On-line monitoring by membrane introduction mass spectrometry of chlorination of organics in water. Mechanistic and kinetic aspects of chloroform formation

An experimental laboratory reactor for quantitative kinetic studies of disinfection byproduct formation using membrane inlet mass spectrometry

RATIONALE

The type and quantity of environmentally problematic disinfection byproducts (DBPs) produced during chlorination of water depend on the natural organic matter and organic contaminants that raw water contains, and on the operational conditions of the drinking water treatment process. There is a need for a fast and quantitative method that determines which DBPs are produced and monitors the chemical dynamics during a drinking water treatment.

METHODS

A small experimental chemical reactor (50 mL) was mounted directly onto the membrane inlet interface of a membrane inlet mass spectrometer (MIMS). In this setup, the membrane was the only separation between the reaction mixture in the chemical reactor and the open ion source of the mass spectrometer 2 cm away. Water samples to be chlorinated were placed in the reactor and the chlorination reaction was initiated by injection of hypochlorite. The formation of intermediates and products was monitored using either full-scan mass spectra or selected ion monitoring of relevant ions.

RESULTS

An algorithm for analyte quantification was successfully developed for analysis of the complex mixtures of phenol (a model for waterborne organic compounds), chlorinated intermediates and trihalomethane products which simultaneously pass the membrane into the mass spectrometer. The algorithm is based upon the combined use of standard addition and an internal standard, and all analytes could be quantified at nanomolar concentrations corresponding to realistic water treatment conditions. Experiments carried out in the temperature range 15-60°C showed that the reaction dynamics change with operational parameters, for example in tap versus deionized water.

CONCLUSIONS

We have successfully shown that an experimental laboratory reactor directly interfaced with a MIMS can be used for quantitative monitoring of the chemical dynamics during a water treatment. This technique could provide rapid assistance in the optimization of operating parameters for minimizing DBP production.

Photosensitized degradation kinetics of trace halogenated contaminants in natural waters using membrane introduction mass spectrometry as an in situ reaction monitor

On-line membrane introduction mass spectrometry used to directly measure the photosensitized reductive dehalogenation kinetics of trace aqueous halocarbons in the presence of naturally occurring dissolved organic matter.

Membrane introduction mass spectrometry (MIMS): a versatile tool for direct, real-time chemical measurements

Membrane introduction mass spectrometry (MIMS) is a direct, continuous, on-line measurement technique. It utilizes a membrane to semi-selectively transfer analyte mixtures from a sample to a mass spectrometer, rejecting the bulk of the sample matrix, which can be a gas, liquid or solid/slurry. Analyte selectivity and sensitivity are affected by optimizations at the membrane, ionization and the mass spectrometer levels. MIMS can be roughly classified by the acceptor phase that entrains analyte(s) to the mass spectrometer after membrane transport, either a gaseous acceptor phase (GP-MIMS) or condensed acceptor phase (CP-MIMS). The aim of this article is to provide an introduction to MIMS as a technique and to explore current variants, recent developments and modern applications, emphasizing examples from our group, the Applied Environmental Research Laboratories as well as selected work from others in this emerging area. Also provided is a synopsis of current and future directions for this versatile analytical technique.

Characterization of a condensed-phase membrane introduction mass spectrometry (CP-MIMS) interface using a methanol acceptor phase coupled with electrospray ionization for the continuous on-line quantitation of polar, low-volatility analytes at trace levels in complex aqueous samples

We report the development and application of a capillary hollow fibre membrane interface using methanol as an acceptor phase to deliver target analytes to an electrospray ionization source and a triple quadrupole mass spectrometer. Superior fluid handling systems lead to greater signal stability and membrane integrity for the continuous on-line monitoring of polar and charged analytes in complex aqueous samples with detection limits in the parts-per-trillion to parts-per-billion range. The system can be operated in either a continuous flow or a stopped acceptor flow mode - the latter giving rise to greater sensitivity. We report detection limits, enrichment factors and signal response times for selected analytes with polydimethylsiloxane and Nafion® polymer membrane interfaces. In addition, we demonstrate the use of this interface to detect pharmaceuticals and other contaminants in natural water and artificial urine. The improved sensitivity and analytical response times of our CP-MIMS system make it possible to continuously monitor dynamic chemical systems with temporal resolutions on the order of minutes. Presented is a comparison of the performance of CP-MIMS versus direct infusion electrospray ionization, demonstrating the potential advantages over direct infusion for trace analyte measurements in complex, high ionic strength samples. Furthermore, by continuously flowing a reaction mixture in a closed loop over the interface, we demonstrate the use of the system as an in situ reaction-monitoring platform for the chlorination of a model organic compound in aqueous solution.

