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Ulitiie ~IBIIEOOIT~Filli ELSEVIER Marine Chemistry 48 (1995) 271 282

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Ulitiie ~IBIIEOOIT~Filli ELSEVIER Marine Chemistry 48 (1995) 271 282 UliTIIE ~IBIIEOOIT~filli ELSEVIER Marine Chemistry 48 (1995) 271 282 Improved chromatographic analysis of 15N: 14N ratios in ammonium or nitrate for isotope addition experiments 3 Wayne S. Gardnera, Harvey A. Bootsma , Christine Evansb, Peter A. St. Johnc aNOAA Great uJkes Environmental Research Laboratory. 2205 Commonwealth Blvd.. Arm Arbor, M/48105, USA bUnivertity of Michigan, Department of Chemi.llry, Ann Arbor, M/48109. USA 'St. John Associates, inc., 4805 Prince George's Arerme, Beltsville. MD 20705. USA Received 29 April 1994; revision accepted 14 November 1994 Abstract Estimating nitrogen transformation rates in aquatic ecosystems by isotope dilution techniques is simplified by directly measuring nitrogen isotopic ratios for NHt in the water using high performance cation exchange liquid chromatography (HPLC). Modifications of HPLC conditions and implementation of a median-area method for retention time deter­ mination improved and linearized a previously reported sigmoid relationship between the retention time shift (RTshift) of 15 15 the NHt peak and the ratio of ( NH4 ]: [Total NHtJ in seawater fortified with NHt. Increasing the temperature of the HPLC column from 47 to 85 C increased mobile phase buffer flow rate relative to column back pressure. decreased the retention time for NH4 • and allowed the buffer pH to be optimized relative to the pK of NH4 . The use of median­ area rather than maximum-hetghl to define the retention time of NH4 further improved the linearity (r > 0.995) of the 15 relationship between the ratio ( NH,I) : (Total NH4 ] and RT,h,n over the range of isotope rat•os. Reduction of N03 to NH! by adding ?inc dust to acidified (pH 2) seawater or lakewater samples, followed by pH neutralization, and subsequent analysis of NHt isotope ratios by HPLC, extended application of the method to isotope dilution experi­ ments with N03 . Advantages of this direct-injection method over mass-measurement approaches traditionally used for isotope dilution experiments include small sample size and minimal sample preparation. J. Introduction 1983b; Blackburn, 1979; Caperon et al., 1979: 15 Glibert et al., 1982). N03 (Dugdale and Nutrient transformation rates must be measured Goering, 1967; Goering et al.. 1970) and 15N­ to understand the ecology and biogeochemistry of Iabeled organic nitrogen compounds (Kirchman aquatic ecosystems. The dynamics of C and P are et al., 1989; Bronk and Glibert, 1991; Gardner et often studied by using the radioactive isotopes 14C al., 1993) have provided useful information about and 32P as tracers (e.g. Steeman Nielsen, 1951; the cycling of nitrogen in benthic and pelagic fresh­ Schlinder et al., 1972; Harrison, 1983a; Rai and water and marine ecosystems. Jacobsen, 1990; Bentzen and Taylor, 1991). Tracer Relatively large samples of water are required studies with nitrogen are usually done with the for isotope dilution or enrichment experiments stable isotope 15 N that is measured by mass or with 15 N compounds because mass detection emission spectrometry. Isotope dilution or enrich­ methods used for stable isotopes are not nearly as 15 ment experiments with NH4 (e.g. Harrison, 1978, sensitive as those used for radioactive isotopes. 0304-4203;95,$09.50 SSDI 0304-4203(94)00060-3 272 W.S. Gardner et al.fMarine Chemistry 48 ( 1995) 271-282 Nitrogen in particles can be simply concentrated [Total NHtJ ratios in aqueous samples and and prepared for mass analysis by filtration and describe a NO.J reduction step that extends the Dumas combustion, but dissolved forms of nitro­ capability of the method to isotope dilution experi­ gen (NH!, NO.J, and dissolved organic nitrogen) ments with 15 NO). Application of the method to must first be removed from the water, concen­ isotope dilution and enrichment experiments in the trated, and dried before they can be converted to Gulf of Mexico and Saginaw Bay, Lake Huron, N 2 for mass analysis (Harrison, 1983b). Direct will be reported separately. measurement of nitrogen ion isotope ratios in the aqueous phase would prevent the need for this multistep procedure that requires large volumes 2. Methods of sample water. As an alternative approach to mass analysis, 2.1. High performance liquid chromatographic nitrogen isotope ratios for NHt in isotope system dilution experiments can be determined directly in small water samples by high performance cation High performance liquid chromatographic con­ exchange liquid chromatography (HPLC; ditions were similar to those recently described Gardner et al., 1991, 1993). The HPLC technique (Gardner et al., 1991, 1993) except that column is not efficient enough to separate the two isotopic temperature and method of RTshift determination