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Skidmore College
Stable Isotope and Palæoclimate Analysis Laboratory

Analytical Services: δ13C and δ15N of combustible material

The SPA lab analyzes the δ13C and δ15N composition of combustible material using a Costech Elemental Combustion System (ECS) 4010 elemental analyzer (EA). Combustion gases are scrubbed of sulfur-bearing compounds, halides and water and then reduced to CO2 and N2. Next, the gases are separated in time using a GC column (50°C, 80 mL/min). The EA is coupled to a MAT-253 IRMS for isotopic analysis via a Conflo IV containing an open split interface. The SPA lab uses two-point linear normalization and thus requires a minimum of two standards, ideally with the standards bracketing the expected range in sample isotopic values. These standards are also used to correct for linearity (size) effects and drift. For research analyses, SPA generally uses the international standards USGS-40 (glutamic acid) [NIST 8573], IAEA-CH-6 (sucrose-ANU) [NIST 8542] and IAEA-NO-3 (potassium nitrate) [NIST 8549]. Analyses for class projects can be accomplished with cheaper in-house standards.  SPA cocoa and SPA corn flour provide the greatest spread in both carbon and nitrogen isotopic values, though others may be chosen to better match the expected sample composition (see Standard Materials). All of the in-house standards contain both carbon and nitrogen. A calibrated in-house laboratory standard is run "as a sample" to monitor quality control and long-term performance. Additionally, a conditioning sample and two blank tins are run to correct for background isotopic composition. Normalized delta values are expressed relative to international standards VPDB (Vienna PeeDee Belemnite) for δ13C and Air for δ15N.

δ13C and δ15N of combustible material rates

Isotope Phase Quantity Measurement Precision Internal External
δ13C, δ15N Solid 0.1–10 mg ± 0.2 ‰ $6* $10*

Sample types: combustible solids, dried liquid residue, quartz/glass filters

*Price is for pre-weighed and packed samples. Un-massed material requiring drying, weighing or other processing will incur an additional charge. Massing and packing is a flat $1 fee. If you would elemental mass (and percent composition) an additional $0.50 per sample will cover additional compositional standards and loss of a few unknown positions in the autosampler.

Sample requirements: A minimum mass of an element must be present in the sample to obtain accurate and precise estimates. Generally, the sample should contain a minimum of 50 µg of an element for isotopic composition with better than 0.2 per mil precision. The ideal elemental mass is about 100–200 µg. Most sample material has a mix of elements that might be analyzed for isotopic composition; the minimum sample size will be determined by the scarcest element of the sample material as calculated from an expected percent composition of the sample material. For example, if dried leaves are to be analyzed for δ13C and δ15N and the expected composition of the leaves is roughly 3% N and 50% C, then the minimum sample mass is:

sample mass = 50 µg / 0.03

In the above calculation, the mass of nitrogen was used to determine minimum sample size (1.7 mg) as it is the element expected to be found in the least concentration for a CN analysis. There are various calculators online and lists of usual material composition to assist in calculating the appropriate minimum sample amount. The UC Santa Cruz Stable Isotope Laboratory has an excellent calculator and discussion of sample requirements.

Samples much larger than the minimum sample size only present a linearity effect problem; this can be easily corrected by using the Conflo open-split interface to dilute the combustion gas of the element with the greater mass. Careful use of dilution will allow you to match the peak amplitudes of sample and reference gases for highest precision, even for sample material with widely different elemental compositions. For isotopic and percent composition of CN, the Conflo is usually set to 0% dilution for N and 89% dilution for C on the sample side as we typically measure dried plant and animal material with between 3% and 10% nitrogen and 40–50% carbon and a C:N ratios of around 7 to 10. The following sample size guidelines pertain to this dilution schedule:

USGS-40, glutamic acid (10%N, 40%C): 0.6 to 1.5 mg
Dried leaves (3%N, 50%C): 2 to 3 mg

For sample material with very low percent nitrogen and C:N ratios between 10 and 50 (e.g. corn flour [1%N, 40%C], sediment, soil), a separate method with sample side dilutions of 0% N and 94% is available and a reference peak amplitude for carbon dioxide set at 10 V. Sample sizes would be roughly 3–5 mg for these high C:N materials. For sample material with C:N less than 6, a separate method with only 67% sample dilution for carbon and a peak amplitude for nitrogen reference gas set at 5 V. Typically, animal tissue and nitrogenous compounds fit this method.  Sample sizes for these materials on this method would be:

Dried whole animal tissue (12%N, 40%C): 0.7 to 1.5 mg
Hair (14%N, 45%C): 0.5 to 1.2 mg

Thus the SPA lab should always be apprised of the material being analyzed to ensure a proper method. Material without a known (even roughly known) elemental composition (e.g. soils, sediments) should be shipped with an additional sample should a re-run be required. We can easily adjust dilution schedules to analyze any material if we have enough material with which to experiment.

Samples should be dried (50–60oC) and often ground and homogenized; however, we will analyze material in any way your field finds acceptable. Samples pretreated with acids should be rinsed several times with distilled water.  We can load your natural abundance samples ($1/sample); ship in a secure container. If you have enriched samples or decide to load your natural-abundance samples yourself, please use 96 well plastic trays. If you prefer, you may load your own natural-abundance samples into the proper size tin capsules; use the smallest capsule size that will hold the sample with no leakage. Capsules should be folded and crushed into a cube or ball without any protrusions. Record the final weight after the capsule is sealed. To prevent leakage during shipping, cover the loaded samples (with tin foil, parafilm, index card, etc) under the 96-well full plate cover. When designing your runs, keep in mind that our autosampler has 49 usable positions.

Material captured on filters may be analyzed. Quartz filters are very much preferred, but normal glass fiber filters (Whatman GF/F or Pall A/E) are acceptable. The largest filter we can run whole is 25 mm diameter. If you would like elemental mass results, please provide the mass of material on the 25 mm filter, not the total mass of filter and filtrate. You may trim parts of the filter not containing visible filtrate to facilitate packing into tins if you do not require elemental mass estimates. If you have used larger filters (e.g. 47 mm) and you do not require elemental mass (i.e. only isotopic results), simply may trim away the parts of the filter that do not contain visible filtrate to facilitate lacking into tins. Alternatively if the 47 mm filters are covered in filtrate, you could cut them into two halves and submit them separately; we obviously cannot provide highly precise percent composition on cut filters, as we do not know if the mass being analyzed is still accurate.  

Please contact us about solid hydrogen isotopic analysis. Hydrogen analysis is somewhat complicated because hydrogen exchanges with atmospheric hydrogen and thus sample material and standards must be equilibrated prior to analysis. Additional charges will apply. Sulfur isotopes may be analyzed; however, we have never performed that analysis due to lack of interest.