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

Rapid and sensitive approaches for detecting food fraud: A review on prospects and challenges

Precise and reliable analytical techniques are required to guarantee food quality in light of the expanding concerns regarding food safety and quality. Because traditional procedures are expensive and time-consuming, quick food control techniques are required to ensure product quality. Various analytical techniques are used to identify and detect food fraud, including spectroscopy, chromatography, DNA barcoding, and inotrope ratio mass spectrometry (IRMS). Due to its quick findings, simplicity of use, high throughput, affordability, and non-destructive evaluations of numerous food matrices, NI spectroscopy and hyperspectral imaging are financially preferred in the food business. The applicability of this technology has increased with the development of chemometric techniques and near-infrared spectroscopy-based instruments. The current research also discusses the use of several multivariate analytical techniques in identifying food fraud, such as principal component analysis, partial least squares, cluster analysis, multivariate curve resolutions, and artificial intelligence.

Ambient mass spectrometry imaging of food natural products by angled direct analysis in real time high-resolution mass spectrometry

Direct surface analysis in ambient conditions provides information on the position and chemical composition of an object at the time of investigation. An angled sampling probe is developed in this work for direct analysis in real time (DART) ionization high-resolution mass spectrometry. The DART ion source and the interface were modified for improved surface resolution, increased ion transfer efficiency, as well as enabling two-dimensional surface scanning. The angled probe DART-MS system was used for investigating a variety of food samples including fruit peels, ginseng root, plant leaves and sections of radish. Abundant signals and distinct chemical profiles are obtained in seconds, and spatial distribution of different molecules across the sample surfaces can be observed. In addition, the developed system can quickly identify the chemical changes when the surfaces were treated. The method is capable of directly evaluating food sample surfaces with different shapes, hardness, and conditions, without any sample pretreatments.

Characterisation of hepatic lipid signature distributed across the liver zonation using mass spectrometry imaging

Background & Aims

Lipid metabolism plays an important role in liver pathophysiology. The liver lobule asymmetrically distributes oxygen and nutrition, resulting in heterogeneous metabolic functions. Periportal and pericentral hepatocytes have different metabolic functions, which lead to generating liver zonation. We developed spatial metabolic imaging using desorption electrospray ionisation mass spectrometry to investigate lipid distribution across liver zonation with high reproducibility and accuracy.

Methods

Fresh frozen livers from healthy mice with control diet were analysed using desorption electrospray ionisation mass spectrometry imaging. Imaging was performed at 50 μm × 50 μm pixel size. Regions of interest (ROIs) were manually created by co-registering with histological data to determine the spatial hepatic lipids across liver zonation. The ROIs were confirmed by double immunofluorescence. The mass list of specific ROIs was automatically created, and univariate and multivariate statistical analysis were performed to identify statistically significant lipids across liver zonation.

Results

A wide range of lipid species was identified, including fatty acids, phospholipids, triacylglycerols, diacylglycerols, ceramides, and sphingolipids. We characterised hepatic lipid signatures in three different liver zones (periportal zone, midzone, and pericentral zone) and validated the reproducibility of our method for measuring a wide range of lipids. Fatty acids were predominantly detected in the periportal region, whereas phospholipids were distributed in both the periportal and pericentral zones. Interestingly, phosphatidylinositols, PI(36:2), PI(36:3), PI(36:4), PI(38:5), and PI(40:6) were located predominantly in the midzone (zone 2). Triacylglycerols and diacylglycerols were detected mainly in the pericentral region. De novo triacylglycerol biosynthesis appeared to be the most influenced pathway across the three zones.

Conclusions

The ability to accurately assess zone-specific hepatic lipid distribution in the liver could lead to a better understanding of lipid metabolism during the progression of liver disease.

Impact and Implications

Zone-specific hepatic lipid metabolism could play an important role in lipid homoeostasis during disease progression. Herein, we defined the zone-specific references of hepatic lipid species in the three liver zones using molecular imaging. The de novo triacylglycerol biosynthesis was highlighted as the most influenced pathway across the three zones.

Ambient Ionization Mass Spectrometry: Application and Prospective

Mass spectrometry (MS) is a formidable analytical tool for the analysis of non-polar to polar compounds individually and/or from mixtures, providing information on the molecular weights and chemical structures of the analytes. During the last more than one-decade, ambient ionization mass spectrometry (AIMS) has developed quickly, producing a wide range of platforms and proving scientific improvements in a variety of domains, from biological imaging to quick quality control. These methods have made it possible to detect target analytes in real time without sample preparation in an open environment, and they can be connected to any MS system with an atmospheric pressure interface. They also have the ability to analyze explosives, illicit drugs, disease diagnostics, drugs in biological samples, adulterants in food and agricultural products, reaction progress, and environmental monitoring. The development of novel ambient ionization techniques, such as probe electrospray ionization, paper spray ionization, and fiber spray ionization, employed even at picolitre to femtolitre solution levels to provide femtogram to attogram levels of the target analytes. The special characteristic of this ambient ion source, which has been extensively used, is the noninvasive property of PESI of examination of biological real samples. The results in the current review supports the idea that AIMS has emerged as a pioneer in MS-based approaches and that methods will continue to be developed along with improvements to existing ones in the near future.

