VUV photon‐induced ionization/dissociation of antipyrine and propyphenazone: mass spectrometric and theoretical insights

Two analgesic and anti‐inflammatory drugs, antipyrine and propyphenazone, were investigated with infrared laser desorption/tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (IR LD/VUV PIMS) and theoretical calculations. Mass spectra of the two drugs were measured at various photon energies. Fragment ions were gradually produced as photon energy increases. The structural assignment of the dominant fragment ions was supported by the results from a commercial electron impact time‐of‐flight mass spectrometer (EI‐TOF MS). Primary fragmentation pathways were established from experimental observations combining with theoretical calculations. Methyl radical elimination is a common fragmentation pathway for two analytes. However, for propyphenazone cation, isopropyl group elimination to form antipyrine cation is another competitive pathway. Copyright © 2010 John Wiley & Sons, Ltd.     

Laser desorption lamp ionization source for ion trap mass spectrometry

A two‐step laser desorption lamp ionization source coupled to an ion trap mass spectrometer (LDLI‐ITMS) has been constructed and characterized. The pulsed infrared (IR) output of an Nd:YAG laser (1064 nm) is directed to a target inside a chamber evacuated to ~15 Pa causing desorption of molecules from the target's surface. The desorbed molecules are ionized by a vacuum ultraviolet (VUV) lamp (filled with xenon, major wavelength at 148 nm). The resulting ions are stored and detected in a three‐dimensional quadrupole ion trap modified from a Finnigan Mat LCQ mass spectrometer operated at a pressure of ≥ 0.004 Pa. The limit of detection for desorbed coronene molecules is 1.5 pmol, which is about two orders of magnitude more sensitive than laser desorption laser ionization mass spectrometry using a fluorine excimer laser (157 nm) as the ionization source. The mass spectrum of four standard aromatic compounds (pyrene, coronene, rubrene and 1,4,8,11,15,18,22,25‐octabutoxy‐29H,31H‐phthalocyanine (OPC)) shows that parent ions dominate. By increasing the infrared laser power, this instrument is capable of detecting inorganic compounds. Copyright © 2015 John Wiley & Sons, Ltd.     

Studies on extracting solutions of endohedral rare-earth metallofullerenes by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Thirteen extracting solutions of rare-earth metallofullerenes containing La,Ce,Pr,Nd Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm and Yb respectively have been investigated by means of matrix-assisted laser desorpuon/ ionization time-of-flight mass spectrometry.The influences of the positive-ion/negative-ion mode,laser intensity,ma trix and mass discrimination to the analytical results are studied,based on which the optimal analytical conditions have been determined.The results show that the extracting solutions contain large quantities of rare-earth metallofullerenes besides empty fullerenes.On the basis of comparing their relative intensities,the different structure stabilities and solubilities of metallofullerenes with different rare-earth metals encapsulated into the fullerene cages,as well as some possible reasons to those differences,are discussed.     

Investigation of porphyrins and metalloporphyrins by liquid matrix-assisted laser desorption mass spectrometry

Laser desorption mass spectrometry with liquid matrix-assistance has been used to study a series of selected porphyrins and metalloporphyrins. This work presents the results of using a liquid matrix with fibrous material as the substrate for liquid matrix assisted laser desorption of porphyrins and metalloporphyrins. The liquid matrices used for porphyrin studies were o-nitrophenyl octyl ether (NPOE) and 15-crown-5. The use of a liquid matrix with soft laser ionization enhances molecular ion formation. We also have investigated the use of NPOE as a liquid matrix for identifying mixtures of up to six porphyrins in a single shot spectrum without prior separation. The (M + H)⁺ peak of each metalloporphyrin component in the mixture is clearly indicated in the spectra and no obvious interference effects were observed.     

