Helko Borsdorf

Ion Mobility Spectrometry: Principles and Applications

Abstract General principles are reviewed for ion mobility spectrometry including new methods for ion separation through field dependent mobilities in strong electric fields with high frequency asymmetric waveform. Additionally, recent advances in the instrumentation for the characterization of ion mobilities in air at ambient pressure are described and critically reviewed. Advances in instrumentation, understanding of principles of measurements by IMS, and the development of hyphenated technologies have resulted in an increase in the number of applications in recent years.

Recent Developments in Ion Mobility Spectrometry

Abstract The general principles and technical implementations of traditional time-of-flight ion mobility spectrometers and analyzers with field-dependent mobilities were reviewed in our last article in this journal. Recent advances in instrumentation and new applications since 2006 are highlighted in this review. In addition to traditional applications as military chemical-agent detectors, ion mobility techniques have become popular for different purposes. Though ion mobility spectrometry was solely used as vapor sensor in the past decades, further developments in ionization techniques (especially electrospray ionization) now permit its routine use for the analysis of liquid samples. The coupling of ion mobility spectrometry with selective sample preparation techniques such as molecular-imprinted polymers, coupling with chromatographic techniques, the use of dopants, and application of selective ionization sources has led to an expanded number of applications in industrial and environmental analysis with complex sample matrices due to an improved selectivity in comparison with traditional stand-alone spectrometers. Furthermore, new developments in hyphenated techniques, especially ion mobility–mass spectrometry couplings, has resulted in an increased number of new applications for the analysis of biomolecules and pharmaceutical samples and in clinical diagnostics.

Corona discharge ion mobility spectrometry of aliphatic and aromatic hydrocarbons

Abstract Positive ion mobility spectra of n -alkanes ( n -C 5 to n -C 19 ), branched chain alkanes (C 5 to C 9 ) and aromatic compounds (benzene and alkylated benzenes) were acquired with an ion mobility spectrometer equipped with a corona discharge ionization source. Reduced mobilities and mass-to-mobility correlation curves were determined for these compounds. Depending on their concentration, the n -alkanes form one or two product ion peaks. The mobilities of product ion peaks regularly decrease with increasing chain length. The same behavior was observed for branched chain alkanes. However, characteristic fragment ions are additionally formed in comparison with n -alkanes. The investigated aromatic hydrocarbons show simple spectra consisting of up to three characteristic product ion peaks. The obtained mass-to-mobility correlation curve differs from that determined using photoionization.

Aerated treatment pond technology with biofilm promoting mats for the bioremediation of benzene, MTBE and ammonium contaminated groundwater

A novel aerated treatment pond for enhanced biodegradation of groundwater contaminants was tested under field conditions. Coconut fibre and polypropylene textiles were used to encourage the development of contaminant-degrading biofilms. Groundwater contaminants targeted for removal were benzene, methyl tert-butyl ether (MTBE) and ammonium. Here, we present data from the first 14 months of operation and compare contaminant removal rates, volatilization losses, and biofilm development in one pond equipped with coconut fibre to another pond with polypropylene textiles. Oxygen concentrations were constantly monitored and adjusted by automated aeration modules. A natural transition from anoxic to oxic zones was simulated to minimize the volatilization rate of volatile organic contaminants. Both ponds showed constant reductions in benzene concentrations from 20 mg/L at the inflow to about 1 microg/L at the outflow of the system. A dynamic air chamber (DAC) measurement revealed that only 1% of benzene loss was due to volatilization, and suggests that benzene loss was predominantly due to aerobic mineralization. MTBE concentration was reduced from around 4 mg/L at the inflow to 3.4-2.4 mg/L in the system effluent during the first 8 months of operation, and was further reduced to 1.2 mg/L during the subsequent 6 months of operation. Ammonium concentrations decreased only slightly from around 59 mg/L at the inflow to 56 mg/L in the outflow, indicating no significant nitrification during the first 14 months of continuous operation. Confocal laser scanning microscopy (CLSM) demonstrated that microorganisms rapidly colonized both the coconut fibre and polypropylene textiles. Microbial community structure analysis performed using denaturing gradient gel electrophoresis (DGGE) revealed little similarity between patterns from water and textile samples. Coconut textiles were shown to be more effective than polypropylene fibre textiles for promoting the recruitment and development of MTBE-degrading biofilms. Biofilms of both textiles contained high numbers of benzene metabolizing bacteria suggesting that these materials provide favourable growth conditions for benzene degrading microorganisms.