Characterization of effective phytochemicals in traditional Chinese medicine by mass spectrometry

Traditional Chinese medicines (TCMs) have been widely used in clinical and healthcare applications around the world. The characterization of the phytochemical components in TCMs is very important for studying the therapeutic mechanism of TCMs. In the analysis process, sample preparation and instrument analysis are key steps to improve analysis performance and accuracy. In recent years, chromatography combined with mass spectrometry (MS) has been widely used for the separation and detection of trace components in complex TCM samples. This article reviews various sample preparation techniques and chromatography-MS techniques, including the application of gas chromatography-MS and liquid chromatography-MS and other MS techniques in the characterization of phytochemicals in TCM materials and Chinese medicine products. This article also describes a new ambient ionization MS method for rapid and high-throughput analysis of TCM components.

Protonated acetone ion chemical ionization time-of-flight mass spectrometry for real-time measurement of atmospheric ammonia

Ammonia (NH₃) is ubiquitous in the atmosphere, it can affect the formation of secondary aerosols and particulate matter, and cause soil eutrophication through sedimentation. Currently, the use of radioactive primary reagent ion source and the humidity interference on the sensitivity and stability are the two major issues faced by chemical ionization mass spectrometer (CIMS) in the analysis of atmospheric ammonia. In this work, a vacuum ultraviolet (VUV) Kr lamp was used to replace the radioactive source, and acetone was ionized under atmospheric pressure to obtain protonated acetone reagent ions to ionize ammonia. The ionization source is designed as a separated three-zone structure, and even 90 vol.% high-humidity samples can still be directly analyzed with a sensitivity of sub-ppbv. A signal normalization processing method was designed, and with this new method, the quantitative relative standard deviation (RSD) of the instrument was decreased from 17.5% to 9.1%, and the coefficient of determination was increased from 0.8340 to 0.9856. The humidity correction parameters of the instrument were calculated from different humidity, and the ammonia concentrations obtained under different humidity were converted to its concentration under zero humidity condition with these correction parameters. The analytical time for a single sample is only 60 sec, and the limit of detection (LOD) was 8.59 pptv (signal-to-noise ratio S/N = 3). The ambient measurement made in Qingdao, China, in January 2021 with this newly designed CIMS, showed that the concentration of ammonia ranged from 1 to 130 ppbv.

Effects of dopants in the imaging of mouse brain by desorption electrospray ionization/post-photoionization mass spectrometry

Desorption electrospray ionization/post-photoionization (DESI/PI) is a newly developed ionization method by the combination of DESI and post-photoionization for the simultaneous imaging of polar and nonpolar compounds in biological tissues. Dopants are of great importance in DESI/PI for the enhancement of signal intensities through ion-molecule reactions. In this work, to evaluate the performance of dopants in DESI/PI, an efficient homogenate model was developed, and four kinds of dopants (toluene, chlorobenzene, bromobenzene, and anisole) were tested using homogenate of mouse brain tissue as target sample. The influences of the dopants on the signal enhancements of different compounds were explained reasonably by the ionization mechanism. Then, the dopants with their optimum volume contents were applied to the mass spectrometry imaging (MSI). For a comprehensive imaging of various compounds with different polarities, methanol/toluene/formic acid (7:3:0.1) was chosen as the best choice. Finally, the stronger quantitative ability of DESI/PI with toluene as dopant for a few compounds in mouse brain tissue was demonstrated.

Imaging of Polar and Nonpolar Lipids Using Desorption Electrospray Ionization/Post-photoionization Mass Spectrometry

Desorption electrospray ionization mass spectrometry imaging (DESI-MSI) can record 2D distribution of polar lipids in tissue slices at ambient condition. However, sensitivity of DESI-MSI for nonpolar lipids is restricted by low ionization efficiency and severe ion suppression. Here, a compact post-photoionization assembly combined with DESI (DESI/PI) was developed for simultaneous imaging polar and nonpolar lipids in tissue sections by switching off/on a portable krypton lamp. Compared with DESI, higher signal intensities of nonpolar compounds could be detected with DESI/PI. We describe the fabrication, optimization, implementation, and data transformation for imaging both the polar and nonpolar lipids in mouse brain tissue using an Agilent 6224 Accurate-Mass TOF mass spectrometer. More than ten nonpolar lipids including cholesterol and GalCer lipids were detected by DESI/PI in the positive ion mode, compared with that by DESI. In the negative-ion mode, ion yields of DESI/PI for lipids (HexCer, PE, and PE-P) were also increased by several folds.

Dilute-and-shoot-based direct nano-electrospray ionization tandem mass spectrometry as screening methodology for multivitamins in dietary supplement and human urine

INTRODUCTION

In recent years, analytical screening methods for simultaneous detection of multivitamins have gained substantial attention to ensure quality and public confidence in dietary supplements. Even so, few analytical methods have been proposed for simultaneous analysis of multivitamin constituents due to the large divergence in chemical characteristics.

OBJECTIVES

In the present study, the objective was to develop a simple and rapid direct nano-electrospray ionization-tandem mass spectrometry (DI-nano-ESI-MS/MS) method for targeted detection of water soluble vitamins, fat soluble vitamins, amino acids, royal jelly, ginkgo biloba, and ginseng in a dietary supplement. The applicability of dilute-and-shoot-based DI-nano-ESI-MS/MS to analyze the same tested compounds and their related metabolites in clinical samples was also examined.