Coffee-ring effects in laser desorption/ionization mass spectrometry

This report focuses on the heterogeneous distribution of small molecules (e.g. metabolites) within dry deposits of suspensions and solutions of inorganic and organic compounds with implications for chemical analysis of small molecules by laser desorption/ionization (LDI) mass spectrometry (MS). Taking advantage of the imaging capabilities of a modern mass spectrometer, we have investigated the occurrence of “coffee rings” in matrix-assisted laser desorption/ionization (MALDI) and surface-assisted laser desorption/ionization (SALDI) sample spots. It is seen that the “coffee-ring effect” in MALDI/SALDI samples can be both beneficial and disadvantageous. For example, formation of the coffee rings gives rise to heterogeneous distribution of analytes and matrices, thus compromising analytical performance and reproducibility of the mass spectrometric analysis. On the other hand, the coffee-ring effect can also be advantageous because it enables partial separation of analytes from some of the interfering molecules present in the sample. We report a “hidden coffee-ring effect” where under certain conditions the sample/matrix deposit appears relatively homogeneous when inspected by optical microscopy. Even in such cases, hidden coffee rings can still be found by implementing the MALDI-MS imaging technique. We have also found that to some extent, the coffee-ring effect can be suppressed during SALDI sample preparation.     

Alternative approaches for the detection of various phospholipid classes by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The detection of phospholipids (PLs) by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was demonstrated nearly a decade ago. However, its use as a conventional tool for PL analysis has been hindered by ambiguities in peak assignments caused by spectral overlaps and difficulties in the detection of some PL classes when analytes with positively charged head groups, such as sphingomyelins (SMs) and phosphatidylcholines (PCs) are present. In this work, either a strong cation-exchange resin or CsCl crystals were added directly to the PL samples to reduce spectral complexity and enhance sensitivity. The quantitative exchange resulted in virtually only protonated or Cs+ adducts. To alleviate difficulties in the detection and identification of PL classes with ionization efficiencies lower than those of SMs and PCs, improvements in the sensitivity of negative-ion mass spectra were sought. For this purpose, several neutral and basic matrices were tried. Among them, p-nitroaniline (PNA) proved to be an advantageous alternative to the use of 2,5-dihydroxybenzoic acid (DHB), the most commonly used matrix in PL analysis. Because of its lower acidity, PNA increased the relative amount of deprotonated species and improved the sensitivity of negative-ion mass spectra. It was possible to confirm peak assignments for PL classes that normally give weak signals when DHB is used. Noteworthy is the detection (in both positive and negative modes) and conclusive identification of species in natural mixtures of phosphatidylethanolamines (PEs) and PE plasmalogens (PEps). PNA allowed the identification of PEs and PEps even in mixtures containing SMs and PCs. Although some cations related to PCs and PEs overlapped in positive-ion spectra, these interferences were eliminated in the negative mode as only the deprotonated forms of PEs and PEps were detectable and those of SMs and PCs were absent owing to their neutrality.     

Improved analysis of membrane protein by PVDF-aided, matrix-assisted laser desorption/ionization mass spectrometry

Characterization of membrane proteins remains an analytical challenge because of difficulties associated with tedious isolation and purification. This study presents the utility of the polyvinylidene difluoride (PVDF) membrane for direct sub-proteome profiling and membrane protein characterization by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The hydrophobic adsorption of protein, particularly membrane proteins, on the PVDF surface enables efficient on-PVDF washing to remove high concentrations of detergents and salts, such as up to 5% sodium dodecyl sulfate (SDS). The enhanced spectrum quality for MALDI detection is particularly notable for high molecular weight proteins. By using on-PVDF washing prior to MALDI detection, we obtained protein profiles of the detergent-containing and detergent-insoluble membrane fractions from Methylococcus capsulatus (Bath). Similar improvements of signal-to-noise ratios were shown on the MALDI spectra for proteins electroblotted from SDS-polyacrylamide gel electrophoresis (SDS-PAGE) onto the PVDF membrane. We have applied this strategy to obtain intact molecular weights of the particulate methane monooxygenase (pMMO) composed of three intrinsic membrane-bound proteins, PmoA, PmoB, and PmoC. Together with peptide sequencing by tandem mass spectrometry, post-translational modifications including N-terminal acetylation of PmoA and PmoC and alternative C-terminal truncation of PmoB were identified. The above results show that PVDF-aided MALDI-MS can be an effective approach for profiling and characterization of membrane proteins.     

Study of synthetic aliphatic copolyamides by time-of-flight matrix assisted laser desorption/ionization mass spectrometry

Synthetic copolyamides based on aliphatic diamines (1,3-propanediamine and 1,4-butanediamine) and dichlorides of aliphatic carboxylic acids (adipic and sebacic acid dichlorides) were investigated using time-of-flight matrix assisted laser desorption/ionization mass spectrometry. Their mass spectra showed peaks for cationized (Na⁺ and K⁺) and protonated (less intense peaks) oligomers with NH₂-NH₂, NH₂-COOH, or COOH-COOH end groups. No cyclic oligomers were detected in the samples. The compositions of oligomers were determined, and the relative reactivities of homologous comonomers in polycondensation were estimated. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1320–1324, July, 2007.     

