Deamination of protonated amines to yield protonated imines

AbstractPrimary and secondary amines, when examined in atmospheric pressure chemical ionization, electrospray ionization, or chemical ionization, display protonated imines in their mass spectra. These products arise formally by nucleophilic substitution at the α-carbon with loss of both ammonia and molecular hydrogen. Collision-induced dissociation (CID) is used to characterize the product ions by comparison with authentic protonated imines. Gas-phase ion/molecule reactions of protonated amines with neutral amines also yield products that correspond to protonated imines (deamination and dehydrogenation), as well as providing simple deamination products. The reaction mechanism was investigated further by reacting the deamination product, the alkyl cation, with a neutral amine. The observed dehydrogenation of the nascent protonated secondary amine indicates that the reaction sequence is loss of ammonia followed by dehydrogenation even though the isolated protonated secondary amines did not undergo dehydrogenation upon CID. Formation of the deamination products in the protonated amine/amine reaction is competitive with proton-bound dimer formation. The proton-bound dimers do not yield deamination products under CID conditions in the ion trap or in experiments performed using a pentaquadrupole instrument. This demonstrates that the geometry of the proton-bound dimer, in which the α-carbons of the alkylamines are well separated [CαNHNCα], is an unsuitable entry point on the potential energy hypersurface for formation of the imine [CαNCα]. Isolation of the proton-bound dimers in the quadrupole ion trap is achieved with low efficiency and this characteristic can be used to distinguish them from their covalently bound isomers

Biogenic aldehyde determination by reactive paper spray ionization mass spectrometry

Ionization of aliphatic and aromatic aldehydes is improved by performing simultaneous chemical derivatization using 4-aminophenol to produce charged iminium ions during paper spray ionization. Accelerated reactions occur in the microdroplets generated during the paper spray ionization event for the tested aldehydes (formaldehyde, n-pentanaldehyde, n-nonanaldehyde, n-decanaldehyde, n-dodecanaldehyde, benzaldehyde, m-anisaldehyde, and p-hydroxybenzaldehyde). Tandem mass spectrometric analysis of the iminium ions using collision-induced dissociation demonstrated that straight chain aldehydes give a characteristic fragment at m/. z 122 (shown to correspond to protonated 4-(methyleneamino)phenol), while the aromatic aldehyde iminium ions fragment to give a characteristic product ion at m/. z 120. These features allow straightforward identification of linear and aromatic aldehydes. Quantitative analysis of n-nonaldehyde using a benchtop mass spectrometer demonstrated a linear response over 3 orders of magnitude from 2.5. ng to 5. μg of aldehyde loaded on the filter paper emitter. The limit of detection was determined to be 2.2. ng for this aldehyde. The method had a precision of 22%, relative standard deviation. The experiment was also implemented using a portable ion trap mass spectrometer

Biogenic aldehyde determination by reactive paper spray ionization mass spectrometry

This is the author’s version of a work that was accepted for publication in Analytica Chimica Acta. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published at: http://dx.doi.org/10.1016/j.aca.2015.01.007Ionization of aliphatic and aromatic aldehydes is improved by performing simultaneous chemical derivatization using 4-aminophenol to produce charged iminium ions during paper spray ionization. Accelerated reactions occur in the microdroplets generated during the paper spray ionization event for the tested aldehydes (formaldehyde, n-pentanaldehyde, n-nonanaldehyde, n-decanaldehyde, n-dodecanaldehyde, benzaldehyde, m-anisaldehyde, and p-hydroxybenzaldehyde). Tandem mass spectrometric analysis of the iminium ions using collision-induced dissociation demonstrated that straight chain aldehydes give a characteristic fragment at m/. z 122 (shown to correspond to protonated 4-(methyleneamino)phenol), while the aromatic aldehyde iminium ions fragment to give a characteristic product ion at m/. z 120. These features allow straightforward identification of linear and aromatic aldehydes. Quantitative analysis of n-nonaldehyde using a benchtop mass spectrometer demonstrated a linear response over 3 orders of magnitude from 2.5. ng to 5. μg of aldehyde loaded on the filter paper emitter. The limit of detection was determined to be 2.2. ng for this aldehyde. The method had a precision of 22%, relative standard deviation. The experiment was also implemented using a portable ion trap mass spectrometer

