ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2015, том 70, № 7, с. 715-720
ОРИГИНАЛЬНЫЕ СТАТЬИ =
УДК 543
DETECTION OF HALOGENATED ORGANICS BY THEIR INHIBITORY ACTION IN A CATALYTIC REACTION BETWEEN DIMETHYL ACETYLENEDICARBOXYLATE AND 4-METHYL-2-NITRO ACETATE
© 2015 Isaac Afreh*, Evan K. Wujcik**, Nathaniel Blasdel*, Benjamin Sauer*, Susan Kaya*, Stephen Duirk***, Chelsea N. Monty*, 1
*Monty Research Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron
Akron, OH, USA 44325-3906 1E-mail: cm78@uakron.edu
** Materials Engineering and Nanosensor [MEAN] Laboratory, Dan F. Smith Department of Chemical Engineering,
Lamar University P.O. Box 10053 Beaumont, TX 77710 USA ***Duirk Research Laboratory, Department of Civil Engineering, The University of Akron
Akron, OH, USA 44325 Received 18.03.2013; in final form 11.11.2014
The purpose of this paper is to report on the detection of toxic halogenated organic compounds in water using their inhibitory action on a pyridine-catalyzed reaction between dimethyl acetylenedicarboxylate (DMAD) and 4-methyl-2-nitroacetate (MNA). Previous work has shown that similar techniques can successfully lower the detection limit of sulfides and arsines in water samples, compared to their standard photometric detection methods. This paper highlights the optimization, selectivity, and sensitivity studies of the proposed sensing scheme. Optimization shows that the pyridine-catalyzed reaction is more favorable at 4 mM DMAD and 8 mM MNA. It was also determined that the inhibitory effect of halogenated organic compounds is more pronounced when carried out at 60°C. Using optimized reaction conditions, the limit of quantification for the four regulated tri-halomethanes (THMs) was approximately 80 ppm. In addition, the sensing method is selective to THMs and a few other halogenated organics. These promising results demonstrate the further success of this technique for sensitive, selective detection, and future work will be carried out to incorporate the technique in sensing applications for THMs in drinking water.
Keywords: trihalomethanes, drinking water, Fujiwara reaction, electrochemical detection.
DOI: 10.7868/S0044450215070063
The work presented here focuses on the extension of a previously developed inhibitory technique for the detection of toxic halogenated compounds in water [1, 2]. In this technique, the inhibition of a catalytic reaction is used to amplify the signal from an analyte of interest. The analyte of interest acts as an inhibitor for a given reaction. During the reaction, reactants A and B are catalyzed by a catalyst C, where C selectively binds to a target inhibitor (I), the analyte of interest. When the inhibitor I binds to the catalyst active site, the rate of the reaction, A + B ^ P, decreases. Therefore, the presence ofa small amount of inhibitor will produce a large change in the concentration profiles of the reactants and/or products [3—6].
While there are a number of possible catalysts for this technique, this work is directed at adapting the selective complexation reactions that have been developed for colorimetric detection. In practice, we can begin to develop a library of chemistries by using the active reagents found in Drager-tubes or other related
systems as catalysts. These methods are relatively selective to a class of compounds; however, colorimetric methods often have limited sensitivity and slow detection times. Previous reaction schemes developed by the authors, for sensing toxic sulfides and arsines in water, have successfully lowered the detection limit by over 2 orders of magnitude, compared to colorimetric techniques. Similar to colorimetric techniques, however, these previously developed assays have also been selective to the analytes of interest and few other related compounds [1, 2].
In this work, we present the success of the proposed reaction mechanism for the detection of toxic haloge-nated compounds in water. Detection methods for these compounds are of special interest due to their increased prevalence [7] as drinking water can easily become contaminated by halogenated organic compounds including disinfection by-products (DBPs). DBPs can be formed when decontamination disinfectants react with organic matter to create harmful constit-
O
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Chloroform
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■ Inhibition reaction
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Fig. 1. Scheme for the detection of DBPs (such as chloroform). DMAD and MNA react in the presence of pyridine to form a functionalized isoxazole (top reaction). When the analyte is present, it reacts with pyridine via the inhibition pathway and the formation pathway for isoxazole is shut-off (bottom reaction).
uents. Gem-polyhalogens such as trihalomethanes, a common form of DBPs, are regulated and monitored by the Environmental Protection Agency (EPA) with the maximum exposure limit set at 80 ppb (80 nL/L) for the four chief THMs (chloroform, bromoform, bromod-ichloromethane, and dibromochloromethane) in treated water [8, 9]. DBPs are also known to be carcinogenic even at relatively low levels, and certain groups of people (pregnant women, children, elderly, and the infirmed) are particularly affected by their exposure to these compounds [10]. In addition, despite the best efforts of monitoring using the current technology, the level of metals and DBPs delivered to the end-user will vary based on a variety of factors such as the type of piping system, type of disinfectant, and distance (time since disinfection) from the water treatment plant [11].