METHODS

Intact urine mixed with the ionization solvent was loaded (4-μL aliquot) into a nanospray (NS) capillary of 1-μm tip diameter. The NS capillary was then fitted into an off-line ion source at a distance of 5 mm from MS aperture. The sample was directly injected by applying a voltage of 1.1 kV, producing a numerous of m/z peaks for analysis in mere minutes.

RESULTS

The DI-nano-ESI-MS/MS method successfully identified almost all dietary supplement components, as well as a plethora of component-related metabolites in clinical samples. In addition, a new merit of the proposed method for the detection of index marker and chemical contaminants as well as subspecies identification was investigated for further quality evaluation of the dietary supplement.

CONCLUSIONS

The previous findings illustrated that DI-nano-ESI-MS/MS approach can emerge as a powerful, high throughput, and promising analytical tool for screening and accurate detection of various pharmaceuticals and ingredient in dietary supplements as well as biological fluids.

Desorption Atmospheric Pressure Photoionization Coupled with Ion Mobility-Mass Spectrometry

Desorption atmospheric photoionization (DAPPI) is an ambient mass spectrometry (MS) technique that can be used to analyze both polar and nonpolar compounds. Here, the coupling of DAPPI with traveling wave ion mobility-mass spectrometry (TWIM-MS) and application to analysis of food, multivitamin, and pharmaceutical products is described.

Chemical profiles of birch and alder bark by ambient mass spectrometry

Desorption atmospheric pressure photoionization (DAPPI) is an ambient mass spectrometry (MS) technique that allows the analysis of both polar and nonpolar compounds directly from the surfaces of various sample types. Here, DAPPI was used to study the chemical profiles in different parts of birch and alder tree barks. Four distinct fractions of Betula pendula (silver birch) bark were collected from three different developmental stages of the stem, after which the chemical profiles of the different tissue types were measured. Of special interest were triterpenoids, a class of important defensive substances, which are found in the bark of the silver birch. Additionally, the chemical profiles of lenticels and the surrounding surfaces in the phellem of B. pendula (silver birch), Alnus glutinosa (black alder), and Alnus incana (gray alder) were screened with DAPPI. Another ambient MS technique, laser ablation atmospheric pressure photoionization (LAAPPI), was further used for the mass spectrometry imaging of lenticels on the B. pendula phellem. All the studied birch bark fractions showed individual chemical profiles in DAPPI. The mass spectra from the young apical stem and the transition zone resembled each other more than the mature stem. Instead, the phellem was found to contain a high amount of triterpenoids in all the developmental stages of the stem. The most intense peaks in the DAPPI mass spectra of the birch bark fractions were those of betulin and lupeol. Betulinic and betulonic acid peaks were intense as well, and these compounds were detected especially in the lenticels of the tree samples.

Graphene-Coated Glass Substrate for Continuous Wave Laser Desorption and Atmospheric Pressure Mass Spectrometric Imaging of a Live Hippocampal Tissue

The atmospheric pressure mass spectrometric (AP-MS) imaging technology combined with an inverted optical microscopic system is a powerful tool for determining the presence and spatial distributions of specific biomolecules of interest in live tissues. Efficient desorption and ionization are essential to acquire mass spectrometric (MS) information in an ambient environment. In this study, we demonstrate a new and efficient desorption process using a graphene-coated glass substrate and a continuous wave (CW) laser for high-resolution AP-MS imaging of a live hippocampal tissue. We found that desorption of biomolecules in a live tissue slice was possible with the aid of a graphene-coated glass substrate and indirect application of a 532 nm CW laser on the graphene substrate. Interestingly, the desorption efficiency of a live tissue on the graphene-coated substrate was strongly dependent on the number of graphene layers. Single-layer graphene was found to be the most sensitive substrate for efficient desorption and reproducible high-resolution hippocampal tissue imaging applications. The subsequent ionization process using nonthermal plasma generated sufficient amounts of molecular ions to obtain high-resolution two-dimensional MS images of the cornu ammonis and the dentate gyrus regions of the hippocampus. Therefore, graphene-coated substrates could be a promising platform to induce an efficient desorption process essential for highly reproducible ambient MS imaging.

Sweep Jet Collection Laser-Induced Acoustic Desorption Atmospheric Pressure Photoionization for Lipid Analysis Applications

Laser-induced acoustic desorption coupled to microplasma-based atmospheric pressure photoionization (LIAD-APPI) using a nebulized sweep jet to aid in dopant introduction and ion transmission has been applied to the analysis of model, apolar lipid compounds. Specifically, several sterols, sterol esters, and triacylglycerols were detected using dopants such as anisole and toluene. Additionally, several triacylglycerols, sterols, carboxylic acids, and hopanoids were detected from complex mixtures of olive oil and Australian shale rock extract as a first demonstration of the applicability of LIAD-APPI on real-world samples. Detection limits using a sweep jet configuration for α-tocopherol and cholesterol were found to be 609 ± 61 and 292 ± 29 fmol, respectively. For sterol esters and triacylglycerols with a large number of double bonds in the fatty acid chain, LIAD-APPI was shown to yield greater molecular ion or [M+NH₄]⁺ abundances than those with saturated fatty acid chains. Dopants such as anisole and toluene, with ionization potentials (IPs) of 8.2 and 8.8 eV, respectively, were tested. A greater degree of fragmentation with several of the more labile test compounds was observed using toluene. Overall, LIAD-APPI with a nebulized sweep jet requires minimal sample preparation and is a generally useful and sensitive analysis technique for low-polarity mixtures of relevance to biochemical assays and geochemical profiling. Graphical Abstract.

