Walters Wendell W, Hastings Meredith G
Institute at Brown for Environment and Society, Brown University, Providence, RI 02912, USA.
Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI 02912, USA.
MethodsX. 2023 Oct 7;11:102413. doi: 10.1016/j.mex.2023.102413. eCollection 2023 Dec.
Oxygen stable isotopes (i.e., O, O, O) of nitrite (NO) are useful for investigating chemical processes and sources contributing to this important environmental contaminant and nutrient. However, it remains difficult to quantify the oxygen isotope compositions of NO due to the lack of internationally recognized NO reference materials with a well-known Δ(O) value. Here we have adopted a combination of methodologies to develop a technique for measuring Δ(O) of NO by reducing nitrate (NO) materials with internationally recognized Δ(O) values to NO using activated cadmium catalyzed by chloride in a basic solution while conserving Δ(O). The NO reference materials reduced to NO and sample NO unknowns are converted to NO using sodium azide/acetic acid reagent and decomposed to O by passing through a heated gold tube and introduced into a continuous flow isotope ratio mass spectrometer for analysis at 32, 33, and 34 for Δ(O) quantification. The adapted method involves the following main points:•NO reference materials with internationally recognized oxygen isotope composition are reduced to NO under high pH conditions that conserve Δ(O) values.•The NO reference materials reduced to NO and sample NO with unknown Δ(O) values are reduced to NO using chemical methods involving sodium azide/acetic acid.•The product NO is extracted, purified, decomposed to O, and analyzed for its isotope composition using a continuous flow isotope ratio mass spectrometer for Δ(O) quantification. The Δ(O) of NO samples are calibrated with respect to the NO reference materials with known Δ(O) values.
亚硝酸盐（NO₂⁻）的氧稳定同位素（即¹⁷O、¹⁸O、¹⁶O）有助于研究导致这种重要环境污染物和营养物质产生的化学过程及来源。然而，由于缺乏具有已知Δ¹⁷O值的国际认可的NO₂⁻参考物质，目前仍难以对NO₂⁻的氧同位素组成进行定量分析。在此，我们采用了多种方法相结合的方式，开发出一种测量NO₂⁻的Δ¹⁷O的技术，即在碱性溶液中，利用氯化物催化的活性镉将具有国际认可的Δ¹⁷O值的硝酸盐（NO₃⁻）物质还原为NO₂⁻，同时保留Δ¹⁷O。将还原为NO₂⁻的NO₃⁻参考物质和未知的样品NO₂⁻，使用叠氮化钠/乙酸试剂转化为NO，通过加热的金管分解为O₂，并引入连续流同位素比率质谱仪，在32、33和34处进行分析以定量Δ¹⁷O。改进后的方法主要包括以下几点：
具有国际认可的氧同位素组成的NO₃⁻参考物质在高pH条件下还原为NO₂⁻，以保留Δ¹⁷O值。
将还原为NO₂⁻的NO₃⁻参考物质和具有未知Δ¹⁷O值的样品NO₂⁻，采用涉及叠氮化钠/乙酸的化学方法还原为NO。
提取、纯化产物NO，将其分解为O₂，并使用连续流同位素比率质谱仪分析其同位素组成以定量Δ¹⁷O。根据具有已知Δ¹⁷O值的NO₃⁻参考物质对NO₂⁻样品的Δ¹⁷O进行校准。
