydrogen. The influence of background hydrogen is diminished in larger peaks as background hydrogen is diluted. At the same time, the influence is enhanced for small analyte peaks with strong 2H-enrichment.  Background hydrogen may derive from a combination of column bleed, water in the carrier gas and/or air leaks, and a slow release of resident hydrogen from pyrolysis tubing, all of which are likely to vary between different labs and over the use of instruments. While it is difficult to quantify the abundance and δ2H value of background hydrogen in individual GC-TC-IRMS systems, its isotopic influence on measured δ2H values can be accounted for by using calibration and normalization standards with similar δ2H values and peak sizes as those of peaks of unknown samples. Bilke and Mosandl offered additional information on conditioning, memory effects, and the need for proper reference materials (Bilke and Mosandl, 2002, Measurements by gas chromatography/ pyrolysis/mass spectrometry: fundamental conditions in 2H/1H isotope ratio analysis. Rapid Communications in Mass Spectrometry 16 (5), 468-472. http://dx.doi.org/10.1002/rcm.599).