Article pubs.acs.org/ac
Molecular Scavengers as Carriers of Analytes for Mass Spectrometry Identification Marek Smoluch,*,† Michal Ceglowski,‡ Joanna Kurczewska,‡ Michal Babij,§ Teodor Gotszalk,§ Jerzy Silberring,†,∥ and Grzegorz Schroeder‡ †
AGH University of Science and Technology, Department of Biochemistry and Neurobiology, Faculty of Materials Science and Ceramics, Al. Mickiewicza 30, 30-059 Krakow, Poland ‡ Adam Mickiewicz University in Poznań, Faculty of Chemistry, Umultowska 89b, 61-614 Poznan, Poland § Wroclaw University of Technology, Faculty of Microsystem Electronics and Photonics, Z. Janiszewskiego 11/17, 50-372 Wroclaw, Poland ∥ Centre of Polymer and Carbon Materials, Polish Academy of Sciences, Curie-Sklodowskiej St. 34, 41-819 Zabrze, Poland S Supporting Information *
ABSTRACT: Storage and preconcentration of various molecules by molecular scavengers for thermal desorption and identification by mass spectrometry is presented. A dielectric barrier discharge ionization source combined with a heating element for the chemical characterization of amines and organic acids, initially trapped by molecular scavengers, is described. The developed technique can be applied for preconcentration of minute amounts of molecules in liquid and gaseous phases, as well as their transportation and thorough analysis. The method, operating at ambient pressure, can also be complementary to electron impact ionization, with no need for sample derivatization.
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supported on silica, the analyte can form hydrogen-bonded systems or ionic bonds. Both types of bonds are weak and reversible, which is beneficial for fast and easy thermal desorption of the compounds. Therefore, we selected this group of sorbents as highly suitable for analytical purposes. After preconcentration of the substances, mass spectrometry can be applied for their analysis. The prerequisite step before studies is desorption of the harvested material. Ionization methods that operate at ambient conditions are especially useful for this purpose. They allow for fast and unambiguous analysis with no additional sample preparation, which sometimes might be a time-consuming process. One of the newly developed types of mass spectrometric analysis, based on the ambient plasma ion source, is dielectric barrier discharge ionization (DBDI).14−16 DBDI produces low-temperature plasma by dielectric barrier discharge (DBD). DBD plasma was used to generate mass spectra of compounds desorbed from various surfaces,17 to detect nonvolatile chemicals directly from the functionalized particles.18 Other approaches, such as liquid chromatography−mass spectrometry (LC−MS)19 or thin-layer chromatography, coupled online with MS for mercury speciation, are also used.20 In many cases, analytical methods are not sufficiently sensitive for direct determination of analytes, which are present
ast analysis of chemicals that are present at low concentrations is always challenging. Currently, molecular receptors covalently linked to solid support are applied for selective concentration of analytes, which are then examined quantitatively and qualitatively. For this purpose, molecular scavengers can be used and are defined as substances added to a mixture in order to remove or inactivate impurities and undesired reaction products. They comprise a class of functionalized hybrid molecules that exhibit ability to bind or “scavenge” diluted molecules in liquid form or in the gas phase.1−3 Basic components of such systems are polymer fibers, resins or inorganic materials (e.g., silica, iron oxides)1−6 and they can find applications in organic synthesis, chromatography, extraction of ions or catalytic reactions. Depending on the attached ligand, molecular scavengers can be applied for various purposes, for example, environmental protection, warfare harvesting and neutralization, long-distance transport of toxic agents, enrichment of narcotics in sewage, or in medicine.7−11 Polymeric scavengers are often used in organic synthesis. The polymeric base, ammonium ion-exchange resin, can be used in synthesis of 2,4-pyrrolidinedione as it promotes cyclization in the initial reaction step,12 while highly basic PTBD (1,5,7-triazabicyclo[4.4.0]dec-ene) scavenger is useful in synthesis of aryl esters.13 Molecular scavengers, which use acid−base interactions to bind the analytes, comprise the largest group of hybrid inorganic−organic compounds. The interest in this group of compounds is dictated by the fact that, depending on the acid−base properties of the organic portion © 2014 American Chemical Society
