Department of Marine Sciences, University of Connecticut, Groton, CT, USA.
USGS, Reston, VA, USA.
Rapid Commun Mass Spectrom. 2020 Jan 15;34(1):e8569. doi: 10.1002/rcm.8569.
The isotope ratios of nitrogen ( N/ N) and oxygen ( O/ O) in nitrite (NO ) can be measured by conversion of the nitrite into nitrous oxide (N O) with azide, followed by mass spectrometric analysis of N O by gas chromatography isotope ratio mass spectrometry (GC/IRMS). While applying this method to brackish samples, we noticed that the N and O isotope ratio measurements of NO are highly sensitive to sample salinity and to the pH at which samples are preserved.
We investigated the influence of sample salinity and sample preservation pH on the N and O isotope ratios of the N O produced from the reaction of NO with azide. The N O isotope ratios were measured by GC/IRMS.
Under the experimental reaction conditions, the conversion of NO into N O was less complete in lower salinity solutions, resulting in respective N and O isotopic offsets of +2.5‰ and -14.0‰ compared with seawater solutions. Differences in salinity were also associated with differences in the fraction of O atoms exchanged between NO and water during the reaction. Similarly, aqueous NO samples preserved at elevated pH values resulted in the incomplete conversion of NO into N O by azide, and consequent pH-dependent isotopic offsets, as well as differences in the fraction of O atoms exchanged with water. The addition of sodium chloride to the reaction matrix of samples and standards largely mitigated salinity-dependent isotopic offsets in the N O product, and nearly homogenized the fraction of O atom exchange among samples of different salinity. A test of the hypobromite-azide method to measure N isotope ratios of ammonium by conversion into NO then N O revealed no influence of sample salinity on the N isotope ratios of the N O product.
We outline recommendations to mitigate potential matrix effects among samples and standards, to improve the accuracy of N and O isotope ratios in NO measured with the azide method.
通过将亚硝酸盐（NO ）转化为笑气（N O），并用叠氮化钠进行转换，可以测量亚硝酸盐中氮（N/ N）和氧（O/ O）的同位素比值，然后通过气相色谱同位素比质谱法（GC/IRMS）对 N O进行质谱分析。在将此方法应用于咸水样品时，我们注意到 NO 产生的 N 和 O 同位素比值测量结果对样品盐度和保存样品的 pH 值高度敏感。
我们研究了样品盐度和样品保存 pH 值对 NO 与叠氮化钠反应生成的 N O的 N 和 O 同位素比值的影响。通过 GC/IRMS 测量 N O 的同位素比值。
在实验反应条件下，较低盐度溶液中亚硝酸盐转化为 N O的程度较低，与海水溶液相比，N 和 O 同位素分别出现+2.5‰和-14.0‰的偏移。盐度的差异也与反应过程中亚硝酸盐与水之间交换的 O 原子分数的差异有关。同样，保存在较高 pH 值的水中的 NO 样品会导致 NO 与叠氮化钠不完全反应生成 N O，并导致与 pH 值相关的同位素偏移，以及与水交换的 O 原子分数的差异。向样品和标准品的反应基质中添加氯化钠在很大程度上减轻了 N O产物中盐度依赖性同位素偏移，并使不同盐度样品之间的 O 原子交换分数趋于均匀。通过将次溴酸盐-叠氮化钠法用于将铵转化为 NO ，然后再转化为 N O 来测量氮同位素比值的测试表明，样品盐度对 N O产物的氮同位素比值没有影响。
我们概述了缓解样品和标准品之间潜在基质效应的建议，以提高用叠氮化钠法测量的 NO 中 N 和 O 同位素比值的准确性。