Multiphoton ionization spectroscopy in surface analysis and laser desorption mass spectrometry

The application of resonance-enhanced multiphoton ionization (REMPI) spectroscopy for the ultrasensitive detection of molecules originating from laser desorption experiments performed on a variety of substrates is reviewed. Laser-induced desorption from surfaces is capable of producing intact gas-phase molecules, even from polar, non-volatile, high-molecular-weight and thermally labile substances. REMPI is a highly efficient and optically selective ionization method, which, coupled with laser desorption allows the direct chemical analysis of complex mixtures, without the need for previous sample purification and separation steps. The use of REMPI spectroscopy is discussed in two contexts: (1) for the direct chemical analysis of complex mixtures, e.g., environmental samples, by laser desorption/laser postionization mass spectrometry and (2) for measurements of internal state distribution of molecules laser-desorbed from sub-monolayers surface films to gain insight into the laser desorption mechanism.Presented at the 13th International Symposium on Microchemical Techniques (ISM), held in Montreux, Switzerland, May 16–20,1994     

基于同步辐射光电离质谱的燃烧与能源研究新方法

化石燃料为当今世界提供了超过80％的能源,其大量消耗所引起的能源危机和环境问题已成为全球关注的热点问题．解决此问题的关键在于深入理解燃烧和能源转化过程中的化学反应机理,进而从本质上探寻提高燃烧效率与减少污染物排放的方法．本文回顾了本课题组近年来发展的多种基于同步辐射光电离质谱技术的新方法和新装置,及其在燃烧与能源研究中的应用,并对未来可以发展的新方法和新方向进行了展望．     

Characterization of some synthetic Ru and Ir complexes by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was applied to the analysis of Ru(OCOCF(3))(2)(CO)(PPh(3))(2), Ru(OCOC(3)F(7))(2)(CO)(PPh(3))(2), Ir(tBuppy)(3) and Ir(ppy)(2)(acac) complexes. A troublesome problem in the MALDI-TOFMS characterization of these metal complexes is the possible replacement of complex ligands by matrix. In this contribution, 10 matrices, ranging from acidic to basic, were investigated: alpha-cyano-4-hydroxycinnamic acid (CHCA), 2,5-dihydroxybenzoic acid (DHB), sinapinic acid (SA), dithranol, 2,4,6-trihydroxyactophenone (THAP), 6-azo-2-thiothymine (ATT), norharman, 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB), 4-nitroaniline (NA) and 2-amino-5-nitrophyridine (ANP). With most of the matrices, including the neutral and basic ones, matrix substitution of ligand could clearly be detected. Based on the experimental results, possible mechanisms of matrix substitution were discussed. It was demonstrated that the ligand exchange process might also occur through the gas-phase reactions initiated by laser shots. Among the matrices tested, DCTB was found to be the best one for the complexes that are prone to ligand exchange by matrix.     

Detection of water-soluble vitamins by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry using porphyrin matrices

The detection of water-soluble vitamins B(1), B(2), B(6), B(12) and C by matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry (TOFMS) was attempted by studying 17 porphyrin matrices. Comparative studies of porphyrin matrices, useful mass spectral window, matrix/analyte molar ratio (M/A), ultraviolet-visible absorption characteristics and quantitative results were made. Most porphyrin matrices provide a useful mass spectral window in the low-mass range. The optimal M/A increases with increasing molecular mass of the vitamin. Vitamin B(12) possesses the highest molecular mass and requires a higher M/A. The presence of hydroxyl or carboxyl groups in the porphyrin is an indicator of a useful MALDI matrix. Vitamins B(2) and B(6) were readily ionized upon irradiation with a 337 nm laser without the use of any porphyrin matrix. Improved linearity and sensitivity of the calibration curves were obtained with samples prepared with a constant M/A. The limits of detection and quantitation are at the picomole level. The results indicate that MALDI-TOFMS with porphyrin matrices is a rapid and viable technique for the detection of low molecular mass water-soluble vitamins.     