On-line measurement of oxidative degradation kinetics for trace gasoline contaminants in aqueous solutions and natural water by membrane introduction tandem mass spectrometry

Membrane introduction mass spectrometry (MIMS) was used to directly monitor the loss of trace gasoline contaminants (benzene, toluene, 2-methylthiophene and methylcyclohexane) in nanomolar (ppb) aqueous solutions under a variety of UV-induced advanced oxidation processes (AOP). The decay kinetics of these contaminants were followed simultaneously in "real-time" via tandem mass spectrometric techniques by re-circulating the reaction mixture in a closed loop over a semi-permeable membrane interface. The photocatalyzed degradations were observed to follow pseudo-first-order kinetics with rate constants ranging from 0.006 to 0.2 min⁻¹ depending on the reaction conditions. We report rate enhancements for several UV-based advanced oxidative processes using physiosorbed titanium dioxide (TiO₂/UV, TiO₂/UV/O₂, TiO₂/UV/H₂O₂) and compare these to the direct photolysis of H₂O₂ under otherwise identical conditions. The relative degradation rates of 4 trace contaminants are reported for reactions carried out in the same solution. The degradation kinetics were also monitored directly in a natural surface water spiked with the same contaminant suite. The observed decay kinetics in the presence of TiO₂ in air-saturated natural water were similar to those carried out in deionized water. However, when the photo-oxidation was enhanced by the addition of H₂O₂, the degradation was markedly slower in natural water relative to deionized water due to competition for photons by dissolved organic matter. This work further demonstrates the use of MIMS as a sensitive on-line measurement technique for "in-situ" reaction monitoring of organic contaminants at environmentally relevant concentrations in complex solutions and reactive media.

Evaluation of functional groups responsible for chloroform formation during water chlorination using compound specific isotope analysis

Compound-specific isotope analysis was used to monitor the delta(13)C signature of chloroform produced upon the chlorination of model compounds representing natural organic matter functional groups (resorcinol, acetylacetone, acetophenone, phenol, and 2,4,6-trichlorophenol) and a natural water sample. For each model compound, a different apparent kinetic isotope effect was found for chloroform formation. Normal isotope effects were found for resorcinol, acetylacetone, and acetophenone, and ranged from 1.009 +/- 0.002 to 1.024 +/- 0.004. For the two phenols, an inverse effect was found (0.980 +/- 0.004). Lake Zürich water also had a inverse effect (0.997 +/- < 0.001) indicating that phenols are likely chloroform precursors in NOM, but that other functional groups may also participate. The apparent 13C kinetic isotope effect for the addition/ elimination reaction of 1,1,1-trichloropropanone mediated by OH- to yield chloroform is 1.014 +/- 0.002. A comparison of this value to those found for the chlorination of the model precursors and an evaluation of the differences in chloroform production kinetics for the different model precursors argue against a mechanism in which all NOM precursors react via a common intermediate. Compound specific isotope analysis may give additional insights into chloroform formation mechanisms beyond those allowed by current techniques.

Photolytic degradation of the insecticide thiamethoxam in aqueous medium monitored by direct infusion electrospray ionization mass spectrometry

Photodegradation of the insecticide thiamethoxam (1), 3-[(2-chloro-5-thiazolyl)methyl]tetrahydro-5-methyl-N-nitro-4H-1,3,5-oxadiazin-4-imine, in an aqueous medium was monitored by electrospray ionization mass spectrometry in the positive ion mode, ESI(+)-MS. An aqueous solution of (1) was incessantly exposed to a UV radiation source and aliquots were taken after reaction times of 1, 2, 3, and 4 h. Analysis by GC/NCI-MS revealed that (1) was continuously degraded under these experimental conditions. However, the total organic carbon (TOC) content remained practically constant during the exposition period, thereby indicating that 1 was not mineralized but continuously converted into other compounds. ESI(+)-MS monitoring revealed that whereas the intensity of the ions of m/z 292/294 ([1 + H](+)) constantly decreased, there was the emergence of other ions of m/z 247/249, 197, 168, and 116 whose intensities simultaneously increased. Their structures were proposed on the basis of: (1) the data of their ESI(+)-MS/MS; (2) their high resolution m/z values; and (3) a plausible reactivity of the thiamethoxam molecule exposed to UV radiation in aqueous solution. Finally, these data allowed us to suggest a reaction route for the photodegradation of 1 in an aqueous medium.

