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.     

Critical assessment of ionization patterns and applications of ambient desorption/ionization mass spectrometry using FAPA–MS

Ambient desorption/ionization mass spectrometry (MS) has gained growing interest during the last decade due to its high analytical performance and yet simplicity. Here, one of the recently developed ambient desorption/ionization MS sources, the flowing atmospheric‐pressure afterglow (FAPA) source, was investigated in detail regarding background ions and typical ionization patterns in the positive as well as the negative ion mode for a variety of compound classes, comprising alkanes, alcohols, aldehydes, ketones, carboxylic acids, organic peroxides and alkaloids. A broad range of signals for adducts and losses was found, besides the usually emphasized detection of quasimolecular ions, i.e. [M + H]⁺ and [M ? H]^? in the positive and the negative mode, respectively. It was found that FAPA–MS is best suited for polar analytes containing nitrogen and/or oxygen functionalities, e.g. carboxylic acids, with low molecular weights and relatively high vapor pressures. In addition, the source was used in proof‐of‐principle studies, illustrating the capabilities and limitations of the technique: Firstly, traces of cocaine were detected and unambiguously identified on euro banknotes using FAPA ionization in combination with tandem MS, suggesting a correlation between cocaine abundance and age of the banknote. Secondly, FAPA–MS was used for the identification of acidic marker compounds in organic aerosol samples, indicating yet‐undiscovered matrix and sample surface effects of ionization pathways in the afterglow region. Copyright © 2016 John Wiley & Sons, Ltd.     

Desorption electrospray ionization ‐ orbitrap mass spectrometry of synthetic polymers and copolymers

Desorption ElectroSpray Ionization (DESI) ‐ Orbitrap Mass Spectrometry (MS) was evaluated as a new tool for the characterization of various industrial synthetic polymers (poly(ethylene glycol), poly(propylene glycol), poly(methylmethacrylate), poly(dimethylsiloxane)) and copolymers, with masses ranging from 500 g.mol⁻¹ up to more than 20 000 g.mol⁻¹. Satisfying results in terms of signal stability and sensitivity were obtained from hydrophobic surfaces (HTC Prosolia) with a mixture water/methanol (10/90) as spray solvent in the presence of sodium salt. Taking into account the formation of multiplied charged species by DESI‐MS, a strategy based on the use of a deconvolution software followed by the automatic assignment of the ions was described allowing the rapid determination of M_n, M_w and PDI values. DESI‐Orbitrap MS results were compared to those obtained from matrix‐assisted laser desorption/ionization‐ time‐of‐flight MS and gel permeation chromatography. An application of DESI‐Orbitrap MS for the detection and identification of polymers directly from cosmetics was described. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization and application of a self‐aspirating electrospray source with pneumatic‐assisted ionization

A single gas‐assisted electrospray ion source developed for ambient mass spectrometry is introduced in this paper. Simultaneous self‐aspiration and electrospray could be achieved by using a constant sheath gas flow supplied from a mini air pump. A gas dynamic study of the spray module is carried out for structural optimization. The entire device exhibits a simplified design and has been systematically characterized through both simulated and experimental investigations. According to the results, the ion source exhibited satisfactory stability and the ability for quantitative operation in routine electrospray ionization mass spectrometry. Furthermore, the ion source can be operated as a desorption electrospray ionization source to perform direct desorption/ionization of the solid samples. The versatile source described here appears to provide a practical approach to perform ambient mass spectrometry analysis with unrestricted sampling operation, and the extensive gas dynamic studies together with the experimental characterization are believed to be helpful in building self‐aspirating spray devices. Copyright © 2017 John Wiley & Sons, Ltd.     

