Fragmentation of benzylpyridinium "thermometer” ions and its effect on the accuracy of internal energy calibration

Electrospray ionization mass spectrometry (ESI-MS) is a powerful analytical method to study biomolecules and noncovalent complexes. The prerequisite for their intact observation is soft ionization. In ESI, the internal energy of ions is primarily influenced by collisional activation in the source. The survival yield method is frequently used to probe the energy deposition in ions during the electrospray process. In the present work, we investigate the fragmentation pathways of para-substituted benzylpyridinium ions, the most widely used "thermometer ions” in the survival yield method. In addition to the C-N bond cleavage, alternative fragmentation channels were found for the compounds studied. We consider these pathways to result from intramolecular rearrangements. The effect of these additional fragments on the accuracy of the internal energy calibration is estimated for both collision-cell and in-source collision-induced dissociation (CID). Altogether, results presented suggest that a correction of the energy scale is necessary for the method based on benzylpyridinium ions to precisely quantify ion internal energie

Exploring fluorescence and fragmentation of ions produced by electrospray ionization in ultrahigh vacuum

Fluorescence spectroscopy and mass spectrometry have been extensively used for characterization of biomaterials, but usually separately. An instrument combining fluorescence spectroscopy and Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS) has been developed to explore both fluorescence and mass spectrometric behavior of ions produced by electrospray ionization (ESI) in ultra high vacuum (<5 × 10−9 mbar). Using rhodamine 6G (R6G) as a sample, the instrument was systematically characterized. Gas-phase fluorescence and mass spectral signal of the same ion population are detected immediately after each other. Effects of gas pressure, ion density, and excitation laser power on the fluorescence signal intensity and mass spectral fragmentation patterns are discussed. Characteristic times of ion photodissociation in ultra high vacuum were recorded for different irradiation powers. Photofragmentation patterns of rhodamine 6G ions in the Penning trap of an FTICR spectrometer obtained by photoinduced dissociation (PID) with visible light and sustained off-resonance irradiation collision-induced dissociation (SORI-CID) were compared. The lowest energy dissociation fragment of rhodamine 6G ions was identified by relating PID patterns of rhodamine 6G and rhodamine 575 dyes at various irradiation powers. The unique instrument provides a powerful platform for probing the intramolecular relaxation mechanisms of nonsolvated ions when interacting with light, which is of great fundamental interest for better understanding of their physical and chemical propertie

Sampling analytes from cheese products for fast detection using neutral desorption extractive electrospray ionization mass spectrometry

The development of analytical techniques suitable for sensitive, high-throughput, and nondestructive food analysis has been of increasing interest in recent years. In this study, mass-spectral fingerprints of various cheese products were rapidly recorded in the mass range of m/z 50-300Da without any sample pretreatment, using neutral desorption extractive electrospray ionization mass spectrometry (ND-EESI-MS) in negative ion mode. The results demonstrate that both volatile and nonvolatile analytes on greasy cheese surfaces can be directly sampled by a neutral desorption gas beam. The influence of the neutral desorption gas flow on the analyte signal was systematically investigated. Under optimized experimental conditions, reproducible results were obtained using ND-EESI-MS. Principal component analysis was applied to differentiate a total of 49 individual cheese samples (four different types), which were purchased from three different supermarkets. All samples were successfully classified according to their types; but distributors and sensory properties were not distinguishable from the spectra data. The principal components 2, 3, and 4 scores showed an excellent capacity of distinguishing types of cheese. Molecular markers of interest can be identified using tandem mass spectrometry and matching the data with those from reference compounds. The experimental data show that ND-EESI-MS is able to sensitively and directly detect analytes on greasy surfaces without chemical contamination, providing a convenient method for high-throughput food analysis with a high degree of safet

Transient Ru-Methyl Formate Intermediates Generated with Bifunctional Transfer Hydrogenation Catalysts

Desorption electrospray ionization (DESI) coupled to high-resolution Orbitrap mass spectrometry (MS) was used to study the reactivity of a (β-amino alcohol)(arene)RuCl transfer hydrogenation catalytic precursor in methanol (CH3OH). By placing [(p-cymene)RuCl2]2 on a surface and spraying a solution of β-amino alcohol in methanol, two unique transient intermediates having lifetimes in the submillisecond to millisecond range were detected. These intermediates were identified as Ru (II) and Ru (IV) complexes incorporating methyl formate (HCOOCH3). The Ru (IV) intermediate is not observed when the DESI spray solution is sparged with Ar gas, indicating that O2 dissolved in the solvent is necessary for oxidizing Ru (II) to Ru (IV). These proposed intermediates are supported by high-resolution and high mass accuracy measurements and by comparing experimental to calculated isotope profiles. Additionally, analyzing the bulk reaction mixture using gas chromatography-MS and nuclear magnetic resonance spectroscopy confirms the formation of HCOOCH3. These results represent an example that species generated from the (β-amino alcohol)(arene)RuCl (II) catalytic precursor can selectively oxidize CH3OH to HCOOCH3. This observation leads us to propose a pathway that can compete with the hydrogen transfer catalytic cycle. Although bifunctional hydrogen transfer with Ru catalysts has been well-studied, the ability of DESI to intercept intermediates formed in the first few milliseconds of a chemical reaction allowed identification of previously unrecognized intermediates and reaction pathways in this catalytic system

