Study of a bisquaternary ammonium salt by atmospheric pressure photoionization mass spectrometry

Characterization of hydrophobic peptides in the presence of detergent by photoionization mass spectrometry

The characterization of membrane proteins is still challenging. The major issue is the high hydrophobicity of membrane proteins that necessitates the use of detergents for their extraction and solubilization. The very poor compatibility of mass spectrometry with detergents remains a tremendous obstacle in studies of membrane proteins. Here, we investigated the potential of atmospheric pressure photoionization (APPI) for mass spectrometry study of membrane proteins. This work was focused on the tetraspanin CD9 and the multidrug transporter BmrA. A set of peptides from CD9, exhibiting a broad range of hydropathicity, was investigated using APPI as compared to electrospray ionization (ESI). Mass spectrometry experiments revealed that the most hydrophobic peptides were hardly ionized by ESI whereas all peptides, including the highly hydrophobic one that corresponds to the full sequence of the first transmembrane domain of CD9, were easily ionized by APPI. The native protein BmrA purified in the presence of the non-ionic detergent beta-D-dodecyl maltoside (DDM) was digested in-solution using trypsin. The resulting peptides were investigated by flow injection analysis of the mixture followed by mass spectrometry. Upon ESI, only detergent ions were detected and the ionic signals from the peptides were totally suppressed. In contrast, APPI allowed many peptides distributed along the sequence of the protein to be detected. Furthermore, the parent ion corresponding to the first transmembrane domain of the protein BmrA was detected under APPI conditions. Careful examination of the APPI mass spectrum revealed a-, b-, c- and y- fragment ions generated by in-source fragmentation. Those fragment ions allowed unambiguous structural characterization of the transmembrane domain. In conclusion, APPI-MS appears as a versatile method allowing the ionization and fragmentation of hydrophobic peptides in the presence of detergent.

Carbon disulfide as a dopant in photon-induced chemical ionization mass spectrometry

RATIONALE

The addition of a dopant to an Atmospheric Pressure PhotoIonization (APPI) source of a mass spectrometer has been shown to enhance the degree of analyte ionization. A series of different dopants has been successfully utilized; however, there has been very little published on the characteristics of a good dopant. We have proposed carbon disulfide (CS2) as a novel new dopant based on its absorption cross-section for the VUV photon's energy used and its unique gas-phase ion chemistry, notably the fact that it does not contain a proton.

METHODS

The ability of CS2 to enhance the ionization effectiveness of APPI was tested by using a group of compounds that have different proton affinities (PAs) and electron affinities (EAs). These results were compared to results obtained using the commonly used dopants, toluene and anisole. Particular attention was paid to the formation of [M](+) ions relative to [M+H](+) ions. Mass spectra were collected using a Waters Quattro Premier liquid chromatography/tandem mass spectrometry (LC/MS/MS) system equipped with a commercial Photomate™ photoionization source.

RESULTS

The results show that CS2 increases the ionization efficiency of most of the analytes studied in this work comparably to toluene and anisole. CS2 promotes both ionization routes of [M](+) and [M+H](+). In addition, due to the higher ionization energy (IE) of CS2 (10.01) compared to the IEs of toluene (8.83) and anisole (8.20), CS2 can enhance the ionization efficiency of analytes that cannot be enhanced with toluene and anisole.

CONCLUSIONS

We have determined that CS2 is a viable dopant for use in APPI sources. For some analytes, significant [M+H](+) ion signals are observed; therefore, the donated proton must come from either water clusters or solvents. In addition, CS2 promotes the ionization of analytes with low PAs and higher IEs than that of toluene and anisole.

Atmospheric pressure photoionization as a powerful tool for large-scale lipidomic studies

Lipidomic studies often use liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) for separation, identification, and quantification. However, due to the wide structural diversity of lipids, the most apolar part of the lipidome is often detected with low sensitivity in ESI. Atmospheric pressure (APPI) can be an alternative ionization source since normal-phase solvents are known to enhance photoionization of these classes. In this paper, we intend to show the efficiency of APPI to identify different lipid classes, with a special interest on sphingolipids. In-source APPI fragmentation appears to be an added value for the structural analysis of lipids. It provides a detailed characterization of both the polar head and the non polar moiety of most lipid classes, and it makes possible the detection of all lipids in both polarities, which is not always possible with ESI.

An electropneumatic-heated nebulizer for enhancing spray ionization in PhotoSpray atmospheric pressure photoionization sources for liquid chromatography/mass spectrometry

We introduce a novel electropneumatic-heated nebulizer (EPn-HN), incorporating an electrified internal pneumatic nebulizer, to enhance the yield of sprayed ions from PhotoSpray atmospheric pressure photoionization (APPI) sources for liquid chromatography/mass spectrometry (LC/MS). Spray ionization from the pneumatic-heated nebulizers used in APPI sources provides a supplemental, complementary ionization method to be used for involatile and thermally labile compounds, otherwise intractable to APPI. Details of the construction and operation of the EPn-HN device are provided. The performance of the EPn-HN is demonstrated using two model compounds: substance P, a peptide used as a standard in studies of ion fragmentation mechanisms, and aztreonam, a thermally labile antibiotic. At the optimum voltage for spray ionization, improvements in sensitivity of two orders of magnitude are obtained relative to when the sprayer is grounded, the conventional case. Since both substance P and aztreonam cannot be detected using the APPI method alone, the results demonstrate how spray ionization from the EPn-HN may be used to extend the range of compounds amenable to PhotoSpray sources.