Direct monitoring of drug degradation by easy ambient sonic-spray ionization mass spectrometry: the case of enalapril

Using enalapril maleate as a test case, the ability of ambient mass spectrometry, namely, via easy ambient sonic‐spray ionization mass spectrometry (EASI‐MS), to perform direct monitoring of drug degradation has been tested. Two manufacturing processes were investigated (direct compression and wet granulation), and the formation of degradation products was measured via both EASI‐MS and high‐performance liquid chromatography with ultraviolet detection for a total period of 18 months. Both techniques provide comparable results, which indicate that direct analysis by ambient mass spectrometric techniques presents a viable alternative for drug degradation monitoring with superior simplicity, throughput, and reliability (no sample manipulation), and comparable quantitative results. In terms of qualitative monitoring, the full mass spectra with intact species provided by EASI‐MS allow for comprehensive monitoring of known and unknown (or unexpected) degradation products. Copyright © 2011 John Wiley & Sons, Ltd.     

Fundamental properties of a touchable high‐power pulsed microplasma jet and its application as a desorption/ionization source for ambient mass spectrometry

Plasma‐based ambient desorption/ionization mass spectrometry (ADI–MS) has attracted considerable attention in many fields because of its capacity for direct sample analyses. In this study, a high‐power pulsed microplasma jet (HPPMJ) was developed and investigated as a new plasma desorption/ionization source. In an HPPMJ, a microhollow cathode discharge is generated in a small hole (500 µm in diameter) using a pulsed high‐power supply. This system can realize a maximum power density of 5 × 10⁸ W/cm³. The measured electron number density, excitation temperature and afterglow gas temperature of the HPPMJ were 3.7 × 10¹⁵ cm^‐3, 7000 K at maximum and less than 60 °C, respectively, which demonstrate that the HPPMJ is a high‐energy, high‐density plasma source that is comparable with an argon inductively coupled plasma while maintaining a low gas temperature. The HPPMJ causes no observable damage to the target because of its low gas temperature and electrode configuration; thus, we can apply it directly to human skin. To demonstrate the analytical capacity of ADI–MS using an HPPMJ, the plasma was applied to direct solid sample analysis of the active ingredients in pharmaceutical tablets. Caffeine, acetaminophen, ethenzamide, isopropylantipyrine and ibuprofen were successfully detected. Application to living tissue was also demonstrated, and isopropylantipyrine on a finger was successfully analyzed without damaging the skin. The limits of detection (LODs) for caffeine, isopropylantipyrine and ethenzamide were calculated, and LODs at the picogram level were achieved. These results indicate the applicability of the HPPMJ for high‐sensitivity analysis of materials on a heat‐sensitive surface. Copyright © 2014 John Wiley & Sons, Ltd.     

Structural motifs in primary oxidation products of palmitoyl‐arachidonoyl‐phosphatidylcholines by LC‐MS/MS

Oxidative modifications to phospholipids (OxPL) play a major role in modulating signaling events in inflammation and infection, and complete understanding on the induced biological effects can only be understood based on knowledge of the oxidative motifs present. Specific neutral losses observed in tandem mass spectrometry data (LC‐MS/MS) of primary peroxidation products in oxidized palmitoyl‐arachidonoyl‐phosphatidylcholines (OxPAPC) provide information on the prevailing structural motifs regarding the oxidized acyl carbon chain, the nature of oxidized group and the site of carbon oxidation. The higher hydrophobicity of hydroperoxides compared to di‐hydroxy derivatives under reverse‐phase conditions together with specific fragmentation patterns enabled the identification of 12 structurally different OxPAPC structural (di‐hydroxy and hydroperoxide derivatives) and positional isomers as well as the presence of poly‐hydroxy together with isoprostanes derivatives. The fragmentation patterns described in quadrupole time‐of‐flight and linear ion trap instruments complement the m/z value and retention time parameters in the identification of oxidative composition in OxPAPC products becoming a valuable tool for the exploratory screening of oxidized phosphatidylcholines in OxPAPC extracts, distinction of native and modified PC isobaric structures in complex samples contributing to the increased understanding of redox lipidomics in inflammation and infection. Copyright © 2013 John Wiley & Sons, Ltd.     

