Measurement of dissolved organic carbon and nitrogen in natural waters: Workshop report

Total organic carbon (TOC) is a well-known and reliable parameter that acts as an indicator to estimate the amount of organic content in a water sample. Since the introduction of TOC analyzers in the 1960s, these devices have proven to be very critical in water quality assessment & process monitoring. The universal method is to measure TOC by oxidation of the organic content in the water sample to convert it into quantifiable forms. This article summarizes the commercially available methods of TOC analysis in water samples. The methods reviewed are combustion with non-dispersive infrared (NDIR) detection, catalytic oxidation with NDIR detection, persulphate oxidation and ultraviolet oxidation. We have highlighted the advantages of the various methods and their relative shortcomings. This study also compares the performance of the current TOC analyzers and TOC analysis of ultra-pure water that deserves specific attention as it contains organic carbon in trace levels. Based on the review, it is suggested that high-temperature combustion is quite an expensive oxidation method and is usually used for high-TOC water samples. Chemical-based oxidation (e.g., Persulphate) and UV oxidation methods provide reliable results in a lab for a wide range of water samples, including portable and ultra-pure water for semiconductor and pharmaceutical industries.

Quantifying trace dissolved organic nitrogen (DON) in water is currently challenging for conventional methods not only because of the presence of dissolved inorganic nitrogen (DIN) species, which usually dominate in total nitrogen (TN), but also because of the high method detection limits (MDLs) of current TN methods, which are ≥100 μg/L. In order to overcome these barriers, an earlier study has applied electrodialysis to eliminate DINs from water, thereby reducing the interference of DIN. However, it remains difficult to analyze DON at nanomolar level. To address this issue, this study proposes to convert DON completely into DIN by vacuum ultraviolet irradiation (VUV) and then to measure DON by the sum of DINs (i.e., ammonia, nitrite, and nitrate). A number of verification tests show that the proposed method had comparable results with conventional methods for selected model compounds and real samples. The average nitrogen recovery for sixteen model DON compounds (each at 2.0 mg-N/L) with varying structures and molecular weights was 89 ± 16%, similar to those obtained by thermal-activated persulfate oxidation (87 ± 18%) and high temperature catalytic oxidation (86 ± 20%) methods. Moreover, this method features an advantage that it reached a MDL of 1.5 μg-N/L, much lower than conventional TN methods. With a prolonged 12-h irradiation, this approach was able to convert ammonia completely into nitrate, thus enabling detection of DON by using ion chromatography only.

Carbon is fundamental to the functioning of the Earth, especially in the transfers of C between inorganic and organic C pools, and the resultant effects on the heterotrophic/autotrophic balance of ecosystems and the enrichment of CO₂ in the atmosphere. Understanding the fate and behavior of large C pools, including dissolved organic carbon (DOC), within the oceans and ocean margins, is key in this respect. Accurate (low bias, high precision) measurement of DOC is a prerequisite to this understanding. The measurement of DOC is described, including sample collection, treatment, and preservation. The evolving analysis of DOC, in particular by the now preferred technique of high temperature catalytic oxidation (HTCO), is described in detail, including the main analytical challenges and improvements in quality control. The importance of using reference materials (RMs) to maintain quality control is described and providers of seawater and river water RMs for use in DOC analysis are reported.

Rainwater is a very low concentrated matrix and, for dissolved organic matter (DOM) characterization, an efficient extraction procedure is essential. Isolation procedures based on the adsorption onto XAD-8 and C-18 sorbents have been used in the literature for rainwater DOM isolation, but a comparison between these procedures is lacking. In this work, UV–visible and molecular fluorescence spectroscopies highlighted differences between rainwater DOM isolated by DAX-8 (replacement for XAD-8) and by C-18. It was possible to recover higher rainwater DOM percentage by the C-18 based procedure than by the DAX-8 one. Rainwater protein-like compounds were better concentrated by the C-18 procedure than by the DAX-8 one, while humic-like compounds were similarly concentrated by both procedures. Furthermore, rainwater DOM extracted by the C-18 procedure was more representative of the global matrix, while DAX-8 preferentially extracted humic-like compounds.

Very different filtration and preservation procedures may be found in the literature on the study of the rainwater dissolved organic fraction. Thus, the influence of sample filtration and preservation procedures on the fluorescence of rainwater dissolved organic matter (DOM) was studied in this work. Rainwater was filtered through different filters (quartz 0.22 μm or PVDF 0.45 μm) and excitation (λ_em = 415 nm) and synchronous (Δλ = 70 nm) fluorescence spectra were obtained at the same day of collection, or after preservation by refrigeration (1–7 days) or by freezing (1–4 weeks). The excitation–emission matrix (EEM) spectra of rainwater showed six types of fluorescent bands: two corresponding to humic-like bands, and four resembling proteins. Then, the excitation and synchronous spectra were chosen in order to monitor changes in the humic-like and protein-like bands, respectively. The filtration procedures adopted in this work did not affect the fluorescence properties of the rainwater samples. However, these properties were differently preserved by refrigeration or freezing: after refrigeration, filtered rainwater maintained the original fluorescent properties for at least 4 days, while after freezing fluorescent properties were not always preserved since it occurred a decrease of protein-like fluorescence intensity.

This article summarizes the most important and current methods for the determination of total organic carbon (TOC) in solid samples. The methods presented are chemical oxidation, combustion, acid treatment and ashing. We discuss the advantages of the different methods as well as problems that can be encountered during analysis. We give additional information on the detection techniques used for TOC determination and on the topic of volatile organic carbon (VOC) that deserves particular attention when carrying out TOC determinations, in particular with solid samples. We have compiled a concise survey of the literature covering 60 reports and tabulated the relevant analytical parameters.