Matrix-assisted laser desorption/ionization mass spectrometry analysis of glycans with co-derivatization of asparaginyl-oligosaccharides

As one of the most prevalent and complex post-translational modifications in biological systems, proteins glycosylation has drawn considerable attention in recent decades. Dissociation of the carbohydrates from glycoproteins may be the prerequisite step of glycomics experiments, which commonly performed by specific proteolysis. In this study, an alternative strategy was reported with nonspecific proteolysis in coupling with co-derivatization of TMPP-Ac and methylamidation for glycan moieties analysis by MALDI-MS. With the co-derivatization, a permanent positive charge was introduced to the Asn-glycans and the carboxylic groups were neutralized by methylamidation simultaneously. As a result, approximately 20 and 50-fold enhancement in the detection sensitivity was achieved for asialo-Asn and disialo-Asn respectively in comparison to their native counterparts. Ultimately, this developed strategy was successfully validated using three model glycoproteins, including ribonuclease B, ovalbumin and transferrin.     

Depth profiling of inks in authentic and counterfeit banknotes by electrospray laser desorption ionization/mass spectrometry

Electrospray laser desorption ionization is an ambient ionization technique that generates neutrals via laser desorption and ionizes those neutrals in an electrospray plume and was utilized to characterize inks in different layers of copy paper and banknotes of various currencies. Depth profiling of inks was performed on overlapping color bands on copy paper by repeatedly scanning the line with a pulsed laser beam operated at a fixed energy. The molecules in the ink on a banknote were desorbed by irradiating the banknote surface with a laser beam operated at different energies, with results indicating that different ions were detected at different depths. The analysis of authentic $US100, $100 RMB and $1000 NTD banknotes indicated that ions detected in ‘color‐shifting’ and ‘typography’ regions were significantly different. Additionally, the abundances of some ions dramatically changed with the depth of the aforementioned regions. This approach was used to distinguish authentic $1000 NTD banknotes from counterfeits. Copyright © 2015 John Wiley & Sons, Ltd.     

Analysis of lipids with desorption atmospheric pressure photoionization‐mass spectrometry (DAPPI‐MS) and desorption electrospray ionization‐mass spectrometry (DESI‐MS)

In this article, the effect of spray solvent on the analysis of selected lipids including fatty acids, fat‐soluble vitamins, triacylglycerols, steroids, phospholipids, and sphingolipids has been studied by two different ambient mass spectrometry (MS) methods, desorption electrospray ionization‐MS (DESI‐MS) and desorption atmospheric pressure photoionization‐MS (DAPPI‐MS). The ionization of the lipids with DESI and DAPPI was strongly dependent on the spray solvent. In most cases, the lipids were detected as protonated or deprotonated molecules; however, other ions were also formed, such as adduct ions (in DESI), [M‐H]⁺ ions (in DESI and DAPPI), radical ions (in DAPPI), and abundant oxidation products (in DESI and DAPPI). DAPPI provided efficient desorption and ionization for neutral and less polar as well as for ionic lipids but caused extensive fragmentation for larger and more labile compounds because of a thermal desorption process. DESI was more suitable for the analysis of the large and labile lipids, but the ionization efficiency for less polar lipids was poor. Both methods were successfully applied to the direct analysis of lipids from pharmaceutical and food products. Although DESI and DAPPI provide efficient analysis of lipids, the multiple and largely unpredictable ionization reactions may set challenges for routine lipid analysis with these methods. Copyright © 2012 John Wiley & Sons, Ltd.     

Identification of formylglycine in sulfatases by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

C(alpha)-Formylglycine, the catalytic amino acid residue in the active site of sulfatases, is generated by post-translational modification of a cysteine or serine residue. We describe a highly sensitive procedure for the detection of C(alpha)-formylglycine-containing peptides in tryptic digests of sulfatase proteins. The protocol is based on the formation of hydrazone derivatives of C(alpha)-formylglycine-containing peptides when using dinitrophenylhydrazine as a matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The hydrazone derivatives desorb and ionize with high efficiency and can be detected in the sub-femtomole range. The presence of C(alpha)-formylglycine is indicated by a mass increment of 180.13 u, corresponding to the hydrazone moiety, and also by a unique C-terminal fragment ion, characteristic of sulfatases, that becomes prominent in MALDI post-source decay mass spectra of the hydrazone derivatives.     