基体辅助激光解吸/电离质谱测定小相对分子质量物质的方法进展

MALD I-MS分析具有高灵敏度、高分辨率、高质量准确度、制样快速、操作简单及高通量等优点,具有使各种不同类型物质离子化的能力。目前已广泛应用于化学、化工、材料、环境、地质、能源、刑侦、药物、生命科学等领域中。MALD I-MS最早主要用于分析各种生物大分子及聚合物。近年     

Analysis of impurities in pharmaceuticals by LC‐MS with cold electron ionization

Pharmaceuticals require careful and precise determination of their impurities that might harm the user upon consumption. Although today, the most common technique for impurities identification is liquid chromatography‐mass spectrometry (LC‐MS/MS), it has several downsides due to the nature of the ionization method. Also, the analyses in many cases are targeted thus despite being present, some of the compounds will not be revealed. In this paper, we propose and show a new method for untargeted analysis and identification of impurities in active pharmaceutical ingredients (APIs). The instrument used for these analyses is a novel electron ionization (EI) LC‐MS with supersonic molecular beams (SMB). The EI‐LC‐MS‐SMB was implemented for analyses of several drug samples spiked with an impurity. The instrument provides EI mass spectra with enhanced molecular ions, named Cold EI, which increases the identification probabilities when the compound is identified with the aid of an EI library like National Institute of Standards and Technology (NIST). We analyzed ibuprofen and its impurities, and both the API and the expected impurity were identified with names and structures by the NIST library. Moreover, other unexpected impurities were found and identified proving the ability of the EI‐LC‐MS‐SMB system for truly untargeted analysis. The results show a broad dynamic range of four orders of magnitude at the same run with a signal‐to‐noise ratio of over 10 000 for the API and almost uniform response.     

Intermediate detection in real time using reactive surface desorption dielectric‐barrier discharge ionization mass spectrometry

Reactive intermediates play key roles for reaction mechanism elucidation. A suitable tool for identifying the key intermediates is crucial and highly desirable. In this study, surface desorption dielectric‐barrier discharge ionization (reactive SDDBDI) was developed for characterization of the reactive intermediates. In reactive SDDBDI, the plasma is doped with a reagent before the plasma ions are directed at a cover slip surface bearing another analyte. Different from SDDBDI, reactive SDDBDI can be used both as an ambient ionization source and as a means to produce reagent ions for ambient ion/molecule reactions. The online derivation of 4‐aminophenol with trifluoroacetic anhydride demonstrated that reactive SDDBDI can be used for chemical analysis where improved specificity or sensitivity is required. The utility of this approach for real‐time detection of reactive intermediate was demonstrated by the Schiff‐base and Eberlin reactions. The formed intermediates and products could be readily detected and identified by tandem mass spectrometry. These results indicate that reactive SDDBDI can be used to generate reagent ions that undergo ion/molecule reactions in the open air with an analyte at condensed phase on a surface. Reactive SDDBDI has high‐efficiency ion transmission and high MS sensitivity. It is thus a potential tool to perform ambient ion/molecule reactions and detect reactive intermediates.     

Laser desorption dual spray post‐ionization mass spectrometry for direct analysis of samples via two informative channels

A laser desorption dual spray post‐ionization mass spectrometry method is described, and its usefulness is demonstrated with the examples of selective detection of food components, manipulation of protein charge state distribution and investigation on the formation of magic number clusters. The method is carried out by adopting two spray emitters for post‐ionization of analytes desorbed by a pulsed infrared laser. Various components in a complex sample or distinct behavior of an analyte in two different spray reagents can be rapidly probed by the method quasi‐simultaneously, highlighting the potential applications of this method for protein characterization, reaction study and food analysis. Copyright © 2013 John Wiley & Sons, Ltd.     

Effective detection of peptides containing cysteine sulfonic acid using matrix-assisted laser desorption/ionization and laser desorption/ionization on porous silicon mass spectrometry