Imaging of Polar and Nonpolar Species Using Compact Desorption Electrospray Ionization/Postphotoionization Mass Spectrometry

Desorption electrospray ionization (DESI) mass spectrometry imaging (MSI) can simultaneously record the 2D distribution of polar biomolecules in tissue slices at ambient conditions. However, sensitivity of DESI-MSI for nonpolar compounds is restricted by low ionization efficiency and strong ion suppression. In this study, a compact postphotoionization assembly combined with DESI (DESI/PI) was developed for imaging polar and nonpolar molecules in tissue sections by switching off/on a portable krypton lamp. Compared with DESI, higher signal intensities of nonpolar compounds could be detected with DESI/PI. To further increase the ionization efficiency and transport of charged ions of DESI/PI, the desorption solvent composition and gas flow in the ionization tube were optimized. In mouse brain tissue, more than 2 orders of magnitude higher signal intensities for certain neutral biomolecules like creatine, cholesterol, and GalCer lipids were obtained by DESI/PI in the positive ion mode, compared with that of DESI. In the negative ion mode, ion yields of DESI/PI for glutamine and some lipids (HexCer, PE, and PE-O) were also increased by several-fold. Moreover, nonpolar constituents in plant tissue, such as catechins in leaf shoots of tea, could also be visualized by DESI/PI. Our results indicate that DESI/PI can expand the application field of DESI to nonpolar molecules, which is important for comprehensive imaging of biomolecules in biological tissues with moderate spatial resolution at ambient conditions.

Hydralazine derivative of aldehyde: A new type of [M - H]+ ion formed in electrospray ionization mass spectrometry

Hydralazine has been widely employed in the development of drugs, derivatization reagents, and ligands. In the present work, we reported a new type of dehydrogenated ion [M - H]⁺ that was produced from the hydralazine derivative of hexanal in electrospray ionization mass spectrometry (ESI-MS). The formation of [M - H]⁺ ions in the ESI-MS was found to be independent on the mobile phase composition of the liquid chromatography and ESI source parameters. A series of hydralazine derivatives of aldehyde were investigated to confirm this phenomenon. The results showed that hydralazine derivatives of aldehydes that contained an sp³ hybridization carbon with a hydrogen at the α-position of aldehydes could form the unexpected [M - H]⁺ ions, whereas hydralazine derivative of acetone could only generate [M + H]⁺ ion in the ESI-MS. We proposed the possible formation mechanism of [M - H]⁺ ion for the hydralazine derivatives of aldehydes: the [M - H]⁺ ion was possibly formed by the loss a hydrogen molecule (H₂ ) from the protonated ion [M + H]⁺ . The results obtained from density functional theory (DFT) calculations supported this proposed formation mechanism of [M - H]⁺ ion.

A Versatile Integrated Ambient Ionization Source Platform

The pursuit of high-throughput sample analysis from complex matrix demands development of multiple ionization techniques with complementary specialties. A versatile integrated ambient ionization source (iAmIS) platform is proposed in this work, based on the idea of integrating multiple functions, enhancing the efficiency of current ionization techniques, extending the applications, and decreasing the cost of the instrument. The design of the iAmIS platform combines flowing atmospheric pressure afterglow (FAPA) source/direct analysis in real time (DART), dielectric barrier discharge ionization (DBDI)/low-temperature plasma (LTP), desorption electrospray ionization (DESI), and laser desorption (LD) technique. All individual and combined ionization modes can be easily attained by modulating parameters. In particular, the FAPA/DART&DESI mode can realize the detection of polar and nonpolar compounds at the same time with two different ionization mechanisms: proton transfer and charge transfer. The introduction of LD contributes to the mass spectrometry imaging and the surface-assisted laser desorption (SALDI) under ambient condition. Compared with other individual or multi-mode ion source, the iAmIS platform provides the flexibility of choosing different ionization modes, broadens the scope of the analyte detection, and facilitates the analysis of complex samples. Graphical abstract ᅟ.

Nanomaterials and continuous wave laser-based efficient desorption for atmospheric pressure mass spectrometric imaging of live hippocampal tissue slices

Micrometer-resolution mass spectrometric imaging of live hippocampal tissue is achieved with a highly efficient desorption of biomolecules using a 532 nm continuous wave laser and gold nanoparticles or graphene oxide as an energy transporter, which enables clear identification of the distributions of monoacylglycerol, adenine, cholesterol, sphingosine and ceramide.

Micrometer-resolution mass spectrometric imaging of live hippocampal tissue is achieved with a highly efficient desorption of biomolecules using a 532 nm continuous wave laser and gold nanoparticles or graphene oxide as an energy transporter.

