International Journal for Ion Mobility Spectrometry

Amyloid diseases are a serious cause for concern world-wide. To understand the mechanism of formation of the fibrillar structures associated with such disorders, it is necessary to study the progression from soluble protein or peptide monomer through an array of oligomers to the final, insoluble, fibrils. The protein IAPP is found in vivo in the form of insoluble amyloid deposits in the pancreatic islets of diabetes type II sufferers. Here, we have studied the in vitro self-aggregation of three fibril-forming peptides from the amyloidogenic core of IAPP. Using electrospray ionization—mass spectrometry coupled with ion mobility spectrometry, the mass and cross-sectional area of each oligomer present in the heterogeneous assembly mixtures can be determined individually in a single, rapid experiment over time. For the three peptides studied, oligomers ≤20-mer were characterized. Conversely, no oligomers higher than a dimer were detected for a non-assembling peptide control. The rate in which the cross-sectional area of the oligomers increases with increasing number of peptide sub-units indicates that assembly for the amyloid-forming peptides proceeds in a linear fashion until an oligomer of a certain size is attained. After this, a step increase in cross-sectional area occurs for the next higher-order oligomer. This behaviour can be explained by molecular modelling of singly, doubly, triply and quadruply stacked β-stranded structures. Using one peptide as an example, the cross-sectional areas of the lower order oligomers (dimer to pentamer) were found to be consistent with a single β-sheet model, whereas the higher order oligomers were consistent with double-stranded (hexamer to decamer oligomers), triply-stranded (11-mers to 15-mers) and quadruply-stranded (16-mers to 20-mers) β-sheet models.

The introduction of mobility shift reagents (SRs) into the buffer gas of mobility spectrometers yields SR-ion clusters that decrease ion mobilities and allow the separation of overlapping ions. With a large amount of papers on the introduction of SRs in ion mobility spectrometry (IMS) few investigations explain the behavior of the adducts of reactant ions with SRs and it is not clear what type of peaks to expect which obscures the interpretation of spectra. Electrospray-ionization IMS was coupled to quadrupole mass spectrometry, and 2-butanol (B), ethyl lactate (L), and methanol were introduced as SRs into the buffer gas. The hybrid functional X3LYP/6–311++(d,p) with Gaussian 09 was used for theoretical calculations of SR-ion interaction energies. Adducts of the reactant ions with B and L presented different behaviors; even at low flow rates, L consumed all sodium, reactant ions, and water by adduction, because a) in the experimental conditions, SRs were more concentrated in the buffer gas than reactant ions, b) L’s high proton affinity and c) L’s three electron-donor oxygens, increases adduction. Therefore, chemical equilibria in the buffer gas were only between L and LnH+, LmH3O+, or LxNa+ adducts and, consequently, these sets of adducts had different mobilities. The lower mobility of LmH3O+ compared to LnH+ was explained on the base of the lower steric hindrance in LH3O+ for attachment of L molecules. The behavior of reactant ions with B was different: BnH3O+ and BnH+ overlapped because the relatively low proton affinity and the single and weaker interaction site in B allowed protons and water to be exchanged between species. Finally, L4H+, L4H3O+, B4H+ and B5H+ ions, not reported before, were seen for large SR concentrations. This study explains two different behaviors of the adducts of SRs with reactant ions using interaction energies, proton affinities, steric hindrance, and the number of locations for adduction.

Most state of the art gas sensor systems based on atmospheric pressure ionization, such ion mobility spectrometers use radioactive beta-sources (e.g. 3H or 63Ni) to provide free electrons with high kinetic energy to initiate a chemical gas phase ionization of the analytes to be detected. Here, we introduce a non-radioactive electron emitter which generates free electrons at atmospheric pressure. Therefore, electrons are generated in a vacuum by field emission and accelerated towards a 300 nm thin 1 mm2 silicon nitride membrane separating the vacuum from atmospheric pressure. Electron currents of about a few hundred microamps can be reached. High energetic electrons of about 10 keV can easily penetrate the membrane without significant loss of kinetic energy. The concept of proximity focusing avoids complex electron lenses to focus the electron beam onto the membrane. The used field emitter tips are made of multi-walled carbon nanotubes. Another benefit of our system is that no insulated power supply operating at high voltage is needed, as necessary for thermo emitters. Here, we show a first prototype of a proximity focused electron gun with field emitting carbon nanotubes. The system is coupled to our drift tube ion mobility spectrometer for validation. Ion mobility spectra similar to those of a 3H ionization source were achieved.

