N₂ & N₂O Analysis by Trace Gas-IRMS

The SIF provides isotope analysis (¹⁵N, ¹⁸O) of N₂ and N₂O in gas mixtures which may also contain O₂ and CO₂. This method is also used to analyze trace quantities N₂O purged from water samples with helium.

Analysis of ¹⁵N and ¹⁸O in Gas Mixtures

Trace gas isotope ratios are measured using a SerCon Cryoprep trace gas concentration system interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK). Gas samples are purged from vials through a double-needle sampler into a helium carrier stream (20 mL/min). Then, N₂ and N₂O are isolated and concentrated in preparation for isotopic analysis. First, N₂ gas (concentration up to 5000 ppmv) is sampled by a rotary 4-port valve fitted with a 20 µL sampling loop and timed to capture the peak N₂ concentration in the carrier gas stream. This gas sample is passed to the IRMS through a molecular sieve 5A GC column (15m x 0.53mm ID,  25°C, 3 mL/min). A reference N₂ peak is used to calculate provisional isotope ratios of the sample N₂ peak.

As N₂ is analyzed, the rest of the gas sample passes through a CO₂ scrubber (Ascarite) and N₂O is trapped and concentrated in 2 liquid nitrogen cryo-traps operated in series such that the N₂O is held in the first trap until the non-condensing portion of the sample gas has been replaced by helium carrier, then passed to the second, smaller trap. Finally the second trap is warmed to ambient and the N₂O is carried by helium to the IRMS via a Poroplot Q GC column (25m x 0.53 mm,  25°C, 1.8 mL/min). This column separates N₂O from residual CO₂. A reference N₂O peak is used to calculate provisional isotope ratios of the sample N₂O peak.

Final delta ¹⁵N values are calculated by adjusting the provisional values such that correct delta ¹⁵N values for working standards are obtained. Two working standards are analyzed every 10 samples. The working standards are mixtures of N₂ and N₂O (e.g., 3% N₂ + 1 ppm N₂O with the balance He or 1 ppm N₂O with balance N₂). The N₂ is calibrated against an Oztech N₂ standard (delta ¹⁵N vs air = -0.74). The calibration of the N₂O is problematic since there are no suitable international standards. Thus, we calibrated ¹⁵N and ¹⁸O by reacting the N₂O with glassy carbon at 1400°C to convert N₂O to N₂ + CO. The resulting N₂ was calibrated against the Oztech N₂ standard and the CO was calibrated against an Oztech CO₂ standard (after converting CO₂ to CO in a similar manner).

Pricing

*Please contact Emily to schedule preparation of the denitrifying bacteria.

Sample preparation

Gas samples are supplied in septum capped vials preferably at atmospheric pressure (~100 kpa) but pressures up to 200kpa are acceptable. Preferred sample vials are 12mL Exetainers (Labco Ltd, High Wycombe, UK) but others may be acceptable by arrangement.

Gas samples are collected in 12mL exetainer vials (Labco UK) with screw top and septa. The exetainers should be evacuated prior to sample collection. Label each vial with a unique ID# as your data will reference your samples using this ID.

For NO3 in water preparation, please see the denitrifier articles in the Reference section. Water samples requiring preparation by the SIF should be filtered and shipped frozen in 30ml-60ml Nalgene-type bottles. Samples requiring extensive handling by the SIF (ie. filtering) will be charged an additional $1.00 handling fee per sample. Please provide a minimum of 10mLs of each water sample.

Shipping

Carefully package groups of exetainers in ziplock bags, small boxes, or their original boxes. Then package these sets in larger, sturdy boxes with styrofoam peanuts or bubble wrap. Make sure the samples are very secure, loose vials can shatter during shipping. These gas samples do not require refrigeration during shipping.

Water samples for bacteria denitrification preparation should be shipped frozen and packaged in an insulated box with dry ice or blue ice blocks. Coolers will not be returned.

*NEW* Please complete an Analysis Order Form and Sample List for your samples. E-mail the completed forms to sif@ucdavis.edu and include a printed copy with your samples. The SIF uses this form to track your samples and to contact you regarding receipt of samples, data, and invoicing. For mixed analysis requests, we recommend completing a separate form for each type of analysis.

Contact information
UC Davis Stable Isotope Facility
Department of Plant Sciences
One Shields Avenue, Mail Stop 1
Davis, CA 95616
USA

Phone:(530) 754-7517, Fax: (530) 752-4361
E-mail: sif@ucdavis.edu

Supplies

*Note: Labco has a new vial style that ends in ###W/W. We DO NOT recommend these vials when sending samples to the SIF because the vials are too wide for our autosampler rack. Supplier: Labco UK + International or Labco USA + Canada

References for NO3 in Water

15N Analysis of NO3 in water using Pseudomonas chlororaphis bacteria:
D. M. Sigman, K. L. Casciotti, M. Andreani, C. Barford, M. Galanter, and J. K. Böhlke. A Bacterial Method for the Nitrogen Isotopic Analysis of Nitrate in Seawater and Freshwater. Anal. Chem., 73 (17), 4145 -4153, 2001.

Tips

1) Avoid over-tightening exetainer caps. Vials will leak when caps are over-tightened, causing the septa to pucker and pull away from the vial. Turn caps until snug, but not so far that the septa start to pucker.

2) Do not apply silicone grease, adhesives, or sealant to the tops of the vials. These materials clog the autosampler needle. The process of removing these sealants increases the risk of losing your gas sample due to dislodged septum.

Updated June 29, 2009