Penggao Duan

Identification of the aromatic tertiary N-oxide functionality in protonated analytes via ion/molecule reactions in mass spectrometers.

A mass spectrometric method is presented for the rapid identification of compounds that contain the aromatic N-oxide functional group. This method utilizes a gas-phase ion/molecule reaction with 2-methoxypropene that yields a stable adduct for protonated aromatic tertiary N-oxides (and with one protonated nitrone) in different mass spectrometers. A variety of protonated analytes with O- or N-containing functional groups were examined to probe the selectivity of the reaction. Besides protonated aromatic tertiary N-oxides and one nitrone, only three protonated amines were found to form a stable adduct but very slowly. All the other protonated analytes, including aliphatic tertiary N-oxides, primary N-oxides, and secondary N-oxides, are unreactive toward or react predominantly by proton transfer with 2-methoxypropene.

Laser-Induced Acoustic Desorption Coupled with a Linear Quadrupole Ion Trap Mass Spectrometer

In recent years, laser-induced acoustic desorption (LIAD) coupled with a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer has been demonstrated to provide a valuable technique for the analysis of a wide variety of nonvolatile, thermally labile compounds, including analytes that could not previously be analyzed by mass spectrometry. Although FT-ICR instruments are very powerful, they are also large and expensive and, hence, mainly used as research instruments. In contrast, linear quadrupole ion trap (LQIT) mass spectrometers are common due to several qualities that make these instruments attractive for both academic and industrial settings, such as high sensitivity, large dynamic range, and experimental versatility. Further, the relatively small size of the instruments, comparatively low cost, and the lack of a magnetic field provide some distinct advantages over FT-ICR instruments. Hence, we have coupled the LIAD technique with a commercial LQIT, the Thermo Fischer Scientific LTQ mass spectrometer. The LQIT was modified for a LIAD probe by outfitting the removable back plate of the instrument with a 6 in. ConFlat flange (CFF) port, gate valve, and sample lock. Reagent ions were created using the LQITs atmospheric pressure ionization source and trapped in the mass analyzer for up to 10 s to allow chemical ionization reactions with the neutral molecules desorbed via LIAD. These initial experiments focused on demonstrating the feasibility of performing LIAD in the LQIT. Hence, the results are compared to those obtained using an FT-ICR mass spectrometer. Despite the lower efficiency in the transfer of desorbed neutral molecules into the ion trap, and the smaller maximum number of available laser pulses, the intrinsically higher sensitivity of the LQIT resulted in a higher sensitivity relative to the FT-ICR.

Ion–molecule reactions for the differentiation of primary, secondary and tertiary hydroxyl functionalities in protonated analytes in a tandem mass spectrometer

A mass spectrometric method utilizing gas-phase ion-molecule reactions of 1-butanethiol and di-tert-butyl peroxide has been developed for the differentiation of primary, secondary and tertiary hydroxyl functionalities in protonated analytes in a FT-ICR mass spectrometer.

Differentiation of Protonated Aromatic Regioisomers Related to Lignin by Reactions with Trimethylborate in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

Several lignin model compounds were examined to test whether gas-phase ion–molecule reactions of trimethylborate (TMB) in a FTICR can be used to differentiate the ortho-, meta-, and para-isomers of protonated aromatic compounds, such as those formed during degradation of lignin. All three regioisomers could be differentiated for methoxyphenols and hydroxyphenols. However, only the differentiation of the ortho-isomer from the meta- and para-isomers was possible for hydroxyacetophenones and hydroxybenzoic acids. Consideration of the previously reported proton affinities at all basic sites in the isomeric hydroxyphenols, and the calculated proton affinities at all basic sites in the three methoxyphenol isomers, revealed that the proton affinities of the analytes relative to that of TMB play an important role in determining whether and how they react with TMB. The loss of two methanol molecules (instead of one) from the adducts formed with TMB either during ion–molecule reactions, or during sustained-off resonance irradiated collision-activated dissociation of the ion–molecule reaction products, revealed the presence of two functionalities in almost all the isomers. This finding supports earlier results suggesting that TMB can be used to count the functionalities in unknown oxygen-containing analytes.

Liquid chromatography/tandem mass spectrometry utilizing ion-molecule reactions and collision-activated dissociation for the identification of N-oxide drug metabolites.

