Assessment of rapid low‐cost isotope ( <i>δ</i> <sup>15</sup> N, <i>δ</i> <sup>18</sup> O) analyses of nitrate in fruit extracts by Ti(III) reduction to differentiate organic from conventional production
Leonard I. Wassenaar, Simon Kelly, Cedric Douence, Marivil Islam, Lucilena Rebêlo Monteiro, Aiman Abrahim, Peter Rinke
Abstract
Rationale The isotopic composition ( δ 15 N, δ 18 O) of nitrate in fruits and vegetables differentiates organic from conventional food production practices. Organic systems do not use synthetic nitrate fertilizers high in 18 O and low in 15 N and thereby help reveal producers’ fertilization claims. Isotope analyses of nitrate extracted from fruits and vegetables are done by bacterial reduction which is costly and by specialized laboratories. Rapid, low‐cost methods are needed to promulgate nitrate isotope analyses of food products to support organic food product certification and to verify the authenticity of production claims. Methods Fresh strawberry samples were obtained from certified organic and conventional growers in Andalucía, Spain. We applied a new, rapid, one‐step Ti(III) reduction method to convert the nitrate from strawberry extracts to N 2 O gas for headspace isotope analyses using isotope‐ratio mass spectrometry. Using the Ti(III) reduction method, 70 samples, controls and references were prepared and analyzed for NO 3 − , δ 15 N and δ 18 O per 48 h. We also analyzed extracts and solids for anions and cations and for bulk δ 15 N for multivariate chemometric evaluation. Results The Ti(III)‐based isotope analyses of nitrate in strawberry extracts revealed clear differentiation between organic and conventional production with mean δ 18 O and δ 15 N values of +18.3 ± 1.2 ‰ and +17.6 ± 1.2 ‰ versus +28.2 ± 4.5 ‰ and +14.9 ± 3.0 ‰, respectively. The δ 15 N of strawberry dry mass differed slightly (+3.0 ± 1.4 ‰ versus +4.0 ± 1.4 ‰) between organic and conventional samples, respectively. Chemometric analyses of nitrate isotopes and extract chemistry revealed that the δ 18 O of nitrate along with δ 15 N and Ca 2+ fully differentiated organic from conventional strawberry production. Conclusions Our results show the Ti(III) reduction method provides a new low‐cost and rapid analytical method to facilitate compound‐specific δ 15 N and δ 18 O isotope analyses of nitrate in selected fruit types, and likely other food products, for the purposes of assessing nitrate fertilization practices of organic versus conventional production claims and to support authenticity investigations.