Atmospheric pressure mass spectrometric imaging of live hippocampal tissue slices with subcellular spatial resolution

We report a high spatial resolution mass spectrometry (MS) system that allows us to image live hippocampal tissue slices under open-air atmospheric pressure (AP) and ambient temperature conditions at the subcellular level. The method is based on an efficient desorption process by femtosecond (fs) laser assisted with nanoparticles and a subsequent ionization step by applying nonthermal plasma, termed AP nanoparticle and plasma assisted laser desorption ionization (AP-nanoPALDI) MS method. Combining the AP-nanoPALDI with microscopic sample scanning, MS imaging with spatial resolution of 2.9 µm was obtained. The observed AP-nanoPALDI MS imaging clearly revealed the differences of molecular composition between the apical and basal dendrite regions of a hippocampal tissue. In addition, the AP-nanoPALDI MS imaging showed the decrease of cholesterol in hippocampus by treating with methyl β-cyclodextrin, which exemplifies the potential of AP-nanoPALDI for live tissue imaging for various biomedical applications without any chemical pretreatment and/or labeling process.

Ambient mass spectrometry-based approaches have found application in biology and medicine. Here the authors report a mass spectrometric imaging method (ambient nanoPALDI) for live hippocampal tissues, based on gold nanorodassisted femtosecond laser desorption and subsequent non-thermal plasma induced ionization.

Recent developments in atmospheric pressure photoionization-mass spectrometry

Recent developments in atmospheric pressure photoionization (APPI), which is one of the three most important ionization techniques in liquid chromatography-mass spectrometry, are reviewed. The emphasis is on the practical aspects of APPI analysis, its combination with different separation techniques, novel instrumental developments - especially in gas chromatography and ambient mass spectrometry - and the applications that have appeared in 2009-2014. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:423-449, 2017.

Applications of DART-MS for food quality and safety assurance in food supply chain

Direct analysis in real time (DART) represents a new generation of ion source which is used for rapid ionization of small molecules under ambient conditions. The combination of DART and various mass spectrometers allows analyzing multiple food samples with simple or no sample treatment, or in conjunction with prevailing protocolized sample preparation methods. Abundant applications by DART-MS have been reviewed in this paper. The DART-MS strategy applied to food supply chain (FSC), including production, processing, and storage and transportation, provides a comprehensive solution to various food components, contaminants, authenticity, and traceability. Additionally, typical applications available in food analysis by other ambient ionization mass spectrometers were summarized, and fundamentals mainly including mechanisms, devices, and parameters were discussed as well. © 2015 Wiley Periodicals, Inc. Mass Spec Rev. 36:161-187, 2017.

Laser-Induced Acoustic Desorption Atmospheric Pressure Photoionization via VUV-Generating Microplasmas

We demonstrate the first application of laser-induced acoustic desorption (LIAD) and atmospheric pressure photoionization (APPI) as a mass spectrometric method for detecting low-polarity organics. This was accomplished using a Lyman-α (10.2 eV) photon generating microhollow cathode discharge (MHCD) microplasma photon source in conjunction with the addition of a gas-phase molecular dopant. This combination provided a soft desorption and a relatively soft ionization technique. Selected compounds analyzed include α-tocopherol, perylene, cholesterol, phenanthrene, phylloquinone, and squalene. Detectable surface concentrations as low as a few pmol per spot sampled were achievable using test molecules. The combination of LIAD and APPI provided a soft desorption and ionization technique that can allow detection of labile, low-polarity, structurally complex molecules over a wide mass range with minimal fragmentation. Graphical Abstract ᅟ.

Analysis of nitrogen-based explosives with desorption atmospheric pressure photoionization mass spectrometry

RATIONALE

Fast methods that allow the in situ analysis of explosives from a variety of surfaces are needed in crime scene investigations and home-land security. Here, the feasibility of the ambient mass spectrometry technique desorption atmospheric pressure photoionization (DAPPI) in the analysis of the most common nitrogen-based explosives is studied.

METHODS

DAPPI and desorption electrospray ionization (DESI) were compared in the direct analysis of trinitrotoluene (TNT), trinitrophenol (picric acid), octogen (HMX), cyclonite (RDX), pentaerythritol tetranitrate (PETN), and nitroglycerin (NG). The effect of different additives in DAPPI dopant and in DESI spray solvent on the ionization efficiency was tested, as well as the suitability of DAPPI to detect explosives from a variety of surfaces.

RESULTS

The analytes showed ions only in negative ion mode. With negative DAPPI, TNT and picric acid formed deprotonated molecules with all dopant systems, while RDX, HMX, PETN and NG were ionized by adduct formation. The formation of adducts was enhanced by addition of chloroform, formic acid, acetic acid or nitric acid to the DAPPI dopant. DAPPI was more sensitive than DESI for TNT, while DESI was more sensitive for HMX and picric acid.

CONCLUSIONS

DAPPI could become an important method for the direct analysis of nitroaromatics from a variety of surfaces. For compounds that are thermally labile, or that have very low vapor pressure, however, DESI is better suited.

Analysis of neonicotinoids from plant material by desorption atmospheric pressure photoionization-mass spectrometry

RATIONALE

Neonicotinoids are widely used insecticides which have been shown to affect the memory and learning abilities of honey bees, and are suspected to play a part in the unexplainable, large-scale loss of honey bee colonies. Fast methods, such as ambient mass spectrometry (MS), for their analysis from a variety of matrices are necessary to control the use of forbidden products and study the spreading of insecticides in nature.

