Using a nanoelectrospray-differential mobility spectrometer-mass spectrometer system for the analysis of oligosaccharides with solvent selected control over ESI aggregate ion formation.

Differential mobility spectrometry (DMS), also commonly referred to as high field asymmetric waveform ion mobility spectrometry (FAIMS) is a rapidly advancing technology for gas-phase ion separation. The interfacing of DMS with mass spectrometry (MS) offers potential advantages over the use of mass spectrometry alone. Such advantages include improvements to mass spectral signal/noise, orthogonal/complementary ion separation to mass spectrometry, enhanced ion and complexation structural analysis, and the potential for rapid analyte quantitation. In this report, we demonstrate the successful use of our nanoESI-DMS-MS system, with a methanol drift gas modifier, for the separation of oligosaccharides. The tendency for ESI to form oligosaccharide aggregate ions and the negative impact this has on nanoESI-DMS-MS oligosaccharide analysis is described. In addition, we demonstrate the importance of sample solvent selection for controlling nanoESI oligosaccharide aggregate ion formation and its effect on glycan ionization and DMS separation. The successful use of a tetrachloroethane/methanol solvent solution to reduce ESI oligosaccharide aggregate ion formation while efficiently forming a dominant MH(+) molecular ion is presented. By reducing aggregate ion formation in favor of a dominant MH(+) ion, DMS selectivity and specificity is improved. In addition to DMS, we would expect the reduction in aggregate ion complexity to be beneficial to the analysis of oligosaccharides for other post-ESI separation techniques such as mass spectrometry and ion mobility. The solvent selected control over MH(+) molecular ion formation, offered by the use of the tetrachloroethane/methanol solvent, also holds promise for enhancing MS/MS structural characterization analysis of glycans.

Pressure effects in differential mobility spectrometry.

A microfabricated planar differential ion mobility spectrometer operating from 0.4 to 1.55 atm in a supporting atmosphere of purified air was used to characterize the effects of pressure and electric field strength on compensation voltage, ion transmission, peak width, and peak intensity in differential mobility spectra. Peak positions, in compensation voltage as a function of separating rf voltage, were found to vary with pressure in a way that can be simplified by expressing both compensation and separation fields in Townsend units for E/N. The separation voltage providing the greatest compensation voltage and the greatest resolution is ion-specific but often occurs at E/N values that are unreachable at elevated pressure because of electrical breakdown. The pressure dependence of air breakdown voltage near 1 atm is sublinear, allowing higher E/N values to be reached at reduced pressure, usually resulting in greater instrumental resolution. Lower voltage requirements at reduced pressure also reduce device power consumption.

Rapid separation and quantitative analysis of peptides using a new nanoelectrospray- differential mobility spectrometer-mass spectrometer system.

Differential mobility spectrometry (DMS) (see Buryakov, I. A.; Krylov, E. V.; Nazarov, E. G.; Rasulev, U. Kh. Int. J. Mass Spectrom. Ion Processes 1993, 128, 143-148), also commonly referred to as high-field asymmetric waveform ion mobility spectrometry (FAIMS) (see Purves, R. W.; Guevremont, R.; Day, S.; Pipich, C. W.; Matyjaszcyk, M. S. Rev. Sci. Instrum. 1998, 69, 4094-4105), is a rapidly advancing technology for gas-phase ion separation. The interfacing of DMS with mass spectrometry (MS) offers potential advantages over the use of mass spectrometry alone. Such advantages include improvements to mass spectral signal-to-noise, orthogonal/complementary ion separation to mass spectrometry, enhanced ion and complexation structural analysis, and the potential for rapid analyte quantitation. In this report, we investigate the use of our nanoESI-DMS-MS system to demonstrate differential mobility separation of peptides. The formation of higher order peptide aggregate ions (ion complexes) via electrospray ionization and the negative impact this has on DMS peptide separation are examined. The successful use of differential mobility drift gas modifiers (dopants) to reduce aggregate ion size and improve DMS peptide ion separation is presented. Following optimization of DMS peptide separation conditions, we examined next the feasibility of a new analytical platform which uses direct sample infusion with nanoESI-DMS-MS for ultrarapid analyte quantitation. Quantitation of a selected peptide from a semicomplex peptide mixture is presented. Initial feasibility results with this new approach demonstrate good accuracy and reproducibility, as well as an absolute mass sensitivity of 6.8 amol and a minimum dynamic range of 2500 for the peptide of interest. This report offers a first look at utilizing nanoESI-DMS-MS to create an ultrarapid (under 5 s) quantitative analysis platform and its potential in the high-throughput arena. Each ion separation technique, DMS and MS, offers orthogonal ion separation to one another, enhancing the overall specificity for this quantitative approach.

Miniature differential mobility spectrometry using atmospheric pressure photoionization.

Positive and negative ion spectra have been obtained with a miniature differential mobility spectrometer equipped with a photoionization source operating at atmospheric pressure. With benzene as a dopant, providing C6H6+ as reactant ion, protonated molecular ions and proton-bound dimer ions were obtained with dimethyl methylphosphonate and butanone. The spectra obtained from gas chromatographic injections of aromatic hydrocarbons, benzene, toluene, and the xylenes, produced the molecular ions when the moisture level was very low, but at a high level the hydrated proton was also present. Possible mechanisms for the formation of protonated products are discussed. Negative ions were produced from electron capture by sulfur hexafluoride using benzene or acetone as dopant. Photoionization of nitrogen dioxide led to the formation of the nitrate ion whose yield was a nonlinear function of concentration. The use of a suitable dopant enhanced ion formation by up to 2 orders of magnitude, and limits of detection in both the positive and negative modes were all at the sub ppm(v) level. The study makes a strong case for the use of a photoionization source as an alternative to the radioactive 63Ni source.

Characterization of gas-phase molecular interactions on differential mobility ion behavior utilizing an electrospray ionization-differential mobility-mass spectrometer system.

Differential mobility spectrometry (DMS) is a rapidly advancing technology for gas-phase ion separation. The interfacing of DMS with mass spectrometry (MS) offers potential advantages over the use of mass spectrometry alone. Such advantages include improvements to mass spectral signal/noise ratios, orthogonal/complementary ion separation to mass spectrometry, enhanced ion and complexation structural analysis, and potential for rapid analyte quantitation. The introduction of a new ESI-DMS-MS system and its utilization to aid in the understanding of DMS separation theory is described. A current contribution to DMS separation theory is one of an association/dissociation process between ions/molecules in the gas phase during the differential mobility separation. A model study was designed to investigate the molecular dynamics and chemical factors influencing the theorized association/dissociation process, and the mechanisms by which these gas-phase interactions affect an ion's DM behavior. Five piperidine analogues were selected as model analytes, and three alcohol drift gas dopants/modifiers were used to interrogate the analyte ions in the gas phase. Two proposed DMS separation mechanisms, introduced as Core and Façade, corresponding to strong and weak attractions between ions/molecules in the gas phase, are detailed. The proposed mechanisms provide explanation for the observed changes in analyte separation by the various drift gas modifiers. Molecular modeling of the proposed mechanisms provides supportive data and demonstrates the potential for predictive optimization of analyte separation based on drift gas modifier effects.