Major questions of all investigations of analytes using ion mobility spectrometer (IMS) are peak finding and in case of proper finding the reliable referencing. In case of rather complex mixtures like human breath or water impurities, automatic procedures should be found to support peak finding and referencing. A visualisation software tool will be described bringing the summarised results of peak finding methods and the reference lists used as input to data bases together in a single system. The details of the software developed are described briefly and the procedures behind are referenced.

Ion mobility spectrometry is increasingly in demand for medical applications and its potential for implementation in food quality and safety or process control suggest rising use of instruments in this field as well. All those samples are commonly extremely complex and mostly humid mixtures. Therefore, pre-separation techniques have to be applied. As ion mobility spectrometers with gas-chromatographic pre-separation acquire a huge amount of data, effective data processing and automated evaluation by comparison of detected peak pattern with data bases have to be utilised. This requires accurate on-line calibration of the instruments to guarantee reproducible results, in particular with respect to identification of an analyte by determination of its ion mobility and retention time. To reduce environmental and instrumental influence, the reduced ion mobility is used. It is derived from the drift time normalised to electric field, length of the drift region and to temperature and pressure of the drift gas (traditional method). All data required for this normalisation are afflicted with a particular error and thus leading to a deviation of the calculated ion mobility value. Furthermore, this traditional method enables a calculation of the reduced ion mobility only after the measurement. To avoid those errors and to enable on-line calibration of ion mobility, an instrument specific factor is implemented generally representing all relevant variables. This factor can be determined from an initial measurement of few spectra and can thereafter be applied on the following measurement. The application of this approach obtained reproducible reduced ion mobility values for positive and negative ions over a broad drift time range and for common variation of ambient conditions as well for varying instrument conditions such as electric fields respectively drift times and in different drift gases. Moreover, the reduced ion mobility is available already during the measurements with a significantly higher reliability and accuracy which was increased to a factor of 5 compared to the traditional ion mobility determination and enables an on-line identification of analytes for the first time.

Analysis of hair is often applied to assess drug abuse history, exposure to environmental and industrial pollutants, heavy metals, gestational drug exposure and various other screening purposes. This manuscript reports the application of ambient pressure ion mobility spectrometry mass spectrometry (IMMS) with electrospray ionization (ESI) source as a rapid analytical tool for hair analysis. The study demonstrated that ion mobility spectrometry (IMS) as a pre-separation technique prior to analysis by mass spectrometry (MS) provides detection and determination of compounds of interest present in hair at nano-molar concentration level. After extraction of analytes from hair, the ESI-IMMS method of analysis does not require the derivatization or sample treatment that is often required for other separation methods such as gas chromatography. One advantage of IMS over chromatography separation is that resolving powers are similar to those in GC and much greater than those possible by liquid chromatography. In addition, separation speed is faster than both gas and liquid chromatographic methods. Four of the nine hair samples anonymously donated by customers at a local hair salon tested positive for caffeine and two of the four samples that tested positive for caffeine also tested positive for nicotine. A positive response based on mass analysis for methamphetamine was obtained for one of the hair samples. Further investigation using the mobility data demonstrated that the response was a false positive and that it may have occurred from the use of a hair gel. This article reports the potential of IMMS as an analytical technique for rapid and routine screening of hair samples, cosmetics, and pharmaceuticals.

The zero-field mobilities of 46 atomic ions in helium are calculated as functions of the gas temperature in an ion mobility spectrometer. The calculations are based on highly accurate, ab initio potential energy curves obtained in the last few years. In general, they start from a small value at low temperature, rise steadily to a maximum at some specific temperature, T max , and then decline at higher temperatures. The ratio of T max to the dissociation energy (well depth) of the ion-neutral interaction potential is shown to be approximation the same for all singly-charged ions and a few multiply-charged ions.