Cuyler Erin E, Byrne Robert H
College of Marine Science, University of South Florida, 140 7th Avenue South, St. Petersburg, FL 33701, USA.
Anal Chim Acta. 2018 Aug 22;1020:95-103. doi: 10.1016/j.aca.2018.02.071. Epub 2018 Mar 10.
A simple protocol was developed to measure seawater calcium carbonate saturation states (Ω) 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 Ω (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 Ω and Ω calculated conventionally from measured total alkalinity and pH (Ω) 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 Ω was calculated from averages of duplicate samples (n = 56), the precision improved to 0.014 and the average difference between Ω and Ω improved to -0.11% ± 0.73%. Additionally, 97% of the duplicate-based Ω 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.
开发了一种简单的协议，用于通过分光光度法测量海水碳酸钙饱和度状态（Ω）。饱和度状态通常通过分别测量其他两个碳系统参数得出，每个参数都需要独特的仪器，并且测量协议通常很复杂。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，在一对光学池中进行pH（可见吸光度）的分光光度测量，一个添加硝酸，一个不添加硝酸。测量紫外吸光度，根据硝酸盐浓度与紫外吸光度之间的直接比例关系确定添加的酸量。pH与添加硝酸时伴随的碱度变化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度状态。这些成对的吸光度测量得出Ω（和其他碳酸盐系统参数），每个样品需要约12分钟的处理时间。最初，必须确定特定于仪器的硝酸盐摩尔吸光系数（由于仪器波长校准存在微小但显著的差异），但此后无需进一步校准。在这项工作中，文石饱和度状态重复测量的1σ精度为0.020，Ω与根据测量的总碱度和pH（Ω）常规计算的Ω之间的平均差异为-0.11%±0.96%（准确度水平与通过分光光度法测量碳酸根离子浓度获得的相当）。在整个实验条件范围内，0.97 < Ω < 3.17（n = 125），所有测量均达到全球海洋酸化观测网络的“天气水平”准确度目标，90%达到更严格的“气候水平”目标。当从重复样品的平均值（n = 56）计算Ω时，精度提高到0.014，Ω与Ω之间的平均差异提高到-0.11%±0.73%。此外，97%基于重复测量的Ω测量达到了“气候水平”准确度目标。这些结果表明，这项工作中开发的简单测量协议应广泛适用于监测与海洋酸化相关的基本海水变化。
