Stable Isotope Analysis

Department of Biology

Utah State University

5305 Old Main Hill

Logan, Utah 84322-5305

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Who are we?

The USU Stable Isotope Laboratory is a nonprofit research laboratory associated with the Department of Biology and the Ecology Center at Utah State University. As a service to researchers within and outside USU, we analyze soil, plant and animal tissues, and other combustible materials containing natural abundance and enriched concentrations of carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) isotopes.

All analyses are performed by continuous-flow direct combustion and mass spectrometry using a Europa Scientific SL-2020 system. With this system we obtain precise measurements of total C and N contents, as well as C and N isotope ratios.

Preparing Samples for ¹³C and ¹⁵N Analyses

What to send?

Plant, soil, or other combustible solid samples should be dried and finely ground before weighing them into tin capsules. Each tin capsule should be tightly crimped so that it does not leak. The final shape of the wrapped sample should be spherical, cubical, or cylindrical. Avoid thin, flat samples if possible. If information on total C or N concentration is desired in addition to isotope ratios, then a list of sample weights (accurate to at least 0.1 mg) should be sent with the samples.

Solutions containing <500 mg N/L that are to be analyzed for ¹⁵N (e.g. extracts, digests, etc.) must be first prepared by a diffusion procedure (see Stark and Hart 1996, Soil Sci. Soc. Am. J. 60:1846-1855). In this procedure, the N in the solution is concentrated on an acidified filter paper disk. The disk is then dried and wrapped in a tin capsule.

Solutions containing >500 mg N/L can be pipetted directly into tin capsules containing an absorbent material such as chromosorb or a filter paper disk.

Tin capsules used for wrapping samples (8x5 mm; Catalog # D1008) are available from: Elemental Microanalysis Limited; 67 Lincoln Street; Manchester, MA 01944; USA; (Phone: 800 659 9885 and FAX: 978 526 8518).

For shipping, plastic 96-well microtiter plates work very well as sample holders. When shipping plant or soil samples, pack small pieces of tissue paper into each well to keep the tin capsules from bouncing around and leaking. Also send a hard copy and a floppy disk containing a text file (Windows format) with the sample ID and other information requested below. Send samples, a sample list, and a purchase order to:

Dr. John Stark
Stable Isotope Laboratory
Utah State University
5305 Old Main Hill
Logan, UT 84322-5305

How much C? Samples sent for ¹³C analysis should generally contain between 0.2 mg and 1.6 mg of C (0.8 mg C is ideal). Best results are obtained when the samples to be compared contain similar quantities of C.

How much N? The optimum quantity of N depends on the enrichment. For samples that are less than 15 atom % ¹⁵N, we prefer samples containing at least 20 µg N but no more than 120 µg of N (80 to 100 µg of N is ideal). For more highly enriched samples, lower masses are preferred so that we don't saturate the detectors on the mass spectrometer. Most labs just run highly enriched samples at very low sensitivity to avoid this; however, we prefer to optimize the system based on the expected enrichment. We run many highly enriched samples and we feel that we get much better precision and accuracy this way. You can use the following equations to calculate the maximum mass (upper limit) of N to send us for ¹⁵N analyses. In the following equations, APE is the approximate atom % ¹⁵N enrichment expected in the sample. (Note: when it is difficult to estimate enrichments, we suggest making the calculations based on the maximum expected enrichment):

If APE is between 15% and 67%:
maximum mass = 12 / (APE/100 - (APE/100)² )

If APE is > 67%:
maximum mass = 24 / (APE/100)²

For example, if the enrichment is expected to be about 50 atom % ¹⁵N, then the sample sent for analysis should contain no more than 48 µg N.

When you send samples for analysis, please specify the expected enrichment and the expected mass of N in each sample (preferably in a text file plus a hard copy). During the analysis, we use two sets of isotope standards that bracket the expected enrichments. This allows us to maintain a high level of accuracy even when the actual enrichment is different than expected.

Samples with low N mass (8 µg or greater) can be analyzed without much problem; however, the precision obtained with low mass samples is somewhat lower than that obtained with higher mass samples. In addition, when samples are prepared by diffusion procedures, N contamination from diffusion reagents will be on the order of 1 to 12 µg , depending on the matrix (see Table 1 in Stark and Hart 1996). Therefore, we recommend that for diffused samples, you try to diffuse at least 20 µg of sample N, so that the sample mass is several times higher than the blank. We also recommend that you compare enrichments in diffused and non-diffused standards to make blank corrections (as described in Stark and Hart 1996). 

Precision

Carbon: For plant and soil samples containing 1 mg of carbon with natural abundance levels of ¹³C, the standard deviation of 5 replicate samples is typically less than 0.1 per mil.

Nitrogen: For plant and soil samples containing 100 µg of nitrogen with natural abundance levels of ¹⁵N, the standard deviation of 5 replicate samples is typically about 0.5 per mil.

Much of the variation in analyses is associated with subsample error, rather than analytical error. For this reason, we recommend grinding plant and soil samples very finely (to the consistency of flour) prior to weighing the samples into tin capsules.

For samples enriched in ¹⁵N (e.g. 5 atom % ¹⁵N), the standard deviation is typically 0.1 to 0.25% of the mean. Preparation of enriched samples by diffusion increases the variation even further (0.5% to 1.5% of the mean), probably due to variation in N contamination in diffusion reagents.

Cost

For samples pre-wrapped in tin capsules:

analyses	price/sample
total C content and ^13C abundance	$7.00
total N content and ^15N abundance	$7.00
C & N content and ^13C & ^15N on same sample	$11.00

For samples sent from within the US, a valid purchase order or check is required.
For international samples, electronic transfer of funds is required before we can proceed with analyses.

Questions?

If you have any questions regarding ¹³C or ¹⁵N analyses, feel free to email me at jstark@biology.usu.edu.

John M. Stark
Professor and Director, Stable Isotope Laboratory
Dept of Biology
Utah State University
Logan, UT 84322-5305