A dopant for improved sensitivity in easy ambient sonic‐spray ionization mass spectrometry

Recently, 3‐nitrobenzonitrile (3‐NBN) has been used to improve sensitivity of sonic‐spray ionization mass spectrometry. Easy ambient sonic‐spray ionization (EASI) is one of the simplest, gentlest and most used spray‐based desorption/ionization ambient techniques, but limited sensitivity has been commonly taken as its major drawback. Herein we investigate the use of 3‐NBN as a dopant in EASI‐MS for improved sensitivity. Using a few typical EASI samples as test cases, the presence of 10 ppm (µg ml^?1) of 3‐NBN in the spray solvent showed two to fourfold gains in EASI‐MS sensitivity as measured both by total ion current and S/N ratios, accompanied with significant reductions in chemical noise. Sensitivity for DESI using 3‐NBN as a dopant also improved and dopant DESI versus dopant EASI sensitivities were compared. The use of solvent dopants seems therefore to be a promising strategy to improve sensitivity for spray‐based ambient MS techniques. Copyright © 2015 John Wiley & Sons, Ltd.     

Analysis of sexual assault evidence: statistical classification of condoms by ambient mass spectrometry

Desorption electrospray ionization mass spectrometry (DESI‐MS) and easy ambient sonic‐spray ionization mass spectrometry (EASI‐MS) are employed here in the forensic analysis of chemical compounds found in condoms and relative traces, and their analytical performances are compared. Statistical analysis of data obtained from mass spectra only was applied in order to obtain classification rules for distinguishing ten types of condoms. In particular, two supervised chemometric techniques [linear discriminant analysis (LDA) and soft independent modeling of class analogy (SIMCA)] were carried out on absolute and relative intensity values to test the performances of statistical models in terms of predictive capacity. The achieved classification of samples was excellent because of the high prediction percentages of the method used both for DESI and EASI mass spectrometry analyses, confirming these two as potential ambient ionization techniques for forensic analyses in case of sexual assault crimes. EASI‐MS showed 99% prediction ability for LDA using relative data and 100% prediction ability for SIMCA using both absolute and relative ones, while DESI showed 94% prediction ability for both LDA and SIMCA. The absence of any sample preparation technique gives advantages in terms of sample preservation and reduced contamination, allowing successive analyses to be performed on the same sample by other techniques. Copyright © 2015 John Wiley & Sons, Ltd.     

Desorption electrospray ionization and electrosonic spray ionization for solid- and solution-phase analysis of industrial polymers

Desorption electrospray ionization (DESI) and electrosonic spray ionization (ESSI), two new techniques, are used to measure average molecular weights and molecular weight distributions of solid-phase and solution-phase samples of the same polymers.     

Protein analysis by desorption electrospray ionization mass spectrometry and related methods

Desorption electrospray ionization mass spectrometry (DESI‐MS) requires little to no sample preparation and has been successfully applied to the study of biologically significant macromolecules such as proteins. However, DESI‐MS and other ambient methods that use spray desorption to process samples during ionization appear limited to smaller proteins with molecular masses of 25 kDa or less, and a decreasing instrumental response with increasing protein size has often been reported. It has been proposed that this limit results from the inability of some proteins to easily desorb from the surface during DESI sampling. The present study investigates the apparent mass dependence of the instrumental response observed during the DESI‐MS analysis of proteins using spray desorption collection and reflective electrospray ionization. Proteins, as large as 66 kDa, are shown to be quantitatively removed from surfaces by using spray desorption collection. However, incomplete dissolution and the formation of protein–protein and protein–contaminant clusters appear to be responsible for the mass‐dependent loss in sensitivity for protein analysis. Alternative ambient mass spectrometry approaches that address some of the problems encountered by spray desorption techniques for protein analysis are also discussed. Copyright © 2013 John Wiley & Sons, Ltd.     

DESI analysis of mammalian cell cultures – sample preparation and method optimisation

Desorption electrospray ion source (DESI) is widely used as an MS imaging technique. It is a rapid and convenient method of surface analysis, but to date, there are methodological obstacles to its application to the analysis of cell culture. This study reported optimised conditions for the analysis of cell culture samples. Parameters such as the surface, medium removal and sample desiccation techniques were assessed as a function of output data quality. Supercharging agents, surfactants and optimal parameters for the DESI ion source were evaluated for use in cell culture analyses. Data indicated that plastic dishes or sodium glass coated with poly‐l ‐lysine and washing cell cultures with 150 mM ammonium acetate followed by drying with inert gas were superior for DESI analyses. The addition of 1 μM surfactin to the DESI spray solvent significantly improved the results for negative and positive ion modes. Copyright © 2014 John Wiley & Sons, Ltd.     