METHODS

The feasibilities of two ambient MS methods, desorption electrospray ionization (DESI) and desorption atmospheric pressure photoionization (DAPPI), for the analysis of five most used neonicotinoid compounds, thiacloprid, acetamiprid, clothianidin, imidacloprid and thiamethoxam, were tested. In addition, DAPPI was used to analyze fresh rose leaves treated with commercially available thiacloprid insecticide and dried and powdered turnip rape flowers, which had been collected from a field treated with thiacloprid-containing insecticide.

RESULTS

DAPPI was found to be more sensitive than DESI, with 2-11 times better signal-to-noise ratios, and limits of detection at 0.4-5.0 fmol for the standard compounds. DAPPI was able to detect thiacloprid from the rose leaves even 2.5 months after the treatment and from the turnip rape flower samples collected from a field. The analysis of plant material by DAPPI did not require extraction or other sample preparation.

CONCLUSIONS

DAPPI was found to be suitable for the fast and direct qualitative analysis of thiacloprid neonicotinoid from plant samples. It shows promise as a fast tool for screening of forbidden insecticides, or studying the distribution of insecticides in nature.

Localization of double bonds in triacylglycerols using high-performance liquid chromatography/atmospheric pressure chemical ionization ion-trap mass spectrometry

A method for localizing double bonds in triacylglycerols using high-performance liquid chromatography-tandem mass spectrometry with atmospheric pressure chemical ionization (APCI) was developed. The technique was based on collision-induced dissociation or pulsed Q collision-induced dissociation of the C3H5N(+•) adducts ([M + 55](+•)) formed in the presence of acetonitrile in the APCI source. The spectra were investigated using a large series of standards obtained from commercial sources and prepared by randomization. The fragmentation spectra made it possible to determine (i) the total number of carbons and double bonds in the molecule, (ii) the number of carbons and double bonds in acyls, (iii) the acyl in the sn-2 position on the glycerol backbone, and (iv) the double-bond positions in acyls. The double-bond positions were determined based on two types of fragments (alpha and omega ions) formed by cleavages of C-C bonds vinylic to the original double bond. The composition of the acyls and their positions on glycerol were established from the masses and intensities of the ions formed by the elimination of fatty acids from the [M + 55](+•) precursor. The method was applied for the analysis of triacylglycerols in olive oil and vernix caseosa.

The formation of [M-H]+ ions in N-alkyl-substituted thieno[3,4-c]-pyrrole-4,6-dione derivatives during atmospheric pressure photoionization mass spectrometry

RATIONALE

The formation of ions during atmospheric pressure photoionization (APPI) mass spectrometry in the positive mode usually provides radical cations and/or protonated species. Intriguingly, during the analysis of some N-alkyl-substituted thieno[3,4-c]pyrrole-4,6-dione (TPD) derivatives synthesized in our laboratory, unusual [M-H](+) ion peaks were observed. In this work we investigate the formation of [M-H](+) ions observed under APPI conditions.

METHODS

Multiple experimental parameters, including the type of ionization source, the composition of the solvent, the type of dopant, the infusion flow rate, and the length of the alkyl side chain were investigated to determine their effects on the formation of [M-H](+) ions. In addition, a comparison study of the gas-phase tandem mass spectrometric (MS/MS) fragmentation of [M + H](+) vs [M-H](+) ions and computational approaches were used.

RESULTS

[M-H](+) ions were observed under APPI conditions. The type of dopant and the length of the alkyl chain affected the formation of these ions. MS/MS fragmentation of [M-H](+) and [M + H](+) ions exhibited completely different patterns. Theoretical calculations revealed that the loss of hydrogen molecules from the [M + H](+) ions is the most favourable condition under which to form [M-H](+) ions.

CONCLUSIONS

[M-H](+) ions were detected in all the TPD derivatives studied here under the special experimental conditions during APPI, using a halogenated benzene dopant, and TPD containing substituted N-alkyl side chains with a minimum of four carbon atoms. Density functional theory calculations showed that for [M-H](+) ions to be formed under these conditions, the loss of hydrogen molecules from the [M + H](+)  ions is proposed to be necessary.

Ambient Analysis by Thermal Desorption Atmospheric-Pressure Photoionization

Among ambient ionization methods, thermal desorption-ionization stands out because of two attributes: (1) simplicity, rendering the technique suitable for in-field applications, and (2) ability to couple with a variety of gas-phase ionization methods, thereby broadening the range of molecules that can be analyzed with this method. In this chapter we will describe the use of atmospheric-pressure photoionization (APPI) for ambient analysis. The focus will be on uses as standalone sources, such as desorption-APPI (DAPPI), and combination with other ambient sources such as direct analysis in real time-APPI (DART-APPI). Generally, the direct photoionization and charge-transfer mechanisms offered by APPI in addition to a proton-transfer mechanism lead to a higher coverage of small molecules and better sensitivities using APPI and the combined sources. APPI has the advantage of ionizing over a wide range of polarities, in particular extending the range to classes of nonpolar compounds (e.g., PAHs, hydrocarbons, steroids, pesticides, etc.).