Cysteine sulfonic acid-containing peptides, being typical acidic peptides, exhibit low response in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. In this study, matrix conditions and the effect of diammonium hydrogencitrate (DAHC) as additive were investigated for ionization of cysteine sulfonic acid-containing peptides in MALDI. A matrix-free ionization method, desorption/ionization on porous silicon (DIOS), was also utilized to evaluate the effect of DAHC. When equimolar three-component mixtures of peptides carrying free cysteine, cysteine sulfonic acid, and carbamidomethyl cysteine were measured by MALDI using a common matrix, alpha-cyano-4-hydroxycinnamic acid (CHCA), no signal corresponding to cysteine sulfonic acid-containing peptide could be observed in the mass spectrum. However, by addition of DAHC to CHCA, the peaks of cysteine sulfonic acid-containing peptides were successfully observed, as well as when using 2,4,6-trihydroxyacetophenone (THAP) and 2,6-dihydroxyacetophenone with DAHC. In the DIOS mass spectra of these analytes, the use of DAHC also enhanced the peak intensity of the cysteine sulfonic acid-containing peptides. On the basis of studies with these model peptides, tryptic digests of oxidized peroxiredoxin 6 were examined as a complex peptide mixture by MALDI and DIOS. In MALDI, the peaks of cysteine sulfonic acid-containing peptides were observed when using THAP/DAHC as the matrix, but this was not so with CHCA. In DIOS, the signal from cysteine sulfonic acid-containing peptides was suppressed; however, the use of DAHC significantly enhanced the signal intensity with an increase in the number of observed peptides and increased signal-to-noise ratio in the DIOS spectra. The results show that DAHC in the matrix or on the DIOS chip decreases discrimination and suppression effects in addition to suppressing alkali-adduct ions, which leads to a beneficial effect on protonation of peptides containing cysteine sulfonic acid.     

Positive and negative‐mode laser desorption/ionization‐mass spectrometry (LDI‐MS) for the detection of indigoids in archaeological purple

Laser‐based ionization techniques have demonstrated to be a valuable analytical tool to study organic pigments by mass spectrometric analyses. Though laser‐based ionization techniques have identified several natural and synthetic organic dyes and pigments, they have never been used in the characterization of purple. In this work, positive and negative‐mode laser desorption/ionization mass spectrometry (LDI‐MS) was used for the first time to detect indigoids in shellfish purple. The method was used to study organic residues collected from archaeological ceramic fragments that were known to contain purple, as determined by a classical high‐performance liquid chromatography‐based procedure. LDI‐MS provides a mass spectral fingerprint of shellfish purple, and it was found to be a rapid and successful tool for the identification of purple. In addition, a comparison between positive and negative mode ionization highlighted the complementarity of the two ionization modes. On the one hand, the negative‐ion mode LDI‐MS showed a better selectivity and sensitivity to brominated molecules, such as 6,6'‐dibromoindigo, 6‐monobromoindigo, 6,6'‐dibromoindirubin, 6‐ and 6’‐monobromoindirubin, thanks to their electronegativity, and produced simpler mass spectra. On the other hand, negative‐ion mode LDI‐MS was found to have a lower sensitivity to non‐brominated compounds, such as indigo and indirubin, whose presence can be established in any case by collecting the complementary positive‐ion LDI mass spectrum. Copyright © 2013 John Wiley & Sons, Ltd.     

A quantitative model of ultraviolet matrix-assisted laser desorption/ionization

A quantitative model of primary ionization in ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) is presented. It includes not only photochemical processes such as exciton pooling, but also the effects of the desorption event. The interplay of these two is found to be a crucial aspect of the MALDI process. The desorbing plume is modeled as an adiabatic expansion with entrained clusters. The parameters in the model are defined as much as possible via experiment or by analogy with known effects. The model was applied to the matrix 2,5-dihydroxybenzoic acid and found to reproduce the fluence dependence of the fluorescence yield and key features of the picosecond two-pulse ion generation efficiency curves. In addition, the model correctly predicts a fluence rather than irradiance threshold, the magnitude of the threshold, the magnitude of the ion yield, laser wavelength effects, plume temperatures, plume expansion velocities and the spot size effect.     

Radical Desorption Kinetics in Emulsion Polymerization, 2 – Brownian Dynamics Simulation of Radical Desorption in Non‐Homogeneous Particles

One of the most important events in free‐radical emulsion polymerization is desorption of radicals from the polymer particles to the aqueous phase. Desorption takes place by diffusion of radicals inside the particle toward the surface and transfer to the aqueous phase. The rate of desorption can be determined theoretically for homogeneous spherical particles. For more complex cases, analytical solutions become difficult or impossible to obtain and a numerical approach is better suited for estimating desorption rate coefficients. In this paper, Brownian dynamics simulation is used for the estimation of desorption rate coefficients in emulsion polymerization systems of increased complexity, in particular for non‐homogeneous polymer particles.