Surface-induced dissociation of molecular ions in a quadrupole ion trap mass spectrometer

AbstractA method is reported by which surface-induced dissociation is used to activate ions stored in a quadrupole ion trap mass spectrometer. The method employs a short (< 5 μs), fast-rising (< 20-ns rise time), high voltage direct current (dc) pulse, which is applied to the endcaps of a standard Paul-type quadrupole ion trap. This is in contrast to the application of an alternating current (ac) signal normally used to resonantly excite and dissociate ions in the trap. The effect of the dc pulse is to cause the ions rapidly to become unstable in the radial direction and subsequently to collide with the ring electrode. Sufficient internal energy is acquired in this collision to cause high energy fragmentations of relatively intractable molecular ions such as pyrene and benzene. The dissociations of limonene are used to demonstrate that high energy demand processes increase in relative importance in the dc pulse experiment compared with the usual resonance excitation method used to cause activation. The fragments are scanned out of the ion trap using the conventional mass-selective instability scan mode. Simulations of ion motion in the trap provide evidence that surface collisions occur at kinetic energies in the range of tens to several hundred electronvolts. The experiments also demonstrate that production of fragment ions is sensitive to the phase of the main radiofrequency drive voltage at the point when the dc is initiated

Polar [4 + 2+] Diels-alder Cycloadditions Of Acylium Ions In The Gas Phase

Acylium ions react with neutral isoprene and other 1,3-dienes in the gas phase to form covalently bound adducts by polar [4 + 2+] Diels-Alder cycloaddition. This general reaction was previously unknown either in the gas phase or as the corresponding solution reaction, but the mechanism is supported by ab initio calculations and experimental studies. Proton transfer competes with cycloaddition, as does fragmentation of both the acylium ion and the nascent cycloaddition product. Structural information on the ion/molecule product is obtained by performing multiple-stage (MS3) experiments using a pentaquadrupole mass spectrometer. Cycloreversion dominates the fragmentation behavior of these adducts. Both alkyl- and aryl-substituted acylium ions, as well as the thioacetyl cation, undergo facile cycloaddition. The α,β-unsaturated acylium ions (13 and 17) undergo a novel cycloaddition reaction at the double bond, the positively charged C≡O+ acting as an activating group. Substitution at the C=C double bond (14-16) can change the regioselectivity redirecting cycloaddition to the C≡O+ bond, as shown by MS3 experiments which display cycloreversion only when addition is on the carbonyl.115209226923

Membrane introduction mass spectrometry in environmental analysis

A semi-permeable membrane, mounted in a direct insertion probe, is used to introduce aqueous samples into a triple quadrupole mass spectrometer while a simpler version of this device is used to introduce samples into an ion trap detector. Both instruments are equipped with flow injection analysis fluid handling systems and this allows on-line monitoring of aqueous solutions at low levels. Low ppb level detection limits are achieved for some typical compounds of environmental interest, and response to changes in analyte concentration is very rapid, especially when the membrane is heated. This means that reacting systems can be monitored on-line, a capability demonstrated for the interconversion of the chloramines NH2Cl, NHCl2 and NCl3. This methodology is also well-suited to determination of organochloramines at sub-ppm levels in water and the successive chlorinations can be followed as the solution pH is adjusted. Typical aliphatic amines are chlorinated at nitrogen but aniline is ring chlorinated. The reactions which accompany ozonation of contaminated water are also studied and the pentafluorobenzyl hydroxylamine derivatives of aldehydes are studied by negative chemical ionization using membrane introduction mass spectrometry. Membrane introduction mass spectrometry is also demonstrated to be applicable in air monitoring