Imaging with mass spectrometry: Which ionization technique is best?

The use of mass spectrometry (MS) to acquire molecular images of biological tissues and other substrates has developed into an indispensable analytical tool over the past 25 years. Imaging mass spectrometry technologies are widely used today to study the in situ spatial distributions for a variety of analytes. Early MS images were acquired using secondary ion mass spectrometry and matrix-assisted laser desorption/ionization. Researchers have also designed and developed other ionization techniques in recent years to probe surfaces and generate MS images, including desorption electrospray ionization (DESI), nanoDESI, laser ablation electrospray ionization, and infrared matrix-assisted laser desorption electrospray ionization. Investigators now have a plethora of ionization techniques to select from when performing imaging mass spectrometry experiments. This brief perspective will highlight the utility and relative figures of merit of these techniques within the context of their use in imaging mass spectrometry.     

Coupling desorption electrospray ionization with ion mobility/mass spectrometry for analysis of protein structure: evidence for desorption of folded and denatured States

A desorption electrospray ionization (DESI) source has been coupled to an ion mobility time-of-flight mass spectrometer for the analysis of proteins. Analysis of solid-phase horse heart cytochrome c and chicken egg white lysozyme proteins with different DESI solvents and conditions shows similar mass spectra and charge state distributions to those formed when using electrospray to analyze these proteins in solution. The ion mobility data show evidence for compact ion structures [when the surface is exposed to a spray that favors retention of "nativelike" structures (50:50 water:methanol)] or elongated structures [when the surface is exposed to a spray that favors "denatured" structures (49:49:2 water:methanol:acetic acid)]. The results suggest that the DESI experiment is somewhat gentler than ESI and under appropriate conditions, it is possible to preserve structural information throughout the DESI process. Mechanisms that are consistent with these results are discussed.     

Combined Fourier-transform infrared imaging and desorption electrospray-ionization linear ion-trap mass spectrometry for analysis of counterfeit antimalarial tablets

This paper reports use of a combination of Fourier-transform infrared (FTIR) spectroscopic imaging and desorption electrospray ionization linear ion-trap mass spectrometry (DESI MS) for characterization of counterfeit pharmaceutical tablets. The counterfeit artesunate antimalarial tablets were analyzed by both techniques. The results obtained revealed the ability of FTIR imaging in non-destructive micro-attenuated total reflection (ATR) mode to detect the distribution of all components in the tablet, the identities of which were confirmed by DESI MS. Chemical images of the tablets were obtained with high spatial resolution. The FTIR spectroscopic imaging method affords inherent chemical specificity with rapid acquisition of data. DESI MS enables high-sensitivity detection of trace organic compounds. Combination of these two orthogonal surface-characterization methods has great potential for detection and analysis of counterfeit tablets in the open air and without sample preparation.     

Desorption electrospray ionization tissue imaging using heated nebulizing gas and high‐resolution accurate mass spectra

A new method for tissue imaging using desorption electrospray ionization (DESI) mass spectrometry is described. The technique utilizes a DESI source with a heated nebulizing gas and high‐resolution accurate mass data acquired with an LTQ‐Orbitrap mass spectrometer. The two‐dimensional (2D) automated DESI ion source creates images using the ions that are collected under high‐resolution conditions. The use of high‐resolution mass detection significantly improves the image quality due to exclusion of interfering ions. The use of a heated nebulizing gas increases the signal intensity observed at lower gas pressure. The technique developed is highly compatible with soft tissue imaging due to the minimal surface destruction. Copyright © 2010 John Wiley & Sons, Ltd.     

Characterization of barley leaf tissue using direct and indirect desorption electrospray ionization imaging mass spectrometry

Chemical profiling of barley (Hordeum vulgare) leaves was demonstrated using direct and indirect desorption electrospray ionization (DESI) imaging mass spectrometry. Direct DESI analysis of the untreated leaves was not possible despite a significant content of hydroxynitrile glucosides known to reside in the epidermis of the leaves. Instead, the epidermis was stripped off the leaves, thus allowing direct DESI imaging to be performed on the back of the epidermis. Furthermore, indirect DESI imaging was performed by making imprints in porous Teflon of the intact leaves as well as of the stripped epidermis. The DESI images reveal accumulation of hydroxynitrile glucosides in the leaf epidermis, homogeneously distributed throughout the surface. The indirect DESI approach enables relative quantitation, confirming variations of hydroxynitrile glucosides content in primary leaves of three different cultivars of barley seedlings. The study presents an example of how to overcome the morphological barriers from the plant surface and perform rapid and repeatable DESI imaging. In addition, a comparison is made of direct and indirect DESI imaging, contributing to the characterization of the recently developed method of indirect DESI imaging of plant material via porous Teflon imprints.     

