Article pubs.acs.org/ac
Online Coupling of In-Tube Solid-Phase Microextraction with Direct Analysis in Real Time Mass Spectrometry for Rapid Determination of Triazine Herbicides in Water Using Carbon-Nanotubes-Incorporated Polymer Monolith Xin Wang, Xianjiang Li, Ze Li, Yiding Zhang, Yu Bai, and Huwei Liu* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, Institute of Analytical Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China S Supporting Information *
ABSTRACT: Online coupling of in-tube solid phase microextraction (IT-SPME) with direct analysis in real time mass spectrometry (DART-MS) was realized for the first time and applied in the analysis of triazine herbicides in lake water and orange juice. We incorporated single-wall carbon nanotubes (SWNTs) into a polymer monolith containing methacrylic acid (MAA) and ethylene dimethacrylate (EDMA) to form a novel poly(methacrylic acid-co-ethylene dimethacrylate-co-single wall carbon nanotubes) (poly(MAA-EDMA-SWNT)) monolith, which was then used in ITSPME for enrichment of six triazine herbicides from water samples. With the online combination of IT-SPME with DART-MS, the analytes desorbed from the monolith were directly ionized by DART and transferred into MS for detection, thus rapid determination was achieved. Compared with regular DART-MS method, this online ITSPME-DART-MS method was more sensitive and reproducible, because of the IT-SPME procedures and the isotope-labeled internal standard used in the experiment. Six triazine herbicides were determined simultaneously using this method with good linearity (R2 > 0.998). The limit of quantification (signal-to-noise ratio of S/N = 10) of the six herbicides were only 0.06−0.46 ng/mL. The proposed method has been applied to determine triazine herbicides in lake water and orange juice, showing satisfactory recovery (85%−106%) and reproducibility (relative standard deviation of RSD = 3.1%−10.9%).
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DART-MS analysis for the cleanup of matrix and enrichment of the desired analytes at the same time. In-tube solid phase microextraction (IT-SPME) is a very attractive technique for sample preparation with great preconcentration ability and minimum matrix interferences.13 To date, several types of capillaries have been developed for ITSPME, such as inner-wall-coated capillary,14,15 fiber-inserted capillary,16,17 and monolithic capillary.18 Among them, the monolithic capillary has been widely utilized, because of the advantages of easy preparation, large surface area, and good control of porosity.19,20 Over the last two decades, carbon nanotubes (CNTs) have formed part of extensive and multidisciplinary research due to their superior properties and wide range of applications over other materials.21 The marriage of unique properties of CNTs and the excellent features of polymer monoliths to develop novel CNTs incorporated polymer monoliths make it possible to immobilize CNTs
mbient mass spectrometry (AMS), introduced by Cooks et al. in 2004,1 greatly simplifies mass spectrometry (MS) analysis and provides a rapid and straightforward way to analyze samples without or with less preseparation. To date, dozens of AMS ionization techniques have been reported, among which direct analysis in real time (DART)2 is one of the most widely used and commercial available AMS ion sources. The ionization of DART is performed by the interactions of excited metastable helium (He) or nitrogen (N2) with the analytes and the atmospheric gases,3,4 and this technique has been successfully used for the analysis of various analytes.5−8 The studies on the combination of DART with chromatography 9 and the application of substrates in DART-MS are also popular.10,11 Although DART-MS analysis is rapid and straightforward, its sensitivity is often lower than traditional gas chromatography− mass spectrometry (GC-MS) or liquid chromatography−mass spectrometry (LC-MS) methods.12 Moreover, the introduction of real samples containing complicated matrices may result in high background noise in DART-MS, making qualitative and quantitative analysis of trace analytes difficult.4 To solve these problems, suitable sample pretreatment is required prior to © XXXX American Chemical Society
Received: November 2, 2013 Accepted: April 18, 2014
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dx.doi.org/10.1021/ac500382x | Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
Article
Figure 1. Experimental protocol and configuration of the online IT-SPME-DART-MS system. The entire experimental procedure involved four parts: (A) activation, (B) sampling, (C) washing, and (D) online desorption and detection (top view, where d1 = d2 = 3 mm).
