Daniel Riebe

An electrospray ionization-ion mobility spectrometer as detector for high-performance liquid chromatography

The application of electrospray ionization (ESI) ion mobility (IM) spectrometry on the detection end of a high-performance liquid chromatograph has been a subject of study for some time. So far, this method has been limited to low flow rates or has required splitting of the liquid flow. This work presents a novel concept of an ESI source facilitating the stable operation of the spectrometer at flow rates between 10 μL min−1 and 1500 μL min−1 without flow splitting, advancing the T-cylinder design developed by Kurnin and co-workers. Flow rates eight times faster than previously reported were achieved because of a more efficient dispersion of the liquid at increased electrospray voltages combined with nebulization by a sheath gas. Imaging revealed the spray operation to be in a rotationally symmetric multijet mode. The novel ESI-IM spectrometer tolerates high water contents (≤90%) and electrolyte concentrations up to 10 mM, meeting another condition required of high-performance liquid chromatography (HPLC) detectors. Limits of detection of 50 nM for promazine in the positive mode and 1 μM for 1,3-dinitrobenzene in the negative mode were established. Three mixtures of reduced complexity (five surfactants, four neuroleptics, and two isomers) were separated in the millisecond regime in stand-alone operation of the spectrometer. Separations of two more complex mixtures (five neuroleptics and 13 pesticides) demonstrate the application of the spectrometer as an HPLC detector. The examples illustrate the advantages of the spectrometer over the established diode array detector, in terms of additional IM separation of substances not fully separated in the retention time domain as well as identification of substances based on their characteristic IMs.

Laser-based ion mobility spectrometer for the direct analysis of aromatic compounds in liquids

A laser-based ionization source for the direct analysis of liquid samples in ion mobility (IM) spectrometry is presented and characterized. Ionization of aromatic substances in liquids is achieved, analogous to atmospheric pressure laser ionization (APLI) in mass spectrometry, by vaporizing the liquid and subsequently ionizing the aromatic substances by resonance-enhanced multiphoton ionization (REMPI). The effects of parameters, such as composition and flow rate of the solvent as well as laser wavelength and pulse energy, are systematically investigated. The characterization of the IM spectrometer is carried out by means of selected substances from diverse fields of applications, e.g., polycyclic aromatic hydrocarbons (PAH), pesticides, wood preservatives and drug compounds. Limits of detection (LOD) down to 10 fmol and linear ranges up to three orders of magnitude are established. In addition to direct laser ionization, indirect laser ionization via dopants (toluene) for substances with low ionization efficiencies is investigated. Ionization occurs as a result of proton transfer from toluene radical cations to substances of sufficiently high proton affinities. As a result of indirect laser ionization, LOD could be decreased by up to two orders of magnitude. Ionization products are investigated by means of a combination of IM and mass spectrometer. Depending on the substance investigated primary ions (radical cations) and secondary ions (protonated molecules) resulting from ion molecule reactions are formed.

Atmospheric pressure chemical ionization of explosives induced by soft X-radiation in ion mobility spectrometry: mass spectrometric investigation of the ionization reactions of drift gasses, dopants and alkyl nitrates: Atmospheric pressure chemical ionization of explosives induced by soft X-radiation