Desorption electrospray ionization mass spectrometry of DNA nucleobases: implications for a liquid film model

Desorption electrospray ionization (DESI) mass spectrometry has been implemented on a commercial ion‐trap mass spectrometer and used to optimize mass spectrometric conditions for DNA nucleobases: adenine, cytosine, thymine, and guanine. Experimental parameters including spray voltage, distance between mass spectrometer inlet and the sampled spot, and nebulizing gas inlet pressure were optimized. Cluster ions including some magic number clusters of nucleobases were observed for the first time using DESI mass spectrometry. The formation of the cluster species was found to vary with the nucleobases, acidification of the spray solvent, and the deposited sample amount. All the experimental results can be explained well using a liquid film model based on the two‐step droplet pick‐up mechanism. It is further suggested that solubility of the analytes in the spray solvent is an important factor to consider for their studies by using DESI. Copyright © 2009 John Wiley & Sons, Ltd.     

Characterization of DESI-FTICR mass spectrometry - from ECD to accurate mass tissue analysis

Implementation of desorption electrospray ionization (DESI) technique on a 9.4 T Fourier transform ion cyclotron resonance (FTICR) mass spectrometer is described. Desorption electrospray technique is capable of the direct investigation of natural samples without any need for sample preparation or chromatographic separation. Since the DESI mass spectra of natural samples are very complex owing to the lack of preseparation or cleanup, the ideal mass spectrometric analyzer for these applications is a high-resolution instrument such as FTICR mass spectrometer. DESI was implemented by constructing an electronically controlled source framework comprising six linear moving stages and one rotating stage. A three-dimensional linear stage was used to accommodate samples, while another 3D linear stage equipped with rotating stage was used as a spray mount. A modified electrosonic sprayer was used as a primary electrospray device. DESI-FTICR setup was characterized with regard to geometrical, electrical and flow conditions using deposited peptide samples in range of 1-100 pmol gross deposited amount on glass and polymer surfaces. Optimized conditions enabled the routine acquisition of DESI-MS spectra on the instrument at 130 000 resolution in the broadband mode and with comparable sensitivity to data reported in the literature. Since the main significance of DESI-FTICR MS is the combination of intact tissue analysis, the capabilities of the technique were demonstrated by analyzing murine liver samples. Presence of lysophospholipids in the liver tissue was tentatively associated with the lipid metabolism taking place in liver. DESI-FTICR is also a promising technique in the field of peptide analysis due to capability of top-down sequencing using electron capture dissociation. As a proof-of-principle experiment, a small synthetic polypeptide containing 36 amino acids was ionized using DESI and was sequenced in the FTICR by means of ECD (electron capture dissociation) fragmentation. Spectra gave almost full sequence information in agreement with the known amino acid sequence of the species.     

Design and performance of air flow-assisted ionization imaging mass spectrometry system

The imaging mass spectrometry（IMS） technology has experienced a rapid development in recent years.A new IMS technology which is based on air flow assisted ionization（AFAI） was reported.It allows for the convenient pretreatment of the samples and can image a large area of sample in a single measurement with high sensitivity.The AFAI in DESI mode was used as the ion source in this paper.The new IMS method is named AFADESI-IMS.The adoption of assisted air flow makes the sample pretreatment easy and convenient.An optimization of the distance between the ion transport tube and MS orifice increases the sensitivity of the system.For data processing,a program based on MATLAB with the function of numerical analysis was developed.A theoretical imaging resolution of a few hundred microns can be achieved.The composite AFAI-IMS images of different target analytes were imaged with high sensitivity.A typical AFAI-IMS image of the whole-body section of a rat was obtained in a single analytical measurement.The ability to image a large area for relevant samples in a single measurement with high sensitivity and repeatability is a significant advantage.The method has enormous potentials in the MS imaging of large and complicated samples.     

Internal energy distributions in desorption electrospray ionization (DESI)

The internal energy distributions of typical ions generated by desorption electrospray ionization (DESI) were measured using the "survival yield" method, and compared with corresponding data for electrospray ionization (ESI) and electrosonic spray ionization (ESSI). The results show that the three ionization methods produce populations of ions having internal energy distributions of similar shapes and mean values (1.7-1.9 eV) suggesting similar phenomena, at least in the later stages of the process leading from solvated droplets to gas-phase ions. These data on energetics are consistent with the view that DESI involves "droplet pick-up" (liquid-liquid extraction) followed by ESI-like desolvation and gas-phase ion formation. The effects of various experimental parameters on the degree of fragmentation of p-methoxy-benzylpyridinium ions were compared between DESI and ESSI. The results show similar trends in the survival yields as a function of the nebulizing gas pressure, solvent flow rate, and distance from the sprayer tip to the MS inlet. These observations are consistent with the mechanism noted above and they also enable the user to exercise control over the energetics of the DESI ionization process, through manipulation of external and internal ion source parameters.     