Desorption atmospheric pressure photoionization and direct analysis in real time coupled with travelling wave ion mobility mass spectrometry

RATIONALE

Ambient mass spectrometry (MS) is a tool for screening analytes directly from sample surfaces. However, background impurities may complicate the spectra and therefore fast separation techniques are needed. Here, we demonstrate the use of travelling wave ion mobility spectrometry in a comparative study of two ambient MS techniques.

METHODS

Desorption atmospheric pressure photoionization (DAPPI) and direct analysis in real time (DART) were coupled with travelling wave ion mobility mass spectrometry (TWIM-MS) for highly selective surface analysis. The ionization efficiencies of DAPPI and DART were compared. Test compounds were: bisphenol A, benzo[a]pyrene, ranitidine, cortisol and α-tocopherol. DAPPI-MS and DART-TWIM-MS were also applied to the analysis of chloroquine from dried blood spots, and α-tocopherol from almond surface, and DAPPI-TWIM-MS was applied to analysis of pharmaceuticals and multivitamin tablets.

RESULTS

DAPPI was approximately 100 times more sensitive than DART for bisphenol A and 10-20 times more sensitive for the other compounds. The limits of detection were between 30-290 and 330-8200 fmol for DAPPI and DART, respectively. Also, from the authentic samples, DAPPI ionized chloroquine and α-tocopherol more efficiently than DART. The mobility separation enabled the detection of species with low signal intensities, e.g. thiamine and cholecalciferol, in the DAPPI-TWIM-MS analysis of multivitamin tablets.

CONCLUSIONS

DAPPI ionized the studied compounds of interest more efficiently than DART. For both DAPPI and DART, the mobility separation prior to MS analysis reduced the amount of chemical noise in the mass spectrum and significantly increased the signal-to-noise ratio for the analytes.

The unexpected formation of [M - H]+ species during MALDI and dopant-free APPI MS analysis of novel antineoplastic curcumin analogues

Unusual ionization behavior was observed with novel antineoplastic curcumin analogues during the positive ion mode of matrix-assisted laser desorption ionization (MALDI) and dopant-free atmospheric pressure photoionization (APPI). The tested compounds produced an unusual significant peak designated as [M - H](+) ion along with the expected [M + H](+) species. In contrast, electrospray ionization, atmospheric pressure chemical ionization and the dopant-mediated APPI (dopant-APPI) showed only the expected [M + H](+) peak. The [M - H](+) ion was detected with all evaluated curcumin analogues including phosphoramidates, secondary amines, amides and mixed amines/amides. Our experiments revealed that photon energy triggers the ionization of the curcumin analogues even in the absence of any ionization enhancer such as matrix, solvent or dopant. The possible mechanisms for the formation of both [M - H](+) and [M + H](+) ions are discussed in this paper. In particular, three proposed mechanisms for the formation of [M - H](+) were evaluated. The first mechanism involves the loss of H2 from the protonated [M + H](+) species. The other two mechanisms include hydrogen transfer from the analyte radical cation or hydride abstraction from the neutral analyte molecule.

New MALDI matrices based on lithium salts for the analysis of hydrocarbons and wax esters

Lithium salts of organic aromatic acids (lithium benzoate, lithium salicylate, lithium vanillate, lithium 2,5-dimethoxybenzoate, lithium 2,5-dihydroxyterephthalate, lithium α-cyano-4-hydroxycinnamate and lithium sinapate) were synthesized and tested as potential matrices for the matrix-assisted laser desorption/ionization (MALDI)-mass spectrometry analysis of hydrocarbons and wax esters. The analytes were desorbed using nitrogen laser (337.1 nm) and ionized via the attachment of a lithium cation, yielding [M + Li](+) adducts. The sample preparation and the experimental conditions were optimized for each matrix using stearyl behenate and n-triacontane standards. The performance of the new matrices in terms of signal intensity and reproducibility, the mass range occupied by matrix ions and the laser power threshold were studied and compared with a previously recommended lithium 2,5-dihydroxybenzoate matrix (LiDHB) (Cvačka and Svatoš, Rapid Commun. Mass Spectrom. 2003, 17, 2203). Several of the new matrices performed better than LiDHB. Lithium vanillate offered a 2-3 times and 7-9 times higher signal for wax esters and hydrocarbons, respectively. Also, the signal reproducibility improved substantially, making this matrix a suitable candidate for imaging applications. In addition, the diffuse reflectance spectra and solubility of the synthesized compounds were investigated and discussed with respect to the compound's ability to serve as MALDI matrices. The applicability of selected matrices was tested on natural samples of wax esters and hydrocarbons.