Atmospheric Pressure Chemical Ionization Studies of Non-Polar Isomeric Hydrocarbons Using Ion Mobility Spectrometry and Mass Spectrometry with Different Ionization Techniques

The ionization pathways were determined for sets of isomeric non-polar hydrocarbons (structural isomers, cis/trans isomers) using ion mobility spectrometry and mass spectrometry with different techniques of atmospheric pressure chemical ionization to assess the influence of structural features on ion formation. Depending on the structural features, different ions were observed using mass spectrometry. Unsaturated hydrocarbons formed mostly [M − 1]+ and [(M − 1)2H]+ ions while mainly [M − 3]+ and [(M − 3)H2O]+ ions were found for saturated cis/trans isomers using photoionization and 63Ni ionization. These ionization methods and corona discharge ionization were used for ion mobility measurements of these compounds. Different ions were detected for compounds with different structural features. 63Ni ionization and photoionization provide comparable ions for every set of isomers. The product ions formed can be clearly attributed to the structures identified. However, differences in relative abundance of product ions were found. Although corona discharge ionization permits the most sensitive detection of non-polar hydrocarbons, the spectra detected are complex and differ from those obtained with 63Ni ionization and photoionization.

Development and application of dynamic air chambers for measurement of volatilization fluxes of benzene and MTBE from constructed wetlands planted with common reed

Phytoremediation of industrially contaminated groundwater has been a proven technique for several decades. However, mass balances of contaminants are often focused in laboratory investigations. The evaluation of the transfer of volatile organic compounds (VOCs) under field conditions from the saturated and vadose soil zone into the atmosphere, directly or via plants, is rarely part of the research scope. This can provoke problems--particularly with regard to legal issues--if large-scale phytoremediation sites are situated near residential areas. In this study volatilization of VOCs was quantified in a horizontal-flow constructed wetland planted with reed grass. For this purpose, a specially designed air chamber was constructed, validated, and routine sampling campaigns were performed over the course of one year. Results indicate that the overall volatilization of the observed contaminants benzene and methyl tert-butyl ether (MTBE) depended on seasonal variations with the highest volatilization fluxes measured in summer, when the detected volatilization fluxes of 846+/-116 and 252+/-11 microg m(-2) h(-1) for MTBE and benzene, respectively, accounted for 2.4% and 5.6% of the respective overall contaminant mass loss in the planted wetland. Furthermore, chamber data give strong evidence for the increased volatilization of VOCs through vegetation by direct comparison of planted and unplanted wetlands.

Selective mixed-bed solid phase extraction of atrazine herbicide from environmental water samples using molecularly imprinted polymer.