A promising replacement for the radioactive sources commonly encountered in ion mobility spectrometers is a miniaturized, energy-efficient photoionization source that produce the reactant ions via soft X-radiation (2.8 keV). In order to successfully apply the photoionization source, it is imperative to know the spectrum of reactant ions and the subsequent ionization reactions leading to the detection of analytes. To that end, an ionization chamber based on the photoionization source that reproduces the ionization processes in the ion mobility spectrometer and facilitates efficient transfer of the product ions into a mass spectrometer was developed. Photoionization of pure gasses and gas mixtures containing air, N2 , CO2 and N2 O and the dopant CH2 Cl2 is discussed. The main product ions of photoionization are identified and compared with the spectrum of reactant ions formed by radioactive and corona discharge sources on the basis of literature data. The results suggest that photoionization by soft X-radiation in the negative mode is more selective than the other sources. In air, adduct ions of O2- with H2 O and CO2 were exclusively detected. Traces of CO2 impact the formation of adduct ions of O2- and Cl- (upon addition of dopant) and are capable of suppressing them almost completely at high CO2 concentrations. Additionally, the ionization products of four alkyl nitrates (ethylene glycol dinitrate, nitroglycerin, erythritol tetranitrate and pentaerythritol tetranitrate) formed by atmospheric pressure chemical ionization induced by X-ray photoionization in different gasses (air, N2 and N2 O) and dopants (CH2 Cl2 , C2 H5 Br and CH3 I) are investigated. The experimental studies are complemented by density functional theory calculations of the most important adduct ions of the alkyl nitrates (M) used for their spectrometric identification. In addition to the adduct ions [M + NO3 ]- and [M + Cl]- , adduct ions such as [M + N2 O2 ]- , [M + Br]- and [M + I]- were detected, and their gas-phase structures and energetics are investigated by density functional theory calculations. Copyright

High-Resolution Spectrometer Using Combined Dispersive and Interferometric Wavelength Separation for Raman and Laser-Induced Breakdown Spectroscopy (LIBS)

In this paper the concept of a compact high-resolution spectrometer based on the combination of dispersive and interferometric elements is presented. Dispersive elements are used to spectrally resolve the light in one direction with coarse resolution (Δλ < 0.5 nm), while perpendicular to that direction an etalon provides high spectral resolution (Δλ < 50 pm). This concept for two-dimensional spectroscopy has been implemented for the wavelength range λ = 350–650 nm. Appropriate algorithms for reconstructing spectra from the two-dimensional raw data and for wavelength calibration were established in an analysis software. Potential applications for this new spectrometer are Raman and laser-induced breakdown spectroscopy (LIBS). Resolutions down to 28 pm (routinely 54 pm) could be realized for these applications.

Detection of volatile organic compounds in the headspace above mold fungi by GC-soft X-radiation-based APCI-MS

Mold fungi on malting barley grains cause major economic loss in malting and brewery facilities. Possible proxies for their detection are volatile and semivolatile metabolites. Among those substances, characteristic marker compounds have to be identified for a confident detection of mold fungi in varying surroundings. The analytical determination is usually performed through passive sampling with solid phase microextraction, gas chromatographic separation, and detection by electron ionization mass spectrometry (EI-MS), which often does not allow a confident determination due to the absence of molecular ions. An alternative is GC-APCI-MS, generally, allowing the determination of protonated molecular ions. Commercial atmospheric pressure chemical ionization (APCI) sources are based on corona discharges, which are often unspecific due to the occurrence of several side reactions and produce complex product ion spectra. To overcome this issue, an APCI source based on soft X-radiation is used here. This source facilitates a more specific ionization by proton transfer reactions only. In the first part, the APCI source is characterized with representative volatile fungus metabolites. Depending on the proton affinity of the metabolites, the limits of detection are up to 2 orders of magnitude below those of EI-MS. In the second part, the volatile metabolites of the mold fungus species Aspergillus, Alternaria, Fusarium, and Penicillium are investigated. In total, 86 compounds were found with GC-EI/APCI-MS. The metabolites identified belong to the substance classes of alcohols, aldehydes, ketones, carboxylic acids, esters, substituted aromatic compounds, terpenes, and sesquiterpenes. In addition to substances unspecific for the individual fungus species, characteristic patterns of metabolites, allowing their confident discrimination, were found for each of the 4 fungus species. Sixty-seven of the 86 metabolites are detected by X-ray-based APCI-MS alone. The discrimination of the fungus species based on these metabolites alone was possible. Therefore, APCI-MS in combination with collision induced dissociation alone could be used as a supervision method for the detection of mold fungi.