Atmospheric pressure photoionization mass spectrometry of oligodeoxyribonucleotides

Small oligonucleotides (di- and trimers) were investigated by atmospheric pressure photoionization (APPI) with focus on the fragmentation mechanisms. The fragmentation patterns of these biomolecular ions have been monitored under dopant-assisted photoionization (DA-APPI) conditions. Our results reveal new aspects of the gas- phase chemistry of ions formed from such biomolecules. They illustrate that the reaction between low-energy electrons released from photoionization processes and di- and trinucleotides lies in dissociative electron attachment processes leading to phosphodiester bond cleavages and to the formation of numerous fragments in the ion source. The conditions of DA-APPI, which involve protic solvents and atmospheric pressure conditions, seem to be relevant for the study of radiation damages to biological molecules.

Study of the metabolites of bencycloquidium bromide racemate, a novel anticholinergic compound, in rat bile by liquid chromatography-tandem mass spectrometry

A sensitive and specific liquid chromatographic-electrospray ionization (ESI) tandem ion trap mass spectrometric method has been developed for identification of bencycloquidium bromide (BCQB) and its metabolites in rat bile. Six healthy rats were administrated a single dose (3.0 mg kg(-1)) of BCQB by intraperitoneal (i.p.) injection. The bile were sampled from 0 h to 24 h and purified by using a C(18) solid- phase extraction (SPE) cartridge, then the purified bile samples were separated on a reversed-phase C(18) column using acetonitrile/40 mM ammonium acetate buffer (containing 0.1% formic acid) as mobile phase at gradient elution and detected by an on-line MS(n) detector. Identification and structural elucidation of the metabolites were performed by comparing the changes in molecular weight (Deltam) and full scan MS(n) spectra with those of the parent drug. Eight metabolites (such as hydroxylated and oxidized metabolites) and the parent drug were found in rat bile. Eight metabolites of BCQB were identified and hydroxylated metabolites were the major metabolites. The metabolic pathways of BCQB in vivo are proposed for the first time.

Photon-Independent Gas-Phase-Ion Formation in Capillary Electrophoresis−Mass Spectrometry Using Atmospheric Pressure Photoionization

In a previous study on capillary electrophoresis-atmospheric pressure photoionization mass spectrometry (CE-APPI-MS), it was observed that the formation of gas-phase ions does not always proceed through photon-induced mechanisms (Hommerson, P.; Khan, A. M.; De Jong, G. J.; Somsen, G. W. Electrophoresis 2007, 28, 1444-1453). That is, analyte signals were observed when the VUV excitation source was switched off. The aim of the present study was to further explore this photon-independent ionization (PII) process. Parameters such as MS capillary voltage, compound nature, background electrolyte (BGE) composition, and presence of dopants were studied using a CE-APPI-MS setup. Infusion experiments showed a relatively low MS capillary voltage of approximately 600 V to be the main prerequisite for PII. Quaternary ammonium compounds showed strong responses in PII-MS but could not be observed in dopant-assisted APPI. Basic amines could be ionized by both photoionization (PI) and PII, whereas neutral compounds (steroids) could only be observed using PI. Nonvolatile BGEs appeared to cause substantial ionization suppression in PII, while PI signals remained largely unaffected. Selection of the proper interface and MS settings allowed PI and PII to proceed simultaneously, which broadened the range of compounds that could be analyzed in a single CE-APPI-MS run. Based on the observed characteristics, it is concluded that PII most probably occurs by a liquid-phase ionization mechanism, which appears to arise in the APPI source when specific conditions are selected.

Electrospray ionization/atmospheric pressure photoionization multimode source for low-flow liquid chromatography/mass spectrometric analysis

Analysis of several polar and non-polar compounds is performed with a newly developed dual electrospray ionization/atmospheric pressure photoionization (ESI/APPI) or ESPI source. Several variables are considered in the source, such as ESI probe heater temperature, solvent flow, dopant effects, repeller plate voltage, source geometry and photon energy (Kr vs. Ar lamp). Direct photoionization resulting in a molecular radical cation [M](*+) dominates at high temperatures (>400 degrees C) and low flow rates (<200 microL/min). Indirect photo-induced chemical ionization (PCI) involving solvent molecules becomes important at lower temperatures and higher solvent flow rates. Indirect PCI is enhanced using an Ar lamp, which yields comparable [M+H](+) signal but poorer [M](*+) signal than the Kr lamp at lower temperatures and higher flow rates. This is in support of our recent finding that the Ar lamp results in a solvent-dependent enhancement of analyte molecules via PCI. Analysis of 12 compounds in methanol under low-flow conditions (10 microL/min) demonstrates that the dual ESPI source performs favorably for most compounds versus the standard ESCI source, and significantly better than ESCI for the analysis of unstable drugs, like flurbiprofen. Several factors contributing to the benefits of the ESPI source are the shared optimal geometry for ESI and APPI sources and soft ionization of APPI versus APCI.

Fragmentation induced in atmospheric pressure photoionization of peptides

In this work, the fragmentation of peptides under atmospheric pressure photoionization conditions is investigated. Intensive fragmentations into b/y- and c-sequence ions are reported. Abundance of these c-ions appeared to be related to the quantity of dopant infused and to the disappearance of the doubly protonated peptide ion. A careful analysis of the role of the dopant indicates that the fragmentations are not dependent on the nature of the dopant but on their ionization efficiencies. This result shows that the fragmentation arises from the reaction of the protonated peptide with photoelectrons released upon ionization of the dopant in an electron capture dissociation/electron transfer dissociation (ECD/ETD) type mechanism. Experiments with peptides bearing a single proton indicate that additional mechanisms are involved. H-atom transfer reactions are suggested to be responsible for the fragmentations as well. Those atoms could arise either from the dopant ions or from negatively charged solvent nanodroplets. This is the first report of an ECD/ETD mechanism in a dense medium and at atmospheric pressure.