Regional phospholipid analysis of porcine lens membranes by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The applicability of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) to the qualitative and quantitative analysis of most mammalian phospholipid (PL) classes was demonstrated in a crude extract of porcine lens membranes. When 2,5-dihydroxybenzoic acid (DHB) was used as the matrix, positive-ion spectra allowed the accurate quantification of phosphatidylcholines (PCs) and sphingomyelins (SMs). Other PLs such as phosphatidylethanolamines (PEs), phosphatidylethanolamine plasmalogens (PEps), phosphatidylethanolamine ethers (PEes) and phosphatidylserines (PSs), could also be detected, but their lower ionization efficiency led to negative errors in their quantification. Despite this limitation, it was possible to determine relative changes among PLs extracted from cortical and nuclear regions. Negative-ion spectra were acquired with the use of p-nitroaniline (PNA) as the matrix. Because neither PCs nor SMs produce negative ions, other PL classes can be analyzed selectively. The absolute quantification of the various PL classes detectable in negative-ion spectra was also affected by differences in ionization efficiencies. However, the trends in compositional changes between cortical and nuclear-fiber PLs were in agreement with those obtained by (31)P NMR spectroscopy. MALDI-TOFMS also offers the possibility of studying variations in the acyl-chain distribution of the various species comprising each PL class. For porcine lenses, PCs, PEs and phosphatidylinositols (PIs) exhibited the greatest depletions in going from cortical to nuclear membranes. Among their individual species, those with two or more sites of unsaturation suffered the most significant reduction.     

Phosphopeptide detection and sequencing by matrix-assisted laser desorption/ionization quadrupole time-of-flight tandem mass spectrometry

A prototype matrix-assisted laser desorption/ionization quadrupole time-of-flight (MALDI-TOF) tandem mass spectrometer was used to sequence a series of phosphotyrosine-, phosphothreonine- and phosphoserine-containing peptides. The high mass resolution and mass accuracy of the instrument allowed the localization of one, three or four phosphorylated amino acid residues in phosphopeptides up to 3.1 kDa. Tandem mass spectra of two different phosphotyrosine peptides permitted amino acid sequence determination and localization of one and three phosphorylation sites, respectively. The phosphotyrosine immonium ion at m/z 216.04 was observed in these MALDI low-energy CID tandem mass spectra. Elimination of phosphate groups was evident from the triphosphorylated peptide but not from the monophosphorylated species. The main fragmentation pathway for the synthetic phosphothreonine-containing peptide and for phosphoserine-containing peptides derived from beta-casein and ovalbumin was the beta-elimination of phosphoric acid with concomitant conversion of phosphoserine to dehydroalanine and phosphothreonine to 2-aminodehydrobutyric acid. Peptide fragment ions of the b- and y-type allowed, in all cases, the localization of phosphorylation sites. Ion signals corresponding to (b-17), (b-18) and (y-17) fragment ions were also observed. The abundant neutral loss of phosphoric acid (-98 Da) is useful for femtomole level detection of phosphoserine-peptides in crude peptide mixtures generated by gel in situ digestion of phosphoproteins.     

Derivatization of small biomolecules for optimized matrix-assisted laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) is a powerful tool for the measurement of low molecular mass compounds of biological interest. The limitations for this method are the volatility of many analytes, possible interference with matrix signals or bad ionization or desorption behavior of the compounds. We investigated the application of well-known and straightforward one-pot derivatization procedures to circumvent these problems. The derivatizations tested allow the measurement and the labeling of alcohols, aldehydes and ketones, carboxylic acids, alpha-ketocarboxylic acids and amines.     

The characterization of selected drugs with infrared laser desorption/tunable synchrotron vacuum ultraviolet photoionization mass spectrometry

Some selected drugs including captopril, fudosteine and racecadotril have been analyzed by infrared (IR) laser desorption/tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (PIMS). The molecular ions of captopril and racecadotril are exclusively observed without any fragments at near threshold single-photon ionization (SPI). However, fudosteine easily forms fragments even at a photon energy near the ionization threshold, indicating the instability of its molecular ion. For these drugs, a number of fragments are yielded with the increase of photon energy. The structures of such fragments proposed by IR LD/VUV PIMS are supported by electron ionization time-of-flight mass spectrometry (EI-TOFMS) results. Fragmentation pathways are discussed in detail.