A novel approach for the selective extraction of organic target compounds from water samples has been developed using a mixed-bed solid phase extraction (mixed-bed SPE) technique. The molecularly imprinted polymer (MIP) particles are embedded in a network of silica gel to form a stable uniform porous bed. The capabilities of this method are demonstrated using atrazine as a model compound. In comparison to conventional molecularly imprinted-solid phase extraction (MISPE), the proposed mixed-bed MISPE method in combination with gas chromatography-mass spectrometry (GC-MS) analysis enables more reproducible and efficient extraction performance. After optimization of operational parameters (polymerization conditions, bed matrix ingredients, polymer to silica gel ratio, pH of the sample solution, breakthrough volume plus washing and elution conditions), improved LODs (1.34 µg L(-1) in comparison to 2.25 µg L(-1) obtained using MISPE) and limits of quantification (4.5 µg L(-1) for mixed-bed MISPE and 7.5 µg L(-1) for MISPE) were observed for the analysis of atrazine. Furthermore, the relative standard deviations (RSDs) for atrazine at concentrations between 5 and 200 µg L(-1) ranged between 1.8% and 6.3% compared to MISPE (3.5-12.1%). Additionally, the column-to-column reproducibility for the mixed-bed MISPE was significantly improved to 16.1%, compared with 53% that was observed for MISPE. Due to the reduced bed-mass sorbent and at optimized conditions, the total amount of organic solvents required for conditioning, washing and elution steps reduced from more than 25 mL for conventional MISPE to less than 2 mL for mixed-bed MISPE. Besides reduced organic solvent consumption, total sample preparation time of the mixed-bed MISPE method relative to the conventional MISPE was reduced from more than 20 min to less than 10 min. The amount of organic solvent required for complete elution diminished from 3 mL (conventional MISPE) to less than 0.4 mL with the mixed-bed technique shows its inherent potential for online operation with an analytical instrument. In order to evaluate the selectivity and matrix effects of the developed mixed-bed MISPE method, it was applied as an extraction technique for atrazine from environmental wastewater and river water samples.

Gas-phase ion mobility studies of constitutional isomeric hydrocarbons using different ionization techniques

Abstract We have investigated the influence of structural differences on the ionization pathways and drift behavior in ion mobility spectrometry for constitutional isomeric hydrocarbons. The ion mobility spectra of isomeric alkylated benzenes (C 9 H 12 and C 10 H 14 ) were recorded using photoionization, 63 Ni ionization and corona discharge ionization. Defined spectra for almost all isomeric compounds were observed using photoionization. When using this technique, no differences in ion mobility spectra of isomers can be observed. However, the methods of atmospheric-pressure chemical ionization ( 63 Ni and corona discharge ionization) produce differences in ion mobility spectra or in the detectable concentration ranges of isomeric compounds depending on the number and position of aliphatic side chains.

Accuracy of Ion Mobility Measurements Dependent on the Influence of Humidity

Measurements with sensor techniques in field analytical chemistry can be considerably affected by varying ambient conditions such as humidity. We systematically investigated the way in which ion mobility measurements are influenced by moisture. Both the peak positions of product ions within the spectrum and their relative abundance can vary depending on humidity. The transportation of humidity via the carrier gas into the ion mobility spectrometer causes changes in ionization pathways. Additional reactant ion species are formed and a lower relative abundance of product ions from halogenated compounds was generally observed. The peak position within the ion mobility spectrum is comparatively unaffected for the same ions. In contrast, considerable differences in drift times detected were found with increasing humidity of drift gas, while the influence on calibration was not as significant for chlorinated and brominated substances as observed for humid carrier gases.

Comparative evaluation of pilot scale horizontal subsurface-flow constructed wetlands and plant root mats for treating groundwater contaminated with benzene and MTBE

In order to evaluate technology options for the treatment of groundwater contaminated with benzene and MTBE in constructed wetlands (CWs), a scarcely applied plant root mat system and two horizontal subsurface-flow (HSSF) CWs were investigated. The inflow load of benzene and MTBE were 188-522 and 31-90 mg d(-1)m(-2), respectively. Higher removal efficiencies were obtained during summer in all systems. The benzene removal efficiencies were 0-33%, 24-100% and 22-100% in the unplanted HSSF-CW, planted HSSF-CW and the plant root mat, respectively; the MTBE removal efficiencies amounted to 0-33%, 16-93% and 8-93% in the unplanted HSSF-CW, planted HSSF-CW and the plant root mat, respectively. The volatilisation rates in the plant root mat amounted to 7.24 and 2.32 mg d(-1)m(-2) for benzene and MTBE, which is equivalent to 3.0% and 15.2% of the total removal. The volatilisation rates in the HSSF-CW reached 2.59 and 1.07 mg d(-1)m(-2), corresponding to 1.1% and 6.1% of the total removal of benzene and MTBE, respectively. The results indicate that plant root mats are an interesting option for the treatment of waters polluted with benzene and MTBE under moderate temperatures conditions.