USA). Methacrylic acid (MAA) and ethylene dimethacrylate (EDMA) were obtained from Alfa-Aesar (Ward Hill, MA, USA). Analytical-grade toluene, dodecanol, azobis(isobutyronitrile) (AIBN), and sodium chloride (NaCl) were obtained from Sinopharm Chemical Reagent Co. (Beijing, China). Acetonitrile (ACN), methanol, acetone, isopropanol (IPA), and formic acid (FA) of LC grade were from Dikma Technology (Richmond, VA, USA). Simazine, atrazine, prometon, ametryn, propazine, and prometryne were all supplied by AccuStandard (New Haven, CT, USA). Individual stock solutions of triazine herbicides were prepared in methanol at the concentration of 1 μg/mL and stored at 4 °C in darkness. Atrazine-d5 was purchased from Dr. Ehrenstorfer GmbH (Augsburg, Germany) and used as the internal standard. Purified water was provided by Hangzhou Wahaha Group (Zhejiang, China) and fused-silica capillaries with 530 μm i.d. × 760 μm o.d. were obtained from SINO Sumtech (Handan, China). Equipment and Conditions. (1) Characterization of Monolith. The microscopic morphology of the monoliths was characterized by scanning electron microscopy (SEM) using a Hitachi S-4800 instrument (Hitachi, Tokyo, Japan). Before measurement, the monolithic capillary was cut into 2-mm-long pieces, placed on an aluminum stub and sputter-coated with gold.20 The surface area and mesopore size distribution of the monoliths were determined using nitrogen sorption experiments with an accelerated surface area and porosimetry analysis system (Model ASAP2020, Micromeritics, Norcross, GA, USA). The through-pore properties of the monoliths were measured using an Autopore IV 9500 mercury porosimeter (Micromeritics, Norcross, GA, USA). (2) DART-MS. A DART-SVP source (IonSense, Saugus, MA, USA) was fitted to an Agilent MSD ToF MS (Agilent Technologies, Palo Alto, CA, USA) after removing the original Agilent electrospray ionization source. The working gas of the DART ion source was nitrogen at 350 °C; the potential on the
onto structured supports, which will make CNTs widely available in various applications. Recently, CNTs have been incorporated into polymer monolith for the development of new stationary-phase for chromatography.22,23 However, to the best of our knowledge, no CNT-incorporated monolith has been used in IT-SPME. Because of the unique physicochemical properties of CNTs, they may offer interesting opportunities for the development of new porous polymer monoliths for SPME. On the other hand, IT-SPME has found many applications in the analysis of various compounds when coupled to GC, LC, and capillary electrophoresis (CE);13,24 however, literature focusing on the coupling of SPME with DART-MS is quite rare. Only three reports have been based on the combination of SPME with DART-MS: membrane solidphase microextraction with DART-MS,25 stir bar sorptive extraction with DART-MS,26 and packed sorbent microextraction coupled to DART-MS.12 There is no publication focusing on the online coupling of IT-SPME with DART-MS. In this study, the online IT-SPME-DART-MS was realized for the first time. First, a novel poly(MAA-EDMA-SWNT) monolith was prepared for IT-SPME and the extraction conditions were optimized systematically. Then online desorption was followed and the analytes were directly ionized and determined by DART-MS. The combination of IT-SPME with DART-MS provides a rapid and sensitive method for the determination of trace triazine herbicides in complicated matrices with satisfactory detection sensitivity. Compared with previous works,12,25,26 the proposed method exhibited enhanced extraction capacity, improved matrix tolerance, and remarkable preconcentration ability.
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EXPERIMENTAL SECTION Reagents and Standards. 3-(Trimethoxysilyl) propyl methacrylate, ammonium formate, and single-wall carbon nanotubes (SWNTs) ( 0.998. Limits of detection (LODs) and limits of quantification (LOQs) were calculated as signal-to-noise ratios of S/N = 3 and 10 (in the range of 0.02−0.14 ng/mL and 0.06−0.46 ng/mL, respectively). The reproducibility of the method was assessed by determining the intraday and interday relative standard deviations (RSDs) of 5 ng/mL standard solutions, showing that the RSDs were all