An alternative field switching ion gate for ESI-ion mobility spectrometry

In electrospray ionization (ESI)-ion mobility spectrometry, continuously generated ions must be desolvated in a first tube before short ion pulses are introduced into a second (drift) tube. Both tubes are separated by an ion-gate. The resolving power of the resulting drift time spectrum is strongly influenced by the design of the ion gate. In the case of the Bradbury-Nielsen gates typically used, an orthogonal field between oppositely charged, parallel wires blocks ions from entering the drift tube. However, the blocking field also distorts the entering ion cloud. One alternative, which eliminates these effects and therefore enables a potentially higher resolving power, is already known for spectrometers with small ionization volumes, where ions are formed between two electrodes and subsequently transferred into the drift tube by a high voltage pulse. Based on this setup, we introduce an alternative ion gate design for liquid samples, named field switching ion gate (FSIG). The continuous flow of ions generated by ESI is desolvated in the first tube and introduced into the space between two electrodes (repeller and transfer electrodes). A third (blocking) electrode prevents the movement of ions into the drift tube in the closed state. Ions are transferred during the open state by pulsing the voltages of the repeller and blocking electrodes. First results demonstrate an increase of the resolving power by 100% without intensity losses and further changes in the spectrometer setup. The parameters of the FSIG, such as electrode voltages and pulse width, are characterized allowing the optimization of the spectrometer’s resolving power.

Characterization of Rhodamine 6G Release in Electrospray Ionization by Means of Spatially Resolved Fluorescence Spectroscopy

Abstract In the present work, the density distribution of rhodamine 6G ions (R6G) in the gas phase and the droplets of an electrospray plume was studied by spatial and spectral imaging. The intention is to contribute to the fundamental understanding of the release mechanism of gaseous R6G in the electrospray ionization (ESI) process. Furthermore, the influence of ESI-parameters on the release efficiency of R6G, e.g. solvent flow, R6G and salt concentration were examined via direct fluorescence imaging of R6G. A solvent-shift of the fluorescence maximum, λmax = 555 nm in methanolic solution and λmax = 505 nm in gas phase, allows the discrimination between solvated and gaseous R6G. Two experimental setups were used for our measurements. In the first experiment, the R6G fluorescence and the light scattered from the spray plume were imaged in two spatial dimensions using a tunable wavelength fil ter. The second experiment was designed for obtaining 1-dimensional spatially resolved emission spectra of the spray. Here, the intensity distribution of solvated and gaseous R6G as well as scattered light (λ=355 nm) were measured simultaneously. The results show the distribution of gaseous R6G in the plane, orthogonal to the ESI capillary, decreasing slightly towards the spray center and showing maxima at the cone margins. The distribution of gaseous R6G confirms the preferred release of gaseous ions from nano-droplets, indicating the ion evaporation model (IEM) to be the dominating release mechanism. Up to now, only a few fluorescence spectra of ionic compounds in the gas phase were published because the measurement of emission spectra of mass-selected ions in an ion trap is experimentally challenging. The fluorescence spectrum of gaseous lucigenin at atmospheric pressure is reported for the first time. This spectrum of lucigenin in the gas phase exhibits a blue shift of about Δλ=10 nm in comparison to the corresponding spectrum in methanol.

Detection of explosive related nitroaromatic compounds (ERNC) by laser-based ion mobility spectrometry

In this study two issues are addressed, namely laser ionisation of selected nitroaromatic compounds (NAC) and the characterisation of their anions by photodetachment (PD) spectroscopy. Laser ionisation of the NAC at λ = 226.75 nm is investigated by ion mobility (IM) spectrometry at atmospheric pressure. The main product after laser ionisation is the reactive NO+ ion formed in a sequence of photofragmentation and multiphoton ionisation processes. NO+ is trapped by specific ion molecule reactions (IMR). Alternatively, NO, added as laser dopant, can directly be ionised. The formed NO+ reacts with the NAC under complex formation. This allows fragmentless NAC detection. The combination of IM spectrometry and PD spectroscopy provides real-time characterisation of the anions in the IM spectrum. This is useful to differentiate between NAC and interfering substances and, thus, to reduce false-positive detections of NAC. The electrons detached by the PD laser at λ = 532 nm are detected in the same spectrum as the anions. The potential of PD-IM spectrometry in terms of cross section determination, analytical improvements, tomographic mapping, spatial hole burning etc., is outlined.