     

Mass spectrometric analysis of low molecular mass polyesters by laser desorption/ionization on porous silicon

A low molecular mass polyester was analyzed by desorption/ionization on porous silicon (DIOS) mass spectrometry. The results were compared with those of matrix-assisted laser desorption ionization (MALDI) mass spectrometry using matrixes of alpha-cyano-4-hydroxycinnamic acid (CHCA) and 10,15,20-tetrakis(pentafluorophenyl)porphyrin (F20TPP). The CHCA matrix was not suitable for characterization of low molecular mass components of the polyester because the matrix-related ions interfered with the component ions. On the other hand, the F20TPP matrix showed no interference because no matrix-related ions appeared below m/z 822. However, the solvent selection for determining optimal conditions of sample preparation was limited, because F20TPP does not dissolve readily in any of the available organic solvents. In the DIOS spectra, the polymer ions were observed at high sensitivity without a contaminating ion. No matrix is needed for DIOS spectra of low molecular mass polyesters, facilitating sample preparation and selectivity of a precursor ion in post-source decay measurements.     

人参皂苷的基质辅助激光解吸质谱研究

利用MALDI-TOFMS测定了八种人参皂苷的分子量, 并分析了西洋参总皂苷的组成。同时, 进行了灵敏度实验, 并探讨了基质及碱金属离子的影响, 证明该方法灵敏度高, 重复性好, 结果准确。是测定极性小分子分子量的有效方法。     

Two-laser infrared and ultraviolet matrix-assisted laser desorption/ionization

Matrix-assisted laser desorption/ionization (MALDI) was performed using two pulsed lasers with wavelengths in the IR and UV regions. A 10.6 micro m pulsed CO(2) laser was used to irradiate a MALDI target, followed after an adjustable delay by a 337 nm pulsed nitrogen laser. The sample consisted of a 2,5-dihydroxybenzoic acid matrix and bovine insulin guest molecule. The pulse energy for both of the lasers was adjusted so that the ion of interest, either the matrix or guest ion, was not produced by either of the lasers alone. The delay time for maximum ion yield occurs at 1 micro s for matrix and guest ions and the signal decayed to zero in approximately 400 micro s. A mechanism is presented for enhanced UV MALDI ion yield following the IR laser pulse based on transient heating.     

Desorption/ionization on porous silicon mass spectrometry (DIOS) of model cationized fatty acids

Desorption/ionization on porous silicon (DIOS) is a very useful technique in the case of small molecular weight compounds, compared to the matrix-assisted laser desorption ionization (MALDI). This is because MALDI generates matrix-related ions that overlap with the mass range of interest. The aim of our work was to investigate the suitability of the DIOS technique in the case of fatty acids in negative ion mode. The analysis of the chosen fatty acid models, nonadecanoic acid (C(19)H(38)O(2)) and heneicosanoic acid (C(21)H(42)O(2)), gave rise to the observation of the deprotonated monomeric species and selective cationized multimeric species. This cation selectivity was further elucidated by complementary studies based on the addition of various metals such as Ag(I), Zn(II), Fe(II), and also Cu(II). Specific behavior, depending upon the introduced metal, was highlighted by different redox reaction processes and also metastable decompositions (in PSD mode).     

基体辅助激光解吸/电离质谱法对核酸分子的分析测定

基体辅助激光解吸／电离质谱（ＭＡＬＤＩＭＳ）法自８０年代末由Ｋａｒａｓ,Ｈｉｌｅｎｋａｍｐ报道以来,已获得极大的发展．近几年来,其应用范围迅速扩大,各种生物大分子如蛋白质,核酸（ＤＮＡ）,多糖等都已能用ＭＡＬＤＩＭＳ法进行分子量测定．１９９３年,...