This is the second post in a three-part series that examines how Picarro analyzers, systems, and accessories ensure precise, accurate measurements of challenging seawater and high-saline water samples. The first post, Water Stable Isotope Measurements of Seawater, presents results from an inter-laboratory study designed to evaluate the quality of cavity ring-down spectroscopy (CRDS) derived measurements compared with the consistency and values of isotope ratio mass spectrometry (IRMS) measurements. This post reports on the evaluation of CRDS for the analysis of high-saline water.

An article in Environmental Science & Technology titled Stable Isotope Analysis of Saline Water Samples on a Cavity Ring-down Spectroscopy Instrument by Grzegorz Skrzypek and Douglas Ford of the University of West Australia evaluates the use of CRDS for the analysis of highly saline water. (The information cited in this post is copyright 2014 Environmental Science & Technology.)

The article states that CRDS is "a novel and low-cost alternative to traditional dual inlet techniques or continuous flow systems for stable hydrogen and oxygen isotope analysis of water." But that "the influence of various salt content in water samples on CRDS performance during liquid water analysis has not been reported in the scientific literature, despite the large interest of the scientific community in using CRDS systems for the analysis of saline ground, surface, and seawater samples."

As the article also states that, in contrast to other techniques, measurement in the Picarro analyzer system is performed directly on vapor yielded from a water sample. The water sample (≈2 µL) undergoes rapid evaporation under vacuum conditions in a vaporizer operating at a relatively low temperature (110 °C). Water vapors are then flushed by a stream of dry air or nitrogen to a laser analyzer chamber.

The study experimentally tested for the influence of three major effects on the stable isotope analysis of saline water on the Picarro analyzer system:

1. an effect related to incomplete extraction/evaporation of water from individual samples;
2. a progressive increase of isotope fractionation due to water reabsorption on salt and other chemical reactions during distillation leading, for example, to the formation of NaOH or HCL (this effect likely will escalate as salt load in the vaporizer increases);
3. a memory effect due to water absorption on accumulated salt, however, without necessarily significant isotope fractionation.

EXPERIMENT No. 1 – Salt Mixture Solutions

A test range of ratios between different known salt (NaCl, KCl, MgCl₂, and CaCl₂) concentrations similar to those observed in the Indian Ocean at the Western Australia coast and saline groundwater from northwestern Western Australia were prepared with deionized (DI) water. The total dissolved solids (TDS) in the prepared solutions varied from 0 to 339.4 g/L.

Measurement of the stable isotope composition of the solutions of mixed salts using the Picarro CRDS analyzer system varied in a narrow range. The δ-values of DI water and water of saline solutions were different by no more than 0.09⁰/₀₀ for δ¹⁸O and 0.90⁰/₀₀ for δ²H. Therefore, even though the addition of salt resulted in a departure of the stable isotope composition of water from its original values, the difference in δ-values over the studied concentration range and salt types was negligible.

EXPERIMENT No. 2 – Individual Salt Solutions

For this experiment, various solutions of individual salts and salt mixtures were prepared in a similar manner to experiment no. 1. However, only new anhydrous chemicals (NaCl, KCl, MgCl₂, and CaCl₂) were used, and they were weighed in a glovebox purged with nitrogen gas.

All results for NaCl, KCl, and CaCl₂ solutions varied within ±0.10⁰/₀₀ for δ¹⁸O and ±1.0⁰/₀₀ for δ²H in relation to the δ-values of DI water used for the preparation of solutions (with one exception 1.5⁰/₀₀ for δ²H in 5.2 g/L solution of CaCl₂). The results for MgCl₂ exhibited similar variations for low concentrations (within ±0.10⁰/₀₀ for δ¹⁸O solutions up to 49.7 g/L and ±1.0⁰/₀₀ for δ²H for solutions up to 9.3 g/L). However, δ¹⁸O and δ²H varied from the δ-values of DI water for higher concentrations of MgCl₂ (δ¹⁸O was 1.21⁰/₀₀ more negative than DI water for 150.5 g/L and δ²H was 1.2⁰/₀₀ and 1.5⁰/₀₀ higher than DI water for concentrations of 49.7 g/L and 150.5 g/L, respectively).

The results confirmed that highly saline water samples can be successfully analyzed with a CRDS system, excluding samples with extremely high concentrations of MgCl₂ (>30 g/L).

Salt Accumulation in the Vaporizer

The final test evaluated if salt accumulation in the CRDS vaporizer significantly increases uncertainty. Eighty-two (82) samples of ocean water (TDS 35 – 40 g/L) were analyzed after vaporizer cleaning (sequential flushing with DI water following the Picarro vaporizer cleaning procedure). Despite the increasing load of salt in the vaporizer caused by evaporation of the saline samples, the measured isotopic composition of the water did not vary above the range of expected analytical uncertainty (one standard deviation was 0.03⁰/₀₀ for δ¹⁸O and 0.6⁰/₀₀ for δ²H). In addition, a "memory effect" was tested by analyzing a freshwater laboratory standard with contrasting δ-values. The measured values only differed by 0.03⁰/₀₀ for δ¹⁸O and 0.5⁰/₀₀ for δ²H from the true δ-values of the standard.

Even when a large salt load accumulated within the vaporizer (6.0 mg of MgCl₂ and 38.5 mg of total salt), the results were not compromised. Results were only affected when highly saline solutions with very high MgCl₂ concentrations (exceeding almost 10 times ocean water) were analyzed.

The author does state at the end of the article that care should be taken to minimize salt accumulation in the vaporizer through regular cleaning. So, in the next post (the last in this series) I’ll will report on the use of our Picarro Salt Liner accessory. It catches salt precipitates as the injected high total dissolved solids (TDS) water sample is vaporized to protect the analyzer and maintain optimum performance.

Click the link for information on Picarro water isotope analyzers and system peripherals.