Marcela Nefliu

Ambient analysis of saturated hydrocarbons using discharge-induced oxidation in desorption electrospray ionization

Saturated nonfunctionalized hydrocarbons can be oxidized in situ by initiating an electrical discharge during desorption electrospray ionization (DESI) to generate the corresponding alchohols and ketones. This form of reactive DESI experiment can be utilized as an in situ derivatization method for rapid and direct analysis of alkanes at atmospheric pressure without sample preparation. Betaine aldehyde was incorporated into the DESI spray solution to improve the sensitivity of detecting the long-chain alcohol oxidation products. The limit of detection for alkanes (C15H32 to C30H62) from pure samples is ∼20 ng. Multiple oxidations and dehydrogenations occurred during the DESI discharge, but no hydrocarbon fragmentation was observed, even for highly branched squalane. Using exact mass measurements, the technique was successfully implemented for analysis of petroleum distillates containing saturated hydrocarbons.

Internal energy distributions in desorption electrospray ionization (DESI).

The internal energy distributions of typical ions generated by desorption electrospray ionization (DESI) were measured using the “survival yield” method, and compared with corresponding data for electrospray ionization (ESI) and electrosonic spray ionization (ESSI). The results show that the three ionization methods produce populations of ions having internal energy distributions of similar shapes and mean values (1.7–1.9 eV) suggesting similar phenomena, at least in the later stages of the process leading from solvated droplets to gas-phase ions. These data on energetics are consistent with the view that DESI involves “droplet pick-up” (liquid-liquid extraction) followed by ESI-like desolvation and gas-phase ion formation. The effects of various experimental parameters on the degree of fragmentation of p-methoxy-benzylpyridinium ions were compared between DESI and ESSI. The results show similar trends in the survival yields as a function of the nebulizing gas pressure, solvent flow rate, and distance from the sprayer tip to the MS inlet. These observations are consistent with the mechanism noted above and they also enable the user to exercise control over the energetics of the DESI ionization process, through manipulation of external and internal ion source parameters.

Serine sublimes with spontaneous chiral amplification

Sublimation of near-racemic samples of serine yields a sublimate which is highly enriched in the major enantiomer; this simple one-step process occurs under relatively mild conditions, and represents a possible mechanism for the chiral amplification step in homochirogenesis.

Organic Reactions of Ionic Intermediates Promoted by Atmospheric‐Pressure Thermal Activation

Interest in “green chemistry” has made the development of environmentally benign alternatives to organic solvents and conventional acid catalysts an active area of investigation. The replacement of organic solvents with recyclable ionic liquids and supercritical carbon dioxide, as well as the replacement of acid catalysts by supercritical water (400 8C, 200 bar) and near-critical water (275 8C, 60 bar), has already been demonstrated in organic synthesis. These developments, and accompanying studies of reaction mechanisms under unusual conditions, have increased the interest in nontraditional ways of carrying out reactions and exploring their mechanisms. Several such approaches are based on mass spectrometry, the best established of which is the use of chemical ionization and gas-phase Brønsted acid/base reactions to generate the ions of interest. Mass spectrometry can be employed to study the reactivity of organic reactants by using electrospray ionization (ESI) to generate ionic reagents under ambient pressure. ESI has been widely used to ionize organic and biological samples, thus allowing compounds to be protonated at their basic sites or deprotonated at their acidic sites. The protonation process with ESI is known to occur even when the analyte is dissolved in pure protic solvents, such as water or methanol, without any added acids or bases. The dissociation equilibrium H2OQH + + OH and the electrochemical reaction 2H2OQ4H + + O2 + 4e [23] are proton sources. This phenomenon prompted us to use ESI as a mild alternative to acid catalysis for generating ionic intermediates in organic reactions. As the protonated analyte ions (potential reaction intermediates in solution-phase reactions) generated by ESI are thermalized by numerous collisions with third body gaseous molecules at atmospheric pressure, they must be activated to undergo reaction. We recently developed an atmospheric-pressure thermal activation method based on a heated coiled tube, which can be used, among other things, for protein/peptide ion dissociation and hence as an aid in sequencing. Herein we have employed this atmosphericpressure activation method to promote reactions of organic ions generated by electrosonic spray ionization (ESSI), a variant form of ESI. By using this strategy, several organic reactions, including the Fischer indole synthesis, the Borsche– Drechsel cyclization, and the pinacol rearrangement, have been successfully performed under mild conditions (Scheme 1 and Figure 1a).

Enhanced desorption ionization using oxidizing electrosprays

A signal enhancement of two orders of magnitude was achieved when reactive desorption electrospray ionization (DESI) was used to investigate copper(II) dibutyl dithiocarbamate, Cu(II)(bu2dtc)2, found in a specialized polymer. Cu(II) was oxidized to Cu(III) during the DESI experiment by oxidants in the spray solvent. Such oxidants could be present or formed during electrospray (e.g., O2) or deliberately added to the spray solvent (this approach is called reactive DESI). When a strong oxidizing agent (e.g., iodine) was added to the spray solvent, the signal increased by two orders of magnitude relative to the pure solvent spray. The correlation between the standard reduction potential of the oxidant and the signal intensity and signal to noise ratio of the product ion for various reagents, was tested and discussed. The observed DESI enhancements in rates of oxidation are not observed in homogeneous solution. The major peaks in the collision induced dissociation (CID) spectrum of the complex ion Cu(III)(bu2dtc)2]+ were identified using isotopic distributions and MS3 data.