An improved ion-exchange-to-silver-salt procedure for δ¹⁸O measurement of nitrate in natural samples

The oxygen isotope composition (δ¹⁸O) of natural nitrate is a proven effective indicator of its sources and the (bio)geochemical processes that it has experienced. Removing interfering species (in particular oxyanions such as SO₄^2-, PO₄^3-, HPO₄^2- and H₂PO₄^-) from natural samples with the currently established ion-exchange-to-silver-nitrate (IE2AgN) for reliable δ¹⁸O measurement with high temperature conversion-isotope ratio mass spectrometry (HTC-IRMS) involves a lengthy 5-step procedure. A less tedious and lower-cost IE2AgN procedure is highly desirable for high throughput analysis.

We report an improved ion-exchange procedure based on the vast difference in solubilities in n-butyronitrile between AgNO₃ and interfering inorganic oxyanions. Nitrate was isolated from interfering inorganic oxyanions (and organic matter) as its silver salt following a 3-step procedure (anion-exchange, neutralization with Ag₂O and selective dissolution of AgNO₃ in n-butyronitrile). The δ¹⁸O of the purified nitrate was then analyzed by HTC-IRMS. A comparative δ¹⁸O analysis of the certified international nitrate O isotope standards before and after processing through our new 3-step and the current 5-step procedures showed them to yield comparably accurate and precise determinations with minimal isotope fractionation. The time required was more than halved (from 3-5 days to 1-2 days) as was the amount of expensive Ag₂O.

The simplified IE2AgN procedure will prove useful for high throughput and lower cost extraction of nitrate from natural samples for oxygen (and nitrogen) isotope analysis.