Surface oxidation under ambient air--not only a fast and economical method to identify double bond positions in unsaturated lipids but also a reminder of proper lipid processing

A simple, fast approach elucidated carbon-carbon double bond positions in unsaturated lipids. Lipids were deposited onto various surfaces and the products from their oxidation in ambient air were observed by electrospray ionization (ESI) mass spectrometry (MS). The most common oxidative products, aldehydes, were detected as transformations at the cleaved double bond positions. Ozonides and carboxylic acids were generated in certain lipids. Investigations of the conditions controlling the appearance of these products indicated that the surface oxidation depends on light and ambient air. Since the lipid oxidation was slower in a high concentration of ozone, singlet oxygen appeared to be a parallel oxidant for unsaturated lipids. The 3-hydroxyl group in the sphingoid base of sulfatides offered some protection from oxidation for the Δ4,5-double bond, slowing its oxidation rate relative to that of the isolated double bond in the N-linked fatty acyl chain. Direct sampling by thin-layer chromatography (TLC)-ESI-MS provides a powerful approach to elucidate detailed structural information on biological samples. Co-localization of the starting lipids and their oxidation products after TLC separation allowed assignment of the native unsaturation sites. Phosphatidylserine and N,N-dimethyl phosphatidylethanolamine isomers in a bovine brain total lipid extract were distinguished on the basis of their oxidation products. Meanwhile, the findings reported herein reveal a potential pitfall in the assignment of structures to lipids extracted from TLC plates because of artifactual oxidation after the plate development.

The use of dicationic ion-pairing compounds to enhance the ambient detection of surface lipids in positive ionization mode using desorption electrospray ionisation mass spectrometry

RATIONALE

Lipids are typically analysed in negative ionisation mode in desorption electrospray ionisation mass spectrometry (DESI-MS), which can result in reduced sensitivity. In this study we examine the use of dicationic compounds as reactive DESI-MS agents to detect a range of lipid standards from the surface in positive ionisation mode.

METHODS

Nine dicationic compounds were tested for their ability to detect seven representative lipid species (palmitoleic acid, linoleic acid, phosphatidic acid (34:1), phosphoethanolamine (34:2), phosphatidylglycerol (34:1), phosphatidylserine (36:1), and phosphoinositol (34:2)) with a 2D DESI source on hydrophobic surfaces. Two different solvent systems (methanol/chloroform (1:1) and methanol) were tested with each dicationic compound, with the DESI-MS analysis performed in the positive ionisation mode.

RESULTS

Most of the dications tested were able to form stable ion-pairs with the negatively charged lipid species when analysed from the surface with DESI-MS, and were detected readily in positive ionisation electrospray mode as singly charged species. The optimal solvent system was found to be methanol. The dicationic compound [C6(C1Pyrr)2][Br]2 was found to enhance the detection of palmitoleic acid (638%), linoleic acid (304%) and phosphoethanolamine (269%) compared with the negative ionisation mode.

CONCLUSIONS

We demonstrate the first successful application of dicationic compounds in DESI-MS for the ambient surface detection of model lipids in positive electrospray ionisation mode. Dicationic compounds could potentially be used as reactive DESI-MS agents to improve the ambient detection of a number of negatively charged analytes.

Surface analysis of lipids by mass spectrometry: more than just imaging

Mass spectrometry is now an indispensable tool for lipid analysis and is arguably the driving force in the renaissance of lipid research. In its various forms, mass spectrometry is uniquely capable of resolving the extensive compositional and structural diversity of lipids in biological systems. Furthermore, it provides the ability to accurately quantify molecular-level changes in lipid populations associated with changes in metabolism and environment; bringing lipid science to the "omics" age. The recent explosion of mass spectrometry-based surface analysis techniques is fuelling further expansion of the lipidomics field. This is evidenced by the numerous papers published on the subject of mass spectrometric imaging of lipids in recent years. While imaging mass spectrometry provides new and exciting possibilities, it is but one of the many opportunities direct surface analysis offers the lipid researcher. In this review we describe the current state-of-the-art in the direct surface analysis of lipids with a focus on tissue sections, intact cells and thin-layer chromatography substrates. The suitability of these different approaches towards analysis of the major lipid classes along with their current and potential applications in the field of lipid analysis are evaluated.

Detection of layer-by-layer self-assembly multilayer films by low-temperature plasma mass spectrometry

The detection of layer-by-layer self-assembly multilayer films was carried out using low-temperature plasma (LTP) mass spectrometry (MS) under ambient conditions. These multilayer films have been prepared on quartz plates through the alternate assembling of oppositely charged 4-aminothiophenol (4-ATP) capped Au particles and thioglycolic acid (TGA) capped Ag particles. An LTP probe was used for direct desorption and ionization of chemical components on the films. Without the complicated sample preparation, the structure information of 4-ATP and TGA on films was studied by LTP-MS. Characteristic ions of 4-ATP (M) and TGA (F), including [M](+•), [M-NH(2)](+), [M-HCN-H](+), and [F + H](+), [F-H](+), [F-OH](+), [F-COOH](+) were recorded by LTP-MS on the films. However, [M-CS-H](+) and [F-SH](+) could not be observed on the film, which were detected in the neat sample. In addition, the semi-quantitative analysis of chemical components on monolayer film was carried out, and the amounts of 4-ATP and TGA on monolayer surface were 45 ng/mm(2) and 54 ng/mm(2), respectively. This resulted the ionization efficiencies of 72% for 4-ATP and 54% for TGA. In order to evaluate the reliability of present LTP-MS, the correlations between this approach and some traditional methods, such as UV-vis spectroscopy, atomic force microscope and X-ray photoelectron spectroscopy were studied, which resulted the correlation coefficients of higher than 0.9776. The results indicated that this technique can be used for analyzing the films without any pretreatment, which possesses great potential in the studies of self-assembly multilayer films.