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Spectrophotometric calibration procedures to enable calibration-free measurements of seawater calcium carbonate saturation states
Analytica Chimica Acta ( IF 6.2 ) Pub Date : 2018-08-01 , DOI: 10.1016/j.aca.2018.02.071
Erin E. Cuyler , Robert H. Byrne

A simple protocol was developed to measure seawater calcium carbonate saturation states (Ωspec) spectrophotometrically. Saturation states are typically derived from the separate measurement of two other carbon system parameters, with each requiring unique instrumentation and often complex measurement protocols. Using the new protocol, the only required equipment is a thermostatted laboratory spectrophotometer. For each seawater sample, spectrophotometric measurements of pH (visible absorbance) are made in paired optical cells, one with and one without added nitric acid. Ultraviolet absorbance is measured to determine the amount of added acid based on the direct proportionality between nitrate concentration and UV absorbance. Coupled measurements of pH and the alkalinity change that accompanies the nitric acid addition allow calculation of a seawater sample's original carbonate ion concentration and saturation state. These paired absorbance measurements yield Ωspec (and other carbonate system parameters), with each sample requiring about 12 min processing time. Initially, an instrument-specific nitrate molar absorptivity coefficient must be determined (due to small but significant discrepancies in instrumental wavelength calibrations), but thereafter no further calibration is needed. In this work, the 1σ precision of replicate measurements of aragonite saturation state was found to be 0.020, and the average difference between Ωspec and Ω calculated conventionally from measured total alkalinity and pH (Ωcalc) was -0.11% ± 0.96% (a level of accuracy comparable to that obtained from spectrophotometric measurements of carbonate ion concentration). Over the entire range of experimental conditions, 0.97 < Ω < 3.17 (n = 125), all measurements attained the Global Ocean Acidification Observing Network's "weather level" goal for accuracy and 90% attained the more stringent "climate level" goal. When Ωspec was calculated from averages of duplicate samples (n = 56), the precision improved to 0.014 and the average difference between Ωspec and Ωcalc improved to -0.11% ± 0.73%. Additionally, 97% of the duplicate-based Ωspec measurements attained the "climate level" accuracy goal. These results indicate that the simple measurement protocol developed in this work should be widely applicable for monitoring fundamental seawater changes associated with ocean acidification.

中文翻译:

分光光度校准程序,可实现海水碳酸钙饱和状态的免校准测量

开发了一个简单的协议来测量海水碳酸钙饱和状态 (Ωspec) 分光光度法。饱和状态通常来自对另外两个碳系统参数的单独测量,每个参数都需要独特的仪器和通常复杂的测量协议。使用新协议,唯一需要的设备是恒温实验室分光光度计。对于每个海水样品,pH 值(可见吸光度)的分光光度测量是在成对的光学池中进行的,一个加入硝酸,一个不加入硝酸。测量紫外线吸光度以根据硝酸盐浓度和紫外线吸光度之间的正比关系确定添加的酸量。pH 值和碱度变化伴随硝酸添加的耦合测量允许计算海水样品的原始碳酸根离子浓度和饱和状态。这些成对的吸光度测量产生 Ωspec(和其他碳酸盐系统参数),每个样品需要大约 12 分钟的处理时间。最初,必须确定仪器特定的硝酸盐摩尔吸收系数(由于仪器波长校准中存在微小但显着的差异),但此后无需进一步校准。在这项工作中,文石饱和状态重复测量的 1σ 精度被发现为 0.020,并且从测量的总碱度和 pH 值 (Ωcalc) 常规计算的 Ωspec 和 Ω 之间的平均差异为 -0.11% ± 0。96%(准确度与碳酸根离子浓度分光光度测量结果相当)。在整个实验条件范围内,0.97 < Ω < 3.17 (n = 125),所有测量都达到了全球海洋酸化观测网络的“天气水平”目标精度,90% 达到了更严格的“气候水平”目标。当 Ωspec 从重复样本 (n = 56) 的平均值计算时,精度提高到 0.014,Ωspec 和 Ωcalc 之间的平均差异提高到 -0.11% ± 0.73%。此外,97% 的基于重复的 Ωspec 测量达到了“气候水平”精度目标。
更新日期:2018-08-01
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