A liquid chromatography/tandem mass spectrometry (LC/MS(3)) method based on ion-molecule reactions and collision-activated dissociation (CAD) is presented for the identification of analytes with the N-oxide functional group directly in mixtures. Tri(dimethylamino)borane (TDMAB) rapidly and selectively derivatizes protonated N-oxides in a modified commercial linear quadrupole ion trap (LQIT) mass spectrometer to yield a distinct product ion (adduct-(CH(3))(2)NH). The LQIT was outfitted with an external reagent-mixing manifold that allows TDMAB to be mixed with the helium buffer gas used in the trap. The derivatized analytes are readily identified on the basis of a shift of 98 Th (Thomson) relative to the m/z value of the protonated analyte. Further probing of the derivatized analytes via isolation followed by CAD can be used to confirm the presence of an N-oxide, and distinguish between aliphatic and aromatic tertiary N-oxides. Since the ion-molecule reaction is fast, these experiments can be accomplished on the same time scale as typical CAD-based MS(n) experiments, thus maintaining the duty cycle of the instrument for this type of experiment. To demonstrate real world applicability, the method was tested on real active pharmaceutical ingredients and their derivatives.

An Ion/Molecule Reaction for the Identification of Analytes with Two Basic Functional Groups

A mass spectrometric method is presented for the identification of analytes with two basic functionalities and PA between 222 and 245 kcal/mol, including diamines. This method utilizes gas-phase ion-molecule reactions of protonated analytes with neutral 1,1-diethoxyethene (DEE) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR). A variety of protonated mono-, bi-, and trifunctional analytes containing different functional groups, namely, amido, amino, N-oxide, hydroxy, carboxylic acid, keto, thio, thioether, alkene, phosphite, and phosphonate, were tested in the FT-ICR. The results demonstrate that basic protonated bifunctional compounds (PA between 222 and 245 kcal/mol) react selectively with DEE by forming a specific addition/elimination product ion (adduct - EtOH) (this product was also observed for lysine with three functionalities). The diagnostic reaction sequence involves proton transfer from the protonated analyte to the basic vinyl group in DEE, followed by addition of one of the functional groups of the analyte to the electrophilic α-carbon in protonated DEE. The next step involves proton transfer from this functionality to the other analyte functionality, followed by proton transfer to DEE and elimination of ethanol. Since the mechanism involves proton transfer between two functional groups of the analyte, the reaction does not occur for analytes where the two functionalities cannot be in close proximity (i.e., meta-phenylenediamine), and where no proton is available (i.e., dimethylaminoketone).

A novel chemical ionization reagent ion for organic analytes: the aquachloromanganese(II) cation [ClMn(H2O)+]

The reactivity of ClMn(H(2)O)(+) towards small organic compounds (L) was examined in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The organic compounds studied are aliphatic and aromatic alcohols, aliphatic amines, ketones, an epoxide, an ether, a thiol and a phosphine. All the reactions lead to the formation of the ClMn(H(2)O)(L)(+) complex, which dissociates by loss of the H(2)O molecule. In general, the reactions were found to occur with high efficiencies (>85%), indicating them to be exothermic. Electron transfer was also observed between ClMn(H(2)O)(+) and compounds with low ionization energies (IE), to form the molecular ion (L(+•)) of the analyte. Based on these observations, the IE of ClMn(H(2)O)(+) is approximated to be 8.1 ± 0.1 eV. Thus, the utility of ClMn(H(2)O)(+) as a chemical ionization reagent in mass spectrometry is expected to be limited to organic compounds with IEs greater than 8 eV.

Data-Dependent Neutral Gain MS3: Toward Automated Identification of the N-Oxide Functional Group in Drug Metabolites

We report here an automated method for the identification of N-oxide functional groups in drug metabolites by using the combination of liquid chromatography/tandem mass spectrometry (LC/MSn) based on ion-molecule reactions and collision-activated dissociation (CAD). Data-dependent acquisition, which has been readily utilized for metabolite characterization using CAD-based methods, is adapted for use with ion-molecule reaction-based tandem mass spectrometry by careful choice of select experimental parameters. Two different experiments utilizing ion-molecule reactions are demonstrated, data-dependent neutral gain MS3 and data-dependent neutral gain pseudo-MS3, both of which generate functional group selective mass spectral data in a single experiment and facilitate increased throughput in structural elucidation of unknown mixture components. Initial results have been generated by using an LC/MSn method based on ion-molecule reactions developed earlier for the identification of the N-oxide functional group in pharmaceutical samples, a notoriously difficult functional group to identify via CAD alone. Since commercial software and straightforward, external instrument modification are used, these experiments are readily adaptable to the industrial pharmaceutical laboratory.