An electrospray membrane probe for the analysis of volatile and semi-volatile organic compounds in water

A new membrane probe incorporating electrospray ionization (ESI) was designed, built and coupled to an ion trap mass spectrometer to detect low levels of semi-volatile organic compounds (SVOCs) in water. Similar to other membrane introduction mass spectrometry (MIMS) systems, the probe contains a capillary polydimethylsiloxane (PDMS) membrane to allow for the preferential permeation of small molecules but, in contrast, the interface uses a liquid/membrane/liquid interface rather than liquid/membrane/gas. The ESI source allows the probe to be operated at atmospheric pressure in positive or negative ionization mode and the lack of fragmentation in ESI allows for the simultaneous screening of many analytes with high sensitivity. The interface allows for the addition of additives to both the external and the internal liquid mobile phases to selectively improve permeation and/or the ionization efficiency of various classes of compounds. Characterization of the probe with methanol as the internal mobile phase showed that the signal for aniline optimized at 60 degrees C and an internal flow rate between 2-5 microL/min. The transfer of analyte through the membrane from water to methanol ensures a strong signal and robust electrospray for both positive and negative ion mode which is not typical when spraying pure water. Detection limits for aniline, pyridine and pentachlorophenol, and for the herbicides alachlor, atrazine, butachlor, metolachlor and simazine, were in the ppb to pptr range.

Reactions of the flavonoid hesperetin with chlorine: a spectroscopic study of the reaction pathways

The flavonoid hesperetin (Hsp) contains aromatic rings substituted with hydroxyl and methoxyl groups, which activate it toward electrophilic attack and hence make it a potential surrogate for natural organic matter with respect to reactions with chlorine. This paper describes the chlorination pathway of Hsp, based on a combination of electrospray tandem mass spectrometry (MS/MS) and absorbance spectroscopy. When a solution containing Hsp is dosed with NaOCl at pH 7, chlorine substitution into Hsp occurs exclusively into the meta-dihydroxy substituted ring. The first two Cl atoms to enter the ring do so at the highly activated carbons that are each ortho to two oxygenated carbon atoms. These substitutions make the molecule more acidic, but do not change its primary structure or aromaticity. The third Cl atom that substitutes into the molecule does so at one of the previously chlorinated sites, destroying the aromaticity of the ring and altering the molecular properties more dramatically than do the first two. The absorbance spectra of Hsp and mono- and di-chlorinated Hsp are all very similar and are very distinct from that of trichlorinated Hsp. In particular, the latter is the only one of those species that absorbs visible light (in a characteristic band centered at approximately 422nm). Di- and trichloroHsp form even at low molar Cl/Hsp ratios, and can coexist with Hsp and monochloroHsp in neutral, aqueous solutions for at least 24 h in the absence of free chlorine. If free chlorine is present, the less-chlorinated species continue to acquire Cl, and trichloroHsp undergoes further reaction, until either the free chlorine or the trichloroHsp is fully depleted. The appearance of di- and trichloroHsp while substantial amounts of Hsp remain unreacted indicates that substitution of one or two Cl atoms into the ring facilitates the incorporation of yet more Cl into the ring. This autoacceleration of the chlorination process is hypothesized to be induced by the increase in acidity that accompanies Cl incorporation. Specifically, the increase in the acidity of the phenolic groups shifts the equilibrium toward the enolate anion, which is considered to be much more amenable to electrophilic attack than the enol.

Membrane introduction mass spectrometry for monitoring complexation equilibria of beta-cyclodextrin with substituted benzenes

Membrane introduction mass spectrometry (MIMS) was used to monitor complexation reactions between beta-cyclodextrin (CD) and a series of benzene derivatives in aqueous solution. The equilibrium constants for benzene, chlorobenzene, bromobenzene, iodobenzene, toluene, cyanobenzene and nitrobenzene were determined. The suitability of MIMS for monitoring complexation reactions of organic compounds with host molecules was demonstrated. Structure-activity relationship analysis shows that the inclusion phenomena are driven by a variety of chemical forces, of which hydrophobicity is predominant for non-polar compounds, but not the only factor for more polar ones.

Selective trace level analysis of phenolic compounds in water by flow injection analysis--membrane introduction mass spectrometry

Flow injection analysis coupled with membrane introduction mass spectrometry (FIA-MIMS) with on-line derivatization is shown to allow fast, accurate, nearly interference-free, and sensitive (low microgram/L) quantitation of phenolic compounds in water. On-line FIA derivatization of the phenolic compounds is performed by acetic anhydride acetylation in a K2CO3-buffered alkaline medium. The phenol acetates so formed efficiently permeate a silicone membrane and are directly transferred to the mass spectrometer, in which they are analyzed with selectivity and high sensitivity via selected ion monitoring. FIA-MIMS analysis was performed for aqueous solutions of phenol, 2-methylphenol, 4-chlorophenol, 4-chloro-3-methylphenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol, and detection limits in the 0.5-20 micrograms/L (ppb) range were observed for an analytical frequency of six samples/h. FIA-MIMS for phenolic compound analysis is considerably less time-consuming and labor intensive than most chromatographic methods based on liquid-liquid extraction and preconcentration procedures and is therefore applicable for on-line and in-situ monitoring of phenols in wastewaters and in the environment. FIA-MIMS employing acetic anhydride derivatization is also virtually free of interferences since it combines chemical, membrane, and enhanced MS selectivity; hence quantitation of phenolic compounds can be performed in the presence of congeners.