Currently, DBPs are detected using techniques such as the GC—MS with a variety of extraction techniques such as liquid—liquid extraction, direct aqueous injection, head space injection, and purge and trap. These techniques involve sending water samples off-site for testing and analysis, and require expensive equipment and trained personnel. Other techniques such as colori-metric methods and biological toxicity assays suffer from low sensitivity, low reproducibility and shelf-life issues. Therefore, there is the need for point-of-use water monitoring for the detection of toxic DBPs in drinking water.
Our goal was to create a sensitive, selective detection method for use in a point-of-use sensing apparatus.
Our approach was to start with the Fujiwara reaction for the colorimetric detection of halogenated or-ganics in water. The Fujiwara reaction [12], first discovered in 1916, was employed to colorimetrically detection of halogenated organics in drinking water and has been previously used in a number ofschemes to detect halogenated compounds in such media as blood [13], bodily tissues [14], air [15], and water [16]. Through the use of this reaction, halogenated organics have been detected through conjugation with pyridine in the presence of a base. The detection can be seen when the heated re-actants form an intermediate that changes the solution from clear to a purple, red, pink, or yellow color, upon addition ofhalogenated organics.
In the proposed inhibitory detection scheme (shown in Fig. 1), pyridine will act as a catalyst for the synthesis of functionalized isoxazoles from activated MNA and DMAD based on the procedure from Yavari et al. [17]. The sensing mechanism is designed to occur in two steps. First, water samples are mixed with a pyridine/base solution allowing the DBPs and pyridine to react via the Fujiwara reaction method. Once a DBP binds to the pyridine, the catalytic ability of the pyridine decreases due to the breakdown of the pyridine catalyst or further inhibition of the indicator reaction from the breakdown product. The breakdown
product of the pyridine and DBP reaction is shown in Fig. 1 and can be measured via UV absorbance at 364 nm [18]. Next, an aliquot of the pyridine/base/water sample is injected into a catalytic reactor to initiate isoxazole production. When a DBP is present, the active site of pyridine is less reactive and the rate of isox-azole production decrease sausing the inhibition of the indicator reaction (isoxazole formation). For this detection method, the production of isoxazole is measured electrochemically by monitoring the open circuit potential (OCP) of the system over time.
This paper highlights the optimization, selectivity, and sensitivity studies on the detection scheme. The work also demonstrates the success of using this scheme to effectively detect halogenated hydrocarbons in water samples on the beaker scale.
EXPERIMENTAL
Reagents. 1,1,2-Trichloroethane (96%), bromod-ichloromethane, bromoform, carbon tetrachloride (99%), chloroform, dibromochloromethane, dichlo-roacetic acid (99%), dimethyl acetylenedicarboxylate (99%), methyl nitroacetate, pyridine (>99%), tetrachlo-roethylene (99%), trichloroacetic acid (100%), trichlo-roethylene were purchased from Sigma-Aldrich (St. Louis, MO, USA); 1,1,1-trichloroethane (purified grade), acetonitrile (99%), and sodium hydroxide (99%) were purchased from Fisher Scientific (Pittsburgh, PA, USA); 1,1,2,2-tetrabromoethane was purchased from Eastman Kodak Company; bromoethane (98%) was purchased from Alfa Aesar; dichloromethane (HPLC grade) was purchased from Acros Organics.
Detection of THMs in water. The inhibition reaction between analyte and pyridine begins in a test tube filled with 0.5 mL of 20% NaOH placed into a water bath maintained at the desired temperature. Then 0.5 mL of pyridine was added to the stirred NaOH solution. The solution was allowed to reach a steady-state temperature for 5 min and 0.1 mL of the analyte in water (with a known concentration) was added to the test tube in a closed system. The reaction between the pyridine and the analyte was allowed to run for 60 min, after which the reaction was removed from heat and the organic phase was extracted. The
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