Received: July 25, 2014 Accepted: October 22, 2014 Published: October 22, 2014 11226
dx.doi.org/10.1021/ac502776z | Anal. Chem. 2014, 86, 11226−11229
Analytical Chemistry
Article
based on a high voltage power supply generates AC voltage (3 kVpp, 70 kHz), which is adequate to ignite the discharge in helium (flow rate 1L/min., purity 99.996%). The source works fully automatically in a fixed current mode with output overvoltage and short circuit protection. The needle source operates with DC voltage (between 6 and 12 V) and consumes a very low amount of power (approximately 2.5 W). A Bruker Esquire 3000 quadrupole ion trap mass spectrometer (Bruker Daltonics, Bremen, Germany) was used for all measurements. The typical electrospray ionization (ESI)MS source settings were found to be optimal also for the DBD ion source, with the exception of the mass spectrometer entrance glass capillary voltage, where lower potential (1 kV) compared to the standard ESI setting (4.5 kV) was used. The temperature of the glass capillary was set to 200 °C; the drying gas flow was maintained at 3 L/min.; and the nebulizer gas (N2) was not applied. The scan range was set from 80 to 500 m/z. For tandem MS/MS experiments the isolation width was set to 2 m/z and the fragmentation amplitude was in the range of 0.5 to 0.8 units.
at very low concentrations in the gas phase or in liquid form. This creates urgency in the search for sample preconcentration methods that can be applied directly at the sampling place. Interaction of analytes with hybrid molecular scavengers allows for rapid preconcentration of molecules. Moreover, substances bound by molecular scavengers can be safely stored and transported from sites remote from a specialized laboratory. The analyte, deposited on the molecular scavenger, can be kept for a longer time, without significant effect on the results of analysis and at a very low cost. In this Article, selective molecular scavengers were applied for extraction of amines and carboxylic acids from liquid or gas phase. The analysis of compounds bound in such a way was performed using thermal desorption of the harvested molecules, followed by direct identification by DBDI. Moreover, we present the general procedure of analytical determination of the compounds using this method.
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EXPERIMENTAL SECTION All reagents used are commercial products. 3(Diethylenetriamino)propyl-functionalized silica gel (extent of labeling = 1.3 mmol/g loading; particle size = 200−400 mesh; surface area = 500 m2/g), polyethylenimine on silica gel (particle size = 40−200 mesh), 3-(trimethylammonium)propylfunctionalized silica gel, carbonate (extent of labeling = 0.8 mmol/g loading; particle size = 200−400 mesh; surface area = 500 m2/g), triamine tetraacetate, silica-supported (extent of labeling > 0.41 mmol/g loading; particle size = 200−400 mesh; surface area = 500 m2/g), diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, benzoic acid, and 2,4-dichlorophenoxyacetic acid were obtained from Sigma-Aldrich (St. Louis, MO, USA). Structures of the functionalized silica gel materials are presented in Figure 1.
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RESULTS AND DISCUSSION A general procedure for the determination of analytes concentrated on selective molecular scavengers and DBDI mass spectrometric analysis is presented Figure 2.
Figure 1. Structures of functionalized silica gel materials.
Construction of a heater with mini crucible to hold ∼0.5 mg of sample, for temperature-controlled desorption in the range of 20−220 °C linked to the DBDI ion source, allowed for the development of an innovative methodology for the analysis of substances deposited on selective molecular scavengers. The described technique can also be complementary to electron impact ionization, particularly combined with a direct inlet probe but operates at ambient pressure. Instrumental Design. Detailed description of the ion source has been published.21,22 Briefly, the DBD plasma source consists of quartz capillaries (O.D. 0.7 mm and I.D. 0.5 mm), a ring high voltage electrode on the surface of a quartz tube and a grounded (25 μm diameter) platinum needle concentric with the tube. A glow discharge is formed between the needle and outer electrode. The quartz tube makes a barrier for direct current flow between both electrodes. Piezoelectric transformer
Figure 2. General procedure for the determination of analytes using preconcentration on selective molecular scavengers, thermal desorption, and DBDI mass spectrometric analysis.
Analysis of Molecules in Liquid Form. To 10 mL of a solution containing the analyte at a concentration of