Ambient analysis of saturated hydrocarbons using discharge-induced oxidation in desorption electrospray ionization

Saturated nonfunctionalized hydrocarbons can be oxidized in situ by initiating an electrical discharge during desorption electrospray ionization (DESI) to generate the corresponding alchohols and ketones. This form of reactive DESI experiment can be utilized as an in situ derivatization method for rapid and direct analysis of alkanes at atmospheric pressure without sample preparation. Betaine aldehyde was incorporated into the DESI spray solution to improve the sensitivity of detecting the long-chain alcohol oxidation products. The limit of detection for alkanes (C₁₅H₃₂ to C₃₀H₆₂) from pure samples is ∼20 ng. Multiple oxidations and dehydrogenations occurred during the DESI discharge, but no hydrocarbon fragmentation was observed, even for highly branched squalane. Using exact mass measurements, the technique was successfully implemented for analysis of petroleum distillates containing saturated hydrocarbons.     

Enhanced detection of olefins using ambient ionization mass spectrometry: Ag<Superscript>+</Superscript> adducts of biologically relevant alkenes

Spray solvent doped with silver ions increases the ease of olefin detection by desorption electrospray ionization (DESI). Characteristic silver adducts were generated in up to 50 times greater abundance when compared to conventional DESI spray solvents for the biologically significant olefin, arachidonic acid, in the positive ion mode. In the analysis of 26 lipids, silver adduct formation was highly favorable for fatty acids, fatty acid esters and prostaglandins but not applicable to some other classes (e.g., polar lipids such as ceramide and its derivative cerebroside sulfate). An investigation exploring competitive Ag⁺ cationization with a mixture of components demonstrated that polyunsaturated compounds form Ag⁺ adducts most readily. Silver cationization allowed the distinction between three sets of isomers in the course of multiple-stage collision-induced dissociation, so providing insight into the location of the olefin bonds. A silver ion-doped solvent was used in DESI imaging of normal and tumor canine bladder tissue sections. The Ag⁺ fatty acid adducts permitted post facto differentiation between the normal and tumor regions. In addition, silver adduct formation in the course of DESI imaging of tissue sections revealed the presence of triacylglycerides, a class of compounds not previously identified through DESI imaging. A simple silver nitrate spray solvent has the potential to further improve DESI analysis of unsaturated biomolecules and other molecules containing π-bonds through selective silver cationization.     

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.     

Fabric softeners: nearly instantaneous characterization and quality control of cationic surfactants by easy ambient sonic‐spray ionization mass spectrometry

A tiny droplet of typical samples of fabric softeners from different commercial brands placed on a smooth paper surface was subjected to easy ambient sonic‐spray ionization mass spectrometry (EASI‐MS). With no need for sample‐preparation or pre‐separation procedures, EASI‐MS and EASI‐MS/MS identify nearly instantaneously the main surfactants and the homologous series employed in their formulations. Adulterated and low quality samples containing no or less efficient softeners are also easily recognized. Copyright © 2009 John Wiley & Sons, Ltd.     

Recent advances of ambient ionization mass spectrometry imaging in clinical research

The structural information and spatial distribution of molecules in biological tissues are closely related to the potential molecular mechanisms of disease origin, transfer, and classification. Ambient ionization mass spectrometry imaging is an effective tool that provides molecular images while describing in situ information of biomolecules in complex samples, in which ionization occurs at atmospheric pressure with the samples being analyzed in the native state. Ambient ionization mass spectrometry imaging can directly analyze tissue samples at a fairly high resolution to obtain molecules in situ information on the tissue surface to identify pathological features associated with a disease, resulting in the wide applications in pharmacy, food science, botanical research, and especially clinical research. Herein, novel ambient ionization techniques, such as techniques based on spray and solid‐liquid extraction, techniques based on plasma desorption, techniques based on laser desorption ablation, and techniques based on acoustic desorption were introduced, and the data processing of ambient ionization mass spectrometry imaging was briefly reviewed. Besides, we also highlight recent applications of this imaging technology in clinical researches and discuss the challenges in this imaging technology and the perspectives on the future of the clinical research.