forms of NHt into separate chromatographic were modified to linearize the relationship between peaks, but the ratios can be determined from a isotope ratio and Rtshift (see details below). Briefly, shift in NH! retention time (RTshift), caused by the HPLC system consisted of an ISCO 260D 15N Ht, relative to that of an internal standard of syringe pump operated in the constant pressure natural-abundance NH! in mobile phase buffer mode, an Alcott Model 728 Autosampler that is injected at a measured time interval before equipped with a Valco Model EC6W fast electro­ the sample. The RTshift occurs because the ratio of nically-activated injection valve with a 50 111 sample 15 15 [ NHtJ : [ N H 3] is slightly larger than the ratio of loop, a heated (Standard CROCO-CIL HPLC 14 14 [ NHtJ : [ NH 3] at pH's near the pK for NH4 column heater) 30 em x 4mm i.d. stainless steel (about pH 9; Gardner et al., 1991 ). The shape of column containing a strong cation exchange resin the calibration curve is sigmoid in shape, a factor (5 J1m beads of the sodium form of sulfonic acid that makes the method less sensitive at low and cation exchanger with 12% cross-linked poly­ high isotope ratios than in the mid portion of the styrenefdivinylbenzene polymeric matrix; St. John curve where concentrations of 15 NH; and 14N H! Associates), an assembled post-column reaction are similar (Gardner et al., 1991, 1993). It would be system, and a Gilson 121 Fluorometric detector desirable to equalize the response factor over the equipped with a Corning 7-60 excitation light filter whole range of the curve to improve the relative (maximum transmission at 356 nm) and a Corning sensitivity for low (and high) ratios of 3-71 emission filter (sharp cutoff at 482 nm). 15 [ NH!J : [Total NH!J and to linearize the shape Sample signal from the detector was recorded of the calibration curve. either with a Shimadzu Integrator (Model C­ The value of using HPLC to quantify isotope R3A) or with a computerized Galactic software ratios would be further increased if its use could system that determined retention times both by a be extended to doing isotope dilution experiments maximum peak height algorithm (Galactic Center with dissolved NO.J in natural waters. This goal X) or by a post-run program (Galactic program could potentially be accomplished by reducing COL-RT.ABP) designed to determine median­ N03 to NHt in sample water (e.g. Stainton et area retention time, i.e. the position in the peak al., 1977) followed by HPLC analysis of the iso­ where the area of the peak before the retention tope ratios of the resulting NHt ions. time is the same as the area after the retention time. In this paper, we linearize the calibration The mobile phase buffer [12 g boric acid+ 12 g relationship between RTshift's and [15NH!J : NaCl + 0.8 g disodium ethylenediaminetetraacetic W.S. Gardner eta/.{Marine Chemistry 48 (1995) 271-282 273 acid in 1 I water, adjusted to the desired pH with was programmed so that the water in the even­ NaOH, fortified with 0.5 ml of Brij 35, and filtered numbered vials would be injected at a precise (0.2 J.Lm pore size nylon; Gardner et al., 1993)] was time interval (5.0 or 7.0 min) after the internal passed through the column at flow rates ranging standard solutions in the odd-numbered vials 1 between 0.14 and 0.17 ml min- , depending on were injected. Sufficient time (33 to 48 min, depend­ HPLC conditions. The o-phthalaldehyde (OPA) ing on chromatographic conditions) was allowed reagent, modified from Hare (1975) and Gardner for the sample NHt to elute before the next inter­ and St. John (1991), was an aqueous solution of nal standard NHJ solution was injected. Under 1 boric acid (30 g 1- ) adjusted to pH 7.0 with these conditions, the syringe pump (266 ml capa­ KOH, and then mixed with a solution of 0.5 g city) contained sufficient buffer to analyze all stan­ OPA dissolved in 10 ml MeOH and 0.5 ml dards and samples from a filled autoinjector tray. 2-mercaptoethanol. Four ml of Brij 35 was added The tray capacity of 64 vials allowed injection of 10 for pump-seal lubrication and the reagent was triplicate pairs of samples and standards (60 vials). filtered (distilled water-rinsed 0.45 J.Lm pore size Vials in the remaining final open slots were loaded Millipore). In the post-column reaction system, with distilled water that was injected to rinse any OPA reagent was pumped at a flow rate of 0.1 ml deposited salts from the sample injector valve. min- 1 using an Anspec 909 (currently available as After standards and samples were loaded, auto­ an Alcott 760) HPLC pump equipped with a micro­ sampler injections were begun and all the data bore head. After the OPA reagent was mixed with were recorded, on the Shimadzu integrator and/ the column eluate via a T-fitting, the mixture was or the Galactic computerized data system, as one passed sequentially through a heated (ca.
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