Simultaneous sampling of volatile and non-volatile analytes in beer for fast fingerprinting by extractive electrospray ionization mass spectrometry

By gently bubbling nitrogen gas through beer, an effervescent beverage, both volatile and non-volatile compounds can be simultaneously sampled in the form of aerosol. This allows for fast (within seconds) fingerprinting by extractive electrospray ionization mass spectrometry (EESI-MS) in both negative and positive ion mode, without the need for any sample pre-treatment such as degassing and dilution. Trace analytes such as volatile esters (e.g., ethyl acetate and isoamyl acetate), free fatty acids (e.g., caproic acid, caprylic acid, and capric acid), semi/non-volatile organic/inorganic acids (e.g., lactic acid), and various amino acids, commonly present in beer at the low parts per million or at sub-ppm levels, were detected and identified based on tandem MS data. Furthermore, the appearance of solvent cluster ions in the mass spectra gives insight into the sampling and ionization mechanisms: aerosol droplets containing semi/non-volatile substances are thought to be generated via bubble bursting at the surface of the liquid; these neutral aerosol droplets then collide with the charged primary electrospray ionization droplets, followed by analyte extraction, desolvation, ionization, and MS detection. With principal component analysis, several beer samples were successfully differentiated. Therefore, the present study successfully extends the applicability of EESI-MS to the direct analysis of complex liquid samples with high gas content. Figure By gently bubbling nitrogen gas through beer, both volatile and non-volatile compounds can be simultaneously sampled in the form of aerosol for further analysis, allowing fast chemically fingerprinting using extractive electrospray ionization mass spectrometry (EESI-MS

Charge-State-Dependent Variation of Signal Intensity Ratio between Unbound Protein and Protein–Ligand Complex in Electrospray Ionization Mass Spectrometry: The Role of Solvent-Accessible Surface Area

Native electrospray ionization mass spectrometry (ESI-MS) is nowadays widely used for the direct and sensitive determination of protein complex stoichiometry and binding affinity constants (<i>K</i>_a). A common yet poorly understood phenomenon in native ESI-MS is the difference between the charge-state distributions (CSDs) of the bound protein–ligand complex (PL) and unbound protein (P) signals. This phenomenon is typically attributed to experimental artifacts such as nonspecific binding or in-source dissociation and is considered highly undesirable, because the determined <i>K</i>_a values display strong variation with charge state. This situation raises serious concerns regarding the reliability of ESI-MS for the analysis of protein complexes. Here we demonstrate that, contrary to the common belief, the CSD difference between P and PL ions can occur without any loss of complex integrity, simply due to a change in the solvent-accessible surface area (ΔSASA) of the protein upon ligand binding in solution. The experimental CSD shifts for PL and P ions in ESI-MS are explained in relation to the magnitude of ΔSASA for diverse protein–ligand systems using a simple model based on the charged residue mechanism. Our analysis shows that the revealed ΔSASA factor should be considered rather general and be given attention for the correct spectral interpretation of protein complexes

Charge-State-Dependent Variation of Signal Intensity Ratio between Unbound Protein and Protein–Ligand Complex in Electrospray Ionization Mass Spectrometry: The Role of Solvent-Accessible Surface Area

Native electrospray ionization mass spectrometry (ESI-MS) is nowadays widely used for the direct and sensitive determination of protein complex stoichiometry and binding affinity constants (<i>K</i>_a). A common yet poorly understood phenomenon in native ESI-MS is the difference between the charge-state distributions (CSDs) of the bound protein–ligand complex (PL) and unbound protein (P) signals. This phenomenon is typically attributed to experimental artifacts such as nonspecific binding or in-source dissociation and is considered highly undesirable, because the determined <i>K</i>_a values display strong variation with charge state. This situation raises serious concerns regarding the reliability of ESI-MS for the analysis of protein complexes. Here we demonstrate that, contrary to the common belief, the CSD difference between P and PL ions can occur without any loss of complex integrity, simply due to a change in the solvent-accessible surface area (ΔSASA) of the protein upon ligand binding in solution. The experimental CSD shifts for PL and P ions in ESI-MS are explained in relation to the magnitude of ΔSASA for diverse protein–ligand systems using a simple model based on the charged residue mechanism. Our analysis shows that the revealed ΔSASA factor should be considered rather general and be given attention for the correct spectral interpretation of protein complexes

Fragmentation of Benzylpyridinium "Thermometer" Ions and Its Effect on the Accuracy of Internal Energy Calibration

ISSN:1879-1123ISSN:1044-030